The widespread occurrence of coupled carbonate dissolution/reprecipitation in surface sediments on the Bahamas Bank

Abstract

Using two complimentary approaches (pore water advection/diffusion/ reaction modeling and stable isotope mass balance calculations) we show that carbonate dissolution/reprecipitation occurs on early diagenetic time scales across a broad range of sediments on the Great Bahamas Bank. The input of oxygen into the sediments, which strongly controls sediment carbonate dissolution, has two major sources—belowground input by seagrasses (that is, seagrass O2 pumping), and permeability-driven advective pore water exchange. The relative importance of these O2 delivery mechanisms depends on both seagrass density, and on how bottom water flow interacts with the seagrass canopy and leads to this advective exchange. Dissolution appears to involve the preferential dissolution of high-Mg calcite, and the rates of dissolution increase linearly with increasing seagrass density. Isotopic evidence of dissolution/reprecipitation is consistent with the occurrence of Ostwald ripening as the mechanism of reprecipitation, in which smaller crystals dissolve and then reprecipitate as larger crystals, with little or no change in mineralogy. Estimates of the aerially-integrated dissolution flux on the Bahamas Bank suggest that carbonate dissolution is an important loss term in the budget of shallow water carbonate sediments, and that on-bank carbonate dissolution, rather than offshore transport, may represent an important sink for gross shallow water carbonate production. Dissolution in carbonate bank and bay sediments may also be a significant alkalinity source to the surface ocean, and should be considered in global alkalinity/carbonate budget. Finally, coupled dissolution/reprecipitation may have a major impact on the stable isotope composition of carbonate sediments that are ultimately preserved in the rock record. These processes may therefore need to be considered, for example, when using carbon isotope records to obtain information on the operation of the global carbon cycle during the Phanerozoic.