University of California at San Diego, United States

The origin of the Oligocene Braarudosphaera layers over large ocean basins has remained a mystery since the question was raised almost three decades ago by the Deep Sea Drilling Project. A micropalaeontological study, coupled with oxygen and carbon isotope data, from Sites 363 and 522 in the S Atlantic Ocean have shed some new light on this question. Our Braarudosphaera abundance data correlate with oxygen isotope curves, suggesting that Braarudosphaera preferred cold waters. Variations in the relative abundance of sphenoliths (warm-water taxa), and longer Coccolithus pelagicus placoliths, also indicate colder water conditions during times of elevated Braarudosphaera deposition. Upwelling and mid-ocean overturn of cold, nutrient-rich, low-salinity water are suggested to be the most likely palaeoceanographic mechanisms for the deposition of Braarudosphaera layers.