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Precambrian Carbonate Platforms: A Database Approach to Querying Carbonate Deposition Through Time

Abstract

The sedimentary record reveals that the large-scale aspects of carbonate deposition have remained unchanged over > 3.4 Ga of Earth History. This reflects long term commonalities in the sources of DIC and alkalinity to seawater and the processes that generate and fill accommodation in sedimentary basins. Despite this stability, the record also reveals important first order changes in the nature of seafloor carbonate precipitation and early diagenesis through time. This is documented in a range of carbonate lithofacies that have a distinct distribution in time including carbonate crystal fans (large bladed crystals formed at the sediment-water interface), molar tooth calcite, giant ooids and abundant early fabric retentive dolomite. We assembled a database of ~150 carbonate platform successions from Precambrian and Cambrian strata worldwide to better quantify the nature of carbonate sedimentation and environmental change through time. Each platform succession is captured with a high-resolution, decameter to meter scale, integrated vertical column through preserved shallow to deep depositional environments. Details of lithofacies, including a range of microbial fabrics, mineralogy, depositional environmental, age and location are recorded for each platform. We will present an integrated assessment of changes at the small and large scale to carbonate platforms through the Precambrian and Cambrian. We hypothesize the carbonate rock record through this transition can best be explained by a combination of changing oxygen, and sulfate concentrations and temperature, with some influence from the evolving biota. Observed carbonate patterns were likely driven by changing redox conditions at the sediment water interface and in the shallow sediments which altered the relative importance of carbonate dissolving remineralization pathways (i.e. aerobic respiration) and carbonate precipitating remineralization pathways (i.e. anaerobic respiration via iron and manganese reduction). In addition, we propose Precambrian early fabric retentive dolomite requires a temporal model for dolomitization that is dependent on low sulfate conditions. Without sulfate inhibition, in shallow evaporitic settings with significant aragonite precipitation, dolomitization could have closely followed initial sediment formation because of Mg enrichment over Ca.