Middle–late Miocene benthic foraminifera in a western equatorial Indian Ocean depth transect: Paleoceanographic implications

Middle through upper Miocene (17–5 Ma) benthic foraminiferal faunas (> 63 μm) from a depth transect in the western equatorial Indian Ocean (DSDP Site 237, ODP Sites 707, 709 and 710; water depth 1500–3800 m) underwent significant faunal changes at ∼ 14–13 Ma and 11–8 Ma. These faunal changes are not easily interpreted, because of the complex factors controlling benthic foraminiferal distribution and abundance. At ∼ 14–13 Ma, the relative abundance of ‘high-productivity’ taxa (Bolivina, Bulimina, Melonis and Uvigerina species), although highly variable, generally increased at the two shallower sites (237 and 707), as did the percentage of Epistominella exigua (indicative of seasonal productivity) and Nuttallides umbonifera (indicative of corrosive bottom waters) at the two deeper sites (709 and 710); the latter species had a peak in abundance at the deepest site (710) between 11 and 9 Ma. Benthic foraminiferal accumulation rates (BFARs) increased strongly at the shallower sites starting at 11 Ma, peaking between 9 and 8 Ma, and increased weakly at the deeper two sites starting at 11 Ma. Elongate, cylindrical taxa decreased in abundance between 12 and 11 Ma at all sites. The abundance of planktic foraminiferal fragments was, as expected, overall higher at the deeper sites, with some high values after 12–13 Ma at the shallower sites. The faunal changes suggest that overall the food supply to the sea floor increased, but also became more pulsed or seasonal, peaking at 9–8 Ma, while deep and intermediate waters may have become more ventilated. The first phase of faunal change (14–13 Ma) was coeval with the global increase in benthic foraminiferal oxygen isotope values, worldwide turnover in benthic foraminiferal faunas, and possibly increased production of Antarctic Bottom Water (AABW) and Northern Component Water (NCW). The overall faunal turnover may reflect the composite faunal response to increased food input resulting from heightened productivity and increased bottom water ventilation associated with a combination of monsoonal intensification, global cooling and changes in ocean circulation, but there is no unequivocal evidence for the presence of Tethyan Outflow Water (TOW) over the studied time and depth range, in contrast to earlier publications.