Sensitivity of the epeiric sea carbon isotope record to local-scale carbon cycle processes: Tales from the Mohawkian Sea

Ancient marine carbon isotope records derived from epeiric sea carbonates can bear a strong influence of local-scale carbon cycling processes. Across the Late Ordovician Mohawkian Sea of eastern Laurentia, the carbon isotope compositions (δ¹³C) of contemporaneous sediments range from - 2‰ in the interior of the sea to + 2.5‰ along the margin. Here we use a box model to explore the relative importance of carbon fluxes within the Mohawkian Sea from a mass balance perspective. In the model, the Mohawkian Sea carbon reservoir is coupled to but distinct from the ocean and atmosphere carbon reservoirs. At initial steady state conditions, the residence time of carbon in the Mohawkian Sea is less than 10 years. We find that to produce model δ¹³C values representative of the range in δ ¹³C across the Mohawkian Sea we must divide the sea into an inner region and an outer region and limit dissolved inorganic carbon (DIC) exchange between the two to 15% of the initial steady state unrestricted exchange rate. It is also necessary to supply low δ¹³C carbon (- 29‰) to the inner region of the sea at a rate of 17.4 × 10¹² mol C yr^{- 1}. The required flux of low δ¹³C carbon could not have been sustained over the long term by remineralization of organic carbon stored in sediments of the inner region of the Mohawkian Sea, and the geographic δ¹³C pattern is unlikely to have been generated by upwelling along the Sebree Trough. Rather, the low δ¹³C flux might have been derived from bryophyte-dominated terrestrial ecosystems, suggesting an important role for terrestrial carbon cycling prior to the advent of vascular plants. An interesting feature of the model is that the isotopic compositions of both the epeiric sea and the larger ocean-atmosphere system are sensitive to changes in exchange rates within the epeiric sea. This has important implications for interpreting global δ¹³C records.