Is there a component of Pleistocene CO2change associated with carbonate dissolution cycles?

摘要

The only processes which could have decreased atmospheric CO2during 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 CO2changes recorded in the 200‐kyr‐long Vostok ice core. Two residual CO2records are generated by scaling the δ13C difference between planktonic and benthic foraminifera in marine sediment cores to −82 ppm CO2per 1‰ increase in Δδ13C and subtracting from this the measured CO2concentration in the ice core. Both residual CO2records exhibit two broad maximums between about 20–50 ka and 140–200 ka, indicating that during these times, about 40 ppm of the CO2decrease from interglacial levels cannot be explained by the interaction of the ocean's biological and vertical mixing cycles. The shape of the residual CO2curve 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 CO2change. However, recent models of atmospheric CO2change in response to changing alkaline input to the ocean exhibit about a 25 to 35 ppm decrease per 1013mol yr−1increase in dissolved CaCO3input. If compensation for the changing input is occurring mostly within an area of about 40 × 106km² below the lysocline in the Indo‐Pacific, the change in carbonate accumulation rate corresponding to a −40 ppm CO2change would be a minimum of 3 mg cm−2yr−1. This can be compared to glacial increases of 0.5 to 1.0 mg cm−2yr−1during 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 CO2signal, leading one to suspect that solubility plays a greater role than expected on the bas