Do erosion rates control the long-term carbon isotope mass balance?

Abstract:

The long-term marine carbon isotope record responds to changes in the proportional burial rates of organic carbon relative to carbonate carbon on a global scale. For this reason, high

δ¹³C values in marine carbonate rocks are normally interpreted to reflect faster rates of organic burial and increased atmospheric oxygenation. Geochemical redox tracers fail to support this paradigm for sustained deviations from the long-term δ¹³C mean, indicating perhaps that proportionally high organic burial may be associated with lower overall flux rates. Here I propose that ~10⁷-10⁸ year trends in average δ¹³C, as with seawater ⁸⁷Sr/⁸⁶Sr, are driven by changes in the balance between volcanism and denudation (~uplift). In other words, high proportional organic burial may be related to increases in the net CO₂ flux (= organic carbon burial + Ca-Mg silicate weathering) relative to the carbonate weathering flux. According to this model, high baseline δ¹³C values will be associated with periods of heightened volcanic activity and/or diminished tectonic uplift. Conversely, lower baseline δ¹³C values can be related to times when the global carbon cycle was dominated by carbonate and oxidative weathering due to high rates of physical erosion. Shorter 10⁵-10⁶ year positive δ¹³C excursions have also been interpreted as the ‘smoking gun’ to extreme oxygenation events. However, large increases in organic burial are difficult to sustain under steady-state conditions without very high volcanic fluxes, indicating that some of these excursions might be better explained by transient changes to the isotopic composition of carbon sources and sinks.