GERTA KELLER PUBLICATIONS

Foraminifera from the early Danian intertrappean beds in Rajahmundry quarries, Andhra Pradesh

TitleForaminifera from the early Danian intertrappean beds in Rajahmundry quarries, Andhra Pradesh
Publication TypeJournal Article
Year of Publication2010
AuthorsMalarkodi, N, Keller, G, Fayazudeen, PJ, Mallikarjuna, UB
JournalJournal of the Geological Society of India
Volume75
Issue6
Pagination851 - 863
Date PublishedJan-06-2010
ISSN0016-7622
Abstract

Intertrappean beds exposed between upper and lower traps of the Government and Sunnamrayalu quarries of Rajahmundry were analyzed based on benthic and planktic foraminifera, ostracodes and algae observed in thin sections. Planktic foraminifera indicate deposition occurred in the early Danian Parvularugoglobigerina eugubina (P1a) zone shortly after deposition of the lower trap flows. The most diverse planktic assemblages were deposited in limestones of the middle intertrappean interval and indicate an upper P1a age, or subzone P1a(2), as marked by the co-occurrence of P. eugubina, Globoconusa daubjergensis, Parasubbotina pseudobulloides and Subbotina triloculinoides. Reworked late Maastrichtian planktic foraminifera are common in a limestone interval and suggest erosion of uplifted Cretaceous sediments. Benthic foraminiferal assemblages indicate deposition occurred predominantly in shallow inner shelf to brackish environments. Similarly, ostracodes indicate variable environments ranging from inner neritic to brackish with fresh water influx, as also indicated by the presence of fresh water algae. These data confirm an overall deepening from restricted shallow marine to estuarine, lagoonal and finally open marine conditions followed by abrupt emersion and paleosoil deposition prior to the arrival of the upper trap flows at or near the base of C29n.

URLhttp://link.springer.com/article/10.1007%2Fs12594-010-0066-0
DOI10.1007/s12594-010-0066-0
Short TitleJ Geol Soc India

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

Cretaceous Extinctions: Evidence Overlooked

TitleCretaceous Extinctions: Evidence Overlooked
Publication TypeJournal Article
Year of Publication2010
AuthorsKeller, G, Adatte, T, Pardo, A, Bajpai, S, Khosla, A, Samant, B
JournalScience
Volume328
Pagination974–975
Date Publishedmay
Abstract

In their Review “The Chicxulub Asteroid impact and mass extinction at the Cretaceous-Paleogene boundary” (5 March, p. 1214), P. Schulte et al. analyzed the 30-year-old controversy over the cause of the end-Cretaceous mass extinction and concluded that the original theory of 1980 was right: A large asteroid impact on Yucatan was the sole cause for this catastrophe. To arrive at this conclusion, the authors used a selective review of data and interpretations by proponents of this viewpoint. They ignored the vast body of evidence inconsistent with their conclusion—evidence accumulated by scientists across disciplines (paleontology, stratigraphy, sedimentology, geochemistry, geophysics, and volcanology) that documents a complex long-term scenario involving a combination of impacts, volcanism, and climate change. Here, we point out some of the key evidence that Schulte et al. overlooked.

The underlying basis for Schulte et al.'s claim that the Chicxulub impact is the sole cause for the Cretaceous-Paleogene (K-Pg) mass extinction is the assumption that the iridium (Ir) anomaly at the K-Pg boundary and Chicxulub are the same age. There is no evidence to support this assertion. No Ir anomaly has ever been identified in association with undisputed Chicxulub impact ejecta (impact glass spherules), and no impact spherules have ever been identified in the Ir-enriched K-Pg boundary clay in Mexico or elsewhere (12). In rare deep-sea sites where the Ir anomaly is just above impact spherules, it is due to condensed sedimentation and/or nondeposition.

A Chicxulub impact–generated tsunami is another basic assumption of Schulte et al. to account for the impact spherules in late Maastrichtian sediments (including a sandstone complex) in Mexico and Texas. Multiple lines of evidence contradict this assumption and demonstrate long-term deposition before the K-Pg, including burrowed horizons, multiple impact spherule layers separated by limestone, and spherule-rich clasts that indicate the original deposition predates the K-Pg and excludes tsunami deposition (14).

Evidence of the pre–K-Pg age of the Chicxulub impact can also be found in sediments above the sandstone complex in Texas and northeastern Mexico and above the impact breccia in the Chicxulub crater. Evidence shows that the K-Pg boundary is not linked to the sandstone complex and impact spherules (1247).

Evidence that supports the pre–K-Pg age of the Chicxulub impact is also found in the presence of a spherule layer in late Maastrichtian sediments below the sandstone complex in northeastern Mexico and Texas (248).

Deccan volcanism is dismissed by Schulte et al. as much older and of no consequence in the K-Pg mass extinction. Recent Deccan volcanism studies show the contrary (911). These studies link the mass extinction with the main phase of Deccan eruptions.

When this evidence is taken into account, it is clear that the massive Chicxulub and Deccan database indicates a long-term multicausal scenario and is inconsistent with the model proposed by Schulte et al.   PDF

URLhttps://doi.org/10.1126/science.328.5981.974-a
DOI10.1126/science.328.5981.974-a

Middle and late Cenomanian oceanic anoxic events in shallow and deeper shelf environments of western Morocco

TitleMiddle and late Cenomanian oceanic anoxic events in shallow and deeper shelf environments of western Morocco
Publication TypeJournal Article
Year of Publication2010
AuthorsGertsch, B, Adatte, T, Keller, G, Tantawy, AAAM, Berner, Z, Mort, HP, Fleitmann, D
JournalSedimentology
Volume57
Issue6
Pagination1430 - 1462
Date PublishedJan-10-2010
KeywordsMiddle Cenomanian event, Morocco, oceanic anoxic event 2, palaeoclimate, shallow shelf environments
Abstract

The response of shallow-water sequences to oceanic anoxic event 2 and mid-Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep-water sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy, mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach section results show double peaks in δ13Corg for mid-Cenomanian events 1a and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic oceanic anoxic event 2 δ13C excursion (Rotalipora cushmani extinction, top of CC10a biozone) and laminated (anoxic) black shale. In the shallow environment north of Agadir, a fluctuating sea-level associated with dysoxic, brackish and mesotrophic conditions prevailed during the middle to late Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and benthonic foraminiferal assemblages. Anoxic conditions characteristic of oceanic anoxic event 2 (for example, laminated black shales) did not reach into shallow-water environments until the maximum transgression of the early Turonian. Climate conditions decoupled along the western margin of Morocco between mid-Cenomanian event 1b and the Cenomanian–Turonian boundary, as also observed in eastern Tethys. North of Agadir alternating humid and dry seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was dry and seasonal. This climatic decoupling can be attributed to variations in the Intertropical Convergence Zone and in the intensity of the north-east trade winds in tropical areas.  PDF

URLhttp://doi.wiley.com/10.1111/j.1365-3091.2010.01151.x
DOI10.1111/sed.2010.57.issue-610.1111/j.1365-3091.2010.01151.x

Deccan volcanism, the KT mass extinction and dinosaurs

TitleDeccan volcanism, the KT mass extinction and dinosaurs
Publication TypeJournal Article
Year of Publication2009
AuthorsKeller, G, Sahni, A, Bajpai, S
JournalJournal of Biosciences
Volume34
Issue5
Pagination709 - 728
Date PublishedJan-11-2009
ISSN0250-5991
Abstract

Recent advances in Deccan volcanic studies indicate three volcanic phases with the phase-1 at 67.5 Ma followed by a 2 m.y. period of quiescence. Phase-2 marks the main Deccan volcanic eruptions in Chron 29r near the end of the Maastrichtian and accounts for ∼80% of the entire 3500 m thick Deccan lava pile. At least four of the world’s longest lava flows spanning 1000 km across India and out into the Gulf of Bengal mark phase-2. The final phase-3 was smaller, coincided with the early Danian Chron 29n and also witnessed several of the longest lava flows.

The KT boundary and mass extinction was first discovered based on planktic foraminifera from shallow marine intertrappean sediments exposed in Rajahmundry quarries between the longest lava flows of the main volcanic phase-2 and smaller phase-3. At this locality early Danian (zone P1a) planktic foraminiferal assemblages directly overlie the top of phase-2 eruptions and indicate that the masse extinction coincided with the end of this volcanic phase. Planktic foraminiferal assemblages also mark the KT boundary in intertrappean sediments at Jhilmili, Chhindwara, where freshwater to estuarine conditions prevailed during the early Danian and indicate the presence of a marine seaway across India at KT time.

Dinosaur bones, nesting sites with complete eggs and abundant eggshells are known from central India surrounding the hypothesized seaway through the Narmada-Tapti rift zone. A Maastrichtian age is generally assigned to these dinosaur remains. Age control may now be improved based on marine microfossils from sequences deposited in the seaway and correlating these strata to nearby terrestrial sequences with dinosaur remains.

URLhttp://link.springer.com/article/10.1007%2Fs12038-009-0059-6
DOI10.1007/s12038-009-0059-6
Short TitleJ Biosci

Late Maastrichtian Volcanism in the Indian Ocean: Effects on Calcareous Nannofossils and Planktic Foraminifera

TitleLate Maastrichtian Volcanism in the Indian Ocean: Effects on Calcareous Nannofossils and Planktic Foraminifera
Publication TypeJournal Article
Year of Publication2009
AuthorsTantawy, AAAM, Keller, G, Pardo, A
JournalPalaeogeography, Palaeoclimatology, Palaeoecology
Volume284
Issue1-2
Pagination63 - 87
Date PublishedJan-12-2009
ISSN00310182
KeywordsBiotic effects, High stress environments, Indian Ocean Maastrichtian, Volcanism
Abstract

The biotic effects of volcanism have long been the unknown factors in creating biotic stress, and the contribution of the Deccan volcanism to the K–T mass extinction remains largely unknown. Detailed studies of the volcanic-rich sediments of Indian Ocean Ninetyeast Ridge Sites 216 and 217 and Wharton Basin Site 212 reveal that the biotic effects of late Maastrichtian volcanism on planktic foraminifera and calcareous nannofossils are locally as severe as those of the K–T mass extinction. The biotic expressions of these high stress environments are characterized by the Lilliput effect, which includes reduced diversity by eliminating most K-strategy species, and reduction in specimen size (dwarfing), frequently to less than half their normal adult size of both r-strategy and surviving K-strategy species. In planktic foraminifera, the most extreme biotic stress results are nearly monospecific assemblages dominated by the disaster opportunist Guembelitria, similar to the aftermath of the K–T mass extinction. The first stage of improving environmental conditions results in dominance of dwarfed low oxygen tolerant Heterohelix species and the presence of a few small r-strategy species (Hedbergella, Globigerinelloides). Calcareous nannofossil assemblages show similar biotic stress signals with the dominance of Micula decussata, the disaster opportunist, and size reduction in the mean length of subordinate r-strategy species particularly in Arkhangelskiella cymbiformis and Watznaueria barnesiae. These impoverished and dwarfed late Maastrichtian assemblages appear to be the direct consequences of mantle plume volcanism and associated environmental changes, including high nutrient influx leading to eutrophic and mesotrophic waters, low oxygen in the water column and decreased watermass stratification.

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0031018209003496
DOI10.1016/j.palaeo.2009.08.025
Short TitlePalaeogeography, Palaeoclimatology, Palaeoecology

K–T transition in Deccan Traps of central India marks major marine Seaway across India

TitleK–T transition in Deccan Traps of central India marks major marine Seaway across India
Publication TypeJournal Article
Year of Publication2009
AuthorsKeller, G, Adatte, T, Bajpai, S, Mohabey, DM, Widdowson, M, Khosla, A, Sharma, R, Khosla, SC, Gertsch, B, Fleitmann, D
JournalEarth and Planetary Science Letters
Volume282
Issue1-4
Pagination10 - 23
Date PublishedJun-05-2011
ISSN0012821X
Abstract

Deccan intertrappean sediments in central India are generally considered as terrestrial deposits of Maastrichtian age, but the Cretaceous–Tertiary (K–T) position is still unknown. Here we report the discovery of the K–T transition, a marine incursion and environmental changes preserved within the intertrappean sediments at Jhilmili, Chhindwara District, Madhya Pradesh. Integrative biostratigraphic, sedimentologic, mineralogic and chemostratigraphic analyses reveal the basal Danian in the intertrappean sediments between lower and upper trap basalts that regionally correspond to C29r and the C29R/C29N transition, respectively. Intertrappean deposition occurred in predominantly terrestrial semi-humid to arid environments. But a short aquatic interval of fresh water ponds and lakes followed by shallow coastal marine conditions with brackish marine ostracods and early Danian zone P1a planktic foraminifera mark this interval very close to the K–T boundary. This marine incursion marks the existence of a nearby seaway, probably extending inland from the west through the Narmada and Tapti rift valleys. The Jhilmili results thus identify the K–T boundary near the end of the main phase of Deccan eruptions and indicate that a major seaway extended at least 800 km across India.

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0012821X09001034
DOI10.1016/j.epsl.2009.02.016
Short TitleEarth and Planetary Science Letters

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

5000 year sedimentary record of hurricane strikes on the central coast of Belize

Title5000 year sedimentary record of hurricane strikes on the central coast of Belize
Publication TypeJournal Article
Year of Publication2009
AuthorsMcCloskey, TA, Keller, G
JournalQuaternary International
Volume195
Pagination53–68
Date Publishedfeb
Abstract

The central coast of Belize has been subject to hurricane strikes throughout recorded history with immense human and material cost to the Belizean people. What remains unknown is the long-term frequency of hurricane strikes and the effects such storms may have had on the ancient Maya civilization. Our sedimentary study of major hurricane strikes over the past 5000 years provides preliminary insights. We calculate that over the past 500 years major hurricanes have struck the Belize coast on average once every decade. One giant hurricane with probably particularly catastrophic consequences struck Belize sometime before AD 1500. A temporal clustering of hurricanes suggests two periods of hyperactivity between ∼4500 and 2500 14C yr BP, which supports a regional model of latitudinal migration of hurricane strike zones. Our preliminary hurricane data, including the extreme apparent size of the giant event, suggest that prehistoric hurricanes were capable of having exerted significant environmental stress in Maya antiquity.   PDF

URLhttps://doi.org/10.1016/j.quaint.2008.03.003
DOI10.1016/j.quaint.2008.03.003

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

Early Danian Planktic Foraminifera From Cretaceous-Tertiary Intertrappean Beds At Jhilmili, Chhindwara District, Madhya Pradesh, India

TitleEarly Danian Planktic Foraminifera From Cretaceous-Tertiary Intertrappean Beds At Jhilmili, Chhindwara District, Madhya Pradesh, India
Publication TypeJournal Article
Year of Publication2009
AuthorsKeller, G, Khosla, SC, Sharma, R, Khosla, A, Bajpai, S, Adatte, T
JournalThe Journal of Foraminiferal Research
Volume39
Issue1
Pagination40 - 55
Date PublishedJan-01-2009
ISSN0096-1191
Abstract

The main Deccan volcanic province erupted mainly in terrestrial to lacustrine environments of late Maastrichtian to early Paleocene age, and lack age diagnostic fossils to accurately place the Cretaceous-Tertiary (K-T) boundary. As a result, the precise position of the K-T event within the volcanic province has remained speculative, and no realistic assessment of the biotic consequences of Deccan volcanism at K-T time has been possible. Here, we report the discovery of early Danian Zone P1a planktic foraminifera within lacustrine to brackish-marine ostracod assemblages near Jhilmili, Chhindwara District, India. These foraminiferal assemblages identify the K-T boundary in intertrappean sediments bracketed by basalt traps that are regionally correlative with C29R (Ambenali Formation) and the C29R-C29N transition (Mahalabeshwar Formation). The Jhilmili sequence is thus correlative with the shallow-marine, intertrappean Zone P1a assemblage and C29R and C29N of the lower and upper basalt traps exposed in Rajahmundry quarries. The presence of planktic foraminifera in predominantly terrestrial intertrappean sediments some 800 km from the nearest ocean suggests the presence of a seaway along the Narmada and Tapti rift zones with Jhilmili located at the eastern margin.

URLhttp://jfr.geoscienceworld.org/cgi/doi/10.2113/gsjfr.39.1.40
DOI10.2113/gsjfr.39.1.40
Short TitleThe Journal of Foraminiferal Research

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

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

Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco

TitleOrganic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco
Publication TypeJournal Article
Year of Publication2008
AuthorsHaydon, MP, Adatte, T, Keller, G, Bartels, D, Föllmi, KB, Steinmann, P, Berner, Z, Chellai, EH
JournalCretaceous Research
Volume29
Issue5-6
Pagination1008 - 1023
Date PublishedJan-10-2008
ISSN01956671
Abstract

With a multi-proxy approach, an attempt was made to constrain productivity and bottom-water redox conditions and their effects on the phosphorus accumulation rate at the Mohammed Plage section on the Tarfaya coast, Morocco, during the Cenomanian-Turonian Anoxic Event (OAE 2). A distinct δ13Corg isotope excursion of +2.5‰ occurs close to the top of the section. The unusually abrupt shift of the isotope excursion and disappearance of several planktonic foraminiferal species (e.g. Rotalipora cushmani and Rotalipora greenhornensis) in this level suggests a hiatus of between 40–60 kyrs at the excursion onset. Nevertheless, it was possible to determine both the long-term environmental history as well as the processes that took place immediately prior to and during OAE 2. TOC% values increase gradually from the base of the section to the top (from ∼2.5% to ∼10%). This is interpreted as the consequence of a long-term eustatic sea-level rise and subsidence causing the encroachment of less oxic waters into the Tarfaya Basin. Similarly a reduction in the mineralogically constructed ‘detrital index’ can be explained by the decrease in the continental flux of terrigenous material due to a relative sea-level rise. A speciation of phosphorus in the upper part of the section, which spans the start and mid-stages of OAE 2, shows overall higher abundances of Preactive mass accumulation rates before the isotope excursion onset and lower values during the plateau. Due to the probable short hiatus, the onset of the decrease in phosphorus content relative to the isotope excursion is uncertain, although the excursion plateau already contains lower concentrations. The Corg/Ptotal and V/Al ratios suggest that this reduction was mostly likely caused by a decrease in the available bottom oxygen content (probably as a result of higher productivity) and a corresponding fall in the phosphorus retention ability of the sediment. Productivity appears to have remained high during the isotope plateau possibly due to a combination of ocean-surface fertilisation via increased aridity (increased K/Al and Ti/Al ratios) and/or higher dissolved inorganic phosphorus content in the water column as a result of the decrease in sediment P retention. The evidence for decreased P-burial has been observed in many other palaeoenvironments during OAE 2. Tarfaya's unique upwelling paleosituation provides strong evidence that the nutrient recycling was a global phenomenon and therefore a critical factor in starting and sustaining OAE 2.  PDF

URLhttp://linkinghub.elsevier.com/retrieve/pii/S0195667108000761
DOI10.1016/j.cretres.2008.05.026
Short TitleCretaceous Research

Pages