TESTING THE FIDELITY OF THE SILURIAN CARBON ISOTOPE RECORD AS A PROXY FOR THE GLOBAL CARBON CYCLE

Modern restricted carbonate platforms can have sediment δ¹³C_carb values that are decoupled from open ocean dissolved inorganic carbon (DIC) reservoirs. These studies indicate that caution is necessary when using δ¹³C_carb values from marine carbonates to interpret global processes. In light of the proliferation of high-resolution carbon isotope chemostratigraphy from ancient epeiric seas over the last few decades, we test the reliability of δ¹³C_carb records through some of the best-studied Paleozoic δ¹³C_carb excursions as proxies of the global carbon cycle. Published data, bolstered with the data from this study, allow an unprecedented evaluation of these isotope excursions using established methodologies from studies of modern and ancient carbonate depositional environments to evaluate the fidelity of our results as a proxy for processes related to the global carbon cycle.

We present a series of δ¹³C_carb transects spanning shallow ramp sections into their deep-water equivalents in four sedimentary basins, from two isolated paleocontinents. These transacts show a striking consistency in the range of values despite crossing a spectrum of paleogeographic and paleoenvironmental settings over hundreds of kilometers. Similarly, there is a remarkable correspondence between δ¹³C values from whole-rock, low-Mg brachiopod (extensively screened for evidence of diagenetic alteration) and organic carbon from these sections. Finally, the lockstep changes between δ¹³C_carb values, other chemical/redox/climatic proxies and mass extinction events are in keeping those expected with variations in open ocean DIC values.

Combined our new and published high-resolution δ¹³C_carb chemostratigraphy, sedimentology, and chrono-stratigraphic correlations, provide, arguably, some of the most thoroughly evaluated record of Paleozoic δ¹³C_carb excursion events. Given previous approaches developed from studies of modern and ancient carbonates to test the fidelity between measured δ¹³C_carb and open ocean DIC, our evaluation strongly suggests that the observed δ¹³C_carb excursions and coincident paleo-biologic, -geochemical, and -environmental changes are, in fact, responding to changes in the global carbon cycle.