Regulation of atmospheric CO2 by deep‐sea sediments in an Earth system model

[1] We have extended the GENIE-1 Earth system model to include a representation of sedimentary stratigraphy and the preservation of biogenic carbonates delivered to the ocean floor. This has enabled us to take a novel approach in diagnosing modern marine carbon cycling: assimilating observation of the calcium carbonate (CaCO3) content of deep-sea sediments with an ensemble Kalman filter. The resulting calibrated model predicts a mean surface sediment content (32.5 wt%) close to the observed value (34.8 wt%), and a global burial rate of CaCO3 in deep sea sediments of 0.121 PgC yr−1, in line with recent budget estimates of 0.10−0.14 PgC yr−1. We employ the GENIE-1 model in quantifying the multimillennial-scale fate of fossil fuel CO2 emitted to the atmosphere. In the absence of any interaction between ocean and sediments, an equilibrium partitioning of CO2 is reached within ∼1000 years of emissions ceasing, with 34% (645 ppm) remaining in the atmosphere out of a total fossil fuel burn of 4173 PgC. An additional 12% of CO2 emissions (223 ppm) are sequestered as bicarbonate ions (HCO3−) by reaction with deep-sea carbonates (“seafloor CaCO3 neutralization”) on a timescale of ∼1.7 ka. Excess of carbonate weathering on land over deep-sea burial results in a further net transformation of 14% of CO2 emissions (261 ppm) into HCO3− (“terrestrial CaCO3 neutralization”) on a timescale of ∼8.3 ka. We have also assessed the importance of a changing climate in modulating the stabilization of atmospheric CO2 through ocean-sediment interaction. Increased ocean stratification suppresses particulate organic carbon export, which in turn enhances seafloor CaCO3 preservation. The resulting reduction in the sequestration of fossil fuel CO2 represents a new positive feedback on millennial-scale climate change.

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