COOL APTIAN SUBTROPICS AND POSSIBLE ONSET OF GLOBAL CRETACEOUS WARMTH AT THE APTIAN/ALBIAN BOUNDARY

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 d¹⁸O gradient, suggesting that the thermocline was weakly developed throughout this time. Benthic d¹⁸O 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_org deposition in OAE 1b (Hedbergella rischi Zone, lowermost Albian). Assuming that the d¹⁸O 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 7-9ºC during the mid-late Aptian and warmed to 12ºC during OAE 1b peak C_org deposition. Mid-late Aptian upper surface dwelling planktic d¹⁸O values consistently range between +0.2 and -0.1‰, then abruptly shift to -2.0‰ during the maximum OAE 1b C_org deposition event. Using the same assumptions as above and no salinity correction, we estimate that mid-late Aptian upper surface waters ranged from 11-12ºC, then warmed to 20ºC 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 d¹³C 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.