GERTA KELLER PUBLICATIONS

End-Cretaceous Mass Extinction, Climate & Sea Level Changes, and the Chicxulub Impact

Mass wasting and hiatuses during the cretaceous-tertiary transition in the north atlantic: Relationship to the chicxulub impact?

TitleMass wasting and hiatuses during the cretaceous-tertiary transition in the north atlantic: Relationship to the chicxulub impact?
Publication TypeJournal Article
Year of Publication2015
AuthorsMateo, P, Keller, G, Adatte, T, Spangenberg, JE
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Date PublishedJan-02-2015
ISSN00310182
KeywordsChicxulub impact spherules, early Danian, hiatus, KTB, Late Maastrichtian, mass wasting
Abstract

Deep-sea sections in the North Atlantic are claimed to contain the most complete sedimentary records and ultimate proof that the Chicxulub impact is Cretaceous-Tertiary boundary (KTB) in age and caused the mass extinction. A multi-disciplinary study of North Atlantic DSDP Sites 384, 386 and 398, based on high-resolution planktonic foraminiferal biostratigraphy, carbon and oxygen stable isotopes, clay and whole-rock mineralogy and granulometry reveals the age, stratigraphic completeness and nature of sedimentary disturbances. Results show a major hiatus across the KTB at Site 384 with Zones CF1, P0 and P1a missing, spanning at least ~ 540 ky, similar to other North Atlantic and Caribbean localities associated with tectonic activity and Gulf Stream erosion. At Sites 386 and 398, discrete intervals of disturbed sediments with mm-to-cm-thick spherule layers have previously been interpreted as the result of impact-generated earthquakes at the KTB destabilizing continental margins prior to settling of impact spherules. However, improved age control based on planktonic foraminifera indicates spherule deposition in the early Danian Zone P1a(2) (upper Parvularugoglobigerina eugubina Zone) more than 100 ky after the KTB. At Site 386, two intervals of white chalk contain very small (< 63 μm) early Danian Zone P1a(2) assemblages (65%) and common reworked Cretaceous (35%) species. In contrast, the in situ red-brown and green abyssal clays of this core are devoid of carbonate. In addition, high calcite, mica and kaolinite and upward-fining are observed in the chalks, indicating downslope transport from shallow waters and sediment winnowing via distal turbidites. At Site 398, convoluted red to tan sediments with early Danian and reworked Cretaceous species represent slumping of shallow water sediments as suggested by dominance of mica and low smectite compared to in situ deposition. We conclude that mass wasting was likely the result of earthquakes associated with increased tectonic activity in the Caribbean and the Iberian Peninsula during the early Danian well after the Chicxulub impact.

URLhttp://www.sciencedirect.com/science/article/pii/S0031018215000309
DOI10.1016/j.palaeo.2015.01.019
Short TitlePalaeogeography, Palaeoclimatology, Palaeoecology

U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction

TitleU-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction
Publication TypeJournal Article
Year of Publication2014
AuthorsSchoene, B, Samperton, KM, Eddy, MP, Keller, G, Adatte, T, Bowring, SA, Khadri, SFR, Gertsch, B
JournalScience
Date PublishedNov-12-2014
ISSN0036-8075
Abstract

The Chicxulub asteroid impact (Mexico) and the eruption of the massive Deccan volcanic province (India) are two proposed causes of the end-Cretaceous mass extinction, which includes the demise of nonavian dinosaurs. Despite widespread acceptance of the impact hypothesis, the lack of a high-resolution eruption timeline for the Deccan basalts has prevented full assessment of their relationship to the mass extinction. Here we apply U-Pb zircon geochronology to Deccan rocks and show that the main phase of eruptions initiated ~250,000 years before the Cretaceous-Paleogene boundary and that >1.1 million km3 of basalt erupted in ~750,000 years. Our results are consistent with the hypothesis that the Deccan Traps contributed to the latest Cretaceous environmental change and biologic turnover that culminated in the marine and terrestrial mass extinctions.   PDF

URLhttp://www.sciencemag.org/cgi/doi/10.1126/science.aaa0118
DOI10.1126/science.aaa0118
Short TitleScience

Chicxulub impact spherules in the North Atlantic and Caribbean: age constraints and Cretaceous–Tertiary boundary hiatus

TitleChicxulub impact spherules in the North Atlantic and Caribbean: age constraints and Cretaceous–Tertiary boundary hiatus
Publication TypeJournal Article
Year of Publication2013
AuthorsKeller, G, Khozyem, H, Adatte, T, Malarkodi, N, Spangenberg, J, Stinnesbeck, W
JournalGeological Magazine
Volume150
Issue05
Pagination885 - 907
Date Published01/2013
ISSN0016-7568
KeywordsChicxulub, impact spherules, KTB unconformity, North Atlantic
Abstract

The Chicxulub impact is commonly believed to have caused the Cretaceous-Tertiary boundary (KTB) mass extinction and a thin impact spherule layer in the North Atlantic and Caribbean is frequently cited as proof. We evaluated this claim in the seven best North Atlantic and Caribbean KTB sequences based on high-resolution biostratigraphy, quantitative faunal analyses and stable isotopes. Results reveal a major KTB unconformity spans most of Danian subzone P1a(1) and Maastrichtian zones CF1-CF2 (~400 kyr) in the NW Atlantic Bass River core, ODP Sites 1049A, 1049C and 1050C. In the Caribbean ODP Sites 999B and 1001B the unconformity spans from the early Danian zone P1a(1) through zones CF1-CF4  (~3 myr). Only in the Demerara Rise ODP Site 1259B is erosion relatively minor and restricted to the earliest Danian zone P0 and most of subzone P1a(1) (~150 kyr). In all sites examined Chicxulub impact spherules are apparently reworked into the early Danian subzone P1a(1) about 150-200 kyr after the mass extinction. A similar pattern of erosion and redeposition of impact spherules in Danian sediments has previously been documented from Cuba, Haiti, Belize, Guatemala, south and central Mexico. This pattern can be explained by intensified Gulf stream circulation at times of climate cooling and sea level changes. The age of the Chicxulub impact cannot be determined from these reworked impact spherule layers, but can be evaluated based on the stratigraphically oldest spherule layer in NE Mexico and Texas, which indicates this impact predates the KTB by about 130-150 kyr.  PDF

URLhttp://www.journals.cambridge.org/abstract_S0016756812001069
DOI10.1017/S0016756812001069
Short TitleGeol. Mag.

The Cretaceous-Tertiary boundary (KTB) transition in NE Brazil

TitleThe Cretaceous-Tertiary boundary (KTB) transition in NE Brazil
Publication TypeJournal Article
Year of Publication2013
AuthorsGertsch, B, Keller, G, Adatte, T, Berner, Z
JournalJournal of the Geological Society
Volume170
Issue2
Pagination249 - 262
Date Published01/2013
ISSN0016-7649
Abstract

The Cretaceous-Tertiary boundary (KTB) transition of the Poty Quarry near Recife, NE Brazil, is the most distant locality (7800 km from Yucatan) with reported Chicxulub impact-tsunami deposits, impact spherules and Ir anomaly. Investigations based on sedimentology, biostratigraphy, mineralogy, stable isotopes and elemental geochemistry failed to confirm these reports. The KTB is at an unconformity marked by erosion and bioturbation. Latest Maastrichtian planktic foraminiferal zones CF1 below the unconformity and early Danian zone P1a(1) above indicates a short hiatus with the KTB clay (zone P0), Ir anomaly and characteristic negative d13C excursion missing. Above the unconformity is an upward-fining micro-conglomerate with abundant reworked Cretaceous foraminifera, sub-angular phosphate clasts, calcitic and phosphatic spheroids along with an early Danian zone P1a(1) assemblage.

This deposit has previously been interpreted as impact-tsunami with impact spherules. However, the spheroids are common throughout the late Campanian-Maastrichtian and appear to be chamber infillings of the benthic foraminifer Dentalina alternata. The unconformity coincides with the latest Maastrichtian sea level fall, which is widely recognized globally. The upward fining micro-conglomerate is likely a gravity-flow deposit associated with the early Danian sea level rise. Two minor Ir anomalies (<0.7 ppb) in thin clay layers of zone Pla are unrelated to the Chicxulub impact. Although there is no evidence of the Chicxulub impact in the Poty Quarry, this section remains a very important distant example of the complex global environmental and sea level changes, including gravity flows, observed in KT sequences from North America through Mexico, Central America and the Caribbean and commonly misinterpreted as impact tsunami event.  PDF

URLhttp://jgs.geoscienceworld.org/cgi/doi/10.1144/jgs2012-029
DOI10.1144/jgs2012-029
Short TitleJournal of the Geological Society

Volcanism, impacts and mass extinctions (Long Version)

TitleVolcanism, impacts and mass extinctions (Long Version)
Publication TypeWeb Article
Year of Publication2012
AuthorsKeller, G, Armstrong, H, Courtillot, V, Harper, D, Joachimski, M, Kerr, A, MacLeod, N, Napier, W, Palfy, J, Wignall, P
Access Year2012
Access DateNovember
PublisherThe Geological Society
CityLondon
Type of MediumOnline version
Keywordsclimate change, Impacts, Mass extinctions, ocean anoxia, Phanerozoic, Volcanism
Abstract

The nature and causes of mass extinctions in the geological past have remained topics of intense scientific debate for the past three decades.  Central to this debate is the question of whether one or several large bolide impacts, the eruption of large igneous provinces (LIPS), or a combination of the two were the primary mechanism(s) driving the environmental and habitat changes that are universally regarded as the proximate causes for four of the five major extinction events.  Recent years have seen a revolution in our understanding of both the interplanetary environment and LIPS eruptions and their environmental effects such that the current impact-kill scenario no longer seems adequate for the KT (KPg) or any other mass extinction.  Massive sequential volcanic eruptions and the breakup of giant comets leading to rapid climate change, ocean anoxia and ozone destruction emerge as the leading causes in major mass extinctions.  PDF

URLhttp://www.geolsoc.org.uk/Geoscientist/Archive/November-2012/Volcanism-impacts-and-mass-extinctions-2
Alternate TitleVolcanism, impacts and mass extinctions (long version)

The Cretaceous-Tertiary Mass Extinction: Theories and Controversies

TitleThe Cretaceous-Tertiary Mass Extinction: Theories and Controversies
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G
Book TitleEnd-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSpecial Publication
Pagination7-22
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
Abstract

The Cretaceous-Tertiary (KT) mass extinction is primarily known for the demise of the dinosaurs, the Chicxulub impact and the frequently rancorous 30 years old controversy over the cause of this mass extinction. Since 1980 the impact hypothesis has steadily gained support that culminated in 1990 with the discovery of the Chicxulub crater on Yucatan as the KT impact site and ‘smoking gun’ that proved this hypothesis. In a perverse twist of fate this discovery also began the decline of this hypothesis because for the first time it could be tested directly based on the impact crater and impact ejecta in sediments throughout the Caribbean, Central and North America. Two decades of multi-disciplinary studies amassed a database with a sum total that overwhelmingly reveals the Chicxulub impact as predating the KT mass extinction in the impact crater cores, in sections throughout NE Mexico and in Brazos River sections of Texas. This chapter recounts the highlights of the KT controversy, the discovery of facts inconsistent with the impact hypothesis and the resurgence of the Deccan volcanism hypothesis as the most likely cause for the mass extinction.  PDF

Age and Origin of the Chicxulub Impact and Sandstone Complex, Brazos River, Texas: Evidence from Lithostratigraphy and Sedimentilogy

TitleAge and Origin of the Chicxulub Impact and Sandstone Complex, Brazos River, Texas: Evidence from Lithostratigraphy and Sedimentilogy
Publication TypeBook Chapter
Year of Publication2011
AuthorsAdatte, T, Keller, G, Baum, G
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination43-80
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
Abstract

Multidisciplinary investigations based on the lithology, sedimentology, mineralogy and biostratigraphy of upper Maastrichtian to lower Danian boundary (KTB) sequences along 3.5 km of the Brazos River in Falls County, Texas, reveal depositonal sequences, including an impact spherule-rich sandstone complex, characteristic of sequence stratigraphic models applied to shallow shelf areas, such as incised valleys, lag conglomerate, storm deposits and repeated bioturbation. The top of the Corsicana Formation coincides with a channel, which we interpret as incised valley. The erosion surface marks a major depositional sequence boundary (SB) associated with the latest Maastrichtian sea-level fall. Initial channel deposits consist of coarse shelly glauconitic sand with large lithified clasts containing impact spherules and large bored and encrusted phosphatized concretions, which we interpret to indicate that the Chicxulub impact occurred well prior to the lithification, erosion and redeposition at the base of the channel. The primary Chicxulub ejecta layer lies about 40-65 cm below the sandstone complex in a 3 cm-thick yellow clay layer that consists of cheto smectitie (altered impact glass) interbedded in claystones of the Corsicana Formation. Above the sandstone complex, claystones and mudstones are burrowed and correspond to a condensed interval interpreted as a maximum flooding surface (MFS).Based on biostratigraphy and the δ13C shift, the KT boundary is up to 1 m (50-100 ky) above the sandstone complex and coincides with increased sediment accumulation during the early Danian sea level rise (HST). No impact signals are observed in KT deposits. These features are inconsistent with a single catastrophic bolide impact on Yucatan and associated mega-tsunami deposition as commonly interpreted.

 The biostratigraphy and KT characteristic δ13C shift of the Brazos sections indicate that the KTB, sandstone complex and the Chicxulub impact occurrred as three different stratigraphic events during the late Maastrichtian planktic foraminiferal zone CF1. These are represented by : 1) the Chicxulub impact sequence deposited about 200-300 ky prior to the KTB ;  2) the sandstone complex with reworked impact spherules deposited in incised valleys during the latest sea level fall about 100-150 ky prior to the KTB, and 3) the KTB event during the subsequent HST and following the condensed MFS.   PDF

Maastrichtian Planktic Foraminiferal Biostratigraphy and Paleoenvironment of Brazos River, Falls County, Texas, U.S.A.

TitleMaastrichtian Planktic Foraminiferal Biostratigraphy and Paleoenvironment of Brazos River, Falls County, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsAbramovich, S, Keller, G, Berner, Z, Cymbalista, M
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM Special Publication
Pagination123-156
ISBN978-1-56576-308-1
Abstract

Investigation of late Maastrichtian faunal and environmental changes in three subsurface wells spanning over 3 km along the Brazos River, Texas, reveals similar minimum diversity high-stress assemblages associated with shallow shelf conditions. Upper Maastrichtian sediments recovered span planktic foraminiferal (CF) zones CF1 to CF4 in well Mullinax-1 to the north and well KT3 at Cottonmouth Creek, and zones CF1-CF2 in Mullinax-3 at Darting Minnow Creek. Biotic stress conditions are demonstrated by the minimum species richness, near exclusion of larger specialized species, dwarfing, and dominance of small generalist taxa.  Faunal assemblages dominated by alternating abundances of the disaster opportunist Guembelitria cretacea (Cushman) (particularly in zones CF4 and CF2-CF1) and heterohelicid species [e.g., Heterohelix globulosa (Ehrenberg), H. planata (Cushman), Paraspiroplecta navarroensis (Loeblich)].. Other small surface and subsurface mixed layer dwellers are rare to common (e.g., hedbergellids, globigerinellids, pseudoguembelinids).

The coincidence of Guembelitria blooms with lithological changes and oxygen and carbon stable isotope excursions may represent discrete episodes of freshwater runoff related to short -term pulses of the latest Maastrichtian (zone CF1-CF2) global climate warming. Climate warming ended in the upper part of zone CF1 with the return to a cooler climate, lower sea level and the formation of incised valleys in a coastal-lagoonal environment. With the subsequent early transgression, incised valleys were infilled by a sandstone complex with reworked impact spherules, as well as lithified clasts with impact spherules up to 80 cm below the K-Pg Boundary. In this environment the shallow inner neritic setting superimposed by sea level and climate changes is the most probable cause for the observed high biotic stress conditions preceding the K-Pg Boundary in the Brazos area.  PDF

Platinum Group Element (PGE) Geochemistry of Brazos Sections, Texas, U.S.A.

TitlePlatinum Group Element (PGE) Geochemistry of Brazos Sections, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsGertsch, B, Keller, G, Adatte, T, Bartels, D
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination227-249
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsBrazos River, Iridium, K-T boundary, PGE, Texas
Abstract

Geochemical and sedimentological analyses of Platinum Group Element (PGE) patterns across the Cretaceous/Tertiary (K-T) transition of 8 sections along the Brazos River, Texas, reveal possible sources and processes responsible for PGE enrichments. Of the five global KT boundary-defining characteristics (mass extinction in planktic foraminifera, first appearance of Danian species, negative δ13C excursion, Ir anomaly, thin (0.5 cm) red clay layer), the Ir anomaly and red clay layer are not present at the KTB in the Brazos sections. Instead, PGEs and especially Ir show several minor enrichments within the sandstone complex with the largest peak at the top or just above it. Possible mechanisms of PGE enrichments include, low sedimentation rates or sediment starvation that concentrates Ir and other PGEs. Absence of Ir at the KTB is likely linked to dilution effects caused by high sedimentation rates, and other still unknown processes. The source of PGEs remains elusive, but may be linked to an increased input of extraterrestrial dust during the late Maastrichtian, or reworked PGEs from the Chicxulub impact that predates the KTB in these sections.  PDF

Trace-Element Geochemistry of Brazos Sections, Texas, U.S.A.

TitleTrace-Element Geochemistry of Brazos Sections, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsGertsch, B, Keller, G, Adatte, T, Bartels, D
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination251-279
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsBrazos, K-T boundary, Texas, Trace Elements
Abstract

A geochemical study of major (MEs) and trace (TEs) elements across the Cretaceous-Tertiary (KT) transition was carried out on 2 sections of the Brazos area to investigate signals of the Chicxulub impact, as well as redox conditions and weathering processes in these shallow-water environments. Results show that ME and TE patterns are primarily affected by the type of lithology, including claystones, mudstones, a 3 cm thick yellow clay layer, a sandstone complex with 2-3 spherule-rich layers and alternating hummocky cross-bedded and laminated sandstone layers. In the yellow clay layer, TEs (Mn, Ni, Cr, Na) concentrations are comparable to the spherules-rich layers and geochemistry of spherules. Relative abundance of MEs (Al, Ca, Fe, Mg) in both spherule-rich layers and the yellow clay layer shows also a good correlation. This indicates that the yellow clay layer is likely an alteration product of a spherule-rich layer. No other evidence of the Chicxulub impact could be determined based on MEs and TEs values. In the claystone/mudstone intervals, TE concentrations are constant and slightly enriched in redox-sensitive TE (Mo, U, V), which indicates that dysoxic conditions prevailed. Occurrence of rare large pyrite framboids (30-50 μm) below the sandstone complex confirms that redox conditions were dysoxic in the shallow-water Brazos environments. High values for Al and weathering indices show high detrital input dominated by chemical weathering. These results reveal that persistent high stress conditions and high continental runoff prevailed through the late Maastrichtian-early Danian transition. No significant geochemical and environmental change due to the Chicxulub impact is detected. Sudden increases in trace elements (Co, Cr, Ni) possibly related to an impact, are observed only in reworked intervals within the sandstone complex.  PDF

The Sandstone Complex in the Brazos Riverbed Section: Geochemical Contraints on Genesis and Depositional Conditions

TitleThe Sandstone Complex in the Brazos Riverbed Section: Geochemical Contraints on Genesis and Depositional Conditions
Publication TypeBook Chapter
Year of Publication2011
AuthorsMunsel, D, Berner, Z, Stuben, D
Book TitleSEPM, Special Publication
VolumeNo. 100
Pagination281-295
ISBN978-1-56576-308-1
Abstract

The origin and deposition of spherule-bearing, dominantly sandy beds in a sandstone complex (also called ‘‘event deposit’’) below the biostratigraphic Cretaceous–Tertiary (KT) boundary plays a key role in models linking the KT mass extinction to the Chicxulub impact. This study, which focuses on the chemostratigraphy of this complex exposed in a ca. 60-cm-thick succession along the Brazos River, Falls County, Texas, U.S.A., aims to constrain the source of the material as well as the depositional conditions and postdepositional history of this highly controversial stratigraphic unit. Major and trace elements, as well as the isotopic composition of the Ca carbonate, contrast sharply with the underlying Corsicana Formation, indicating a dramatic change in the source of material and depositional conditions. Evaluation of geochemical data by principal-component analysis permits identification of (1) siliciclastic components, (2) ejecta material, consisting of altered impact-glass spherules, and (3) Ca carbonate. The ejecta material, originally represented chiefly by glass spherules with carbonate infill, is strongly altered to clay minerals with dominantly smectitic composition and is characterized by the element association Al2O3, TiO2, Fe2O3, P2O5, and SO2 and the trace elements (TE) V, Cr, Ni, Cu, Zn, Ga, and Mo. The occurrence of two moderately high Ir peaks (0.2 and 1.1 μg/kg) suggests the presence of tiny amounts of extraterrestrial material within the sandstone complex. Based on the contrasting abundance of Ni and Cu in chondritic meteorites and middle crust, the Ni/Cu ratio was used to trace the portion of extraterrestrial material in the sequence. The distribution of this ratio reflects changes in the amount of siliciclastic components added during deposition of the sandstone complex rather than variations in the amount of meteoritic material. The disagreement between evidence suggesting a prevalence of reducing conditions during the alteration of the ejecta material (pyrite inclusions in spherules; accommodation of Mn2+ by secondary calcite) and sedimentologic features which indicate that the sandstone complex was deposited in a dominantly oxic, high-energy environment strongly supports the case that the ejecta material in these deposits was subjected to reworking. PDF

 

Biostratigraphy, Age of Chicxulub Impact, and Depositional Environment of the Brazos River KTB Sequences

TitleBiostratigraphy, Age of Chicxulub Impact, and Depositional Environment of the Brazos River KTB Sequences
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G, Abramovich, T, Adatte, T, Berner, Z
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination81-122
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsBiostratigraphy, Brazos, Chicxulub impact, Cretaceous-Tertiary, d13C shift, Evolution, Ir anomaly, mass extinction, Sandstone complex, Sea level, Texas
Abstract

Integrated biostratigraphy, sedimentology and stable isotopes of 11 outcrops and wells along the Brazos River of Falls County, Texas, U.S.A., reveal the stratigraphic separation and sequential depositional history of the Chicxulub impact, followed by the sandstone complex and associated sea-level fall, which in turn was followed by the Cretaceous-Tertiary boundary (KTB). The KTB was identified up to 1 m above the sandstone complex based on three global standard criteria: the mass extinction in planktic foraminifera, the evolution of first Danian species and the negative d13C shift. No Ir anomaly is associated with the KTB or the Chicxulub impact-ejecta layers. Late Maastrichtian sediment deposition occurred in a middle-shelf environment that shallowed to inner-shelf depth at the time of deposition of the sandstone complex. At this time, Brazos sections show distinct shallowing from inner neritic in the north to subtidal and lagoonal at Cottonmouth Creek, with further shallowing to intertidal swamp or marsh conditions in the Darting Minnow Creek area to the south. The sandstone complex is the most prominent feature of the Brazos sections. At the base of this unit are reworked Chicxulub impact spherules and lithified clasts with impact spherules and mud cracks that bear witness to erosion of an older primary spherule deposit. This primary Chicxulub impact-ejecta layer was discovered between 45 and 60 cm below the sandstone complex in a (3 cm thick) impact-glass layer that is diagenetically altered to yellow clay. The sandstone complex, the reworked impact spherules, the spherule-rich clasts, and the yellow clay layer all clearly predate the KTB. PDF

Nature of the KTB Controversy

TitleNature of the KTB Controversy
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G, Adatte, T
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
ISBN978-1-56576-308-1
Abstract

One of the liveliest debates among scientists concerns potential causes of catastrophic extinction events, but none have garnered the imagination of scientists and public alike as the Cretaceous–Tertiary boundary (KTB) mass extinction including the demise of the dinosaurs 65 million years ago. Over three decades ago the discovery of anomalous concentrations of iridium in a thin clay layer between Cretaceous limestones and Tertiary claystones led Alvarez and collaborators to propose that a large meteorite crashed into Earth and caused the KTB mass extinction (Alvarez et al., 1980). Because iridium is rare on Earth’s surface, relatively common deep in Earth’s interior where it can surface via volcanic eruptions, but most abundant in some meteorites, this hypothesis rapidly gained support. With the discovery of the 175 km diameter Chicxulub impact crater on Yucata´n in 1991 (Hildebrand et al., 1991), followed by discoveries of impact glass spherule ejecta throughout the Caribbean, Central America, and North America in stratigraphic proximity of the KTB mass extinction (Izett et al., l99l; Swisher et al., l992; Smit et al., 1992) there seemed little doubt that the smoking gun had been found in the Chicxulub impact crater and that the impact-kill hypothesis was all but proven. For many scientists, the impact-kill hypothesis became a Eureka moment—a beautiful theory that could be expanded with many corollaries to account for virtually all observations. It was reconfirmed by 41 scientists in a recent Science article (Schulte et al., 2010) and expressed well by Birger Schmitz (2011) in his review of Ted Nield’s new book Incoming—Or why we should stop worrying and learn to love the meteorite. Nield (2011) writes a riveting account on meteorites that begins with fascinating historical facts, heresy, and beliefs through the ages before leading into the scientific geological account of the meteorite theory and an objective treatment of the controversy based on evidence inconsistent with this theory. There is nothing worse than destroying a beautiful theory with facts. Schmitz takes issue with Nield’s suggestion that doubters like Gerta Keller and her small team may have a point—the impact harmed nature, but the mass extinction had more varied causes. Schmitz considers this a compromise that belongs in politics, not in science. He goes on to state that he started his career in the 1980s as a non-believer of the impact theory, but has now seen the KTB clay layer in over 50 localities ‘‘where the iridium enriched layer always occurs exactly at the level at which the microscopic foraminifera typical of Cretaceous oceans disappear almost completely . . . The precise coincidence of these two events is so compelling that it is difficult to understand how anyone can doubt the direct relationship between them’’ (Schmitz, 2011).  PDF

Defining the Cretaceous-Tertiary Boundary: A Practical Guide and Return to First Principles

TitleDefining the Cretaceous-Tertiary Boundary: A Practical Guide and Return to First Principles
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
Number of Volumes23-42
EditionSEPM, Special Publication
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN978-1-56576-308-1
KeywordsCretaceous-Tertiary, d13C shift, Evolution, Guide, Ir anomaly, KT Definition, mass extinction
Abstract

The Cretaceous-Tertiary boundary (KTB) is one of the easiest epoch boundaries to identify, whether based on lithological changes in the field, geochemical analysis in the laboratory, or fossil content. A set of five KTB-identifying criteria, originally proposed by the ICS working group during 1980 -1990s, have proven globally applicable and independently verifiable: (1) mass extinction of Cretaceous planktic foraminifera, (2) evolution of the first Danian species, (3) KTB clay and red layer, (4) Ir anomaly, and (5) δ13C shift. Despite this successful track record, it was recently proposed to reduce the five KTB-identifying criteria to just two, the mass extinction and impact signals, based on the assumption that the Chicxulub impact caused the mass extinction and therefore defines the KTB. Because this assumption is contradicted by stratigraphic data in many places, this has led to contentious arguments, whereas defining the Chicxulub impact as KTB in age has led to circular reasoning. This study demonstrates the contradictions, pitfalls, and erroneous assumptions that accompany the use of these reduced impact-event-based KTB criteria. Returning the definition of the KTB to its GSSP based on all five criteria, and where this is not possible based on the mass extinction, the first appearance of Danian species, and the δ13C shift provide the most reliable KT boundary markers.   PDF

Cretaceous-Tertiary Mass Extinction in Marginal and Open Marine Environments: Texas, U.S.A., and Tunisia

TitleCretaceous-Tertiary Mass Extinction in Marginal and Open Marine Environments: Texas, U.S.A., and Tunisia
Publication TypeBook Chapter
Year of Publication2011
AuthorsKeller, G
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
Pagination197-226
PublisherSEPM (Society for Sedimentary Geology)
CityTulsa
ISBN Number978-1-56576-308-1
KeywordsChicxulub impact, d13C shift, Depositional environment, Evolution, high stress, Ir anomaly, KT Mass Extinction, Sandstone complex, Sea level, Shallow Environment, Texas, Tunisia. Cretaceous-Tertiary
Abstract

The Cretaceous-Tertiary boundary (KTB) sequences along the Brazos River, Texas, U.S.A., have been controversial for over two decades. At issue is whether the KTB and the mass extinction should be placed at the base of a sandstone complex based on the presence of Chicxulub impact spherules or at the mass extinction. This issue goes to the very core of the KTB controversy – did the Chicxulub impact cause the KTB mass extinction? Faunal, stable isotope, platinum group elements (PGEs), and lithological analyses of six Brazos cores and outcrop sections, and comparison of these data with the Elles, Tunisia, parastratotype shed empirical light on these issues. The KTB is well marked by the mass extinction of planktic foraminifera, the first appearance of Danian species, and the δ13C negative shift, which occurs up to 1 m above the sandstone complex that contains two to three impact spherule layers at its base. There is no Ir anomaly at the KTB and mass extinction, but minor Ir enrichments are present in condensed intervals within and slightly above the sandstone complex. Clasts at the base of the sandstone complex contain impact spherules that reveal earlier deposition, lithification, erosion and redeposition. The Chicxulub impact thus predates not only the KTB, but also the sandstone complex. A yellow clay layer consisting of altered impact glass 45-60 cm below the sandstone complex (zone CF1) may represent the original Chicxulub impact ejecta fallout.

 The mass extinction pattern in the Brazos sections appears gradual or progressive compared with patterns documented from open-ocean environments. This is largely the result of high sediment accumulation rates in inner-neritic depositional settings coupled with the sea-level fall that culminated with deposition of the sandstone complex. Comparison of various extinction parameters, such as overall species richness, species abundances, life strategies, and separation into opportunists vs. specialists reveals that the shallow Brazos environment excluded the specialized larger and deeper dwelling species (~40%) that suffered the most abrupt mass extinction at the KTB. The Brazos extinction pattern thus reflects the mass extinction in the most hardy and environmentally most tolerant assemblages, which include several KTB survivors. Similar patterns are observed in shallow-water environments of southern Tunisia, Egypt, Denmark and Argentina. These data strongly show that the Chicxulub impact predates the KTB and caused no species extinctions at the KTB or at the earlier time of the impact.   PDF

KT Mass Extinction: theories and controversies - extended version

TitleKT Mass Extinction: theories and controversies - extended version
Publication TypeWeb Article
Year of Publication2010
AuthorsKeller, G
Access Year2010
Access DateMay 5
PublisherGeoscientist Online
CityLondon
Abstract

The Cretaceous-Tertiary (KT) mass extinction is primarily known for the demise of the dinosaurs, the Chicxulub impact and the frequently rancorous 30 year-old controversy over the cause of this mass extinction. Since 1980 the impact hypothesis steadily gained support that culminated in 1990 with the discovery of the Chicxulub crater on Yucatan as the KT impact site and ‘smoking gun’ that proved this hypothesis. In a perverse twist of fate this discovery also began the decline of this hypothesis because for the first time it could be tested directly based on the impact crater and impact ejecta in sediments throughout the Caribbean, Central and North America. Two decades of multidisciplinary studies amassed a database with a sum total that overwhelmingly reveals the Chicxulub impact as predating the KT mass extinction in the impact crater cores, in sections throughout NE Mexico and in Brazos River sections of Texas.

Most mass extinctions over the past 500Ma occurred during times of major volcanic eruptions, some occurred at times of multiple impacts (Fig. 1) and all were accompanied by major changes in climate, sea level and oxygenation levels of the water column. This first order test favours some direct or indirect causal relationship between mass extinctions, volcanism, large impacts, climate and sea-level changes. But among the five major mass extinctions, only the Cretaceous-Tertiary (KT) boundary mass extinction can be shown to have a close correspondence between an iridium anomaly commonly assumed to represent an impact, an impact crater (Chicxulub), a large igneous province (Deccan Traps) and major climate and sea level changes.  PDF

URLhttp://www.geolsoc.org.uk/keller

KT Mass Extinction: theories and controversies

TitleKT Mass Extinction: theories and controversies
Publication TypeWeb Article
Year of Publication2010
AuthorsKeller, G
Access DateMay 2010
PublisherGeoscientist Online
CityLondon
Abstract

Most mass extinctions that have afflicted life on Earth during the past 500 million years have occurred during times of major volcanic eruption and all were accompanied by major changes in climate, sea level and oxygenation levels in the ocean.  Among the five major mass extinctions, only the end-Cretaceous (KT) displays a close coincidence of four factors - an iridium anomaly (commonly assumed to represent an impact), an impact crater (Chicxulub), a large igneous province (the Deccan Traps) and major climate and sea level changes (Fig. 2). The KT mass extinction also differs in that it follows the longest period (145-65.5Ma) of low background extinction (Fig. 2). Throughout the Cretaceous, generic diversity had increased, accelerating during the Campanian and peaking during the late Maastrichtian, prior to the mass extinction.  PDF

URLhttp://www.geolsoc.org.uk/keller

Lilliput effect in late Maastrichtian planktic foraminifera: Response to environmental stress

TitleLilliput effect in late Maastrichtian planktic foraminifera: Response to environmental stress
Publication TypeJournal Article
Year of Publication2009
AuthorsKeller, G, Abramovich, S
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Volume284
Issue1-2
Pagination47 - 62
Date PublishedJan-12-2009
ISSN00310182
KeywordsBiotic Stress, K-T Mass Extinction, Lilliput effect, Maastrichtian
Abstract

The Lilliput effect marks morphologic and intraspecies size reductions in response to environmental stresses commonly associated with the aftermath of mass extinctions. This study shows that the Lilliput effect is a universal biotic response associated with greenhouse warming, mesotrophic or restricted basins, shallow marginal settings and volcanically active regions during the late Maastrichtian. Sedimentary sequences analyzed from Tunisia, Egypt, Texas, Argentina, the South Atlantic and Indian Ocean reveal that the biotic stress response appears uniform, regardless of the cause, varying only with the degree of biotic stress. Overall, late Maastrichtian environments span a continuum from optimum conditions to the catastrophic (mass extinctions) with a predictable set of biotic responses relative to the degree of stress induced by oxygen, salinity, temperature and nutrient variations as a result of climate and sea level changes and volcanism. Early stages of biotic stress result in diversity reduction and the elimination of large specialized species (k-strategists) leading to morphologic size reduction via selective extinction/disappearances and intraspecies dwarfing of survivors. Later stages of biotic stress result in the complete disappearance of kstrategists, intraspecies dwarfing of r-strategists and dominance by low oxygen tolerant small heterohelicids. At the extreme end of the biotic response are volcanically influenced environments, which cause the same detrimental biotic effects as observed in the aftermath of the K–T mass extinction, including the disappearance of most species and blooms of the disaster opportunist Guembelitria.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0031018209003460
DOI10.1016/j.palaeo.2009.08.029
Short TitlePalaeogeography, Palaeoclimatology, Palaeoecology

New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico

TitleNew evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico
Publication TypeJournal Article
Year of Publication2009
AuthorsKeller, G, Adatte, T, Juez, AP, Lopez-Oliva, JG
JournalJournal of the Geological Society
Volume166
Issue3
Pagination393 - 411
Date PublishedJan-05-2009
ISSN0016-7649
KeywordsBiotic effects, Chicxulub impact, K-T Mass Extinction, Mexico
Abstract

In the 1990s the Chicxulub impact was linked to the K–T boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact-generated tsunami deposit. Since that time a preponderance of evidence has failed to support this interpretation, revealing long-term deposition of the sandstone complex, the K–T boundary above it and the primary impact spherule ejecta interbedded in Late Maastrichtian marls below. Based on evidence from Mexico and Texas we suggested that the Chicxulub impact predates the K–T boundary. Impact-tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in Mexico represents slumps and widespread tectonic disturbance, although no such evidence has been presented. The decades-old controversy over the cause of the K–T mass extinction will never achieve consensus, but careful documentation of results that are reproducible and verifiable will uncover what really happened at the end of the Crectaceous. This study takes an important step in that direction by showing (1) that the stratigraphically older spherule layer from El Peñon, NE Mexico, represents the primary Chicxulub impact spherule ejecta in tectonically undisturbed sediments and (2) that this impact caused no species extinctions.  PDF

URLhttp://jgs.geoscienceworld.org/cgi/doi/10.1144/0016-76492008-116
DOI10.1144/0016-76492008-116
Short TitleJournal of the Geological Society

Biotic effects of the Chicxulub impact, K–T catastrophe and sea level change in Texas

TitleBiotic effects of the Chicxulub impact, K–T catastrophe and sea level change in Texas
Publication TypeJournal Article
Year of Publication2009
AuthorsKeller, G, Abramovich, S, Berner, Z, Adatte, T
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Volume271
Issue1-2
Pagination52 - 68
Date PublishedJan-01-2009
ISSN00310182
Abstract

Biotic effects of the Chicxulub impact, the K–T event and sea level change upon planktic foraminifera were evaluated in a new core and outcrops along the Brazos River, Texas, about 1000 km from the Chicxulub impact crater on Yucatan, Mexico. Sediment deposition occurred in a middle neritic environment that shallowed to inner neritic depths near the end of the Maastrichtian. The sea level fall scoured submarine channels, which were infilled by a sandstone complex with reworked Chicxulub impact spherules and clasts with spherules near the base. The original Chicxulub impact ejecta layer was discovered 45–60 cm below the sandstone complex, and predates the K–T mass extinction by about 300,000 years.

Results show that the Chicxulub impact caused no species extinctions or any other significant biotic effects. The subsequent sea level fall to inner neritic depth resulted in the disappearance of all larger (N150 μm) deeper dwelling species creating a pseudo-mass extinction and a survivor assemblage of small surface dwellers and low oxygen tolerant taxa. The K–T boundary and mass extinction was identified 40–80 cm above the sandstone complex where all but some heterohelicids, hedbergellids and the disaster opportunistic guembelitrids went extinct, coincident with the evolution of first Danian species and the global δ13C shift. These data reveal that sea level changes profoundly influenced marine assemblages in near shore environments, that the Chicxulub impact and K–T mass extinction are two separate and unrelated events, and that the biotic effects of this impact have been vastly overestimated.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0031018208005270
DOI10.1016/j.palaeo.2008.09.007
Short TitlePalaeogeography, Palaeoclimatology, Palaeoecology

Cretaceous climate, volcanism, impacts, and biotic effects

TitleCretaceous climate, volcanism, impacts, and biotic effects
Publication TypeJournal Article
Year of Publication2008
AuthorsKeller, G
JournalCretaceous Research
Volume29
Issue5-6
Pagination754 - 771
Date PublishedJan-10-2008
ISSN01956671
KeywordsBiotic effects, Cretaceous Impacts, Mass extinctions, Volcanism
Abstract

Cretaceous volcanic activities (LIPs and CFBPs) appear to have had relatively minor biotic effects, at least at the generic level. Major biotic stress during the Cretaceous was associated with OAEs and related to nutrient availability largely from weathering, greenhouse warming, drowning of platform areas, and volcanism. The biotic effects of OAEs were often dramatic at the species level, causing the extinction of larger specialized and heavily calcified planktonic foraminifera (rotaliporid extinction) and nannoconids (nannoconid crises), the temporary disappearances of other larger species, and the rapid increase in r-selected small and thin-walled species, such as the low oxygen tolerant heterohelicids and radially elongated taxa among planktic foraminifera and thin walled nannofossils. Biotic diversity increased during cool climates, particularly during the late Campanian and Maastrichtian, reaching maximum diversity during the middle Maastrichtian. High biotic stress conditions began during greenhouse warming and Deccan volcanism about 400 ky before the K-T boundary; it reduced abundances of large specialized tropical planktic foraminiferal species and endangered their survival. By K-T time, renewed Deccan volcanism combined with a large impact probably triggered the demise of this already extinction prone species group.

Evidence from NE Mexico, Texas, and the Chicxulub crater itself indicates that this 170 km-diameter crater predates the K-T boundary by ∼300,000 years and caused no species extinctions. The Chicxulub impact, therefore, can no longer be considered a direct cause for the K-T mass extinction. However, the K-T mass extinction is closely associated with a global Ir anomaly, which is considered too large, too widespread, and too concentrated in a thin layer to have originated from volcanic activity, leaving another large impact as the most likely source. This suggests that a second still unknown larger impact may have triggered the K-T mass extinction.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0195667108000566
DOI10.1016/j.cretres.2008.05.030
Short TitleCretaceous Research

Biotic effects of environmental catastrophes at the end of the Cretaceous and early Tertiary: Guembelitria and Heterohelix blooms

TitleBiotic effects of environmental catastrophes at the end of the Cretaceous and early Tertiary: Guembelitria and Heterohelix blooms
Publication TypeJournal Article
Year of Publication2008
AuthorsPardo, A, Keller, G
JournalCretaceous Research
Volume29
Issue5-6
Pagination1058 - 1073
Date PublishedJan-10-2008
ISSN01956671
KeywordsCatastrophes, eutrophy, Guembelitria & Heterohelix blooms, K-T, Late Maastrichtian
Abstract

In this study we report similar biotic response patterns in planktic foraminiferal assemblages, whether in association with volcanism, impacts or climate change at the end of the Cretaceous and early Tertiary. During and after each type of catastrophe two groups dominate high stress assemblages: (1) the small Guembelitria species, which are interpreted as having thrived in eutrophic surface waters where other species rarely survived; and (2) the low oxygen tolerant small Heterohelix species, which thrived at times of an expanding oxygen minimum zone associated with high nutrients and a stratified water column. The ecosystem collapse appears to be primarily the result of high macro- and micronutrient influx (from impacts, volcanism and erosion) leading to eutrophication and phytoplankton blooms (i.e., primary producers) that result in toxic conditions for foraminifera. Once nutrients decrease due to consumption by phytoplankton, the first opportunistic foraminifera, the Guembelitria, appear and graze on phytoplankton, rapidly reproduce (heterochronic acceleration) and increase populations exponentially. With nutrient depletion Guembelitria populations rapidly decrease leading to ecologic niches for other generalists and ecosystem recovery. Small low O2 tolerant heterohelicid populations mark this second stage, followed by small trochospiral and planispiral species. With further environmental recovery, increasing competition, niche development, and restoration of a well-stratified watermass, oligotrophic conditions are restored opening habitats for large, highly specialized species and a return to normal diverse assemblages. Such highly stressed ecological successions are observed in association with mantle plume volcanism in the Indian Ocean, Andean volcanism in Argentina and shallow inland seas in Egypt and Madagascar during the late Maastrichtian, the K-T impact, volcanism during the early Danian, and intense upwelling and climate extremes. We present a simple model to explain the ecological succession and recovery phases that follow major biotic perturbations.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0195667108000815
DOI10.1016/j.cretres.2008.05.031
Short TitleCretaceous Research

Impact stratigraphy: Old principle, new reality

TitleImpact stratigraphy: Old principle, new reality
Publication TypeJournal Article
Year of Publication2008
AuthorsKeller, G
JournalGeological Society of America Special Papers
Volume437
Pagination147-178
Abstract

Impact stratigraphy is an extremely useful correlation tool that makes use of unique events in Earth's history and places them within spatial and temporal contexts. The K-T boundary is a particularly apt example to test the limits of this method to resolve ongoing controversies over the age of the Chicxulub impact and whether this impact is indeed responsible for the K-T boundary mass extinction. Two impact markers, the Ir anomaly and the Chicxulub impact spherule deposits, are ideal because of their widespread presence. Evaluation of their stratigraphic occurrences reveals the potential and the complexities inherent in using these impact signals. For example, in the most expanded sedimentary sequences: (1) The K-T Ir anomaly never contains Chicxulub impact spherules, whereas the Chicxulub impact spherule layer never contains an Ir anomaly. (2) The separation of up to 9 m between the Ir anomaly and spherule layer cannot be explained by differential settling, tsunamis, or slumps. (3) The presence of multiple spherule layers with the same glass geochemistry as melt rock in the impact breccia of the Chicxulub crater indicates erosion and redeposition of the original spherule ejecta layer. (4) The stratigraphically oldest spherule layer is in undisturbed upper Maastrichtian sediments (zone CF1) in NE Mexico and Texas. (5) From central Mexico to Guatemala, Belize, Haiti, and Cuba, a major K-T hiatus is present and spherule deposits are reworked and redeposited in early Danian (zone P1a) sediments. (6) A second Ir anomaly of cosmic origin is present in the early Danian. This shows that although impact markers represent an instant in time, they are subject to the same geological forces as any other marker horizons—erosion, reworking, and redeposition—and must be used with caution and applied on a regional scale to avoid artifacts of redeposition. For the K-T transition, impact stratigraphy unequivocally indicates that the Chicxulub impact predates the K-T boundary, that the Ir anomaly at the K-T boundary is not related to the Chicxulub impact, and that environmental upheaval continued during the early Danian with possibly another smaller impact and volcanism.  PDF

URLhttp://specialpapers.gsapubs.org/content/437/147.abstract
DOI10.1130/2008.2437(09)

Reply to ‘Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas’ Comment by Schulte et al.

TitleReply to ‘Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas’ Comment by Schulte et al.
Publication TypeJournal Article
Year of Publication2008
AuthorsKeller, G, Adatte, T, Baum, G, Berner, Z
JournalEarth and Planetary Science Letters
Volume269
Issue3-4
Pagination621 - 629
Date PublishedJan-05-2008
ISSN0012821X
Abstract

We appreciate this opportunity for further discussion of the Brazos, Texas, K–T boundary sequences and their timing with respect to the Chicxulub impact. Keller et al. (2007) used a multidisciplinary approach to document the stratigraphy, paleontology, mineralogy and geochemistry of the newly drilled Mullinax-1 core and a new outcrop sequence. Based on this multi-proxy dataset very strong evidence was presented that reveals that the Chicxulub impact predates the K–T mass extinction (Keller et al., 2007). Schulte et al. take issue with this approach and our findings largely because they believe that the Chicxulub impact caused the K–T mass extinction and therefore the K–T boundary must be placed at the impact spherule layer (Schulte et al., 2008-this volume; Schulte et al., 2006; Smit et al., 1996).We welcome this opportunity to clarify misunderstandings, misconceptions and misinterpretations of the K–T record in Texas and elsewhere.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0012821X08000150
DOI10.1016/j.epsl.2007.12.025
Short TitleEarth and Planetary Science Letters

The Chicxulub Impact and K-T Mass Extinction in Texas

TitleThe Chicxulub Impact and K-T Mass Extinction in Texas
Publication TypeReport
Year of Publication2007
AuthorsKeller, G
Series TitleBulletin of the South Texas Geol. Soc.
Document NumberXLVII (9)
Pagination15-44
Date Published05/2007
InstitutionSouth Texas Geol. Soc.
CitySan Antonio
TypeBulletin
Abstract

The K-T sequences along the Brazos River of Falls County, Texas, provide the most important and critical information regarding the age and biotic effects of the Chicxulub impact outside of Mexico. New investigations based on outcrops and new cores drilled by DOSECC and funded by the National Science Foundation reveal a complex history of three tectonically undisturbed and stratigraphically well-separated events: the Chicxulub impact spherule ejecta layer, a sea-level lowstand sandstone complex, and the K-T mass extinction. The newly discovered Chicxulub impact spherule layer is the oldest of the three events and marks the time of the impact about 300,000 years before the K-T boundary (base of zone CF1), consistent with similar observations from NE Mexico and the Chicxulub crater core Yaxcopoil-l. The sea level lowstand sandstone complex predates the K-T boundary by about 100,000 years and contains clasts with Chicxulub impact spherules eroded from the original impact spherule layer. The third event is the K-T boundary mass extinction, which is not linked to the Chicxulub impact. These results indicate that a combination of impacts (Chicxulub and K-T), volcanism and climate changes caused increasingly stressful environmental conditions that culminated in the end-Cretaceous mass extinction.  PDF

Chicxulub impact predates K–T boundary: New evidence from Brazos, Texas

TitleChicxulub impact predates K–T boundary: New evidence from Brazos, Texas
Publication TypeJournal Article
Year of Publication2007
AuthorsKeller, G, Adatte, T, BERNER, ZSOLT, Harting, M, Baum, G, Prauss, M, Tantawy, A, Stüben, D
JournalEarth and Planetary Science Letters
Volume255
Issue3-4
Pagination339 - 356
Date PublishedJan-03-2007
ISSN0012821X
Abstract

Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact, the event deposit and the K–T boundary event. The K–T boundary, as identified by the negative δ13C shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K–T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with recolonization of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K–T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0012821X06009162
DOI10.1016/j.epsl.2006.12.026
Short TitleEarth and Planetary Science Letters

High stress late Maastrichtian – early Danian palaeoenvironment in the Neuquén Basin, Argentina

TitleHigh stress late Maastrichtian – early Danian palaeoenvironment in the Neuquén Basin, Argentina
Publication TypeJournal Article
Year of Publication2007
AuthorsKeller, G, Adatte, T, Tantawy, AAAM, Berner, Z, Stinnesbeck, W, Stüben, D, Leanza, HA
JournalCretaceous Research
Volume28
Issue6
Pagination939 - 960
Date PublishedJan-12-2007
ISSN01956671
KeywordsBiostratigraphy, paleoclimate, paleoecology, planktonic foraminifera, Tunisia, upper Maastrichtian
Abstract

High resolution (V5-10 kyr) planktonic foraminiferal analysis at Elles, Tunisia, reveals major changes in the structure of the Tethyan marine ecosystem during the upper Maastrichtian. During the first 1.5 Myr of the late Maastrichtian (68.3-66.8 Ma) relatively stable environmental conditions and cool temperatures are indicated by diverse planktonic foraminiferal populations with abundant intermediate and surface dwellers. A progressive cooling trend between V66.8-65.45 Ma resulted in the decline of globotruncanid species (intermediate dwellers). This group experienced a further decline at the climax of a rapid warm event about 300 kyr before the K-T boundary. At the same time relative abundances of long ranging dominant species fluctuated considerably reflecting the high stress environmental conditions. Times of critical high stress environments during the late Maastrichtian, and particularly at the K-T boundary, are indicated by low species diversity and blooms of the opportunistic genus Guembelitria at warm^ cool transition intervals. During the last 100 kyr of the Maastrichtian rapid cooling is associated with accelerated species extinctions followed by the extinction of all tropical and subtropical species at the K-T boundary.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0195667107000687
DOI10.1016/j.cretres.2007.01.006
Short TitleCretaceous Research

Planktonic Foraminiferal Biostratigraphy and Faunal Turnover across the Cretaceous-Tertiary Boundary in Southwestern Iran

TitlePlanktonic Foraminiferal Biostratigraphy and Faunal Turnover across the Cretaceous-Tertiary Boundary in Southwestern Iran
Publication TypeJournal Article
Year of Publication2007
AuthorsDarvishzad, B, Ghasemi-Nejad, E, Ghourchaei, S
JournalJournal of Sciences, Islamic Republic of Iran
Volume18
Issue2
Pagination39-149
Type of ArticleUniversity of Tehran
KeywordsCretaceous-Tertiary Boundary; Biostratigraphy; Plankton foraminifera
Abstract

The Kabirkuh section in the Ilam Province of southwestern Iran contains one of the most complete Late Maastrichtian to early Danian sequences similar to those known from the eastern Tethys realm. The Cretaceous-Tertiary boundary is marked by a 1-2 cm thick kidney-red shale in the uppermost Gurpi Formation. All Late Maastrichtian planktonic foraminiferal biozones CF1 to CF4 (equivalent to the Abathomphalus mayaroensis zone) and Danian zones P0 (Parvularugo-globigerina extensa), P1a (Parvularugoglobigerina eugubina) and Parasubbotina pseudobulloides are present. Faunal studies show that all but six of the Cretaceous species identified (22 of 29 species) disappeared at or below the K-T boundary in zone CF1 (P. hantkeninoides). Another 6 species (Heterohelix globulosa, H. navarroensis, H. dentata, Hedbergella monmouthensis, H. holmdelensis, Guembelitria cretacea) appear to have survived into the early Danian. Early disappearances appear to be environmen tally controlled. Coarse ornamented species with small populations disappeared first, whereas small species with little or no ornamentation and generally large populations tended to survive after the environment changing. This indicates a pattern of gradual and selective faunal turnover in planktonic foraminifera during the latest Maastrichtian and into the earliest Danian that is similar to that observed at the El Kef stratotype of Tunisia, as well as K-T sequences in Egypt, Italy, Spain and Mexico.  PDF

URLhttp://jsciences.ut.ac.ir

Late Maastrichtian age of spherule deposits in northeastern Mexico: implication for Chicxulub scenario

TitleLate Maastrichtian age of spherule deposits in northeastern Mexico: implication for Chicxulub scenario
Publication TypeJournal Article
Year of Publication2001
AuthorsStinnesbeck, W, Schulte, P, Lindenmaier, F, Adatte, T, Affolter, M, Schilli, L, Keller, G, üben, D, Berner, Z, Kramar, U, Burns, SJ, ópez-Oliva, JG
JournalCanadian Journal of Earth Sciences
Volume38
Issue2
Pagination229 - 238
Date PublishedJan-02-2001
ISSN0008-4077
Abstract

In the La Sierrita area of Nuevo Leon, Mexico, three spherule layers are present and separated from the overlying siliciclastic deposits by up to 6 m of pelagic marls. The marls are of latest Maastrichtian age (Plummerita hantkeninoides (CF1) Zone, Micula prinsii Zone) and deposited under normal pelagic conditions with no significant evidence of reworking or slumping. Original deposition of the spherule layers occurred during the last 300 ka of the Maastrichtian and well prior to the Cretaceous–Tertiary (K–T) boundary event. Thus, if the spherules in northeastern Mexico provide critical evidence of an impact at Chicxulub, this impact predates the K–T boundary.  PDF

URLhttp://www.nrcresearchpress.com/doi/abs/10.1139/e00-061
DOI10.1139/e00-061
Short TitleCan. J. Earth Sci.

The end-cretaceous mass extinction in the marine realm: year 2000 assessment

TitleThe end-cretaceous mass extinction in the marine realm: year 2000 assessment
Publication TypeJournal Article
Year of Publication2001
AuthorsKeller, G
JournalPlanetary and Space Science
Volume49
Issue8
Pagination817 - 830
Date PublishedJan-07-2001
ISSN00320633
Abstract

The current database indicates that the terminal decline and extinction, or near extinction, of many groups commonly attributed to an asteroid or comet impact at the Cretaceous–Tertiary (K–T) boundary (e.g., ammonites, bivalves, planktic foraminifera) began during the last 500 k:y: of the Maastrichtian. By the time of the K–T boundary, extinction-prone tropical and subtropical marine faunas and 4oras were almost gone, or had severely reduced species populations struggling to survive. The K–T boundary kill-e5ect was largely restricted to these struggling tropical and subtropical populations that accounted for 2=3 of the species among planktic foraminifera, but less than 10% of the total foraminiferal population. No signi7cant extinctions occurred among ecological generalists that dominated across latitudes. No single kill mechanism can account for this mass extinction pattern. The last 500 k:y: of the Maastrichtian were characterized by a series of rapid and extreme climate changes characterized by 3–4◦C warming between 65.4 and 65:2 Ma, major volcanic activity between 65.4 and 65:2 Ma, a spherule-producing event between 65.3 and 65:2 Ma, and an impact at the K–T boundary (65:0 Ma). All of these events caused major environmental perturbations and biotic stresses that resulted in severe reductions in species populations and extinctions that culminated at the K–T boundary. The mass extinction pattern, and the parallel environmental changes during the last 500 k:y: of the Maastrichtian, suggest that both long-term (climate, sea-level) and short-term (impact, volcanism) events contributed to the K–T boundary mass extinction.   PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0032063301000320
DOI10.1016/S0032-0633(01)00032-0
Short TitlePlanetary and Space Science

Age, chemo- and biostratigraphy of Haiti spherule-rich deposits: a multi-event K–T scenario

TitleAge, chemo- and biostratigraphy of Haiti spherule-rich deposits: a multi-event K–T scenario
Publication TypeJournal Article
Year of Publication2001
AuthorsKeller, G, Adatte, T, Stinnesbeck, W, üben, D, BERNER, ZSOLT
JournalCanadian Journal of Earth Sciences
Volume38
Issue2
Pagination197 - 227
Date PublishedJan-01-2001
ISSN00084077
Abstract

Examination of two new expanded K–T transitions and reexamination of road outcrops near Beloc, Haiti, reveals that deposition of the glass spherule-rich deposit (SRD) occurred within the early Danian Parvularugoglobigerina eugubina (Pla(1)) Zone, followed by an Ir anomaly, and 50 cm above it, a second Pd-dominated platinum group element (PGE) anomaly. The K–T boundary is at an erosional unconformity between the base of the SRD and underlying Maastrichtian limestone, where an interval representing about 100–250 thousand years appears to be missing (juxtaposition of planktic foraminiferal zones (Pla(1)) and Plummerita hantkeninoides (CF1)). It is possible that the spherule layers are reworked from original deposits at or below the K–T boundary. The Ir anomaly is of roughly chondritic-type and compatible with an impact event, whereas the Pd-dominated PGE anomaly is a more basalt-type and compatible with a magmatic origin. This suggests a multi-event scenario consistent with one impact followed by a major volcanic event in the Caribbean or elsewhere.  PDF

URLhttp://www.nrc.ca/cgi-bin/cisti/journals/rp/rp2_abst_e?cjes_e00-087_38_ns_nf_cjes38-01
DOI10.1139/cjes-38-2-197
Short TitleCan. J. Earth Sci.Rev. Can. Sci. TerreRevue canadienne des sciences de la Terre

Late Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach

TitleLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach
Publication TypeJournal Article
Year of Publication2000
AuthorsLi, L, Keller, G, Adatte, T, Stinnesbeck, W
JournalJournal of the Geological Society
Volume157
Issue2
Pagination447 - 458
Date PublishedJan-03-2000
ISSN0016-7649
Abstract

A multi‐disciplinary study of sea‐level and climate proxies, including bulk rock and clay mineral compositions, carbon isotopes, total organic carbon (TOC), Sr/Ca ratios, and macro‐ and microfaunal associations, reveals seven major sea‐level regressions in the southwestern Tethys during the last 10 million years of the Cretaceous: late Campanian (c. 74.2  Ma, 73.4–72.5  Ma and 72.2–71.7  Ma), early Maastrichtian (70.7–70.3  Ma, 69.6–69.3  Ma, and 68.9–68.3  Ma), and late Maastrichtian (65.45–65.3  Ma). Low sea levels are generally associated with increased terrigenous influx, low kaolinite/chlorite + mica ratios, high TOC and high Sr/Ca ratios, whereas high sea levels are generally associated with the reverse conditions. These sea‐level changes may be interpreted as eustatic as suggested by the global recognition of at least four of the seven major regressions identified (74.2  Ma, 70.7–70.3  Ma, 68.9–68.3  Ma and 65.45–65.3  Ma). Climatic changes inferred from clay mineral contents correlate with sea‐level changes: warm or humid climates accompany high sea levels and cooler or arid climates generally accompany low sea levels. PDF

URLhttp://jgs.lyellcollection.org/cgi/doi/10.1144/jgs.157.2.447http://jgs.geoscienceworld.org/cgi/doi/10.1144/jgs.157.2.447
DOI10.1144/jgs.157.2.447
Short TitleJournal of the Geological Society

Iridium and the K/T boundary at El Caribe, Guatemala

TitleIridium and the K/T boundary at El Caribe, Guatemala
Publication TypeJournal Article
Year of Publication2000
AuthorsKeller, G, Stinnesbeck, W
JournalInternational Journal of Earth Sciences
Volume88
Issue4
Pagination840 - 843
Date PublishedOct-03-2001
ISSN1437-3254
Abstract

At El Caribe, Guatemala, we reported the presence of up to 10±15% altered glass spherules near the top of the breccia, which is located stratigraphically near the K/T boundary, and linked these to the Chicxulub impact (Stinnesbeck et al. l997). We also noted that analyses of Ir by R. Rocchia failed to show anomalous concentrations at the top of the breccia (Stinnesbeck et al. l997, p. 703). In this short note we amend this statement and elaborate as to the actual Ir values reported by Rocchia and the precise stratigraphic position of the Ir analyses made by him in our section as well as the section collected by Fourcade et al. (l998). We then place these findings within the context of new geological studies in Haiti and Guatemala.  PDF

URLhttp://link.springer.com/10.1007/s005310050310http://www.springerlink.com/index/pdf/10.1007/s005310050310
DOI10.1007/s005310050310
Short TitleInternational Journal of Earth Sciences

Belox, Haiti, revisited: multiple events across the KT boundary in the Caribbean

TitleBelox, Haiti, revisited: multiple events across the KT boundary in the Caribbean
Publication TypeJournal Article
Year of Publication1999
AuthorsStinnesbeck, W, Keller, G, Adatte, T, Stüben, D, Kramar, U, Berner, Z, Desremeaux, C, Moliere, E
JournalTerra Nova
Volume11
Issue6
Pagination303 - 310
Date PublishedOct-12-1999
ISSN0954-4879
Abstract

Examination of new expanded K/T boundary sections near Beloc, Haiti, reveals deposition of a glass spherule-rich deposit (SRD) and two (PGE) anomalies (one Ir-dominated and one Pd-dominated) during the early Danian Parvularugoglobigerina eugubina Zone [Pla(l)]. The presence of the Haiti SRD within the early Danian is interpreted as being due to reworking. Ir is only slightly elevated within the SRD but forms an anomaly at the top of the SRD extending into the overlying pelagic limestones. It is unclear at present whether this Ir anomaly results from mechanical reworking of an impact at the K/T boundary, or an additional impact event in the early Danian. The second PGE anomaly upsection is dominated by Pd and Pt and is more compatible with a magmatic origin. This suggests a multi-event scenario consistent with one (and possibly two) impact(s), followed by a PGE-enriched volcanic event in the Caribbean.  PDF

URLhttp://doi.wiley.com/10.1046/j.1365-3121.1999.00263.x
DOI10.1046/j.1365-3121.1999.00263.x
Short TitleTerra Nova

Variability in Late Cretaceous climate and deep waters: evidence from stable isotopes

TitleVariability in Late Cretaceous climate and deep waters: evidence from stable isotopes
Publication TypeJournal Article
Year of Publication1999
AuthorsLi, L, Keller, G
JournalMarine Geology
Volume161
Issue2-4
Pagination171 - 190
Date PublishedJan-10-1999
ISSN00253227
KeywordsMaastrichtian; Stable isotopes; Climate; Deep-water
Abstract

Strong climatic and temperature fluctuations mark the Late Campanian and Maastrichtian as indicated by stable isotope records from the equatorial Pacific (Site 463) and middle and high latitude South Atlantic (Sites 525, 689 and 690). The first major global cooling decreased intermediate water temperatures (IWT) by 5–6°C between 73–70 Ma. At the same time, sea surface temperature (SST) decreased by 4–5°C in middle and high latitudes. Intermediate waters (IW) temporarily warmed by 2°C in low and middle latitudes between 70–68.5 Ma. Global cooling resumed between 68.5–65.5 Ma when IWT decreased by 3–4°C and SST by 5°C in middle latitudes. About 450 ka before the Cretaceous–Tertiary boundary rapid global warming increased IWT and SST by 3–4°C, though SST in the tropics changed little. During the last 200 ka of the Maastrichtian, climate cooled rapidly with IWT and SST decreasing by 2–3°C. During the global cooling at 71–70 Ma and possibly at 67–65.5 Ma, the sources of cold intermediate waters in the equatorial Pacific, Indo-Pacific and South Atlantic were derived from the high latitude North Pacific. In contrast, during the global climate warming between 65.2–65.4 Ma, the middle latitude South Atlantic was closest to the source of IW production and implies that the low latitude Tethys played a major role in global climate change. Climate changes, sea-level fluctuations and associated restricted seaways appear to be the most likely mechanisms for the alternating sources of IW production.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S002532279900078X
DOI10.1016/S0025-3227(99)00078-X
Short TitleMarine Geology

Diversification and extinction in Campanian-Maastrichtian planktic foraminifera of northwestern Tunisia

TitleDiversification and extinction in Campanian-Maastrichtian planktic foraminifera of northwestern Tunisia
Publication TypeJournal Article
Year of Publication1998
AuthorsLi, L, Keller, G
JournalEclogae Geologicae Helvetiae
Volume91
Issue1
Start Page75 - 102
Abstract

Investigation of Campanian-Maastrichtian planktic foraminifera in north Tunisia reveals that the late Maastrichtian not only ends with a mass extinction, but also attains maximum species diversity during theirevolutionary history. Maximum species diversity is reached during global cooling in the early late Maastrichtian over a 600 kyr interval (69.1-69.7 Ma) when species richness nearly doubled with the evolution of many rugoglobigerinids and globotruncanids. No species extinctions occur at this time and there is little change in the relative abundanceof existing species, whereas new species did not evolve into numerically large populations during the succeeding late Maastrichtian. This suggests that species originations did not result in major competition and that the early-late Maastrichtian climatic cooling may have resulted in increased habitats and nutrient supply for marine plankton. The onset of the permanent decline in Cretaceous species richness began at65.9 Ma and accelerated during the last 50-100 kyr of the Maastrichtian, culminating in the mass extinction of all tropical and subtropicaltaxa at the end of the Maastrichtian. Climate changes appear to be responsible for both the rapid evolutionary activity in the early late Maastrichtian, as well as the gradual decline in species richness near the end of the Maastrichtian, although the additional stress imposed on the ecosystem by a bolide impact is the likely cause for the final demise of the tropical and subtropical fauna at the K-T boundary.

URLhttp://serials.unibo.it/cgi-ser/start/it/spogli/df-s.tcl?prog_art=5567963&language=ITALIANO&view=articoli

Abrupt deep-sea warming at the end of the Cretaceous

TitleAbrupt deep-sea warming at the end of the Cretaceous
Publication TypeJournal Article
Year of Publication1998
AuthorsLi, L, Keller, G
JournalGeology
Volume26
Pagination995-998
Abstract

Climatic and oceanographic variations during the last 2 m.y. of the Maastrichtian inferred from high-resolution (10 k.y.) stable isotope analysis of the mid-latitude South Atlantic Deep Sea Drilling Project Site 525 reveal a major warm pulse followed by rapid cooling prior to the Cretaceous-Tertiary boundary. Between 66.85 and 65.52 Ma, cool but fluctuating temperatures average 9.9 and 15.4 °C in intermediate and surface waters, respectively. This interval is followed by an abrupt short-term warming between 65.45 and 65.11 Ma, which increased temperatures by 2–3 °C in intermediate waters, and decreased the vertical thermal gradient to an average of 2.7 °C. This warm pulse may be linked to increased atmospheric pCO2, increased poleward heat transport, and the switch of an intermediate water source from high to low-middle latitudes. During the last 100 k.y. of the Maastrichtian, intermediate and surface temperatures decreased by an average of 2.1 and 1.4 °C, respectively, compared to the maximum temperature between 65.32 and 65.24 Ma.  PDF

URLhttp://geology.gsapubs.org/content/26/11/995.abstract
DOI10.1130/0091-7613(1998)026<0995:ADSWAT>2.3.CO;2

Maastrichtian climate, productivity and faunal turnovers in planktic foraminifera in South Atlantic DSDP sites 525A and 21

TitleMaastrichtian climate, productivity and faunal turnovers in planktic foraminifera in South Atlantic DSDP sites 525A and 21
Publication TypeJournal Article
Year of Publication1998
AuthorsLi, L, Keller, G
JournalMarine Micropaleontology
Volume33
Issue1-2
Pagination55 - 86
Date PublishedJan-02-1998
ISSN03778398
Abstract

Stratigraphic, faunal and isotopic analyses of the Maastrichtian at DSDP sites 525A and 21 in the South Atlantic reveal a planktic foraminiferal fauna characterized by two major events, an early late Maastrichtian diversification and end-Maastrichtian mass extinction. Both events are accompanied by major changes in climate and productivity. The diversification event which occurred in two steps between 70.5 and 69.1 Ma increased species richness by a total of 43% and coincided with the onset of major cooling in surface and bottom waters and increased surface productivity. The onset of the terminal decline in Maastrichtian species richness began at 67.5 Ma and the first significant decline in surface productivity occurred at 66.2 Ma, coincident maximum cooling to 13°C in surface waters and the reduction of the surface-to-deep temperature gradient to less than 5°C. Major climatic and moderate productivity changes mark the mass extinction and the last 500 kyr of the Maastrichtian. Between 200 and 400 kyr before the K/T boundary surface and deep waters warmed rapidly by 3–4°C and cooled again during the last 100 kyr of the Maastrichtian. Surface productivity decreased only moderately across the K/T boundary. Species richness began to decline during the late Maastrichtian cooling and by K/T boundary time, the mass extinction had claimed 66% of the species. Viewed within the context of Maastrichtian climate and productivity changes, the K/T mass extinction could have resulted from extreme environmental stress even without the addition of an extraterrestrial impact.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0377839897000273
DOI10.1016/S0377-8398(97)00027-3
Short TitleMarine Micropaleontology

Age, stratigraphy, and deposition of near-K/T siliciclastic deposits in Mexico: Relation to bolide impact?

TitleAge, stratigraphy, and deposition of near-K/T siliciclastic deposits in Mexico: Relation to bolide impact?
Publication TypeJournal Article
Year of Publication1997
AuthorsKeller, G, Lopez-Oliva, JG, Stinnesbeck, W, Adatte, T
JournalGeological Society of America Bulletin
Volume109
Pagination410-428
Abstract

Examination of 10 K/T boundary sections in northeastern and east-central Mexico, and new data presented from 7 sections, permit the following conclusions. (1) The globally recognized K/T boundary and mass extinction in planktic foraminifera is stratigraphically above, and separated by a thin marl layer of Maastrichtian age, from the siliciclastic deposit that is commonly interpreted as a short-term (hours to days) K/T-impact–generated tsunami deposit. A similar relationship between the K/T boundary and siliciclastic or breccia deposits is observed at Brazos River in Texas, Beloc in Haiti, and Poty Quarry in Brazil. (2) Stratigraphic control indicates that deposition of the siliciclastic member occurred sometime during the last 150 k.y. of the Maastrichtian, and ended at least several thousand years prior to the K/T boundary. (3) At least four discrete horizons of bioturbation have been observed within the siliciclastic deposit that indicate episodic colonization by invertebrates over an extended time period. (4) The glass- and spherule-rich unit, which has been linked to the Haiti spherule layer and the Chicxulub structure, is at the base of the siliciclastic deposit and thus significantly predates the K/T boundary event.The stratigraphic separation of the K/T boundary and siliciclastic deposits and the evidence of long-term deposition between them, suggests the presence of two events: (1) a globally recognized K/T boundary (impact) event marked by Ir anomaly and the mass extinction, and (2) a Caribbean event (impact or volcanic and probably linked to the Chicxulub structure) that predates the K/T boundary and is marked by glass and siliciclastic or breccia deposits.  PDF

URLhttp://gsabulletin.gsapubs.org/content/109/4/410.abstract
DOI10.1130/0016-7606(1997)109<0410:ASADON>2.3.CO;2

The cretaceous-tertiary transition on the shallow Saharan Platform of southern tunisia

TitleThe cretaceous-tertiary transition on the shallow Saharan Platform of southern tunisia
Publication TypeJournal Article
Year of Publication1997
AuthorsKeller, G, Adatte, T, Stinnesbeck, W, üben, D, Kramar, U, BERNER, ZSOLT, Li, L, Perch-Nielsen, Kvon Salis
JournalGeobios
Volume30
Issue7
Pagination951 - 975
Date PublishedJan-01-1997
ISSN00166995
Abstract

A multidisciplinary approach to the study of a K/T boundary section on the Saharan Platform based on planktic and benthic foraminifera, calcareous nannofossils, lithology, stable isotopes, mineralogy and geochemistry reveals a biota stressed by fluctuating hyposaline, hypoxic littoral and nearshore environments, productivity changes, and a paleoclimate altering between seasonal warm to temperate and warm/humid conditions. Benthic foraminifera indicate that during the last 300 kyr of the Maastrichtian (CF1, Micula prinsii) deposition occurred in a inner nerit~c (littoral) environment that shallowed to a near-shore hyposaline and hypoxic environment during the last 100-200 kyr of the Maastrichtian. These conditions were accompanied by a seasonal warm to temperate climate that changed to warm/humid conditions with high rainfall, by decreasing surface productivity, and significantly decreasing planktic and benthic foraminiferal species richness. The K/T boundary is marked by an undulating erosional contact overlain by a 10 cm thick sandstone layer which is devoid of any exotic minerals or spherules. Their absence may be due to a short hiatus and the fact that the characteristic clay and red layer (zone P0) are missing. During the earliest Danian (Pla), low sea-levels prevailed with continued low oxygen, low salinity, high rainfall, high erosion and terrigenous sediment influx, accompanied by low diversity, low oxygen and low salinity tolerant species. These environmental conditions abruptly ended with erosion followed by deposition of a phosphatic siltstone layer that represents condensed sedimentation in an open (transgressive) marine environment. Above this layer, low sealevels and a return to near-shore, hyposaline and hypoxic conditions prevailed for a short interval [(base of Plc(2)] and are followed by the re-establishment of normal open marine conditions (inner neritic) comparable to the late Maastrichtian. This marine transgression is accompanied by increased productivity, and the first diversified Danian foraminiferal assemblages after the K/T boundary event and represents the return to normal biotic marine conditions. Though the K/T Seldja section represents one of the most shallow marginal sea environments studied to date for this interval, it does not represent isolated or atypical conditions. This is suggested by the similar global trends observed in sea-level fluctuations, hiatuses, as well as faunal assemblages. We conclude that on the Saharan platform of southern Tunisia, longterm environmental stresses beginning 100-200 kyr before the K/T boundary and related to climate, sea-level, nutrient, oxygen and salinity fluctuations, were the primary causes for the eventual demise of the Cretaceous fauna in the early Danian. The K/T boundary bolide impact appears to have had a relatively incidental short-term effect on this marine biota.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0016699597802185
DOI10.1016/S0016-6995(97)80218-5
Short TitleGeobios

The Cretaceous–Tertiary transition in Guatemala: limestone breccia deposits from the South Petèn basin

TitleThe Cretaceous–Tertiary transition in Guatemala: limestone breccia deposits from the South Petèn basin
Publication TypeJournal Article
Year of Publication1997
AuthorsStinnesbeck, W, Keller, G, Cruz, Jd. l., León, Cd., MacLeod, N, Whittaker, JE
JournalGeol Rundsch
Volume86
Start Page686
Date Published05/1997
Abstract

Limestone breccia deposits in southern Mexico, Guatemala and Belize have recently been interpreted as proximal to distal ballistic fallout deposits, generated by a bolide impact that struck Yucatan at K/T boundary time. We review the age, lithology and the depositional environment of Þve K/T boundary sections in the South Pete«n area of Guatemala (Caribe, Aserradero, Chisec, Actela, Chemal) in order to evaluate the nature and origin of K/T limestone breccia deposition. The sections are located 500 km south of the proposed impact site at Chicxulub and trend in an eastÐwest direction from the Guatemala/Mexico border to southern Belize. In four of the Þve sections examined, a breccia unit up to 50 m thick overlies reef-bearing shallow-water limestones of late Cretaceous (Campanian-Maastrichtian) age. Rhythmically bedded limestones, marls and siltstones of early Danian age overlie the breccia and were deposited under middle-to outer-neritic conditions. The breccia consists of di¤erently coloured layers of shallow-water limestones. Clast size generally decreases upsection to thin layers of predominantly rounded clasts, and these Þnegrained rudstones grade into grainstones at the top. In at least one section (EI Caribe) diagenetically altered glass spherules are present in the uppermost layers of the grainstone. These glass spherules are of strati- W. Stinnesbeck ( ) Geologisches Institut der Universita¬ t Karlsruhe, Postfach 6980, D-76128 Karlsruhe, Germany Fax:#0721 608 2138 G. Keller Department of Geosciences, Princeton University, Princeton, NJ 08544, USA J. de la Cruz á Carlos de Leo«n Ministerio de Energõ«a y Minas, Direccio«n de Hidrocarburos, Diagonal 17, 29Ð78, Zona 11, Guatemala 01011, Guatemala N. MacLeod á J. E. Whittaker Department of Paleontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK graphic position and chemical composition similar to black and yellow glass from Beloc, Haiti and Mimbral, Mexico, which some workers have chemically linked to melt glass within the breccia of the Chicxulub cores. We suggest that breccia deposition in Guatemala may have been multi-event, over an extended time period, and related to the collision of the Yucatan and Chortis plates as well as related to a major impact or volcanic event at the end of the Cretaceous.  PDF

URLhttp://link.springer.com/article/10.1007%2Fs005310050171

Lithostratigraphic and mineralogic correlations of near K/T boundary clastic sediments in northeastern Mexico: Implications for origin and nature of deposition

TitleLithostratigraphic and mineralogic correlations of near K/T boundary clastic sediments in northeastern Mexico: Implications for origin and nature of deposition
Publication TypeJournal Article
Year of Publication1996
AuthorsAdatte, T, Stinnesbeck, W, Keller, G
JournalGeological Society of America Special Papers
Volume307
Pagination211-226
Abstract

We interpret the near K/T boundary clastic deposits of northeastern Mexico as deposited over an extended time period, during the last 170 to 200 k.y. of the Maastrichtian and by normal sedimentary processes that include gravity flows and turbidity currents, rather than impact-generated tsunami waves. This deposition scenario is indicated by multiple horizons of bioturbation within the top unit 3 and near the base of the middle unit 2; the presence of thin layers enriched in fine clayminerals and planktic foraminifera, suggesting hemipelagic sedimentation within unit 3; the presence of a marl layer of Maastrichtian age above the clastic deposit; the occurrence of distinct layers enriched in zeolites in unit 3; and the presence of lithologically, sedimentologically, and mineralogically distinct units and subunits that are correlatable over more than 300 km. Such correlations do not support a chaotic deposition as predicted for an impact-generated tsunami event. We interpret the clastic beds of northeastern Mexico as having accumulated during the major eustatic sealevel lowstand near the end of the Maastrichtian. In this scenario, the unconformity at the base of the clastic deposit represents a type 1 sequence boundary, where deltaic sediments were eroded and transported into deeper waters, depositing the spherulerich layer of unit 1. Continued sea-level lowering resulted in erosion and bypass of shelf sediments and the deposition of the sandstone of unit 2. Subsequently, stabilization of the sea-level lowstand resulted in episodes of decreased erosion and sediment transport alternating with normal hemipelagic sedimentation, thus depositing the sand and the silt and shale layers of unit 3. The sea-level rise during the last 50 to 100 k.y. of the Maastrichtian resulted in the normal hemipelagic sedimentation observed in the pre–K/T boundary marl layer above the clastic deposit. PDF

URLhttp://specialpapers.gsapubs.org/content/307/211.abstract
DOI10.1130/0-8137-2307-8.211

Cretaceous-Tertiary boundary clastic deposits in NE Mexico: Bolide impact or sea-level lowstand?

TitleCretaceous-Tertiary boundary clastic deposits in NE Mexico: Bolide impact or sea-level lowstand?
Publication TypeBook Chapter
Year of Publication1996
AuthorsStinnesbeck, W, Keller, G, Adatte, T, Lopez-Oliva, JG, MacLeod, N
Book TitleThe Cretaceous-Tertiary Mass Extinction: Biotic and Environmental Effects
Pagination471-518
PublisherNorton Press
CityNew York

The Cretaceous/Tertiary boundary stratotype section at El Kef, Tunisia: how catastrophic was the mass extinction?

TitleThe Cretaceous/Tertiary boundary stratotype section at El Kef, Tunisia: how catastrophic was the mass extinction?
Publication TypeJournal Article
Year of Publication1995
AuthorsKeller, G, Li, L, MacLeod, N
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Volume119
Issue3-4
Pagination221 - 254
Date PublishedJan-01-1996
ISSN00310182
Abstract

The Cretaceous/Tertiary (K/T) boundary stratotype section at El Kef, Tunisia, represents the most complete and expanded sedimentary record across this important mass extinction horizon presently known. High resolution analysis of planktic foraminifera in two outcrops (El Kef I—stratotype and El Kef II) along with comparisons between planktic and benthic foraminifera, calcareous nannofossils, ostracods, pollen and spores, and dinoflagellates indicate that major changes across the K/T boundary are registered only in benthic and planktic foraminifera and calcareous nannofossils. Biotic changes in benthic foraminifera are unique to El Kef and similarly shallow continental shelf sections and appear to be the result of a sea-level regression in the latest Maastrichtian followed by a sea-level rise across the K/T boundary that was accompanied by expansion of the local oxygen minimum zone (OMZ). Biotic changes in planktic foraminifera appear partly related to these conditions also, but in general reflect more global oceanographic changes. For instance, species extinctions are gradual and selective as observed in K/T sections worldwide, rather than random and abrupt. Although there is a 69% decline in species richness between 25 cm below and 10 cm above the K/T boundary, only rare species disappeared. Their combined relative abundance constitute less than 20% of the total population. About 52% of these extinct taxa (8% of the population) are large, ornate, morphologically complex tropical-subtropical forms that lived at or below the thermocline. No planktic foraminifera from this depth range survived the K/T boundary event. All survivor taxa were surface dwellers living within the photic zone. Their relative abundance (∼80%) dominates both Cretaceous and early Tertiary populations.

These data indicate that the K/T biotic record in the shallow continental shelf section at El Kef was significantly influenced by local conditions which, combined with the latest Maastrichtian sea-level regression and subsequent sea-level rise, resulted in shallowing of the local OMZ relative to the sea-surface. Shallowing of the local OMZ lead to the selective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers. The selective nature of species extinctions, however, appear to be related partly to long-term global oceanographic changes which were accelerated at the K/T boundary possibly by a bolide impact. PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/0031018295000097
DOI10.1016/0031-0182(95)00009-7
Short TitlePalaeogeography, Palaeoclimatology, Palaeoecology

Yucatán subsurface stratigraphy: Implications and constraints for the Chicxulub impact

TitleYucatán subsurface stratigraphy: Implications and constraints for the Chicxulub impact
Publication TypeJournal Article
Year of Publication1995
AuthorsWard, WC, Keller, G, Stinnesbeck, W, Adatte, T
JournalGeology
Volume23
Pagination873-876
Abstract

Much of the discussion about the effects of an end-of-Cretaceous impact by a large extraterrestrial body in northwestern Yucatán has been done in the context of limited and partly erroneous published data on the Mesozoic stratigraphy of that area. Reexamination of cores and geophysical logs taken in several Pemex wells has produced improved lithologic and biostratigraphic correlation of the Jurassic to Maastrichtian section across the northern Yucatán peninsula. These data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near Chicxulub. The only unusual lithologic unit is polymict breccia, which apparently was penetrated at or near the top of the Cretaceous section in all the deep wells of northern Yucatán. This breccia in Pemex wells Yucatán 1, 2, 4, 5A, and 6 is composed predominantly of detrital dolomite, limestone, and anhydrite clasts set in dolomitized carbonate mud matrix, which contains upper Maastrichtian foraminifers. These constituents, mixed with fragments of altered glass or melt rock, shocked quartz and feldspar, and basement rock, suggest an impact as the most likely origin for the breccia. The timing of brecciation is poorly constrained by biostratigraphic data. There is some evidence, however, that the breccia unit is overlain by about 18 m of uppermost Maastrichtian marls, suggesting an impact before the Cretaceous-Tertiary boundary. In addition, there may have been more than one episode of breccia deposition.  PDF

URLhttp://geology.gsapubs.org/content/23/10/873.abstract
DOI10.1130/0091-7613(1995)023<0873:YNSSIA>2.3.CO;2

Is there evidence for Cretaceous-Tertiary boundary-age deep-water deposits in the Caribbean and Gulf of Mexico?

TitleIs there evidence for Cretaceous-Tertiary boundary-age deep-water deposits in the Caribbean and Gulf of Mexico?
Publication TypeJournal Article
Year of Publication1993
AuthorsKeller, G, MacLeod, N, Lyons, JB, Officer, CB
JournalGeology
Volume21
Pagination776-780
Abstract

Over most of the Gulf of Mexico and Caribbean a hiatus is present between the lower upper Maastrichtian and lowermost Tertiary deposits; sedimentation resumed ∼200 ka (upper zone Pla) after the K-T boundary. Current-bedded volcaniclastic sedimentary rocks at Deep Sea Drilling Project (DSDP) Sites 536 and 540, which were previously interpreted as impact-generated megawave deposits of K-T boundary age, are biostratigraphically of pre-K-T boundary age and probably represent turbidite or gravity-How deposits. The top 10 to 20 cm of this deposit at Site 536 contains very rare Micula prinsii, the uppermost Maastrichtian index taxon, as well as low values of Ir (0.6 pbb) and rare Ni-rich spinels. These indicate possible reworking of sediments of K-T boundary age at the hiatus. Absence of continuous sediment accumulation across the K-T boundary in the 16 Gulf of Mexico and Caribbean sections examined prevents their providing evidence of impact-generated megawave deposits in this region. Our study indicates that the most complete trans-K-T stratigraphic records may be found in onshore marine sections of Mexico, Cuba, and Haiti. The stratigraphic records of these areas should be investigated further for evidence of impact deposits.  PDF

URLhttp://geology.gsapubs.org/content/21/9/776.abstract
DOI10.1130/0091-7613(1993)021<0776:ITEFCT>2.3.CO;2

Hiatus distributions and mass extinctions at the Cretaceous/Tertiary boundary

TitleHiatus distributions and mass extinctions at the Cretaceous/Tertiary boundary
Publication TypeJournal Article
Year of Publication1991
AuthorsMacLeod, N, Keller, G
JournalGeology
Volume19
Pagination497-501
Abstract

Much disagreement over the interpretation of data bearing on various Cretaceous/Tertiary (K/T) extinction scenarios results from a failure to view these data within their appropriate stratigraphic context. Combined biostratigraphic and chronostratigraphic analyses of K/T boundary sequences have revealed systematic differences in patterns of sediment accumulation within continental-shelf and deep-sea depositional settings. Although virtually all deep-sea boundary sequences are marked by intervals of nondeposition or hiatus formation during the latest Cretaceous and earliest Tertiary, many continental shelf-slope sequences appear to be temporally complete over this same interval. This differential pattern of sediment accumulation can be related to the latest Maastrichtian-earliest Danian sea-level rise, during which deep-sea sediment- accumulation rates would be expected to drop as the locus of sediment deposition migrated across the continental shelf. Our data suggest that the abrupt shifts in carbon-isotope abundances, single-peak Ir anomalies, and apparently instantaneous mass extinctions of marine plankton—which are routinely reported from deep-sea K/T boundary sequences and used to support a causal relation between Late Cretaceous bolide impacts and K/T mass extinctions—may be artifacts of a temporally incomplete (or extremely condensed) deep-sea stratigraphic record.  PDF

URLhttp://geology.gsapubs.org/content/19/5/497.abstract
DOI10.1130/0091-7613(1991)019<0497:HDAMEA>2.3.CO;2

Extended period of extinctions across the Cretaceous /Tertiary boundary in planktonic foraminifera of continental-shelf sections: Implications for impact and volcanism theories

TitleExtended period of extinctions across the Cretaceous /Tertiary boundary in planktonic foraminifera of continental-shelf sections: Implications for impact and volcanism theories
Publication TypeJournal Article
Year of Publication1989
AuthorsKeller, G
JournalGeological Society of America Bulletin
Volume101
Pagination1408-1419
Abstract

The extinction of planktonic foraminiferal species across the Cretaceous/Tertiary (K/T) boundary has been examined in continental-shelf sections at El Kef, Tunisia, and Brazos River, Texas. These sections are considered to contain the most complete boundary transition record known to date. In both sections, an extended period of species extinctions spans from about 300,000 yr below to about 200,000-300,000 yr above the K/T boundary. Distinct episodes of accelerated extinctions occur below the boundary and about 50,000 yr above the boundary. At El Kef, only 26% of the species extinctions appear directly associated with the K/T boundary and iridium anomaly. At Brazos River, no species extinctions or measurable faunal changes appear directly associated with the K/T boundary and iridium anomaly. Species extinctions selectively affect large, ornate, tropical to subtropical species first and small, primitive, nonornate, subtropical to temperate species last. This pattern of species extinction is likely caused by increased ecological stresses as a result of a late Maastrichtian sea-level regression and global cooling. The extended period of species extinctions and absence of extinctions at the K/T boundary at Brazos River is not entirely compatible with either impact or volcanism theories. Perhaps, multiple unrelated causes should be considered, including a sea-level regression, global cooling, a K/T boundary impact of limited extent, and extensive volcanism.  PDF

URLhttp://gsabulletin.gsapubs.org/content/101/11/1408.abstract
DOI10.1130/0016-7606(1989)101<1408:EPOEAT>2.3.CO;2

Extinction, survivorship and evolution of planktic foraminifera across the Cretaceous/Tertiary boundary at El Kef, Tunisia

TitleExtinction, survivorship and evolution of planktic foraminifera across the Cretaceous/Tertiary boundary at El Kef, Tunisia
Publication TypeJournal Article
Year of Publication1988
AuthorsKeller, G
JournalMarine Micropaleontology
Volume13
Issue3
Pagination239 - 263
Date PublishedJan-10-1988
ISSN03778398
Keywordsmass extinction
Abstract

An expanded sediment record at E1 Kef shows that the K/T boundary extinctions of planktic foraminifera extend over an interval from 25 cm below the geochemical boundary (Ir anomaly) to 7 cm above. Species extinctions appear sequential with complex, large, ornate forms disappearing first and smaller, less ornate, forms surviving longer. The 14 species extinctions below the boundary appear unrelated to an impact event.

Cretaceous species survivorship is greater than previously assumed. About 10 species survive (22%) into Subzone Pla (Globigerina eugubina). All Cretaceous survivors are small primitive forms which are generally smaller than their ancestors in Cretaceous sediments.

Species evolution after the K/T event occurs in two pulses. The first new Paleocene species evolve in the basal black clay (Zone PO ) immediately after the major Cretaceous extinctions. Evolving species are small and primitive similar to Cretaceous survivors. The second pulse in species evolution occurs in the lower part of Subzone Plb with the appearance of larger more diverse species. The first major increase in carbonate sedimentation and productivity occurs at this time and signals the recoveyr of the ecosystem nearly 300,000 years after the K/T event. The species extinctions prior to the generally assumed impact event implied by the Ir anomaly, and the long recovery period of the ecosystem thereafter cannot be explained by a single impact, but suggest that multiple causes may be responsible such as climatic changes, volcanism, a sea level drop, production of warm saline bottom water and the chemical consequences associated with increased salinity.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/0377839888900059
DOI10.1016/0377-8398(88)90005-9
Short TitleMarine Micropaleontology