Close mass balance of long-term carbon fluxes from ice-core CO2 and ocean chemistry records

Feedbacks controlling long-term fluxes in the carbon cycle and in particular atmospheric carbon dioxide are critical in stabilizing the Earth's long-term climate. It has been hypothesized that atmospheric CO2 concentrations over millions of years are controlled by a CO2-driven weathering feedback that maintains a mass balance between the CO2 input to the atmosphere from volcanism, metamorphism and net organic matter oxidation, and its removal by silicate rock weathering and subsequent carbonate mineral burial(1-4). However, this hypothesis is frequently challenged by alternative suggestions, many involving continental uplift and either avoiding the need for a mass balance or invoking fortuitously balanced fluxes in the organic carbon cycle(5-9). Here, we present observational evidence for a close mass balance of carbon cycle fluxes during the late Pleistocene epoch. Using atmospheric CO2 concentrations from ice cores(10-12), we show that the mean long-term trend of atmospheric CO2 levels is no more than 22 p. p. m. v. over the past 610,000 years. When these data are used in combination with indicators of ocean carbonate mineral saturation to force carbon cycle models, the maximum imbalance between the supply and uptake of CO2 is 1-2% during the late Pleistocene. This long-term balance holds despite glacial-interglacial variations on shorter timescales. Our results provide support for a weathering feedback driven by atmospheric CO2 concentrations that maintains the observed fine mass balance.