Pacific Carbonate Dissolution Revisited

In order to understand the global CO2 cycle, it is important to be able to infer past ocean chemistry from the sedimentary carbonate record. Using a simple box model, we show that the modern sedimentary lysocline in the equatorial Pacific appears to be out of steady-state equilibrium with respect to water chemistry, as first suggested by Kier (1984). We attribute large excursions in lysocline depth to changes in carbonate chemistry and/or productivity fluxes. If carbonate ion changes in the deep sea have given rise to these lysocline fluctuations, the implied alkalinity change may explain as much as 22 ppm of atmospheric pCO2 change; about a quarter of the glacial-interglacial amplitude. We attribute the relative invariance of the depth of the CCD over the past 800 Ka to slow accumulation rates accompanying high dissolution fluxes. Relaxation times of the sediment mixed layer in these highly dissolved sediments are considerably longer than the timescale of glacial-interglacial cycles.A radiocarbon dilemma is manifested by an increase in the age of coretop calcite with water depth (and thus also with dissolution flux) below 4500 m in the equatorial Pacific. We present radiocarbon data from the Atlantic that suggest that this phenomenon is not produced by chemical erosion as has been previously suggested (Broecker et al., 1991). A fraction of the dissolution flux must occur at the sediment-water interface or by an equivalent mechanism.