GERTA KELLER PUBLICATIONS

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

Cenomanian–Turonian transition in a shallow water sequence of the Sinai, Egypt

TitleCenomanian–Turonian transition in a shallow water sequence of the Sinai, Egypt
Publication TypeJournal Article
Year of Publication2008
AuthorsGertsch, B, Keller, G, Adatte, T, Berner, Z, Kassab, AS, Tantawy, AAAM, El-Sabbagh, AM, Stüben, D
JournalInternational Journal of Earth Sciences
Volume99
Issue1
Pagination165 - 182
Date PublishedJan-01-2010
ISSN1437-3254
Abstract

Environmental and depositional changes across the Late Cenomanian oceanic anoxic event (OAE2) in the Sinai, Egypt, are examined based on biostratigraphy, mineralogy, δ13C values and phosphorus analyses. Comparison with the Pueblo, Colorado, stratotype section reveals the Whadi El Ghaib section as stratigraphically complete across the late Cenomanian–early Turonian. Foraminifera are dominated by high-stress planktic and benthic assemblages characterized by low diversity, low-oxygen and low-salinity tolerant species, which mark shallow-water oceanic dysoxic conditions during OAE2. Oyster biostromes suggest deposition occurred in less than 50 m depths in low-oxygen, brackish, and nutrient-rich waters. Their demise prior to the peak δ13C excursion is likely due to a rising sea-level. Characteristic OAE2 anoxic conditions reached this coastal region only at the end of the δ13C plateau in deeper waters near the end of the Cenomanian. Increased phosphorus accumulations before and after the δ13C excursion suggest higher oxic conditions and increased detrital input. Bulk-rock and clay mineralogy indicate humid climate conditions, increased continental runoff and a rising sea up to the first δ13C peak. Above this interval, a dryer and seasonally well-contrasted climate with intermittently dry conditions prevailed. These results reveal the globally synchronous δ13C shift, but delayed effects of OAE2 dependent on water depth.  PDF

URLhttp://link.springer.com/article/10.1007%2Fs00531-008-0374-4
DOI10.1007/s00531-008-0374-4
Short TitleInt J Earth Sci (Geol Rundsch)

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)

Oceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco

TitleOceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco
Publication TypeJournal Article
Year of Publication2008
AuthorsKeller, G, Adatte, T, Berner, Z, Chellai, EH, Stüben, D
JournalCretaceous Research
Volume29
Issue5-6
Pagination976 - 994
Date PublishedJan-10-2008
ISSN01956671
Abstract

Profound biotic changes accompanied the late Cenomanian δ13C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid δ13C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding δ13C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in diversity to just two species, the dominant (80–90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0195667108000773
DOI10.1016/j.cretres.2008.05.020
Short TitleCretaceous Research

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

Phosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2

TitlePhosphorus and the roles of productivity and nutrient recycling during oceanic anoxic event 2
Publication TypeJournal Article
Year of Publication2007
AuthorsHaydon, MP, Adatte, T, Föllmi, KB, Keller, G, Steinmann, P, Matera, V, Berner, Z, Stüben, D
JournalGeology
Volume35
Issue6
Pagination483
Date PublishedJan-01-2007
ISSN0091-7613
Abstract

Four sections documenting the impact of the late Cenomanian oceanic anoxic event (OAE 2) were studied in basins with different paleoenvironmental regimes. Accumulation rates of phosphorus (P) bound to iron, organic matter, and authigenic phosphate are shown to rise and arrive at a distinct maximum at the onset of OAE 2, with an associated increase in δ13C values. Accumulation rates of P return to pre-excursion values in the interval where the δ13C record reaches its first maximum. An offset in time between the maximum in P accumulation and peaks in organic carbon burial, hydrogen indices, and Corg/Preact molar ratios is explained by the evolution of OAE 2 in the following steps. (1) An increase in productivity increased the flux of organic matter and P into the sediments; the preservation of organic matter was low and its oxidation released P, which was predominantly mineralized. (2) Enhanced productivity and oxidation of organic matter created dysoxic bottom waters; the preservation potential for organic matter increased, whereas the sediment retention potential for P decreased. (3) The latter effect sustained high primary productivity, which led to an increase in the abundance of free oxygen in the ocean and atmosphere system. After the sequestration of CO2 in the form of black shales, this oxygen helped push the ocean back into equilibrium, terminating black shale deposition and removing bioavailable P from the water column.  PDF

URLhttp://geology.geoscienceworld.org/cgi/doi/10.1130/G23475A.1
DOI10.1130/G23475A.1

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

Biotic effects of late Maastrichtian mantle plume volcanism: implications for impacts and mass extinctions

TitleBiotic effects of late Maastrichtian mantle plume volcanism: implications for impacts and mass extinctions
Publication TypeJournal Article
Year of Publication2005
AuthorsKeller, G
JournalLithos
Volume79
Issue3-4
Pagination317 - 341
Date PublishedJan-02-2005
ISSN00244937
KeywordsBiotic effects, DSDP Site 216, Impacts, Late Maastrichtian, Planktic foraminifera Guembelitria, Volcanism
Abstract

During the late Maastrichtian, DSDP Site 216 on Ninetyeast Ridge, Indian Ocean, passed over a mantle plume leading to volcanic eruptions, islands built to sea level, and catastrophic environmental conditions for planktic and benthic foraminifera. The biotic effects were severe, including dwarfing of all benthic and planktic species, a 90% reduction in species diversity, exclusion of all ecological specialists, near-absence of ecological generalists, and dominance of the disaster opportunist Guembelitria alternating with low O2-tolerant species. These faunal characteristics are identical to those of the K–T boundary mass extinction, except that the fauna recovered after Site 216 passed beyond the influence of mantle plume volcanism about 500 kyr before the K–T boundary. Similar biotic effects have been observed in Madagascar, Israel, and Egypt. The direct correlation between mantle plume volcanism and biotic effects on Ninetyeast Ridge and the similarity to the K–T mass extinction, which is generally attributed to a large impact, reveal that impacts and volcanism can cause similar environmental catastrophes. This raises the inevitable question: Are mass extinctions caused by impacts or mantle plume volcanism? The unequivocal correlation between intense volcanism and high-stress assemblages necessitates a review of current impact and mass extinction theories.

 

 

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0024493704003111
DOI10.1016/j.lithos.2004.09.005
Short TitleLithos

Basinward transport of Chicxulub ejecta by tsunami-induced backflow, La Popa basin, NE Mexico: COMMENT

TitleBasinward transport of Chicxulub ejecta by tsunami-induced backflow, La Popa basin, NE Mexico: COMMENT
Publication TypeJournal Article
Year of Publication2005
AuthorsKeller, G, Adatte, T
JournalGeology
Volume33
Paginatione88
Abstract

Lawton et al. (2005) describe valley-like deposits in the continental to shallow marine La Popa basin northwest of Monterrey, Mexico, and interpret these as the result of Chicxulub impact induced-tsunami backflow. They further speculate that the thick siliciclastic units, known from the deep paleocanyons of the continental slope to the south and southeast, can also be explained as tsunami backflow deposits. We find no evidence to support this notion in their paper or in more than 45 exposures we examined to the south. Below we comment on their tsunami backflow interpretation and the lack of evidence from Mexico to Texas.  PDF

URLhttp://geology.gsapubs.org/content/33/1/e88.1.short
DOI10.1130/0091-7613-33.1.e88

A new lithographic limestone deposit in the Upper Cretaceous Austin Group at El Rosario, county of Múzquiz, Coahuila, northeastern Mexico

TitleA new lithographic limestone deposit in the Upper Cretaceous Austin Group at El Rosario, county of Múzquiz, Coahuila, northeastern Mexico
Publication TypeJournal Article
Year of Publication2005
AuthorsStinnesbeck, W, Ifrim, C, Schmidt, H, Rindfleisch, A, Buchy, M-C, Cavin, L, Keller, G, Smith, K, Vega, F, Frey, E, González, AGonzález
JournalRevista mexicana de ciencias geológicas, ISSN 1026-8774, Vol. 22, Nº. 3, 2005, pags. 401-418
Volume22
Date Published01/2005
Abstract

At El Rosario, 170 km WNW of Múzquiz in northern Coahuila, Mexico, alternating evenly layered platy limestone and . ssile marly limestone of late Turonian-early Coniacian age (Late Cretaceous) contain vertebrate fossils with exceptionally well-preserved anatomical details of their soft tissues, as well as abundant ammonoids, inoceramids and other invertebrates. Deposition was in an open marine shelf environment near the southern opening of the Western Interior Seaway, several hundreds of kilometers south of the North American coastline, in water depths of at least 50-100 m. The present research intends to highlight the enormous preservational potential of this new conservation deposit (Konservat- Lagerstätte) and to analyze the paleoenvironmental conditions present at this locality. Our preliminary data suggest that the El Rosario fossil deposit is a combined result of anoxic bottom conditions, early diagenetic phosphatization, and rapid burial in a soft, micritic lime mud.   PDF

Impacts, volcanism and mass extinction: random coincidence or cause and effect?

TitleImpacts, volcanism and mass extinction: random coincidence or cause and effect?
Publication TypeJournal Article
Year of Publication2005
AuthorsKeller, G
JournalAustralian Journal of Earth Sciences
Volume52
Issue4-5
Pagination725 - 757
Date PublishedJan-09-2005
ISSN0812-0099
Abstract

Large impacts are credited with the most devastating mass extinctions in Earth's history and the Cretaceous – Tertiary (K/T) boundary impact is the strongest and sole direct support for this view. A review of the five largest Phanerozoic mass extinctions provides no support that impacts with craters up to 180 km in diameter caused significant species extinctions. This includes the 170 km-diameter Chicxulub impact crater regarded as 0.3 million years older than the K/T mass extinction. A second, larger impact event may have been the ultimate cause of this mass extinction, as suggested by a global iridium anomaly at the K/T boundary, but no crater has been found to date. The current crater database suggests that multiple impacts, for example comet showers, were the norm, rather than the exception, during the Late Eocene, K/T transition, latest Triassic and the Devonian – Carboniferous transition, but did not cause significant species extinctions. Whether multiple impacts substantially contributed to greenhouse warming and associated environmental stresses is yet to be demonstrated. From the current database, it must be concluded that no known Phanerozoic impacts, including the Chicxulub impact (but excluding the K/T impact) caused mass extinctions or even significant species extinctions. The K/T mass extinction may have been caused by the coincidence of a very large impact (> 250 km) upon a highly stressed biotic environment as a result of volcanism. The consistent association of large magmatic provinces (large igneous provinces and continental flood-basalt provinces) with all but one (end-Ordovician) of the five major Phanerozoic mass extinctions suggests that volcanism played a major role. Faunal and geochemical evidence from the end-Permian, end-Devonian, end-Cretaceous and Triassic/Jurassic transition suggests that the biotic stress was due to a lethal combination of tectonically induced hydrothermal and volcanic processes, leading to eutrophication in the oceans, global warming, sea-level transgression and ocean anoxia. It must be concluded that major magmatic events and their long-term environmental consequences are major contributors, though not the sole causes of mass extinctions. Sudden mass extinctions, such as at the K/T boundary, may require the coincidence of major volcanism and a very large Impact.  PDF

URLhttp://www.tandfonline.com/doi/abs/10.1080/08120090500170393
DOI10.1080/08120090500170393
Short TitleAustralian Journal of Earth Sciences

High-resolution geochemical record of Cretaceous-Tertiary boundary sections in Mexico: New constraints on the K/T and Chicxulub events

TitleHigh-resolution geochemical record of Cretaceous-Tertiary boundary sections in Mexico: New constraints on the K/T and Chicxulub events
Publication TypeJournal Article
Year of Publication2005
AuthorsStüben, D, Kramar, U, Harting, M, Stinnesbeck, W, Keller, G
JournalGeochimica et Cosmochimica Acta
Volume69
Issue10
Pagination2559 - 2579
Date PublishedJan-05-2005
ISSN00167037
Abstract

The investigation of eight Cretaceous–Tertiary (K/T) sections in Mexico, based on major and trace element, platinum group element (PGE), stable isotope, and multivariate statistical analysis, reveals a complex depositional history across the Chicxulub and K/T boundary events. At the biostratigraphically determined K/T boundary, a minor but significant Ir-dominated PGE anomaly (0.2– 0.8 ng/g) is present in most sections. This Ir anomaly originated from an impact event and is always stratigraphically and geochemically decoupled from the underlying spherule-rich ejecta deposit related to the Chicxulub event. In all sections examined, one to three glass spherule ejecta layers and one or two chondrite-dominated PGE anomalies are separated by a bioturbated siliciclastic deposit and/or hemipelagic marl, which indicates the occurrence of at least two impact events separated by a considerable amount of time. In addition, bentonite layers and Pt and Pd-dominated PGE anomalies below and above the K/T boundary indicate volcanic activity. Above the K/T boundary, reduced bioproductivity is documented by a decrease in the biogenically bound fraction of nutrients and fluctuating ratios of immobile elements (e.g., Ti/Zr). Variations in detrital elements reflect changes in the depositional environment. Carbon and oxygen isotope and trace element distribution patterns indicate a gradually changing climate during the latest Maastrichtian, an abrupt change at the K/T boundary, and a slight recovery during the lowermost Paleocene.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0016703704008622
DOI10.1016/j.gca.2004.11.003
Short TitleGeochimica et Cosmochimica Acta

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