Dolomite and nahcolite formation in organic rich sediments: isotopically heavy carbonates

High positive C isotopic ratios in many carbonates coincide with high organic carbon contents. Nahcolites in the Green River oil shales have carbon isotopic ratios up to +20%. Some samples of organic rich Monterey and Pismo Formation dolomites also are ¹³C-rich (δ¹³C to +14%.). Combining the observations of unusual isotopic ratios and organic richness has led to proposed mechanisms of formation of nahcolites and dolomites. Bacterial fermentation produces ¹³C-rich carbon dioxide which in the organic rich sediments is formed in large amounts. This leads to chemical reasons for the occurrence of dolomite rather than calcite and nahcolite rather than trona in some shallow burials.