MODELING THE CALCITE LYSOCLINE

A numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea. Model results are compared with lysocline data from 13 regions in the Atlantic, Pacific, and Indian Oceans. The model lysocline shape is sensitive to the calcite dissolution rate constant, the calcite, organic carbon, and refractory material rates, and the rates of oxic versus anoxic organic carbon degradation in the sediment. The model is able to reproduce the observed lysocline, Within the constraints of the sediment trap and calcite accumulation data, using a calcite dissolution rate constant of 30-100%d-1, a molar ratio of organic carbon to calcite rain rates of 0.5-1.0, and an initial CaCO3 fraction of 90% (excluding organic carbon). This rate constant is consistent with microelectrode results presented by Archer et al. [1989]. The model predicts that 30-50% of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration, consistent with previous modeling studies. The range in the best fit value of the organic-inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material, rather than from scatter in the data. Lysocline data from the western equatorial Atlantic are anomalous to the rest of the data; this anomaly may be explained by high rates of refractory material sedimentation from the Amazon River.