BENTHIC FORAMINIFERAL PALEOENVIRONMENTAL RECONSTRUCTION FROM THE MIS 13-12 TIME INTERVAL IN THE WESTERN NORTH ATLANTIC (BLAKE OUTER RIDGE, ODP SITE 1057)

We have examined benthic foraminifera assemblages in sediments from ODP Site 1057 (Blake Outer Ridge, 2600 m water depth), spanning interglacial MIS 13 and MIS12, one of the most severe glacials of the last 600 ky. The ridge is a sediment drift produced by the interaction between the northward flowing surface waters of the Gulf Stream and the southerly flow of the Western Boundary Undercurrent (WBUC). Sedimentation rates in this area are high, due to the abundant supply of terrigenous material from northern sources via the WBUC, and possibly from southern areas via the Gulf Stream.

Our benthic foraminifera assemblage data show the dominance of epifaunal species during MIS 13 (mainly Nuttallides umbonifer, Gyroidina spp., Gyroidinoides spp., and lesser amounts of Cibicidoides wuellerstorfi). We interpret this assemblage as indicative of oligotrophic environments, similar to those found today in this area. During glacial MIS 12 instead, the benthic assemblage is dominated by infaunal taxa, predominantly Uvigerina peregrina, Bulimina alazanensis, and Bolivina spp. In modern environments, these taxa are associated with organic carbon-rich sediments, and/or lower oxygen conditions in pore water. The MIS 12-11 transition is marked by a distinct increase in the percentages of Bulimina marginata and Bulimina aculeata (>40 %). These are opportunistic species adapted to elevated but intermittent organic carbon fluxes, and suggest episodes of recurrent phytoplankton blooms at the surface and consequent export of labile organic matter to the seafloor.