Oxygen and carbon isotope data for benthic foraminifera from DSDP Site 94-608 and ODP Site 208-1264

Small biserial foraminifera were abundant in the early Miocene (ca. 18.9-17.2 Ma) in the eastern Atlantic and western Indian Oceans, but absent in the western equatorial Atlantic Ocean, Weddell Sea, eastern Indian Ocean, and equatorial Pacific Ocean. They have been assigned to the benthic genus Bolivina, but their high abundances in sediments without evidence for dysoxia could not be explained. Apertural morphology, accumulation rates, and isotopic composition show that they were planktic (genus Streptochilus). Living Streptochilus are common in productive waters with intermittent upwelling. The widespread early Miocene high Streptochilus abundances may reflect vigorous but intermittent upwelling, inducing high phytoplankton growth rates. However, export production (estimated from benthic foraminiferal accumulation rates) was low, possibly due to high regeneration rates in a deep thermocline. The upwelled waters may have been an analog to Subantarctic Mode Waters, carrying nutrients into the eastern Atlantic and western Indian Oceans as the result of the initiation of a deep-reaching Antarctic Circumpolar Current, active Agulhas Leakage, and vigorous vertical mixing in the Southern Oceans.

Geochemical traces across the Paleocene-Eocene thermal maximum on Walvis Ridge

An understanding of sediment redox conditions across the Paleocene-Eocene thermal maximum (PETM) (?55 Ma) is essential for evaluating changes in processes that control deep-sea oxygenation, as well as identifying the mechanisms responsible for driving the benthic foraminifera extinction. Sites cored on the flanks of Walvis Ridge (Ocean Drilling Program Leg 208, Sites 1262, 1266, and 1263) allow us to examine changes in bottom and pore water redox conditions across a ~2 km depth transect of deep-sea sediments of PETM age recovered from the South Atlantic. Here we present measurements of the concentrations of redox-sensitive trace metals manganese (Mn) and uranium (U) in bulk sediment as proxies for redox chemistry at the sediment-water interface and below. All three Walvis Ridge sites exhibit bulk Mn enrichment factors (EF) ranging between 4 and 12 prior to the warming, values at crustal averages (Mn EF = 1) during the warming interval, and a return to pre-event values during the recovery period. U enrichment factors across the PETM remains at crustal averages (U EF = 1) at Site 1262 (deep) and Site 1266 (intermediate depth). U enrichment factors at Site 1263 (shallow) peaked at 5 immediately prior to the PETM and dropped to values near crustal averages during and after the event. All sites were lower in dissolved oxygen content during the PETM. Before and after the PETM, the deep and intermediate sites were oxygenated, while the shallow site was suboxic. Our geochemical results indicate that oxygen concentrations did indeed drop during the PETM but not sufficiently to cause massive extinction of benthic foraminifera.