Abstract

Dissolved organic carbon (DOC) is the largest reduced carbon reservoir in modern oceans. Its dynamics regulate marine ecosystems and atmospheric CO2 levels, whereas 13C compositions track autotrophic metabolism. However, the geologic history of marine DOC remains entirely unconstrained. 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 25 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. Furthermore, Proterozoic DOC was significantly 13C-depleted relative to today. I will discuss how trends likely reflect coupled changes in ocean oxygenation, particle export flux, and autotrophic 13C fractionation through geologic time.