41-18 Deciphering marine versus terrestrial organic matter to apply organic carbon isotope stratigraphy for recognizing global carbon cycle perturbations: a Devonian case study

Session: Paleontology, Stratigraphy, Sediments, and Soils (Posters)

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Stable carbon isotope stratigraphy is a key tool in chronostratigraphy and provides important insights into carbon cycle fluctuations and climate in Earth’s past. Perturbations in the carbon cycle, recognized as marine carbon isotopic profile excursions and interpreted to represent changes in the ocean’s dissolved inorganic carbon (DIC) content, have temporal extents that in many cases are shorter than biozone durations and therefore extremely useful for stratigraphic correlation and chronostratigraphy. In the Geologic Time Scale 2020 (GTS2020), variations in carbonate carbon isotopic composition (δ¹³C_carb) are compiled for the last 2.3 billion years and this compilation serves as an important reference for placing newly generated carbon isotope profiles within a global chronostratigraphic framework. In comparison, the organic carbon isotopic record through this same time interval is much less complete, possess multi-million-year gaps, and a comprehensive reference profile does not exist. Another complication of using organic carbon isotopic composition (δ¹³C_org) for chronostratigraphy is that in any given study area it is not always clear what component of the organic carbon being analyzed is marine versus terrestrial origin, which complicates using δ¹³C_org as a proxy for changes in the ocean’s DIC. This is especially problematic given that many major carbon isotopic excursions in Earth’s history are linked to eutrophication and black shale deposition, which is a carbonate-poor lithofacies in which δ¹³C_carb analysis cannot be easily applied.

In this study, elemental data collected by pXRF are used as proxies for detrital sediment input and paleoredox and then compared with δ¹³C_org chemostratigraphy trends to constrain when δ¹³C_org values most likely reflect DIC of the ocean and therefore represent a global carbon cycle record. This method is then applied to Devonian black shale successions in the Appalachian, Illinois, and Michigan basins, which when combined with independent biostratigraphic data, allows recognition of carbon isotopic excursions associated with global Devonian bioevents. This study presents a novel approach for reconstructing the global carbon cycle in carbonate-poor black shale facies that has applications throughout Earth’s history and implications for δ¹³C_org chemostratigraphy inclusion in future editions of the GTS.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

doi: 10.1130/abs/2026SE-13292

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