Aftermath of a snowball Earth

Abstract

Using a simple 3‐box model of the ocean‐atmosphere system, we simulate the cycling of carbon and strontium in the aftermath of a global glaciation. Model simulations include the delivery of alkalinity to seawater from intense carbonate and silicate weathering under high pCO2 conditions as well as ocean mixing, air‐sea gas exchange, and biological productivity. The δ13C of the first carbonate precipitated after the glaciation depends on the pCO2, temperature, the saturation state of the surface ocean, and kinetic effects associated with mineral precipitation. With no biological productivity, the model produces δ13C values between +1‰ and −3‰, consistent with observations. This is in direct contradiction with arguments by Kennedy et al. [2001a], who suggest that the δ13C value of dissolved carbon in a snowball ocean (and directly afterward) must be −5‰. Kennedy et al. assume the carbon isotope cycle is in steady state, which does not apply to a global glaciation, and also neglect any effect of high pCO2 on the carbonate chemistry of seawater. A major difference between our findings and the qualitative predictions of Hoffman et al. [1998] is our interpretation of the cap dolostone as representing an interval dominated by carbonate weathering of exposed continental shelves. As a result, the ∼2‰ drop in the δ13C observed in the cap dolostone is unlikely to be the product of Rayleigh distillation of atmospheric CO2 via silicate weathering. Instead, we interpret the ∼2‰ drop in the δ13C values as indicative of an increase in sea surface temperature which lowers the fractionation between CO2 and carbonate. Kinetic isotope effects associated with rapid precipitation from a highly supersaturated surface ocean may also be important. Rayleigh distillation of atmospheric CO2 via silicate weathering is a viable explanation for the continued drop in the δ13C values in the limestone sequence above the cap dolostone, with biological productivity and carbonate weathering driving a slow increase in δ13C values once pCO2 levels decline. Our study also simulates the cycling of strontium in seawater. In contrast to the finding of Jacobsen and Kaufman [1999] and Kennedy et al. [2001a], model simulations show a drop in 87Sr/86Sr of less than 0.001 during 5 million years of global glaciation and an increase of less than 0.001 over the entire episode of silicate weathering. Our calculations emphasize the importance of considering the changes in seawater chemistry due to high pCO2 in evaluating the Snowball Earth hypothesis.