264-6 The Role of Production, Dilution, and Decomposition of Marine Organic Matter in the decoupling of δ13CTOC and δ13CCARB patterns during the Givetian-Frasnian Transition, Northern Appalachian Basin

Abstract:

Sageman et al.’s seminal paper “A Tale of Shales” outlined the roles of production, dilution, and decomposition in controlling the formation of organic-rich strata in the Devonian Appalachian Basin, with key insights into black shale deposition throughout Earth history. As part of this and Sageman et al.’s companion papers, unique geochemical profiles associated with the Geneseo Formation in the Akzo #9455 Core were noted. For example, a ~2‰ negative shift in δ¹³C_TOC during Geneseo black shale deposition was not related to changing proportions of marine versus terrestrial organic matter fluxes even though the detrital sediment influx proxy Ti/Al showed a slight increase through the Geneseo. This negative shift in δ¹³C_TOC values has since been recognized as a global shift to some of the lightest δ¹³C values in the Devonian coincident with evidence for warming during the Givetian-Frasnian transition.

We build upon Sageman et al.’s work on the Akzo #9455 Core Geneseo Formation with a comparable dataset from the more proximal Cargill #17 Core that also includes a δ¹³C_CARB profile that shows a negative shift to relatively extreme values near -5‰. However, in the Lansing #17 Core δ¹³C_TOC values show a shift to lighter values only in the lower Geneseo equivalent. A decoupling of δ¹³C_TOC and δ¹³C_CARB patterns occurs in the upper Geneseo-equivalent, coarser-grained Sherburne Formation with a δ¹³C_TOC shift to more positive values while δ¹³C_CARB values become further depleted to extremely light values. Unlike the basinal settings of the Akzo #9455 Core, in proximal settings the influx of terrestrial organic matter, and dilution of marine organic matter, imparts enough of a signal to influence δ¹³C_TOC values as seen in the Lansing #17 Core. Furthermore, the proximal record shows an increase in the detrital influx ratios of Si/Al and Ti/Al as well as a decrease in TOC values and Mo concentrations coincident with the transition to the coarser-grained Sherburne Formation and influx of terrestrial organic matter. Within the framework of Sageman et al.’s model for the formation of organic-rich strata, the decoupling of δ¹³C_TOC and δ¹³C_CARB patterns in combination with detrital influx and redox proxies suggests marine organic matter was likely influenced by increased dilution and possibly increased decomposition and decreased production during Sherburne Formation progradation in proximal settings.

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