Carbon cycle models based on extreme changes in δ13C: an example from the lower Mississippian

Abstract

The Lower Mississippian δ13Ccarb excursion is one of the largest in the Phanerozoic, reaching ≥+7‰ in parts of western North America. Two new sections analyzed in Belgium and the Ural Mountains record the event, but peak values are ≤+5.5‰. The presence of key conodont zones allows for good correlation between Europe and North America, and indicates that a major time gap cannot account for the differences in δ13Ccarb. Patterns of diagenetic alteration are similar among sections and also do not appear to fully explain the lower values in European carbonate. The observed variability of several per mil, which is typical for Paleozoic excursions that are also recorded in epeiric sea carbonates, likely reflects a significant role for local carbon cycling in δ13C of dissolved inorganic carbon. Enhanced regeneration of phosphate under anoxic conditions in western North American basins is interpreted to have increased biological pumping of 12C and locally elevated δ13C relative to mean surface ocean water. The lower δ13C in the European sections is interpreted to be more representative of the global surface oceans, rather than having been influenced by influx of respired 12C during increased water mass residence time on the shelf. The interpretation of the influence of local carbon cycling on δ13Ccarb has implications for calculations of global organic carbon burial rates. Model input of a shift to values of ≥+7‰ predicts steady-state increases in the fraction of carbon buried as organic matter of as much as 50–75‰, assuming no changes in the riverine input. This would require a rapid buildup of extensive global sinks for Corg that could accommodate ∼1.5×1020 g of excess burial above and beyond the steady state during the ∼2 Myr event. However, use of a lower value for δ13Ccarb (closer to the +5.5‰ in European sections) in global models makes it easier to account for carbon storage in basins such as the Antler and related foreland basins of the western margin of Euramerica, in addition to the unknown amounts buried in deep-sea environments during the Early Mississippian.