Authigenic carbonate burial and its impact on the global carbon cycle: a case study from Late Devonian strata of the Western Canada Sedimentary Basin




Gazdewich, Sean

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It has been hypothesized that authigenic carbonate minerals, formed within the pore spaces of marine siliciclastic formations during early diagenesis, may have had a substantial influence on the global carbon cycle, particularly in times of low oxygen in Earth history. According to this idea, alkalinity is generated via anaerobic organic matter degradation, resulting in carbonate oversaturation and the precipitation of low δ13C carbonate cements. If a substantial amount of 13C-depleted carbonate was sequestered in this authigenic sink, the δ13C of dissolved inorganic carbon (DIC) in the global ocean would be driven to more positive values without significant organic carbon burial - a signal which would be recorded in marine carbonates. Research presented herein tests this hypothesis from newly acquired lithostratigraphic and coupled stable carbon and oxygen isotope data of Upper Devonian limestone and black shale formations preserved within the Western Canada Sedimentary Basin. The Late Devonian includes a mass-extinction event, and is characterized by pervasive ocean anoxia and a dramatic reduction in platformal carbonate sediment deposition. As such, it has been hypothesized to represent an ideal time for the emergence of an active authigenic carbonate sink. Results show that both basinal shale (Besa River and Exshaw formations) and platform carbonates (Wabamun Group and its equivalents), record a δ13C signal that is within the expected range of Devonian seawater (3‰ to -2‰), signifying that precipitated authigenic carbonate had no influence on the isotopic composition of DIC. It was observed, however, that evaporitic depositional settings can accumulate carbonate sediment with low δ13C values (down to -8.4‰), potentially caused by local water column organic matter respiration during prolonged water-mass residence in a restricted marginal marine setting. If such depositional environments were globally pervasive, such as during during global sea-level lows, it is plausible that the carbon isotope mass balance would be affected.



Authigenic Carbonate, Western Canada Sedimentary Basin, Global Carbon Cycle, Late Devonian