Central North Atlantic paleoceanography during Marine Isotope Stage 16 (ca. 610-665 ka, Middle Pleistocene): insights from the dinoflagellate cyst record at IODP Site U1313

Dinoflagellate cyst (dinocyst) analyses from Marine Isotope Stage (MIS) 16 and transition to bounding interglacials are providing new information on changes in sea-surface salinity (SSS) and temperature (SST) in the central North Atlantic. IODP Hole U1313A, from the northern margin of the North Atlantic subtropical gyre, is being analyzed at a resolution of approximately 1600 yrs. The dinocyst assemblages are dominated by the cosmopolitan species Operculodinium centrocarpum, along with Nematosphaeropsis labyrinthus and Impagidinium aculeatum. Bitectatodinium tepikiense, characteristic of cold/temperate regions, increases in abundance towards the later part of MIS 16, possibly as a result of a southward shifting of the polar front. High frequencies of Cretaceous and Paleozoic specimens are additionally recorded as a result of the first Hudson Strait Heinrich (-like) layers. Transfer functions using the modern analogue technique on a North Atlantic subset of the n=1419 Northern Hemisphere dataset have been used to obtain a quantitative reconstruction of SSS and SST. Our preliminary results show salinity values ranging from 33 to 34.6 (±1.2) and a mean sea-surface temperature between 9 and 14.6 degrees Celsius (±1.3). The SST values drop to 9 degrees only by the end of MIS 16. Alkenone paleotemperatures show a muted peak cooling in mid-MIS 16 followed by warming, whereas Mg/Ca results measured on the planktonic foraminifer Globigerina bulloides, suggest increasing cooling until just before the termination. The dinocyst data are more in accord with the cooling trend showed by Mg/Ca records, although the match is imperfect. Our study is aiming to reconstruct in high resolution the paleoceanography of North Atlantic Site U1313 during MIS 16, and will constitute the first detailed dinocyst study of this critical phase of the Middle Pleistocene transition.