The radiocarbon age of calcite dissolving at the sea floor: Estimates frompore water data

We measured the radiocarbon content and stable isotopic composition of pore water and bottom water Sigma CO2, sedimentary organic carbon, and CaCO3 at two sites on the Ceara Rise, one underlying bottom water that is supersatu rated with respect to calcite (Site B), the other underlying undersaturated bottom water (Site G). The results were combined with pore water O-2, Sigm a CO2, and Ca2+ profiles (Martin and Sayles, 1996) to estimate the radiocar bon content of the CaCO3 that is dissolving in the sediment mixed layer. At Site G, the CaCO3 that is dissolving in the upper 2 cm of the sediments is clearly younger (richer in C-14) than the bulk sedimentary CaCO3, indicati ng that nonhomogeneous CaCO3 dissolution occurs there. The case for nonhomo geneous dissolution is much weaker at the site underlying supersaturated bo ttom water. The results indicate that nonhomogeneous dissolution occurs in sediments underlying undersaturated bottom water, that the dissolution is r apid relative to the rate of homogenization of the CaCO3 in the mixed layer by bioturbation, and that the dissolution rate of CaCO3 decreases as it ag es in the sediment mixed layer. The results support the hypothesis, based o n solid phase analyses, that the preferential dissolution of young (i.e., r adiocarbon-rich) CaCO3 leads to a pattern of increasing radiocarbon age of mixed-layer CaCO3 as the degree of undersaturation of bottom water increase s (Keir, 1984; Broecker et al., 1991). Copyright (C) 2000 Elsevier Science Ltd.