BIOLOGICAL OVERPRINT OF THE GEOLOGICAL CARBON CYCLE

The isotopic composition of the global carbon reservoir integrates large kinetic fractionations from photosynthesis with small thermodynamic fractionations from carbonate precipitation. It often is assumed that these two pools approach a steady state on time scales >100 million years (myr). We present the first concordant d¹³C records of carbonates (d¹³C_carb) and organic matter (d¹³C_org) together with a new compilation of photosynthetic phytoplankton diversity for the past 205 million years. Our analysis reveals that d¹³C_carb increased by 1.1‰ over ~190 myrs beginning in the Jurassic, while the photosynthetic fractionation factor remained ~-29.5‰ until ~30 Ma. This requires a net depletion of ¹²C from mobile carbon reservoirs as a consequence of an organic carbon burial fraction increase of ~0.05-0.1. This depletion corresponds to the radiation of major eucaryotic phytoplankton in the Mesozoic. In the latter part of the Cenozoic, the development of b-carboxylation and C₄ photosynthetic pathways in phytoplankton and terrestrial plants increasingly influenced d¹³C_org, ultimately contributing to the reversal of the long-term trend in d¹³C_carb. Hence, biologically driven isotopic fractionation of the carbon reservoir has overprinted the geological carbon cycle.