Late Pliocene–Early Pleistocene North Atlantic Circulation: Integrating Dinocyst Assemblages and Foraminiferal Geochemistry

During the Late Pliocene, between 3.3 Ma and 2.6 Ma, tectonic events changed ocean basin interactions against a background of shifting orbital forcing mechanisms and a global cooling trend. A climate system that had been locked in a stable, warm state gradually transformed into one typified by the high-amplitude glacial–interglacial fluctuations characteristic of the later Quaternary. The onset of Northern Hemisphere glaciations in the Late Pliocene marks an important step in this transition, due to the role of feedback mechanisms including ice albedo. A crucial factor in this Northern Hemisphere ice sheet expansion is the North Atlantic surface ocean circulation. To evaluate how they are linked, a ca. 200-kyr time slab spanning 2782-2520 ka (Late Pliocene–Early Pleistocene) was analyzed at millennial scale resolution from eastern North Atlantic Deep Sea Drilling Project (DSDP) Site 610 and Integrated Ocean Drilling Program (IODP) Site U1313. The causes of the Plio-Pleistocene climatic turnover are compared to that of the well-documented Marine Isotope Stage (MIS) M2 occurring in the Late Pliocene (3.3 Ma). MIS M2, a severe glacial event seen as a precursor to later Quaternary-style glaciations, was investigated from western North Atlantic DSDP Site 603. Utilizing a same-sample methodology, two paleoceanographic proxies were used: (1) dinocyst assemblages, and (2) foraminiferal geochemistry (δ18O and Mg/Ca). Dinocysts are proven tracers of sea-surface temperature (SST), salinity, nutrient supply, and sea ice cover, and are analyzed here to characterize the overlying water masses at the studied sites. Strong dinocyst assemblage fluctuations attest to variations in the influence of the North Atlantic Current (NAC). Using Mg/Ca ratios for the planktonic foraminifer Globigerina bulloides to determine absolute SSTs allows salinity changes to be reconstructed when combined with stable oxygen isotopes. This study shows a persistent Gulf Stream–NAC in the western North Atlantic during MIS M2, favoring a southern shift of the NAC over a shutdown of the thermohaline circulation. At the newly established Plio-Pleistocene boundary, a profound turnover in dinoflagellate cyst assemblages reveals a shift in ocean mode during MIS 104 (2.6 Ma). Three distinct dinocyst ecozones demonstrate this fundamental reorganization of the North Atlantic circulation.Ph