We measured the radiocarbon content and stable isotopic composition of pore water and bottom water ΣCO 2 , sedimentary organic carbon, and CaCO 3 at two sites on the Ceara Rise, one underlying bottom water that is supersaturated with respect to calcite (Site B), the other underlying undersaturated bottom water (Site G). The results were combined with pore water O 2 , ΣCO 2 , and Ca 2 + profiles (Martin and Sayles, 1996) to estimate the radiocarbon content of the CaCO 3 that is dissolving in the sediment mixed layer. At Site G, the CaCO 3 that is dissolving in the upper 2 cm of the sediments is clearly younger (richer in 1 4 C) than the bulk sedimentary CaCO 3 , indicating that nonhomogeneous CaCO 3 dissolution occurs there. The case for nonhomogeneous dissolution is much weaker at the site underlying supersaturated bottom water. The results indicate that nonhomogeneous dissolution occurs in sediments underlying undersaturated bottom water, that the dissolution is rapid relative to the rate of homogenization of the CaCO 3 in the mixed layer by bioturbation, and that the dissolution rate of CaCO 3 decreases as it ages in the sediment mixed layer. The results support the hypothesis, based on solid phase analyses, that the preferential dissolution of young (i.e., radiocarbon-rich) CaCO 3 leads to a pattern of increasing radiocarbon age of mixed-layer CaCO 3 as the degree of undersaturation of bottom water increases (Keir, 1984; Broecker et al., 1991).