LATE PLIOCENE VARIATION IN NORTHERN HEMISPHERE ICE SHEETS AND NORTH ATLANTIC DEEP WATER CIRCULATION

High-resolution records of delta O-18, delta C-13, and percent CaCO3 from the late Pliocene North Atlantic (Deep Sea Drilling Project sites 607 and 609) are presented and oxygen isotope stages are formalized back to stage 116 at 2.73 Ma. From 2.8 to 1.6 Ma, the interval studied, variations in these records were dominated by the 41-kyr component of orbital obliquity. Significant variation at the orbital frequencies of eccentricity (96-kyr) and precession (23-kyr) are observed in the delta O-18 record between 1.6 and 2.1 Ma, but not before. Prior to 2.4 Ma (stage 100), delta O-18 variations suggest ice sheet growth 1/4 to 1/2 as large as late Pleistocene ice volumes; however, these events are below the threshold needed to result in extensive ice-rafting to the open North Atlantic Ocean. After 2.4 Ma, ice sheets appear to be, on average, 1/2 as large as those of the late Pleistocene. The delta C-13 record indicates that some glacial suppression of North Atlantic Deep Water occurred both before and after 2.4 Ma and that glacial-interglacial transfers of C-12 between the continents and oceans appear to have been larger in the late Pliocene relative to the late Pleistocene. In addition, the strong 23-kyr power observed in delta C-13 between 2.75 and 2.10 Ma suggests that deep-sea circulation (or changes in biomass) is controlled, in part, by climatic variations unrelated to ice sheets.