Foraminiferal assemblage and stable isotopic change across the Cenomanian-Turonian boundary in the subtropical North Atlantic

Smithsonian Institution, Department of Paleobiology, Washington, DC, United States

The Cenomanian/Turonian boundary interval (CTBI) at Site 1050 (30 degrees 6'N, 76 degrees 14'W) was investigated to characterize climatic and oceanographic changes during the Oceanic Anoxic Event that was associated with it (OAE 2). Because of unusually good foraminiferal preservation for sediments of this age, we have obtained an unaltered oxygen and carbon isotope profile and an accompanying record of planktic and benthic foraminifer abundance changes across the OAE 2 interval. Biostratigraphic, sedimentologic, and chemostratigraphic analyses indicate that more than 0.5 m.y. between the onset and tail end of OAE 2 are missing. This explains why organic-rich sediments are absent from the Site 1050 sequence and why the planktic and benthic carbon isotope shifts are minor ( approximately 0.8 per mil) compared with the most complete OAE 2 sections. While planktic species diversity shows relatively minor change across the OAE 2 interval, extinction of the rotaliporids and Globigerinelloides bentonensis, a shift to Heterohelix-dominated assemblages, and increased abundance of helvetoglobotruncanids at the onset of OAE 2 cause a dramatic change in the planktic foraminifer assemblage composition. The rotaliporid extinction occurs at the level where middle bathyal temperatures are estimated to have increased from 15 to 19 degrees C, which is warmer than any other time during the Cretaceous and Cenozoic. This deep water warming may have caused a breakdown in the vertical structure of the water column, and could explain the extinction of deeper dwelling planktic species, including Rotalipora and G. bentonensis. On the other hand, sea surface temperature estimates based on planktic foraminiferal delta ¹⁸ O values (corrected for salinity) remain steady throughout the CTBI, varying between 23 to 26 degrees C. The presence of volcaniclastic sediments within the OAE 2 interval at Site 1050 is consistent with previous suggestions that the CTBI was a time of anonymously high rates of CO ₂ flux into the atmosphere and oceans during a major phase of explosive volcanic activity and large igneous province emplacement in the Caribbean and other regions worldwide. Further investigation of the CTBI is needed to establish whether increased PCO ₂ can be accepted as the primary forcing mechanism for the middle Cretaceous "supergreenhouse".

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