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

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 δ 13 C, whereas the amplitude of the δ 13 C signal at Site 1088 (∼2100 m water depth) is small. Unlike the deep sites, at no time during the Pleistocene were benthic δ 13 C values at Site 1088 lower than those of the deep Pacific. Reconstruction of intermediate‐to‐deep δ 13 C gradients (Δ 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 ∼2500 m from poorly ventilated water below. The Δ 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 pCO 2 . The emergence of a strong 100‐kyr cycle in Δ 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 CO 2 ‐forcing of this climate transition.