New mid-Cretaceous stable isotope (δ 18 O and δ 13 C) records of multiple planktonic foraminiferal species and coexisting coccoliths from Blake Nose (western North Atlantic) document a major depth-ecology reorganization of planktonic foraminifera. Across the Albian/Cenomanian boundary, deep-dwelling Praeglobotruncana stephani and Rotalipora globotruncanoides adapted to living at a shallower depth, while, at the same time, the population of surface-dwelling Paracostellagerina libyca declined. Subsequently, the opportunistic species Hedbergella delrioensis shifted to a deep environment, and the deep-dwelling forms Rotalipora montsalvensis and Rotalipora reicheli first appeared. The primary paleoenvironmental cause of the observed changes in planktonic adaptive strategies is uncertain, yet their coincidence with an earliest Cenomanian cooling trend reported elsewhere implicates the importance of reduced upper-ocean stratification. Although there has been an implicit assumption that the species-specific depth habitats of fossil planktonic foraminifera were invariant through time, planktonic paleoecology is a potential variable. Accordingly, the possibility of evolutionary changes in planktonic foraminiferal depth ecology should be a primary consideration (along with other environmental parameters) in paleoceanographic interpretations of foraminiferal stable isotope data.
The global distribution of Maastrichtian inoceramids is now known in enough detail that the patterns of disappearance can be used to place first-order constraints on paleoceanographic changes that may have occurred during that age. The Inoceramidae is an excellent group to focus on in a study of Maastrichtian events for the following reasons: (1) they were globally distributed in the early Maastrichtian; (2) they did not survive the age (i.e., they undergo change during the interval); and (3) they have left a rich microfossil and macrofossil record. Some inoceramids grew to be very large; however, even the largest often passively disaggregated and are preserved as hundreds of millions of characteristic, columnar, polygonal prisms of calcite approximately 100 microns across. This taphonomic process has greatly increased the inoceramid fossil record and provides a means of objectively estimating changes in their standing population. In addition, because these prisms commonly occur in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) cores, it is relatively easy to generate a truly global database. The existing macrofossil record of inoceramids has less temporal and spacial resolution but greater taxonomic resolution than the microfossil record. In concert the microfossil and macrofossil records of inoceramids demonstrate that important changes occurred during the Maastrichtian. These changes are distinct from the KT boundary catastrophe but are part of the larger KT transition.
Abstract The search into Earth's mid‐Cretaceous greenhouse conditions has recently been stimulated by the Tanzania Drilling Project (TDP) which has recovered exceptionally well‐preserved biogenic carbonates from subsurface pre‐Neogene marine sediments in the eastern margin of central Africa. Published Tanzanian oxygen isotope records measured on exquisitely preserved foraminiferal tests, dating to as old as ~93 Ma, provided evidence for a Turonian “hot greenhouse” with very high and stable water‐column temperatures. We have generated a comparable data set of exceptionally well‐preserved foraminifera from a lower Cenomanian interval of TDP Site 24 spanning 99.9–95.9 Ma (planktonic foraminiferal Thalmanninella globotruncanoides Zone; nannofossil Corollithion kennedyi to Lithraphidites eccentricus Zones), thereby extending the age coverage of the Tanzanian foraminiferal δ 18 O record back by ~7 million years. Throughout the interval analyzed, the new foraminiferal δ 18 O data are consistently around −4.3‰ for surface‐dwelling planktonic taxa and −1.9‰ for benthic Lenticulina spp., which translate to conservative paleotemperature estimates of >31°C at the surface and >17°C at the sea floor (upper bathyal depths). Considering the ~40°S Cenomanian paleolatitude of TDP Site 24, these estimates are higher than computer simulation results for accepted “normal” greenhouse conditions (those with up to 4X preindustrial p CO 2 level) and suggest that the climate mode of the early Cenomanian was very similar to the Turonian hot greenhouse. Taking account of other comparable data sources from different regions, the hot greenhouse mode within the normal mid‐Cretaceous greenhouse may have begun by the latest Albian, but the precise timing of the critical transition remains uncertain.
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