angquan Li

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

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.

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

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.

Abrupt deep-sea warming at the end of the Cretaceous

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 pCO 2 , increased poleward heat transport, and the switch of an intermediate water source from high to lowmiddle 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.

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

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.

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

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.

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

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 formaminifera indicate that during the last 300 kyr of the Maastrichtian (CF1, Micula prinsii) deposition occurred in a inner neritic (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.