High Planktonic Foraminiferal Species Turnover, Enhanced Vertical Ocean Mixing and “Coolhouse” Climatic Conditions across the Aptian-Albian Boundary Interval
Our taxonomic investigation of this turnover event involves study of well preserved Aptian-Albian foraminifera from five deep-sea sites in the subtropics and southern high latitudes. We estimate that at least 70% of species present in the late Aptian did not survive into the Albian, and lowest Albian assemblages consist almost entirely of two, tiny trochospiral species, one of which is described as new. In addition, stable isotopic analyses of pristinely preserved foraminifera from ODP Site 1049 reveal that the turnover event is associated with a negative shift in carbon isotope values but little change in benthic or planktonic foraminiferal oxygen isotopes. Paleotemperature estimates are quite cool, with middle bathyal temperatures ranging from 9 to 11°C and sea-surface temperatures estimated as 13 to 16°C, and the vertical oxygen and carbon isotopic gradients are quite low, ranging between 2 and 4°C. A minimum in 87Sr/86Sr values near the time of the Aptian-Albian boundary suggests increased ocean crust production. Furthermore, a sharp drop in foraminiferal Mg/Ca ratios across this boundary suggests increased hydrothermal activity leading to a secular change in ocean carbonate chemistry, which may have been partially responsible for the dramatic planktonic foraminiferal extinction event.