Redox conditions and depositional environment of the Lower Jurassic Bächental bituminous marls (Tyrol, Austria)

A suite of trace elements (TEs) characterized by an affinity to reducing environments, including molybdenum (Mo), uranium (U), vanadium (V), copper (Cu), and nickel (Ni), were used to investigate secular variations in environmental redox conditions during deposition of the Bachental bituminous marls (Bachentaler Bitumenmergel). These marls, which belong to the Sachrang Member of the Lower Jurassic Middle Allgau Formation in the Northern Calcareous Alps, are subdivided on the basis of Al-normalized TE concentrations and concentration ratios into three units that correspond to major shifts in depositional redox conditions. The lower and upper units (Units 1 and 3, respectively) were deposited under suboxic to transiently anoxic conditions. The removal of TEs to the sediment was connected to siliciclastic input, diagenetic processes, and organic-matter accumulation in the sediment. In contrast, the middle unit (Unit 2) was characterized by intense anoxia as reflected in strong TE enrichments and peak total organic carbon (TOC) contents up to 13 %. Enrichment of TEs was linked mainly to adsorption onto organic matter in Unit 2, reflecting control of TE enrichment by the availability of an organic substrate. Comparison of inorganic and organic (e. g., pristane/phytane ratio, gammacerane index) geochemical proxies for environmental conditions permits detection of periods with elevated surface-water bioproductivity. A flourishing of algal and planktonic organisms contributed to the significant increase of TOC content at the base of Unit 2. This increase corresponds to the time-equivalent early Toarcian oceanic anoxic event characterized by the global occurrence of organic-rich sediments.