Is there a component of Pleistocene CO₂ change associated with carbonate dissolution cycles?

The only processes which could have decreased atmospheric CO 2 during glacial climates without appreciably changing the carbon isotope distribution in the sea appear to be (1) dissolved calcium carbonate input to the ocean, e.g., coral reef buildup and erosion, (2) the ocean solubility pump, due to changes in surface temperature or air‐sea exchange, or (3) decreased biological production of calcium carbonate. It is assumed here that one of these mechanisms caused part of the atmospheric CO 2 changes recorded in the 200‐kyr‐long Vostok ice core. Two residual CO 2 records are generated by scaling the δ 13 C difference between planktonic and benthic foraminifera in marine sediment cores to −82 ppm CO 2 per 1‰ increase in Δδ 13 C and subtracting from this the measured CO 2 concentration in the ice core. Both residual CO 2 records exhibit two broad maximums between about 20–50 ka and 140–200 ka, indicating that during these times, about 40 ppm of the CO 2 decrease from interglacial levels cannot be explained by the interaction of the ocean's biological and vertical mixing cycles. The shape of the residual CO 2 curve is similar qualitatively to the variation of calcium carbonate in central equatorial Pacific sediments during this time period, which would imply that changes in dissolved carbonate input to the ocean contributed the added component of CO 2 change. However, recent models of atmospheric CO 2 change in response to changing alkaline input to the ocean exhibit about a 25 to 35 ppm decrease per 10 13 mol yr −1 increase in dissolved CaCO 3 input. If compensation for the changing input is occurring mostly within an area of about 40 × 10 6 km² below the lysocline in the Indo‐Pacific, the change in carbonate accumulation rate corresponding to a −40 ppm CO 2 change would be a minimum of 3 mg cm −2 yr −1 . This can be compared to glacial 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 accumulation of carbonate does not seem to match quantitatively with the 40 ppm amplitude of the residual CO 2 signal, leading one to suspect that solubility plays a greater role than expected on the basis of a 2°C cooler surface ocean.