RECOGNIZING A GLOBAL NEGATIVE CARBON ISOTOPE EXCURSION IN THE EARLY DEVONIAN

Carbon isotope stratigraphy is routinely utilized to understand changes in the global carbon budget, as a proxy for atmospheric oxygen levels, and as a high-resolution stratigraphic correlation tool. In this study, we test whether the Early Devonian (Lochkovian-Pragian) carbon isotope trends in central Nevada represent a global carbon isotope signature and thus can be used in subsequent global ocean redox studies.

Deep-water limestones in the Simpson Park Range were sampled every 1-5 meters over a 280 meter-thick interval. The Lochkovian stage (~6 My) is characterized by a >4‰ negative excursion and the Pragian stage (<3 My) records relatively constant values of about 2‰. These trends are similar to previously reported Early Devonian carbon isotope curves from Spain, the Czech Republic, Ukraine, and the eastern U.S. indicating that the central Nevada record represents a global signal. The origin of the excursion is not well understood because a eustatic sea-level rise occurred during the Lochkovian and Pragian so erosion of depleted carbon during a regression cannot explain the observed negative shift. The ~6 My duration of the excursion also cannot be explained by methane release or volcanism because these processes are not likely to continue for such long time intervals.