Reduced CaCO3 Flux to the Seafloor and Weaker Bottom Current Speeds Curtail Benthic CaCO3 Dissolution Over the 21st Century. Issue 12 (10th December 2019)

• Record Type: Journal Article
• Authors: Sulpis, Olivier; Dufour, Carolina O.; Trossman, David S.; Fassbender, Andrea J.; Arbic, Brian K.; Boudreau, Bernard P.; Dunne, John P.; Mucci, Alfonso
• Is Part Of: Global biogeochemical cycles. Volume 33:Issue 12(2019:Dec.)
• Page Start: 1654
• Page End: 1673
• Publication Date: 2019-12-10
• Subjects: CaCO3 -- dissolution -- ocean acidification -- RCP8.5 -- bottom currents; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2019GB006230
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Results from a range of Earth System and climate models of various resolution run under high‐CO2 emission scenarios challenge the paradigm that seafloor CaCO3 dissolution will grow in extent and intensify as ocean acidification develops over the next century. Under the "business as usual, " RCP8.5 scenario, CaCO3 dissolution increases in some areas of the deep ocean, such as the eastern central Pacific Ocean, but is projected to decrease in the Northern Pacific and abyssal Atlantic Ocean by the year 2100. The flux of CaCO3 to the seafloor and bottom‐current speeds, both of which are expected to decrease globally through the 21st century, govern changes in benthic CaCO3 dissolution rates over 53% and 31% of the dissolving seafloor, respectively. Below the calcite compensation depth, a reduced CaCO3 flux to the CaCO3 ‐free seabed modulates the amount of CaCO3 material dissolved at the sediment‐water interface. Slower bottom‐water circulation leads to thicker diffusive boundary layers above the sediment bed and a consequent stronger transport barrier to CaCO3 dissolution. While all investigated models predict a weakening of bottom current speeds over most of the seafloor by the end of the 21st century, strong discrepancies exist in the magnitude of the predicted speeds. Overall, the poor performance of most models in reproducing modern bottom‐water velocities and CaCO3 rain rates coupled with the existence of large disparities in predicted bottom‐water chemistry across models hampers our ability to robustly estimate the magnitude and temporal evolution of anthropogenic CaCO3 dissolution rates and the associated anthropogenic CO2 neutralization. Plain language summary: Carbon dioxide (CO2 ), produced and released to the atmosphere by human activities, has been accumulating in the oceans for two centuries and will continue to do so well beyond the end of this century if emissions are not curbed. One direct consequence of CO2 buildup in the ocean is the acidification of seawater. Calcite, a mineral secreted by many organisms living in the surface ocean to produce their shells and skeletons, covers a large part of the seafloor and acts as a natural antacid, neutralizing this excess CO2 . Model projections for the 21st century, under a "business as usual" scenario, reveal that seawater will become more corrosive to this mineral, but calcite dissolution at the seafloor will only increase slightly due to reductions in bottom‐current speeds and in the amount of calcite particles delivered to the seafloor over that period. These results indicate that the neutralization of human‐made CO2 by calcite dissolution at the seafloor may take longer than previously anticipated. Key Points: Reduced CaCO3 flux to the seafloor and weaker bottom‐current speeds curtail benthic CaCO3 dissolution over the 21st century Modeled bottom currents underestimate current meter observations by up to 90% Under RCP8.5, the mean calcite compensation depth may rise by ~800 m by the end of this century