IS THERE A COMPONENT OF PLEISTOCENE CO2 CHANGE ASSOCIATED WITH CARBONATE DISSOLUTION CYCLES

The only processes which could have decreased atmospheric CO2 during g lacial climates without appreciably changing the carbon isotope distri bution in the sea appear to be (1) dissolved calcium carbonate input t o the ocean, e.g., coral reef buildup and erosion, (2) the ocean solub ility pump, due to changes in surface temperature or air-sea exchange, or (3) decreased biological production of calcium carbonate. It is as sumed here that one of these mechanisms caused part of the atmospheric CO2 changes recorded in the 200-kyr-long Vostok ice core. Two residua l CO2 records are generated by scaling the delta(13)C difference betwe en planktonic and benthic foraminifera in marine sediment cores to -82 ppm CO2 per 1 parts per thousand increase in Delta delta(13)C and sub tracting from this the measured CO2 concentration in the ice core. Bot h residual CO2 records exhibit two broad maximums between about 20 - 5 0 ka and 140 - 200 ka, indicating that during these times, about 40 pp m of the CO2 decrease from interglacial levels cannot be explained by the interaction of the ocean's biological and vertical mixing cycles. The shape of the residual CO2 curve is similar qualitatively to the va riation of calcium carbonate in central equatorial Pacific sediments d uring this time period, which would imply that changes in dissolved ca rbonate input to the ocean contributed the added component of CO2 chan ge. However, recent models of atmospheric CO2 change in response to ch anging alkaline input to the ocean exhibit about a 25 to 35 ppm decrea se per 10(13) mol yr(-1). increase in dissolved CaCO3 input. If compen sation for the changing input is occurring mostly within an area of ab out 40 x 10(6) km(2) below the lysocline in the Indo-Pacific, the chan ge in carbonate accumulation rate corresponding to a -40 ppm CO2 chang e would be a minimum of 3 mg cm(-2) yr(-1). This can be compared to gl acial increases of 0.5 to 1.0 mg cm(-2) yr(-1) during the last 200 kyr in central equatorial Pacific sediments. Thus, the added glacial accu mulation of carbonate does not seem to match quantitatively with the 4 0 ppm amplitude of the residual CO2 signal, leading one to suspect tha t solubility plays a greater role than expected on the basis of a 2 de grees C cooler surface ocean.