Glacial water mass geometry and the distribution of δ 13 C of ΣCO 2 in the western Atlantic Ocean

Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin. The cores span water depths from about 400 to 3000 m and intersect the major water masses in this region. These new data fill a critical gap in the South Atlantic Ocean and provide the motivation for updating the classic glacial western Atlantic δ 13 C transect of Duplessy et al. (1988). The distribution of δ 13 C of ΣCO 2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (∼1000 m), southern source water mass with an end‐member δ 13 C value of about 0.3–0.5‰ VPDB, a middepth (∼1500 m), northern source water mass with an end‐member value of about 1.5‰, and a deep (>2000 m), southern source water with an end‐member value of less than −0.2‰, and perhaps as low as the −0.9‰ values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal δ 18 O. A revised glacial section of deep water δ 13 C documents the positions and gradients among these end‐member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.