A diurnal carbon engine explains¹³C-enriched carbonates without increasing the global production of oxygen

Significance We present stable carbon isotope ( δ13 C) data from modern carbonate sediment that require a decoupling of the carbon cycles in the global ocean versus shallow carbonate shelves. This realization is important because, for the first 97% of Earth history, many inferences about global paleoclimate and seawater chemistry rely on interpretations of shallow carbonates. We use modern observations and a simple model to show how ordinary diurnal carbon cycling in shallow waters is sufficient to produce anomalously positive δ13 C on shelves today, and in the geological record. Our results alleviate the need to interpret positive δ13 C excursions in the geological record as global reorganizations of the carbon cycle and instead link δ13 C to local and/or global paleoenvironmental and paleoecological controls.