Authigenic carbonate burial in the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA)

Abstract

Late Devonian (Famennian) marine successions globally are typified by organic-rich black shales deposited in anoxic and euxinic waters and the cessation of shelf carbonate sedimentation. This global 'carbonate crisis', known as the Hangenberg Event, coincides with a major extinction of reefbuilding metazoans and perturbations to the global carbon cycle, evidenced by positive carbon-isotope excursions of up to 4&. It has been suggested that authigenic carbonate, formed as cements in sedimentary pore spaces during early burial diagenesis, is a significant mass fraction of the total global carbon burial flux, particularly during periods of low oxygen concentration. Because some authigenic carbonate could have originated from remineralization of organic carbon in sediments, it is possible for this reservoir to be isotopically depleted and thereby drive changes in the carbon isotopic composition of seawater. This study presents bulk isotopic and elemental analyses from fine-grained siliciclastics of the Late Devonian-Early Mississippian Bakken Formation (Williston Basin, USA) to assess the volume and isotopic composition of carbonates in these sediments. Carbonate in the Bakken black shales occurs primarily as microscopic disseminated dolomite rhombs and calcite cements that, together, comprise a significant mass-fraction (ca 9%). The elemental composition of the shales is indicative of a dynamic anoxic to sulphidic palaeoenvironment, likely supported by a fluctuating chemocline. Despite forming in an environment favourable to remineralization of organic matter and the precipitation of isotopically depleted authigenic carbonates, the majority of carbon isotope measurements of disseminated carbonate fall between À3& and +3&, with systematically more depleted carbonates in the deeper-water portions of the basin. Thus, although there is evidence for a significant total mass-fraction of carbonate with contribution from remineralized organic matter, Bakken authigenic carbonates suggest that Famennian black shales are unlikely to be sufficiently 13 C-depleted relative to water column dissolved inorganic carbon to serve as a major lever on seawater isotopic composition.

Chemotrophy-based phosphatic microstromatolites from the Mississippian at Drewer, Rhenish Massif, Germany

“…2019; Barnes et al . 2020). A lack of positive δ 13 C excursions during the Middle and Late Devonian of the Drewer section was noted by Buggisch & Joachimski (2006), who attributed this lack to a stratigraphic gap where no carbonates were deposited.…”

“…Carbon isotope stratigraphy from the Devonian-Carboniferous boundary suggests high rates of organic carbon burial and drawdown of atmospheric carbon dioxide as evidenced by positive δ 13 C excursions across this interval from localities in Europe, China and the USA (Buggisch & Joachimski, 2006;Myrow et al 2011;Kumpan et al 2014;Kaiser et al 2015;Qie et al 2015). These excursions show a variety of δ 13 C peaks between þ2 and þ6‰ and are difficult to correlate globally as they differ between various regions and basins, which makes a reliable reconstruction of the carbon isotope composition of global ocean DIC difficult (Kaiser et al 2015;Paschall et al 2019;Barnes et al 2020). A lack of positive δ 13 C excursions during the Middle and Late Devonian of the Drewer section was noted by Buggisch & Joachimski (2006), who attributed this lack to a stratigraphic gap where no carbonates were deposited.…”

“…2019; Barnes et al . 2020). Facies changes are recorded at Drewer starting with the latest Famennian nodular Wocklum Limestone and the calcareous, laminated Drewer Sandstone (Luppold et al .…”

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

“…Mean wt % IC (both directly measured via coulometry and inferred from IRMS peak sizes) for all sections is 0.55% (the average for coulometry-measured is 0.76% and for IRMS-inferred is 0.42%; Figure 6a). These values, however, are low when compared to the Lower Bakken Formation, which overlaps in age with the Exshaw and was deposited in an adjacent intracratonic sedimentary basin (Barnes et al, 2020) and shows much higher wt % IC values (average = 6.8%; Figure 6b). This comparison suggests that intracratonic basins could host a higher burial flux of authigenic carbonate relative to more open marine depositional settings.…”

“…The presence of positive δ 13 C excursions in globally dispersed basins (Cramer et al., 2008; Myrow et al., 2011) and the widespread deposition of organic‐rich black shales at the Devonian‐Carboniferous boundary, often referred to as the Hangenberg Crisis (Becker et al., 2016; Kaiser et al., 2006), suggest a significant perturbation of the carbon cycle. Supplementary geochemical proxies indicate pervasive global ocean anoxia at this time, including high concentrations of framboidal pyrite in black shales (Barnes et al., 2020; Marynowski et al., 2012; Schieber & Baird, 2001) and trace metal enrichment (Barnes et al., 2020; Riquier et al., 2006; Scott et al., 2017), in particular redox‐sensitive elements that accumulate in anoxic sediments (e.g., Mo, U, and V). The crash in shelf carbonates is especially significant because mass balance dictates that if alkalinity is not being buried as marine carbonates, then another alkalinity sink (potentially authigenic carbonate) must compensate to balance the input from chemical weathering.…”