Extreme polar warmth during the Cretaceous greenhouse? Paradox of the late Turonian δ18O record at Deep Sea Drilling Project Site 511

Abstract

Oxygen isotope data for upper Turonian planktonic foraminifera at Deep Sea Drilling Project Site 511 (Falkland Plateau, 60°S paleolatitude) exhibit an ∼2‰ excursion to values as low as −4.66‰ (Vienna Peedee belemnite standard; PDB) coincident with the warmest tropical temperature estimates yet obtained for the open ocean. The lowest planktonic foraminifer δ18O values suggest that the upper ocean was as warm as 30–32°C. This is an extraordinary temperature for 60°S latitude but is consistent with temperatures estimated from apparently coeval mollusc δ18O from nearby James Ross Island (65°S paleolatitude). Glassy textural preservation, a well‐defined depth distribution in Site 511 planktonics, low sediment burial temperature (∼32°C), and lack of evidence of highly depleted pore waters argue against diagenesis (even solid state diffusion) as the cause of the very depleted planktonic values. The lack of change in benthic foraminifer δ18O suggests brackish water capping as the mechanism for the low planktonic δ18O values. However, mixing ratio calculations show that the amount of freshwater required to produce a 2‰ shift in ambient water would drive a 7 psu decrease in salinity. The abundance and diversity of planktonic foraminifera and nannofossils, high planktonic:benthic ratios, and the appearance of keeled foraminifera argue against lower‐than‐normal marine salinities. Isotope calculations and climate models indicate that we cannot call upon more depleted freshwater δ18O to explain this record. Without more late Turonian data, especially from outside the South Atlantic basin, we can currently only speculate on possible causes of this paradoxical record from the core of the Cretaceous greenhouse.