The geologic history of marine dissolved organic carbon from iron oxides

Dissolved organic carbon (DOC) is the largest reduced carbon reservoir in modern oceans¹,². Its dynamics regulate marine communities and atmospheric CO₂ levels³,⁴, whereas ¹³C compositions track ecosystem structure and autotrophic metabolism⁵. However, the geologic history of marine DOC remains entirely unconstrained⁶,⁷, hindering our ability to mechanistically reconstruct coupled ecological and biogeochemical evolution. To address this, we developed the first direct proxy for past DOC signatures using co-precipitated organic carbon in iron ooids, and we applied this to 26 marine iron ooid-containing formations deposited over the past 1650 million years. Predicted DOC concentrations were near modern levels in the Paleoproterozoic then decreased by 90-99% in the Neoproterozoic before sharply rising in the Cambrian. We interpret these dynamics to reflect three distinct states: (i) small, single-celled organisms combined with severely hypoxic deep oceans; (ii) larger, more complex organisms and little change in oxygenation; (iii) continued organism growth and a transition to fully oxygenated oceans⁸,⁹. Furthermore, modern DOC is significantly ¹³C-enriched relative to the Proterozoic, likely due to changing autotrophic fractionation driven by biological innovation; together with isotopically invariant carbon inputs to Earth's surface¹⁰, this implies increasing relative organic carbon burial through time. Our results reveal new connections between the carbon cycle, ocean oxygenation, and the evolution of complex life.