This study investigates the covariation between carbonate and organic δ 13 C values in a proximal to distal transect of four outcrops in the Madison Limestone in the Western United States Rockies, combined with δ 34 S values of carbonate associated sulphate, the concentration of acid-insoluble material and measurements of total organic carbon. These new geochemical datasets not only allow for an evaluation of carbon isotope covariance during one of the largest perturbations to the global carbon cycle over the past 550 Myr, but also constrain the cause of the excursion in carbonate δ 13 C values. The results support the hypothesis that a period of anoxia did not play a role in generating the positive carbonate δ 13 C values, but rather favour interpretations by previous workers that the proliferation of land plants destabilized the Carboniferous carbon cycle, setting the stage for a significant change in the carbonate δ 13 C values of contemporaneous marine carbonates. These results also demonstrate that one of the largest perturbations to the global carbon cycle did not produce synchronous variations in carbonate and organic δ 13 C values, emphasizing the importance of local depositional controls on carbon isotope covariance in the geological record in both modern and ancient environments.
- carbonate associated sulphate
- organic carbon
- stable isotopes
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