Trace metal enrichment in 1.98 Ga black shales of the Zaonega Formation

Abstract

The trace metal Molybdenum (Mo) has seen widespread use in estimating ocean redox conditions throughout the rock record, due to its bimodal behaviour in euxinic versus non-euxinic waters. The size of the marine Mo pool is determined by the extent of sea bottom euxinia and the rate of terrestrial oxidative weathering of sulfide minerals (Robbins et al. 2016). Large-scale studies of the rock record have found an uptick in the Mo concentrations of black shales up to 80 ppm in the Paleoproterozoic, which has been attributed to an increase in oxidative weathering and oxic marine conditions following the rapid rise of oxygen during the Great Oxidation Event (C. Scott et al. 2008). While the basic principles of Mo cycling are relatively well-understood, the specific mechanisms controlling the removal of Mo into sediments are a subject of much debate, mainly involving the roles of iron oxides and organic matter in providing adsorption sites for Mo. Furthermore, it is not known whether organic matter acts as a shuttle of Mo from the water column to the sediments or whether Mo is preferentially removed below the sediment-water interface (Dahl et al. 2016). Here are presented data on the upper section of the NW Russian Zaonega Fm organic-rich shales, which contain unprecedented Mo concentrations (up to 2000 ppm) for this age. Whole-rock and in-situ measurements of δ 98 Mo and [Mo] coupled with detailed lithological and geochemical controls allow for a thorough evaluation of the mechanisms controlling the redox-sensitive trace metal budget in Precambrian black shales. The contributions of different Mo transport mechanisms, the role of hydrothermal input, the recrystallization of sulfide species and the input of methanotrophic organic matter are considered, with important implications for global paleoredox models based off of the trace metal systematics of black shales.