INVITED: UNDERSTANDING PROTEROZOIC CARBON CYCLE EVOLUTION: IMPLICATIONS OF RESERVOIR SIZE

The Proterozoic C-isotopic record reveals two distinct, interrelated trends: stepwise increases in average δ¹³C and a concomitant increase in the magnitude of isotopic excursions. A combination of steady-state and time-dependent models for isotopic change provides a unique perspective by which to view Proterozoic carbon cycle evolution. Steady-state models suggest that stepwise increases in average δ¹³C represent primary reorganizations of the marine carbon cycle resulting from changes in C_org and C_carb partitioning, driven by a combination of organic evolution, pO₂ change, and a secular decrease in marine carbonate saturation.

Time-dependent modeling suggests that a decreasing marine C reservoir size (M_o) through the Proterozoic also acted to increase sensitivity of the global isotopic system to biogeochemical perturbations. By examining patterns of isotopic change across individual excursions, this time-dependent model provides first-order constraints on M_o. Short-lived isotopic excursions marking the terminal Proterozoic are constrained to have lasted ~1 Myr, suggesting that M_o was unlikely to have been more than 2x present levels at the end of the Proterozoic. C-isotope excursions in the latter Mesoproterozoic and early Neoproterozoic, however, appear to record protracted recovery from a geologically instantaneous input of isotopically light C. This protracted recovery suggests that, by the latter Mesoproterozoic, M_o was at least 7-10x that of the modern ocean. Subdued isotopic variation recorded in older sedimentary successions suggests that M_o >10x modern levels characterized the earlier Proterozoic.

Finally, a single large C-isotope anomaly ~2.1 Ga stands out against this background. Not only does this excursion represent the largest recorded in Earth history, but it occurred when high M_o would be expected to have buffered the C-isotopic system against any but the largest perturbations. This excursion has been linked to initial atmosphere oxidation, yet other geochemical proxies (e.g., S-isotopes) indicate only minimal oxygenation of marine waters, suggesting a possible link between this excursion and lithospheric oxidation. In any case, this event may have marked the onset of the fundamental linkages between C_org burial and pO₂ that characterize the modern carbon cycle.