Abstract:
ODP Hole 919A in the western Irminger Basin yielded a continuous section of Pleistocene sediments with high rates of deposition (8-35 cm/kyr) from a combination of glaciated terrigenous, marine biogenic, and volcanic sources. As part of a sediment census of the >150 micrometer size fraction, planktonic and benthic biogenic diversity and abundance trends were examined. Age control is based on Neogloboquadrina pachyderma oxygen isotope data, supplemented by ash marker beds (Ash Zones 1 and 2) and by magnetostratigraphic data. Major groups of planktonic foraminifera occurring at this site include the polar species N. pachyderma (s), the subpolar species N. pachyderma (d), Globigerina quinqueloba, and Globigerina bulloides, and the temperate species Globorotalia scitula and Globorotalia inflata. Increases in the abundance and diversity of non-polar species are most prominent between 490-470, 430-420 ka, 405-360 ka, and 225-195 ka. Each of these periods corresponds to light excursions in the oxygen isotope record and to times of low IRD accumulation. We interpret this data to represent episodic influxes of warmer waters (and related lower ice volumes) within glacial marine isotope stages (MIS) 14 and 12, and interglacial MIS 15, 13 and 9. Conversely, the non-polar species are rare to absent after MIS 9, and diatoms and radiolarian accumulation is strongly reduced after this point in the record as well. The decrease of this biogenic sediment cannot be attributed to dilution by IRD or ash based on the accumulation patterns for these components. Therefore either environmental changes affecting productivity (e.g., more persistent sea ice) or problems of preservation (dissolution) are likely responsible. Site 919 benthic foraminifera are generally less abundant than the planktonic forms, but more diverse. Preliminary data of the benthic groups occurring at this site include Bulimina aculeata, Cassidulina spp., Cibicides spp., Elphidium spp. (including E. excavatum), Eponides spp., Melonis spp., and Pullenia bulloides. The occurrence of shallow water Elphidium spp. at deepwater site 919 (2086 mbsl) indicates redeposition of shelf sediments into the basin. The Elphidium spp. accumulation data suggests that such redeposition is rare overall, but more common in the older part of the record (MIS 14-16).