Equatorial Pacific Calcite Preservation Cycles: Production or Dissolution?

Abstract

In the Equatorial Pacific Ocean, the depth and shape of the calcite lysocline appears to have changed significantly in response to climate forcing (Farrell and Prell, 1989). Taking the Farrell and Prell record at face value, the lysocline apparently became steeper during each of the last eight glacial stages. Also, the accumulation rate of calcite during the last glacial maximum was 2–4 times higher than it was during the previous interglacial (Arrhenius; 1952,1988; Broecker, 1971; Farrell, 1991). A numerical model for calcite dissolution in sediment is used to interpret these observations. The model was validated by comparison with observations from the present‐day ocean; these results are presented elsewhere. If the lysocline fluctuations in the equatorial Pacific are primarily a dissolution record, then changes in the glacial Pacific [CO3] of 20–40 µM can be inferred. Here I offer the alternative explanation that cycles in equatorial production are responsible for the observations. Higher rates of calcite and organic carbon rain to the sediment in the equatorial region would have depressed the calcite lysocline and increased the calcite accumulation rate, as observed. A twofold increase in glacial production appears to be adequate to explain the observations, but a precise determination is prevented by uncertainties in some of the model parameters.