Refining vanadium hyper-enrichment in black shales of the Late Devonian–Early Mississippian Bakken Formation (USA)

Abstract

Vanadium hyper-enrichment (V ≥ 500 μg g⁻¹) is widespread in ancient marine organic-rich sedimentary rocks and is controlled by its source and accumulation. To understand how V source and accumulation control V hyper-enrichment, the V hyper-enriched organic-rich black shales of the Late Devonian–Early Mississippian Bakken Formation (USA) were investigated here. The results imply that V in the Bakken Formation black shales is mainly derived from detritus and open ocean. Its hyper-enrichment was always related to the complexation of V(IV) species by organic matter and/or fine clay mineral adsorption, though extremely reducing (i.e. euxinic) depositional conditions sometimes assisted in accumulating V through forming V(IV)–sulphur structure and/or V(III). Vanadium is a frequently used seawater redox indicator, but its hyper-enrichment in marine sediments/sedimentary rocks does not always need to be redox driven. This study, thus, reminds us to be more careful when reconstructing palaeoceanographic redox states using V-involved proxies.