Authigenic Carbonate Burial Within the Late Devonian Western Canada Sedimentary Basin and Its Impact on the Global Carbon Cycle

Stable carbon isotope ratios (delta 13C values) of marine carbonates are widely used to infer the relative burial rates of organic carbon, a source of oxygen to the ocean-atmosphere system. This inference, however, is based on the assumption that ocean-atmospheric carbon is buried either as organic carbon or as marine carbonate minerals. The burial of authigenic carbonate minerals formed within sediments after deposition, with low delta 13C values (i.e., similar to organic carbon), has been proposed to explain high delta 13C values in marine carbonates without the need for high burial fluxes of organic carbon. To test this hypothesis, we focus on the Late Devonian, a time period with both pervasive ocean anoxia and a severe reduction in shallow-water carbonate deposition-conditions hypothesized to promote authigenic carbonate formation. We present sedimentological and geochemical data from limestones and black shales of the Wabamun Group, Besa River and Exshaw formations of the Western Canada Sedimentary Basin. These data are compared to inorganic and organic weight percent measurements of North American shales acquired from the USGS National Geochemical Database (N = 4,437). Results show that basinal shale lack authigenic carbonate with low delta 13C values and that the mean delta 13C value of carbonate in these shales (-0.3 parts per thousand) do not differ substantially from mean delta 13C of carbonates in platform carbonates of a similar age (0.4 parts per thousand). Furthermore, inorganic carbon content in Late Devonian shales (mean weight percent = 0.55%, N = 54) is lower than average Phanerozoic North American shale (mean of 1.95%, N = 4,055). Lastly, organic carbon-to-inorganic carbon ratios (OC:IC) of North American shales are well above 1 (mean = 3.72 for Late Devonian shales (N = 374), 2.25 for shales (N = 3,653) of all other ages). Therefore, even if the burial of fine-grained siliciclastic formations carrying authigenic carbonates were to increase, the concomitant increase in organic carbon burial would be even larger. Together, data from this study do not provide evidence that the burial of authigenic carbonate would have a significant effect on global carbon isotope mass balance. Stable carbon isotope ratios (delta 13C) of marine carbonate rocks are widely used as a proxy for estimating the relative amount of organic carbon buried throughout geological time. Certain instances in the geologic record indicate pronounced delta 13C excursions for which there is no corroborating evidence for enhanced organic carbon burial. It has been proposed that these excursions may be the result of high burial rates of authigenic carbonate-carbonate minerals formed in situ within marine sediments. We test this hypothesis by analyzing the stable carbon isotope composition of Late Devonian limestones and shales of the Western Canada Sedimentary Basin. We also compare the organic and inorganic carbon compositions of shales from this field study with those from the USGS Geochemical database, an archive of North American shale geochemistry data from across the Phanerozoic. Results show that carbonates found in Devonian shales do not have low average delta 13C values, and average organic carbon content was shown to be significantly higher than inorganic carbon in shale samples across the Phanerozoic. These results signify that authigenic carbonate burial had a negligible effect on the global carbon cycle, and furthermore, an increase in shale burial would result in a higher increase in organic carbon compared to authigenic carbonate. CaCO3 found in Late Devonian shales has average carbon isotope values similar to Late Devonian shallow marine carbonates Weight percent values of inorganic carbon content in Late Devonian shales are low compared to average North American shale The mass ratio of organic to inorganic carbon in Late Devonian shales is much higher than average North American shale