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

a r t i c l e i n f o Keywords: paleoceanography carbonate sedimentology aqueous geochemistry Observed changes in the source of CaCO 3 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 CaCO 3 precipitation on the seafloor is caused by a decrease in CaCO 3 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 CaCO 3 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 CaCO 3. Oxic oceans like the modern are characterized by large gradients in CaCO 3 saturation. Calcium carbonate precipitates where CaCO 3 saturation is high (surface ocean) and dissolves where CaCO 3 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 CaCO 3 saturation in the ocean and sediments. This effect suppresses CaCO 3 dissolution and promotes CaCO 3 precipitation on the seafloor. Our results suggest that the growth or contraction of gradients in CaCO 3 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 CaCO 3 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 CaCO 3 cycle. The abundance of carbonate minerals in marine sedimentary rocks since 3.43 Ga (Allwood et al., 2006) suggests that precipitation of CaCO 3 has been an important sink of alkalinity and CO 2 over much of Earth history. Reconstructions of ancient and modern carbonate depositional systems indicate that the large-scale features of shallow-water carbonate sedimentation (paleogeography, platform architecture , and sequence stratigraphy; Grotzinger and James, 2000) have changed little over the last 3 billion yrs, suggesting similar controls on global patterns of CaCO 3 deposition (e.g. climate, weathering, sedimentary accommodation and accumulation). Importantly, the continuity of CaCO 3 deposition has been maintained in spite of large changes in the sources of …