Biological overprint of the geological carbon cycle

The oxidation of Earth's atmosphere is coupled to the net sequestration of organic matter, which is related to the relative fractions of organic carbon (f_org) and carbonate (f_carb) buried in marine sediments. These fractions can be inferred from carbon isotope data. We present bulk sediment δ¹³C records of carbonate (δ¹³C _carb) and organic carbon (δ¹³C_org) with a compilation of evolutionary trajectories of major eucaryotic phytoplankton for the past 205 million years. Our analysis indicates that changes in phytoplankton community structure, coupled with the opening of the Atlantic Ocean basin and global sea-level rise, increased the efficiency of organic carbon burial beginning in the Early Jurassic; in turn, this organic carbon burial increased the oxidation state of Earth's surface while drawing down atmospheric CO ₂ levels (assuming no substantial negative feedbacks). The net oxidation and CO₂ drawdown appear to be related to the opening phase of the current Wilson cycle, where the newly formed passive plate margins store organic matter for hundreds of millions of years. This process should reverse during the closing phase of the Wilson cycle, when the continents reassemble and the Atlantic Ocean basin closes. The associated oxidation and storage of organic matter have contributed to the long-term depletion of CO₂, which was a key factor that selected C₄ photosynthetic pathways in marine and terrestrial ecosystems in the latter part of the Cenozoic; these pathways increasingly influenced δ¹³C_org, and ultimately contributed to the reversal of the long-term trend in δ¹³C_carb.