The Cenomanian/Turonian boundary interval (CTBI) at Site 1050 (30°6'N, 76°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 (~0.8%) 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°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 d18O values (corrected for salinity) remain steady throughout the CTBI, varying between 23 to 26°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 CO2 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 pCO2 can be accepted as the primary forcing mechanism for the middle Cretaceous "supergreenhouse".
Reprinted with permission from the Cushman Foundation.
ODP Leg 171B investigated the sediments of the Blake Plateau off northern Florida and recovered 36 Upper Albian ammonites - one from Site 1050C, the others from Site 1052E. This unusually large number of specimens from an ODP site permits the dating of the interval between 668 to 621 m below sea-floor at Site 1052E as late Late Albian, Stoliczkaia (S.) dispar ammonite zone. This zone is indicated by the genera Mortoniceras and Stoliczkaia (S.). Site 1050C (Interval 171B-1050C-31R-3, 0.80-0.86 m) cannot be dated more precisely than Late Aptian to Mid Cenomanian by ammonites. The fauna is cosmopolitan. Tetragonites jurinianus and Puzosia mayoriana are widely distributed forms. Kossmatella muhlenbecki was thought to be restricted to a fairly small area around the Mediterranean, but the record off northern Florida presented here, indicates that it is not an endemic specied; this is also true for Hemiptychoceras subgaultinum in the Albian. The event-like character of the ammonite-bearing interval at Site 1052E is unique. It is overlain by a laminated claystone succession; the top of this sequence is considered to represent maximum flooding (Oceanic Anoxic Event, OAE 1d). Ammonites perhaps profited from an increased nutrient supply derived from flooded coastal plains during a continuous transgression.
Reprinted with permission from The Palaeontological Association.