Recognition of global carbon cycle perturbations in the Middle-Late Devonian New Albany Shale, south-central Indiana, U.S.A.: Deciphering local versus global influences on δ13CTOC patterns

Abstract

Stable carbon isotope stratigraphy is an integral tool for identifying Devonian global events. Carbon-cycle perturbations (excursions) tend to have geologically short durations that aid in chemo- and chronostratigraphic correlation, but also provide insight as a proxy for extinction-causing anoxia (eutrophication) and climate change. In carbonate-dominated successions, carbonate carbon isotopes (δ¹³C_carb) provide a relatively reliable record of sea-water isotopic composition. However, since many Devonian global events are associated with anoxia, most geological records consist of black shale-dominated successions and therefore analysis of organic carbon isotopes (δ¹³C_TOC) may be the only method available for reconstructing the carbon cycle. Interpreting δ¹³C_TOC results is complicated, though, by the presence of marine and terrestrial sources of organic matter in many depositional settings; furthermore, differentiating these sources can be time-consuming and costly. This study utilizes δ¹³C_TOC, elemental detrital input and paleoredox proxies, and analysis of total organic carbon from the SDH-474 Core from Bartholomew County, Indiana, to recognize Devonian global events in the marine Middle-Late Devonian New Albany Shale. A novel schematic model is presented for integrating detrital input proxy data with δ¹³C_TOC patterns to decipher the role of local versus global carbon fluxes. Global carbon cycle perturbations are then identified within the context of conodont and palynomorph biostratigraphic data. In the SDH-474 Core, the Late Frasnian semichatovae Event and Late Frasnian into Early Famennian Lower and Upper Kellwasser events are recognized. The Early Frasnian Middlesex and/or Middle Frasnian Rhinestreet Events, as well as the Famennian Nehden, Condroz, and annulata events may also be represented in the core, but their identification is hindered by potential local terrestrial influence on the reconstructed δ¹³C_TOC record and/or a lack of a high-resolution global δ¹³C reference for these events.