Benthic foraminifera across the Cretaceous/Paleogene boundary in the Southern Ocean

The impact of an asteroid at the Cretaceous/Paleogene (K/Pg) boundary triggered dramatic biotic, biogeochemical and sedimentological changes in the oceans that have been intensively studied. Paleo-biogeographical differences in the biotic response to the impact and its environmental consequences, however, have been less well documented. We present a high-resolution analysis of benthic foraminiferal assemblages at Southern Ocean ODP Site 690 (Maud Rise, Weddell Sea, Antarctica). At this high latitude site, late Maastrichtian environmental variability was high, but benthic foraminiferal assemblages were not less diverse than at lower latitudes, in contrast to those of planktic calcifiers. Also in contrast to planktic calcifiers, benthic foraminifera did not suffer significant extinction at the K/Pg boundary, but show transient assemblage changes and decreased diversity. At Site 690, the extinction rate was even lower (~3%) than at other sites. The benthic foraminiferal accumulation rate varied little across the K/Pg boundary, indicating that food supply to the sea floor was affected to a lesser extent than at lower latitude sites. Compared to Maastrichtian assemblages, Danian assemblages have a lower diversity and greater relative abundance of heavily calcified taxa such as Stensioeina beccariiformis and Paralabamina lunata. This change in benthic foraminiferal assemblages could reflect post-extinction proliferation of different photosynthesizers (thus food for the benthos) than those dominant during the Late Cretaceous, therefore changes in the nature rather than in the amount of the organic matter supplied to the seafloor. However, severe extinction of pelagic calcifiers caused carbonate supersaturation in the oceans, thus might have given competitive advantage to species with large, heavily calcified tests. This indirect effect of the K/Pg impact thus may have influenced the deep-sea dwellers, documenting the complexity of the effects of major environmental disturbance.

Strontium isotope ratios of planktonic foraminifera from sediment samples acroos the K/T boundary

Seawater 87Sr/86Sr values increase abruptly by 28 * 10**-6 across the Cretaceous/Tertiary boundary (KTB). This small, but rapid shift is superimposed on the larger scale structure of the seawater Sr isotope curve. The time scale of radiogenic Sr addition appears to be too rapid to reconcile with sources associated with volcanism, and we show that the amount of Sr required to produce even this small increase is too large to be derived from: (1) a KT bolide of the size constrained by the Ir anomaly, (2) continental crust ejecta from the impact of such a bolide, (3) soot from global wildfires initiated by an impact, or (4) any combination of these sources. The probable source of the radiogenic Sr is enhanced continental weathering, but the high rate of increase appears to rule out processes such as sea level regression, glaciation or tectonism. A plausible mechanism for rapid addition of radiogenic Sr to the oceans is enhanced weathering associated with globally distributed acid rain (pH c. 1) which is a proposed by-product of a bolide impact (Prinn and Fegley, 1987, doi:10.1016/0012-821X(87)90046-X).

Benthic foraminifera across the Cretaceous/Paleogene boundary in ODP Hole 198-1210A

Deep-sea benthic foraminifera show important but transient assemblage changes at the Cretaceous/Paleogene (K/Pg) boundary, when many biota suffered severe extinction. We quantitatively analyzed benthic foraminiferal assemblages from lower bathyal-upper abyssal (1500-2000 m) northwest Pacific ODP Site 1210 (Shatsky Rise) and compared the results with published data on assemblages at lower bathyal (~ 1500 m) Pacific DSDP Site 465 (Hess Rise) to gain insight in paleoecological and paleoenvironmental changes at that time. At both sites, diversity and heterogeneity rapidly decreased across the K/Pg boundary, then recovered. Species assemblages at both sites show a similar pattern of turnover from the uppermost Maastrichtian into the lowermost Danian: 1) The relative abundance of buliminids (indicative of a generally high food supply) increases towards the uppermost Cretaceous, and peaks rapidly just above the K/Pg boundary, coeval with a peak in benthic foraminiferal accumulation rate (BFAR), a proxy for food supply. 2) A peak in relative abundance of Stensioeina beccariiformis, a cosmopolitan form generally more common at the middle than at the lower bathyal sites, occurs just above the buliminid peak. 3) The relative abundance of Nuttallides truempyi, a more oligotrophic form, decreases at the boundary, then increases above the peak in Stensioeina beccariiformis. The food supply to the deep sea in the Pacific Ocean thus apparently increased rather than decreased in the earliest Danian. The low benthic diversity during a time of high food supply indicates a stressed environment. This stress might have been caused by reorganization of the planktic ecosystem: primary producer niches vacated by the mass extinction of calcifying nannoplankton may have been rapidly (<10 kyr) filled by other, possibly opportunistic, primary producers, leading to delivery of another type of food, and/or irregular food delivery through a succession of opportunistic blooms. The deep-sea benthic foraminiferal data thus are in strong disagreement with the widely accepted hypothesis that the global deep-sea floor became severely food-depleted following the K/Pg extinction due to the mass extinction of primary producers ("Strangelove Ocean Model") or to the collapse of the biotic pump ("Living Ocean Model").

Stable organic carbon isotope record and total organic carbon content from the Cenomanian-Turonian boundary

Ocean Drilling Program (ODP) Leg 207 recovered expanded sections of organic-carbon-rich laminated shales on Demerara Rise (western tropical Atlantic). High-resolution organic carbon isotope and total organic carbon (TOC) records are presented, which span the Cenomanian-Turonian boundary interval (CTBI), including the Oceanic Anoxic Event (OAE) 2, from four sites oriented along a NW striking depth transect. These records represent the first high-resolution carbon isotope records across OAE 2 from the South American margin of the tropical Atlantic. Due to the scarcity of age significant fossils, the main purpose of this study was to develop a detailed carbon isotope stratigraphy in order to correlate the CTBI across the depth transect and to tie this to biostratigraphically well-defined sections in the Western Interior Basin (Pueblo, USA), boreal shelf seas (Eastbourne, England), and western Tethys (Oued Mellegue, Tunisia). All four sections studied document a 6 per mil increase of d13Corg values at the base of the CTBI, which is followed by an interval of elevated d13Corg values and a subsequent decrease. Our results supply an important stratigraphic base for subsequent paleoceanographic studies on Late Cenomanian to Early Turonian sediments from Demerara Rise and elsewhere.

Carbonate and barite content accross the Eocene/Oligocene boundary of ODP Leg 198 sites, Shatsky Rise

The barite and CaCO3 content (in weight percent) of marine sediments can be used to determine spatial and temporal changes in export production (organic and carbonate carbon flux) and/or CaCO3 preservation (inorganic carbon burial). Here we report barite and CaCO3 content in Eocene/Oligocene (E/O) boundary sediments from locations drilled on Shatsky Rise during Ocean Drilling Program Leg 198. Records of these indexes may be used along with other data to determine how the major E/O boundary climatic transition (initiation of Antarctic glaciation and resultant ocean-climate system changes) affected marine export production/preservation at Shatsky Rise. Such data are necessary to elucidate the timing and phasing of changes in the carbon cycle relative to fluctuations in oceanographic conditions across this climatically important interval.

Os isotope ratios of sediments from the Eocene-Oligocene boundary

Osmium (Os) isotope analyses of bulk sediments from the South Atlantic, Equatorial Pacific, and the Italian Apennines yield a well-dated and coherent pattern of 187Os/188Os variation from the late Eocene to the early Oligocene. The resulting composite record demonstrates the global character of two prominent features of the low-resolution LL44-GPC3 Os isotope record (Pegram and Turekian, 1999, doi:10.1016/S0016-7037(99)00308-7). These are: (1) a pronounced minimum in 187Os/188Os (0.22-0.27) in the late Eocene, between 34 and 34.5 Ma, and (2) a subsequent rapid increase in 187Os/188Os, to approximately 0.6 by 32 Ma. An ultramafic weathering event and an increased influx of extraterrestrial particles to the Earth are discussed as alternative explanations for the late Eocene 187Os/188Os minimum. Comparison of the 187Os/188Os to benthic foraminiferal oxygen isotope records demonstrates that the nearly three-fold increase in 187Os/188Os from the late Eocene minimum coincides with the growth and decay of the first large ice sheet of the Oligocene (Oi1 (Miller et al., 1991, doi:10.1029/90JB02015)). The fine structure of the Os isotope record indicates that enhanced release of radiogenic Os, unrelated to the recovery from late Eocene minimum, lagged the initiation of the Oi1 event by roughly 0.5 Myr. This record, in conjunction with weathering studies in modern glacial soils (Blum, in: W.F. Ruddiman (Ed.), Tectonic Uplift and Climate Change, Plenum Press, New York, 1997, pp. 259-288; Peucker-Ehrenbrink and Blum, 1998, doi:10.1016/S0016-7037(98)00227-0), suggests that exposure of freshly eroded material during deglaciation following Oi1 enhanced chemical weathering rates, and may have contributed to ice sheet stabilization by drawing down atmospheric carbon dioxide. The improved temporal resolution and age control of the refined Eocene-Oligocene Os isotope record also makes it possible to illustrate the late Eocene Os isotope excursion as a tool for global correlation of marine sediments.

Abrupt shifts of the Sahara-Sahel boundary during Heinrich stadials demonstrated on a sediment core transect

Relict dune fields that are found as far south as 14° N in the modern-day African Sahel are testament to equatorward expansions of the Sahara desert during the Late Pleistocene. However, the discontinuous nature of dune records means that abrupt millennial-timescale climate events are not always resolved. High-resolution marine core studies have identified Heinrich stadials as the dustiest periods of the last glacial in West Africa although the spatial evolution of dust export on millennial timescales has so far not been investigated. We use the major-element composition of four high-resolution marine sediment cores to reconstruct the spatial extent of Saharan-dust versus river-sediment input to the continental margin from West Africa over the last 60 ka. This allows us to map the position of the sediment composition corresponding to the Sahara-Sahel boundary. Our records indicate that the Sahara-Sahel boundary reached its most southerly position (13° N) during Heinrich stadials and hence suggest that these were the periods when the sand dunes formed at 14° N on the continent. Heinrich stadials are associated with cold North Atlantic sea surface temperatures which appear to have triggered abrupt increases of aridity and wind strength in the Sahel. Our study illustrates the influence of the Atlantic meridional overturning circulation on the position of the Sahara-Sahel boundary and on global atmospheric dust loading.

(Tabls S1) Bulk geochemistry for sediment samples collected from the Cenomanian-Turonian boundary section at Pont d'Issole

Oceanic Anoxic Event 2 (OAE2), spanning the Cenomanian-Turonian boundary (CTB), represents one of the largest perturbations in the global carbon cycle in the last 100 Myr. The d13Ccarb, d13Corg, and d18O chemostratigraphy of a black shale-bearing CTB succession in the Vocontian Basin of France is described and correlated at high resolution to the European CTB reference section at Eastbourne, England, and to successions in Germany, the equatorial and midlatitude proto-North Atlantic, and the U.S. Western Interior Seaway (WIS). Delta13C (offset between d13Ccarb and d13Corg) is shown to be a good pCO2 proxy that is consistent with pCO2 records obtained using biomarker d13C data from Atlantic black shales and leaf stomata data from WIS sections. Boreal chalk d18O records show sea surface temperature (SST) changes that closely follow the Delta13C pCO2 proxy and confirm TEX86 results from deep ocean sites. Rising pCO2 and SST during the Late Cenomanian is attributed to volcanic degassing; pCO2 and SST maxima occurred at the onset of black shale deposition, followed by falling pCO2 and cooling due to carbon sequestration by marine organic productivity and preservation, and increased silicate weathering. A marked pCO2 minimum (~25% fall) occurred with a SST minimum (Plenus Cold Event) showing >4°C of cooling in ~40 kyr. Renewed increases in pCO2, SST, and d13C during latest Cenomanian black shale deposition suggest that a continuing volcanogenic CO2 flux overrode further drawdown effects. Maximum pCO2 and SST followed the end of OAE2, associated with a falling nutrient supply during the Early Turonian eustatic highstand.

Total number of benthic foraminifera in sediments below the K/T boundary in the Atlantic and Pacific Ocean

Most species of Late Cretaceous deep-sea benthic foraminifera are believed to be cosmopolitan and therefore to exhibit only minor biogeographical differences. In this preliminary report, six Deep Sea Drilling Project (DSDP) sites from different oceans, paleolatitudes, and paleodepths were analyzed for terminal Cretaceous abyssal-bathyal benthic foraminifera in order to investigate their assumed cosmopolitan distribution and the question of whether different faunal compositions are related to time, different paleolatitudes, and/or different paleodepths. The material studied was obtained from the low-latitude Site 465 (Pacific Ocean), and the intermediate-latitude Sites 384 (North Atlantic) and 356, 516, 525, and 527 (South Atlantic). The material analyzed represents a time slice encompassing the last 20-50 k.y. of the Cretaceous. The faunas contain numerous "Velasco-type" species, such as Gavelinella beccariiformis (White), Cibicidoides velascoensis (Cushman), Nuttallides truempyi (Nuttall), Gaudryina pyramidata Cushman, and various gyroidinoids and buliminids. The results contradict the general assumption of the cosmopolitan nature of Late Cretaceous deep-sea benthic foraminifera advocated in the literature. Only about 9% of the taxa identified were found to be truly "cosmopolitan" through their occurrence at all the sites analyzed. On the basis of correspondence analysis and relative abundance data, three assemblages and three subassemblages were recognized: (1) a bathyal-abyssal assemblage [Nuttallinella sp. A, Cibicidoides hyphalus (Fisher), Valvalabamina sp. evolute form, and Gyroidinoides spp.] at the South Atlantic Sites 356, 516, 525, and 527, divided into three subassemblages, namely (a) a middle bathyal subassemblage [Eouvigerina subsculptura McNeil and Caldwell, Truaxia aspera (Cushman), and G. pyramidata] at Sites 516 and 525, (b) a lower bathyal subassemblage [Osangularia? sp., Pyramidina rudita (Cushman and Parker), and Quadrimorphina camerata (Brotzen)] at Site 356, and (c) an abyssal subassemblage [Gyroidinoides sp. C, Hyperammina-Bathysiphon, Gyroidinoides beisseli (White), and Globorotalites sp. B] at Site 527; (2) an abyssal assemblage [Buliminella cf. plana (Cushman and Parker) and Bulimina incisa Cushman] at the North Atlantic Site 384; and (3) a middle bathyal assemblage [Vulvulina sp. A, Osangularia navarroana (Cushman), Alabamina? sp., Bulimina velascoensis (Cushman), Spiroplectammina spp. calcareous forms, and Bulimina trinitatensis Cushman and Jarvis] at the Pacific Site 465.

Geochemistry of sediments from the Cretaceous-Tertiary boundary

Siderophilic element concentrations are high in sediments from the Cretaceous-Tertiary boundary. An extraterrestrial source is indicated. Concentrations are too high to be understood in terms of the impact of a chondritic asteroid. Either the projectile was a metal-sulphide core or the infalling material (probably weak cometary matter) was slowed down during atmospheric passage.