CaCO3 content in ocean surface sediments

Reconstructions based on back-tracked depths of the transition between CaCO3-bearing and CaCO3-depleted sediment suggest that over the last 65 million years the ocean's CaCO3 compensation depth (CCD) has remained within the range 4.0 ± 0.6 km. Taken at face value, these results indicate that depth of the calcite saturation horizon and hence also the product of the Ca2+ and [CO3]2- concentrations have remained nearly constant. In turn, this suggests that the ratio of calcite production to ingredient supply has not undergone large changes. In this paper, the method used to reconstruct the temporal fluctuations and interocean differences in the depth of the calcite saturation horizon is scrutinized. The basis for my concern is that offset between the depth of the calcite saturation horizon and the depth where dissolution reduces the CaCO3 content to 20% (i.e., the CCD) can vary by a half kilometer or so depending on the rain rates of CaCO3 and non CaCO3. When observations on modern ocean sediments are used to test the reliability of these reconstructions, the result is that when averaged over a full glacial cycle, the depth of the CCDs in the Pacific and Indian oceans fall within the long-term range. Reliable documentation of fluctuations around this long-term average requires that additional measurements, such as extent of fragmentation, be used to supplement those of CaCO3 content.