EFFECT OF MARINE CARBON RESERVOIR SIZE ON THE DURATION OF CARBON ISOTOPE EXCURSIONS: INTERPRETING THE MESOPROTEROZOIC CARBON ISOTOPE RECORD

The Proterozoic C-isotope record shows two distinct, interrelated trends: (1) a series of stepwise increases in average δ¹³C, and (2) concomitant increases in the magnitude of isotopic excursions. This large-scale pattern can be understood in terms of a combination of steady-state and time-dependent models for isotopic change, in which fundamental shifts in C_org and C_carb partitioning reflect a combination of biological innovation and a decrease in the size of the marine dissolved inorganic carbon (DIC) reservoir. Because DIC reservoir size strongly affects the sensitivity of the marine system to transitory biogeochemical perturbations, time-dependent modelling of both the magnitude of isotopic excursions and the duration of these excursions can help us to constrain changes in the size of the marine carbon reservior through Proterozoic time.

Here we examine a single carbon isotope excursion recorded in late Mesoproterozoic (~1200 Ma) carbonate strata from the Bylot Supergroup (Arctic Canada), the Turukhansk Group (Siberia), and the Atar Group (Mauritania). The excursion is marked by an abrupt 5‰ drop over ~25 meters of section, followed by a stepwise increase toward initial δ¹³C values. In contrast to the initial perturbation, recovery to pre-excursion isotopic compositions occurs over 250-400 meters of stratigraphic section. The characteristic shape of this excursion suggests a geologically instantaneous input of isotopically light carbon to the marine system (i.e. input occurring over much less than one residence time of carbon), followed by a systematic recovery to original marine compositions (i.e. recovery over 3-5x the residence time of carbon). Estimated accumulation rates of 35-50 meters per million year for peritidal platformal carbonates of the Turukhansk and Atar groups suggests a duration of 5-7 million years for this excursion (thicker strata preserved in the Bylot Supergroup likely reflect increased subsidence associated with localized basin rifting). The extended duration of isotopic recovery suggest that the Mesoproterozoic marine DIC reservior size was at least 10x that of the modern ocean. Similar excursions in the terminal Proterozoic are constrained in duration to ~1 My, suggesting that marine DIC levels may have dropped considerably during the Neoproterozoic.