PACIFIC CARBONATE DISSOLUTION REVISITED

In order to understand the global CO2 cycle, it is important to be abl e to infer past ocean chemistry from the sedimentary carbonate record. Using a simple box model, we show that the modern sedimentary lysocli ne in the equatorial Pacific appears to be out of steady-state equilib rium with respect to water chemistry, as first suggested by Kier (1984 ). We attribute large excursions in lysocline depth to changes in carb onate chemistry and/or productivity fluxes. If carbonate ion changes i n the deep sea have given rise to these lysocline fluctuations, the im plied alkalinity change may explain as much as 22 ppm of atmospheric p CO2 change; about a quarter of the glacial-interglacial amplitude. We attribute the relative invariance of the depth of the CCD over the pas t 800 Ka to slow accumulation rates accompanying high dissolution flux es. Relaxation times of the sediment mixed layer in these highly disso lved sediments are considerably longer than the timescale of glacial-i nterglacial cycles. A radiocarbon dilemma is manifested by an increase in the age of coretop calcite with water depth (and thus also with di ssolution flux) below 4500 m in the equatorial Pacific. We present rad iocarbon data from the Atlantic that suggest that this phenomenon is n ot produced by chemical erosion as has been previously suggested (Broe cker et al., 1991). A fraction of the dissolution flux must occur at t he sediment-water interface or by an equivalent mechanism.