University of Missouri at Columbia, United States

Stable isotope data generated from glassy (diagenetically unaltered) foraminifera from the subtropical North Atlantic (ODP Site 1049) reveal abrupt paleoceanographic and faunal changes that coincide with the Aptian/Albian boundary and the onset of Oceanic Anoxic Event (OAE) 1b. At least three planktic and one benthic species per sample, selected at closely spaced intervals from the Globigerinelloides algerianus Zone (mid-Aptian) through uppermost Ticinella bejaouaensis Zone (uppermost Aptian), reveal a consistently low (<1.1%) vertical delta (super 18) O gradient, suggesting that the thermocline was weakly developed throughout this time. Benthic delta (super 18) O values show a slight positive increase from +0.5% during the mid-Aptian to +1.1% during the latest Aptian, then decrease to -0.2% during peak C (sub org) deposition in OAE 1b (Hedbergella rischi Zone, lowermost Albian). Assuming that the delta (super 18) O composition of Cretaceous seawater averaged -1.2% and polar ice sheets were absent or very small, we estimate that middle bathyal waters at this site ranged from 7-9 degrees C during the mid-late Aptian and warmed to 12 degrees C during OAE 1b peak C (sub org) deposition. Mid-late Aptian upper surface dwelling planktic delta (super 18) O values consistently range between +0.2 and -0.1%, then abruptly shift to -2.0% during the maximum OAE 1b C (sub org) deposition event. Using the same assumptions as above and no salinity correction, we estimate that mid-late Aptian upper surface waters ranged from 11-12 degrees C, then warmed to 20 degrees C during OAE 1b. An abrupt turnover of planktonic foraminifera, including extinction of large T. bejaouaensis and H. trocoidea and disappearance of species bearing distinctive pore mound wall textures, exactly corresponds with a 2% negative delta (super 13) C shift at the Aptian/Albian boundary and the base of OAE 1b. The simultaneous change in planktic foraminifer assemblages and stable isotope values indicates that the onset of OAE 1b involved major changes in the North Atlantic climate and oceanography. Determination whether these changes heralded the onset of global warmth will require further investigation.