Oxygenation of the ocean and sediments: Consequences for the seafloor carbonate factory

Observed changes in the source of CaCO₃ sediments since Archean time suggest a first order pattern of decreasing abundance of carbonate cements precipitated directly on the seafloor. We propose that the observed reduction in CaCO₃ precipitation on the seafloor is caused by a decrease in CaCO₃ saturation in sediments related to increased oxic cycling of organic carbon and a decline in the size of the marine DIC reservoir. Using a simple model of CaCO₃ saturation in the ocean, we show that changes in ocean-atmosphere redox and the size of the marine carbon reservoir strongly influence the ability of sediments to dissolve or precipitate CaCO₃. Oxic oceans like the modern are characterized by large gradients in CaCO₃ saturation. Calcium carbonate precipitates where CaCO₃ saturation is high (surface ocean) and dissolves where CaCO₃ saturation is low (sediments). In contrast, anoxic respiration of organic carbon and/or a large ocean carbon reservoir leads to a more homogeneous distribution of CaCO₃ saturation in the ocean and sediments. This effect suppresses CaCO₃ dissolution and promotes CaCO₃ precipitation on the seafloor. Our results suggest that the growth or contraction of gradients in CaCO₃ saturation in the ocean and sediments may explain the observed trends in carbonate precipitation on the seafloor in the Precambrian and changes in the global CaCO₃ cycle, such as the reappearance of seafloor precipitates and the drowning of carbonate platforms during episodes of widespread anoxia in the Phanerozoic marine basins. Our work provides novel insights into the consequences of the long-term geochemical evolution of the ocean and atmosphere for the global CaCO₃ cycle.