Dalhousie University, Canada

Palynomorphs record the establishment of modern conditions in the subtropical North Atlantic during the early Pleistocene. Prior to approximately 1.4 Ma, muds on both the New Jersey shelf and the Iberia Abyssal Plain contained relatively few terrestrial palynomorphs, and had a dinocyst flora rich in Operculodinium israelianum and other dinocyst taxa recording warmer surface waters than at present (e.g., Tectatodinium pellitum, Lingulodinium machaerophorum, and Polysphaeridium zoharyi). Over a span of approximately 250 ka, this palynological assemblage was succeeded by one rich in pollen and with a dinocyst flora similar to the "modern" flora, i.e. rich in Operculodinium centrocarpum, Bitectatodinium tepikiense, Spiniferites spp. (predominantly S. ramosus), and Brigantedinium spp. (predominantly B. simplex). The synchronous palynological changes in such different geological settings in the eastern and western subtropical North Atlantic are attributed to global climatic deterioration and the expansion of ice sheets in the northern hemisphere. Climatic cooling increased the velocity of the Gulf Stream and other surface currents in the subtropical gyre, causing the gyre to contract and pull away from the continents. Glacioeustatically lowered sea levels also exposed a large percentage of the continental shelf areas, and together with other bathymetric highs like the Charleston Bump, deflected the gyre boundary currents (like the Gulf Stream) offshore. Consequently, surface waters of polar origin were able to penetrate between the warm waters of the gyre and the North American continent north of Cape Hatteras in the west, and Iberia in the east, around 1.4 Ma. This paleoceanographic change increased the area of "neritic" sedimentation at mid latitudes in the North Atlantic, allowing greater terrestrial influx beyond the shelfbreak.