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

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 (CaCO 3 ) 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 CaCO 3 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 CO 2 emitted to the atmosphere. In the absence of any interaction between ocean and sediments, an equilibrium partitioning of CO 2 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 CO 2 emissions (223 ppm) are sequestered as bicarbonate ions (HCO 3 − ) by reaction with deep‐sea carbonates (“seafloor CaCO 3 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 CO 2 emissions (261 ppm) into HCO 3 − (“terrestrial CaCO 3 neutralization”) on a timescale of ∼8.3 ka. We have also assessed the importance of a changing climate in modulating the stabilization of atmospheric CO 2 through ocean‐sediment interaction. Increased ocean stratification suppresses particulate organic carbon export, which in turn enhances seafloor CaCO 3 preservation. The resulting reduction in the sequestration of fossil fuel CO 2 represents a new positive feedback on millennial‐scale climate change.