Effects of Organic Carbon/Carbonate Burial Ratios and Biological Carbon Fixation on the Global Carbon Cycle Over the Past ~200 myr

The isotopic composition of the global carbon reservoir integrates large kinetic fractionations from photosynthesis with small thermodynamic fractionations from carbonate precipitation. We present concordant δ ¹³C records of carbonates (δ ¹³C_carb) and organic matter (δ ¹³C_org), along with new carbonate (C_carb) and organic carbonate (C_org) fluxes for the past ∼205 myrs (Jurassic-Cenozoic) generated from bulk sediment samples from the Atlantic. The new δ ¹³C_org record greatly refines previous compilations (Hayes et al., 1999) by providing a sample resolution of ∼100-300 kyrs. Model simulations using these δ ¹³C_carb and δ ¹³C_org data provide constraints on carbon sources (mantle and weathering) and sinks (carbonate and organic carbon sedimentation); comparisons with the flux records provide insight on the components of the geological carbon cycle. Stable isotope records indicate that long-term net depletion of ¹²C from mobile carbon reservoirs was a consequence of an organic carbon burial fraction increase of ∼0.05-0.1 that began in the Jurassic ( ∼200 Ma). Superimposed on the long-term trend are higher-order variations (5-10s of myrs) in δ ¹³C_carb and δ ¹³C_org that show episodic intervals of elevated values. In contrast to paleoceanographic convention, organic carbon burial is often decoupled from global δ 13C variations on the 5-10s of myrs scale. Brief episodes of elevated C_org flux tend to occur near the onset and cessation of these intervals of elevated δ ¹³C_carb and δ ¹³C_org values; prolonged episodes of elevated C_carb flux tend to correspond to the cessation of extended intervals of elevated δ ¹³C values. In the latter part of the Cenozoic, the development of β carboxylation and C₄ photosynthetic pathways in phytoplankton and terrestrial plants increasingly influenced δ ¹³C_org, ultimately contributing to the reversal of the long-term trend in δ ¹³C_carb. Thus, the geologic record of the global carbon cycle over the past 205 myr has been influenced by a combination of changes in carbonate burial, organic carbon burial, and biological fixation.