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

Abstract

[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 (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 À1 , in line with recent budget estimates of 0.10À0.14 PgC yr À1 . 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 À ) 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 À (''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.Citation: Ridgwell, A., and J. C. Hargreaves (2007), Regulation of atmospheric CO 2 by deep-sea sediments in an Earth system model, Global Biogeochem. Cycles, 21, GB2008,

Presentation, calibration and validation of the low-order, DCESS Earth System Model (Version 1)

“…12d). For the case above with constant weathering, the calcite burial decrease accounts for all the increase of this flux in the "terrestrial neutralization" process Ridgwell and Hargreaves, 2007). Model calcite burial decrease is accompanied by decreasing sedimentation velocities (Fig.…”

“…For example, comprehensive Earth System Models of high complexity and spatial resolution built around AtmosphereOcean General Circulation Models can nowadays be integrated over thousands of years and have been extended to include land and ocean biogeochemical cycling, important over these time scales (Cox et al, 2000;Doney et al, 2006;Schmittner et al, 2008). Only very recently have comprehensive Earth System Models of intermediate complexity and spatial resolution been developed that can be integrated over tens of thousands of years and that have been extended to include ocean sediments, important over these longer time scales (Ridgwell and Hargreaves, 2007;Brovkin et al, 2007).…”

Carbonate compensation dynamics

“…[ [Ridgwell and Hargreaves, 2007]. If, however, [CO 3 ] D increases, as in the geological past, the distance between z sat and z cc will asymptote to $0.68 km, or a ratio of about 0.9.…”

“…[5] Previously z cc has been computed laboriously by modelling the calcite content of sediments with ocean depth using diagenetic models [e.g., Schink and Guinasso, 1977;Takahashi and Broecker, 1977;Sundquist, 1990;Archer and Maier-Reimer, 1994;Ridgwell and Hargreaves, 2007]. These models predict that the zone of dissolution in the sediment is on-the-order-of a few millimeters at most.…”

Geoengineering climate by stratospheric sulfur injections: Earth system vulnerability to technological failure

“…The worst-case scenario would be to burn all the available fossil fuel within the next several centuries, increasing atmospheric CO 2 concentration up to 1,400-1,800 ppmv. Globally-averaged annual surface air temperature could be warmer than today by 5 • C for thousands of years (Archer and Brovkin 2008;Ridgwell and Hargreaves 2007), leading to the collapse of the major ice sheets and sea level rise ultimately of tens of meters. To avoid extreme climate change, current policy debates focus on a reduction in carbon emissions through increased energy efficiency and a shift from fossil fuels to renewable sources of energy, such as wind, solar, and geothermal energy, and biofuels, possibly complemented by large-scale use of nuclear energy.…”