UPPER MANTLE OXIDATION AS A MECHANISM FOR INITIATING COUPLING BETWEEN BIOSPHERIC OXYGEN AND CARBON RESERVOIRS

The Proterozoic C-isotopic record reveals two distinct, interrelated trends: (1) stepwise increases in average d ¹³C from ~0‰_PDB in the late Paleoproterozoic to values near +3.5‰ in the mid-Mesoproterozoic to values >+5‰ in the mid-Neoproterozoic, and (2) a concomitant increase in isotopic excursion magnitude. These trends can be understood in terms of carbon production and burial in an ocean-atmosphere system evolving toward increasing pO₂ and decreasing carbon reservoir size, and can examined using a combination of steady-state and time-dependent models for isotopic change. Steady-state models suggest that increases in d ¹³C reflect a reorganization of carbon cycling associated with global oxygenation and a concomitant decrease in marine carbon reservoir size, resulting in a secular shift in C_org and C_carb partitioning. Time-dependent models suggest that decreases in marine carbon reservoir size also caused increased sensitivity of the global isotopic system to biogeochemical perturbations. Together, these trends produced the alternation of strongly positive values (exceeding +8‰) and strongly negative values (to less than –6‰) preserved in late Neoproterozoic successions worldwide.

A single large, positive carbon isotopic excursion at ~2.1 Ga stands out against this background. Not only does this excursion represent the largest recorded in Earth history (to values exceeding +10‰), but it occurred at a time when a large marine carbon reservoir would have been expected to buffer the carbon isotopic system against any but the largest perturbations. Additionally, carbon isotopic values before and after the event are markedly similar, indicating that carbon partitioning returns to pre-2.1 Ga equilibria. This excursion has been linked to initial oxidation of the Earth’s atmosphere and, perhaps, to oxidation of the upper mantle. We suggest that this event may have also initiated the fundamental linkages between C_org burial and atmospheric pO₂ that characterize the modern carbon cycle; prior to the Paleoproterozoic excursion, the capacity of volcanogenic oxygen sinks may have outstripped net biospheric oxygen production. After the event, atmospheric O₂ was intimately coupled to variation in C_org and C_carb partitioning at the Earth’s surface.