Uranium and V isotope variations in Archean sediments: Evidence for metal mobilization and enzymatic reduction in an overall anoxic world?

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In the proposed study we want to analyse U and V isotope compositions of black shales, carbonates and banded iron formations (BIFs) to explore the redox evolution of the oceans prior to the great oxidation event (GOE). We also aim to obtain evidence for the first occurrence of metal reducing microorganisms on Earth. Microorganisms that gain energy by the oxidation of organic carbon, while reducing sulfur, sulfate or oxidized metal, may be among the oldest life forms on Earth. These may have existed in relatively oxygen-rich lagoons that may have existed already a few 100 Ma before the GOE as indicated by several studies. Many of these studies used concentrations and isotope compositions of redox sensitive elements that are immobile at anoxic conditions, but are mobilized and enriched in the oceans and sediments under more oxic conditions, including e.g. S, Cr, Mo and also U. Uranium isotopes may be a particular exciting redox indicator, as, according to several investigations (including recent findings of the groups around Bernier Latmani and Weyer), enzymatic U reduction appears to result in a distinct U isotope signature that is different to that of abiotic U reduction. Little is yet known about V isotope fractionation during low-temperature redox reactions; however, our preliminary results on black shales indicate that significant isotope fractionation occurs during V reduction and that isotopic signatures of V can be coupled with those of U.In the proposed project, we want to use the variations of U and V isotope in Archean and early Proterozoic black shales, carbonates and banded iron formations to search for the first evident of redox-related mobilization of redox sensitive metals. In combination with previous findings from other redox tracers, we want to characterize the conditions during metal mobilization and subsequent reduction and deposition. In particular, we will search for evidence, if metal-reduction was enzymatically-driven. We will focus on samples across the Archean-Proterozois transition and the GOE, including black shales, for which other metal isotopic redox tracers (e.g. Mo, Cr) indicate mobilization and subsequent reduction of these metals, carbonates which have previously been investigated (or will be in a parallel project) for C and S isotopes, as well as Archean BIFs which have previously been characterized by other redox proxies. Furthermore, we will also investigate older samples, e.g. the 3.5 Ga old sediments from the Dresser formation (Western Australia), for which S isotopes indicate microbial S reduction. The overall goal will be to examine the coupling of U and V isotopes with the isotopic signature of other redox-sensitive elements to gain information on (1) redox-related metal mobilization and (2) the contribution of biotic versus abiotic reduction.