A diurnal carbon engine explains 13C-enriched carbonates without increasing the global production of oxygen

In the past 3 billion years, significant volumes of carbonate with high carbon-isotopic (delta C-13) values accumulated on shallow continental shelves. These deposits frequently are interpreted as records of elevated global organic carbon burial. However, through the stoichiometry of primary production, organic carbon burial releases a proportional amount of O-2, predicting unrealistic rises in atmospheric pO(2) during the 1 to 100 million year-long positive delta C-13 excursions that punctuate the geological record. This carbon-oxygen paradox assumes that the delta C-13 of shallow water carbonates reflects the delta C-13 of global seawater-dissolved inorganic carbon (DIC). However, the delta C-13 of modern shallow-water carbonate sediment is higher than expected for calcite or aragonite precipitating from seawater. We explain elevated delta C-13 in shallow carbonates with a diurnal carbon cycle engine, where daily transfer of carbon between organic and inorganic reservoirs forces coupled changes in carbonate saturation (Omega(A)) and delta C-13 of DIC. This engine maintains a carbon-cycle hysteresis that is most amplified in shallow, sluggishly mixed waters with high rates of photosynthesis, and provides a simple mechanism for the observed delta C-13-decoupling between global seawater DIC and shallow carbonate, without burying organic matter or generating O-2.