The hyper-enrichment of V and Zn in black shales of the Late Devonian-Early Mississippian Bakken Formation (USA)

Abstract

Black shales of the Late Devonian to Early Mississippian Bakken Formation are characterized by high concentrations of organic carbon and the hyper-enrichment (>500 to 1,000s of mg/kg) of V and Zn. Deposition of black shales resulted from shallow seafloor depths that promoted rapid development of euxinic conditions. Vanadium hyper-enrichments, which are unknown in modern environments, are likely the result of very high levels of dissolved H 2 S (~10 mM) in bottom waters or sediments. Because modern hyper-enrichments of Zn are documented only in Framvaren Fjord (Norway), it is likely that the biogeochemical trigger responsible for Zn hyper-enrichment in Framvaren Fjord was also present in the Bakken basin. With Framvaren Fjord as an analogue, we propose a causal link between the activity of phototrophic sulfide oxidizing bacteria, related to the development of photic-zone euxinia, and the hyper-enrichment of Zn in black shales of the Bakken Formation.

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

“…The elemental data are presented normalized to the conservative element Al, which reduces error from absolute measurement variation (Calvert & Pedersen, ), and are compared to an ‘average’ shale composition from Wedepohl () for reference (Tables and S1). In the black shales each of the redox proxies is significantly enriched and Mn/Al is depleted; of particular note are the Mo and Zn data, which exhibit ‘hyper‐enrichments’ two to three orders of magnitude higher than the reference shale (Scott et al ., ). These proxies’ stratigraphic trends through the black shale members display noticeable fluctuations and generally covary (Fig.…”

“…The Bakken Formation certainly meets the criterion for organic‐rich sediments: more than 600 compiled TOC analyses from the black shale members (Table ) average 10 wt.% and range up to 25%. This richness is likely due to a combination of elevated primary productivity driven by nutrient upwelling (Caplan & Bustin, ; Smith & Bustin, ) and increased preservation afforded by rapid delivery of organic matter (OM) to the sea floor in a shallow anoxic basin (Scott et al ., ). Water‐column productivity in the Williston Basin also accounts for large silica enrichments in the black shales (Figs and A) which suggest blooms of radiolarian zooplankton (Egenhoff & Fishman, ).…”

“…Most palaeoenvironmental interpretations propose that the Bakken black shales were deposited in a stratified, restricted basin where bottom‐water anoxia was maintained by increased primary productivity and a coincident marine transgression (Lineback & Davidson, ; Smith & Bustin, ). An alternative model for the black shales’ organic preservation is water‐column euxinia – oxygen‐depleted and sulphide‐rich conditions – throughout the Williston Basin (Scott et al ., ). Long‐term anoxia in the bottom waters may have promoted sulphate‐reducing microbial (SRM) communities which drove sulphide concentrations higher, forcing euxinic waters up into the photic zone.…”

“…The shoaling of the chemocline has been previously proposed as a Late Devonian extinction mechanism (Kump et al ., ; Marynowski & Filipiak, ) and hypothesized in neighbouring epeiric basins (Formolo et al ., ). Euxinic conditions account for the Bakken enrichment of trace metals which are sensitive to anoxic–sulphidic sedimentation (Algeo & Tribovillard, ; Scott et al ., ), as well as abundant aryl isoprenoids (Requejo et al ., ; Jiang et al ., ; Aderoju & Bend, ). These biomarkers are a proxy for Chlorobiaceae , obligate phototrophic green sulphur bacteria which live in photic‐zone sulphidic environments (Summons & Powell, ).…”

Global biogeochemical cycle of vanadium

“…The 25-fold increase in the V concentration of coal, compared with the plant tissue from which it forms, likely results from the affinity of V for organic material under reducing conditions and its retention as these materials are compacted and lithified to form coal (e.g., ref. 41). Using this average V concentration in coal, and estimates of global increases in coal production, we estimate that V extraction associated with coal mining has increased from ∼100 × 10 9 g V/y at the end of the 20th century to 180 × 10 9 g V/y to 190 × 10 9 g V/y in 2014-2016 (Fig.…”