Saturday, November 07, 2015

Evidence of an Early Cenomanian Cretaceous “Hot Greenhouse” Paleoclimate

Early Cenomanian “hot greenhouse” revealed by oxygen isotope record of exceptionally well-preserved foraminifera from Tanzania


Ando et al


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 dataset 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 δ18O record back by ~7 million years. Throughout the interval analyzed, the new foraminiferal δ18O 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 greater than 31 °C at the surface and greater than 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 4× preindustrial pCO2 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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