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Constraining the Importance of Authigenic Carbonate in the Global Carbon Cycle: A Case Study From the Bakken Formation

Abstract

The Late Devonian is characterized by several important environmental perturbations and includes the Hangenberg event, characterized by a mass extinction, marine anoxia, and a ~+5‰ positive carbon isotopic excursion (+CIE) in marine carbonates. The Hangenberg is also associated with the global occurrence of black shale deposits. Recent work has hypothesized that authigenic carbonates, often formed diagenetically during the remineralization of sedimentary organic matter, are a volumetrically significant and isotopically depleted sink for carbon, and thus important for interpreting the environmental implications of isotopic records. Because black shales are a likely host for authigenic carbonates, it is possible that the Hangenberg +CIE was driven by increased rates of organic carbon and authigenic carbonate burial, a model that has not yet been treated quantitatively. Here we estimate the mass and isotopic composition of carbonates in the Upper Devonian-Lower Mississippian Bakken Formation. Powdered samples were collected from three cores covering the Williston Basin depocenter and margin and were analyzed for bulk mineralogical and stable carbon and oxygen isotopic composition. Carbonates represent a significant mass within the unit – the Lower and Upper black shale members are, on average, ~9 wt.% authigenic carbonate and the Middle member calcareous siltstone is ~48 wt.% carbonate, of which approximately 60% is detrital grains and 40% is carbonate cements. Carbon isotopic measurements from gas-source mass spectrometry range from -6.5‰ to +3.9‰ (VPDB). The basin center and basin margin cores show a positive d13C shift of +3.0‰ and +3.3‰, respectively, from the Lower Black shale into the Middle siltstone. This positive excursion may reflect the Hangenberg +CIE. However, the basin center core yields d13C values that are isotopically lighter by approximately 2‰ relative to the basin margin core. There is also a weak negative correlation between TOC and d13C, suggesting that authigenic carbonate produced from remineralized organic matter is mixing with marine carbonate to cause systematic but small negative offsets in the d13C of the basin center relative to the basin margin. Thus, authigenic carbonates in Late Devonian black shales are unlikely to be a strong lever for driving positive isotopic shifts in the carbon isotopic composition of seawater, but they remain a significant sink for inorganic carbon that must be considered in global carbon cycle models.