Early Pliocene deep-water circulation: stable isotope evidence for enhanced northern component deep water

Abstract

High resolution benthic (Cibicidoides spp.) stable isotope and percent sand fraction records were constructed for the early Pliocene (3.3-4.7 Ma) at Ocean Drilling Program (ODP) Sites 925 (3042 m water depth) and 929 (4361 m water depth). These sites are located in the modern mixing zone of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW). We evaluate changes in deep-water circulation between the two sites using the vertical stable isotope gradient. The oxygen isotope records of both sites have well deÞned maxima that correspond to established glacial isotope stages for this time interval. Between 4.2 and 3.7 Ma, average d 18 O values from the shallower Site 925 are higher than at the deeper Site 929 by 0.2ä, indicating that NADW may have been relatively warm and salty compared to today. Site 929 d 18 O values correspond well to previously published records, suggesting an average decrease in the isotopic composition of ocean water of ~0.4ä with respect to the late Holocene. At the deeper Site 929, there is considerable d 13 C variability with extremely low minima. These results indicate that Site 929 was highly sensitive to changes in the relative ßux of northern vs. southern component deep water. Site 929 d 13 C minima coincide with glacial stages suggesting a deep-water circulation link similar to that observed during the late Pliocene/Pleistocene. At Site 925, d 13 C values are consistently high and minima can be entirely accounted for by global d 13 C variability. These data indicate a relatively strong NADW ßux and suggest that the shallower Site 925 remained within the core of NADW throughout the majority of the time interval. The prevailing inßuence of northern component deep water evident at these sites is consistent with climate models that attribute early Pliocene warmth to increased northward heat transport.