Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean

[1] We demonstrate that the carbon isotopic signal of mid-depth waters evolved differently from deep waters in the South Atlantic sector of the Southern Ocean during the Pleistocene. Deep sites (>3700 m) exhibit large glacial-to-interglacial variations in benthic delta(13)C, whereas the amplitude of the delta(13)C signal at Site 1088 ( 2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic delta(13)C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate-to-deep delta(13)C gradients (Delta(13)C(I-D)) supports the existence of a sharp chemocline between 2100 and 2700 m during most glacial stages of the last 1.1 myr. This chemical divide in the glacial Southern Ocean separated well-ventilated water above similar to2500 m from poorly ventilated water below. The Delta(13)C(I-D) signal parallels the Vostok atmospheric pCO(2) record for the last 400 kyr, lending support to physical models that invoke changes in Southern Ocean deep water ventilation as a mechanism for changing atmospheric pCO2. The emergence of a strong 100-kyr cycle in Delta(13)C(I-D) during the mid-Pleistocene supports a change in vertical fractionation and deep-water ventilation rates in the Southern Ocean, and is consistent with possible CO2-forcing of this climate transition.