Stable W and Mo isotopic evidence for increasing redox-potentials from the Paleoarchean towards the Paleoproterozoic deep ocean

The scavenging of dissolved trace metals such as chromium (Cr), molybdenum (Mo) and tungsten (W) and their authigenic enrichment in sedimentary archives mainly occur by particle shuttling that cause resolvable isotope fractionation. Because their scavenging is also dependent on the local marine redox potential and the overall marine chemical environment, the stable isotope composition of these elements in Archean sediments is widely used as paleoredox proxy. Tungsten, a new element in this tool box, is dissolved as the oxyanion tungstate (WO42−) at extremely low marine redox potentials and can thus help to reconstruct the earliest changes in the marine redox state. We tested the applicability of stable W isotopes as a new paleoredox proxy of the atmosphere–ocean system. We analyzed the δ186/184W of Precambrian igneous rocks to investigate the detrital background δ186/184W signature. This Precambrian igneous inventory (PII) shows δ186/184W values between −0.007 and +0.097‰, identical to the range of modern igneous crustal rocks, which indicates that the average Earth’s crust δ186/184W remained constant over billions of years. Furthermore, we present δ186/184W values of euxinic sediments from the Black Sea as a modern sedimentary analog, which we then compare with δ186/184W data of Archean and Proterozoic black shales (3.47–2.3 Ga). Modern Black Sea sapropels reveal crustal-like δ186/184W values between +0.050 and +0.071‰ suggesting limited authigenic enrichment of W in euxinic environments. Similarly, post-Great Oxidation Event shales (2.3 Ga) show crustal- or PII-like δ186/184W values indicating the deposition in euxinic or oxic environment. However, the Archean shale sample suites show elevated W concentrations and fractionated δ186/184W values up to +0.246‰, which we attribute to authigenic enrichment of heavy seawater W in a ferruginous setting. Thus, black shales δ186/184W values can help to distinguish between anoxic-ferruginous and euxinic depositional environments. Furthermore, we combine previously published Cr and Mo isotope data with new Cr and W isotope data to quantify the evolution of the marine redox state during the Archean. By combining the Eh-pH stability fields of W and Mo with the δ186/184W and δ98/95Mo values of Precambrian sediments, we suggest a 3-step evolution of the Precambrian ocean. (1) From 3.47 to 3.0 Ga, the Eh of a dominantly ferruginous ocean was between −0.4 V and −0.25 V, enabling the persistence of soluble WO42− but not MoO42− and thus only the authigenic enrichment of isotopically fractionated W. (2) Starting from 3.0 Ga, the Eh of some shallow-marine environments increased above −0.25 V, as indicated by δ98/95Mo values above the detrital background in shallow-marine sediments, while the Eh of the deep ocean still remained below −0.25 V indicated by the lack of authigenic Mo enrichment and fractionated δ98/95Mo values in deep-sea shales. (3) After 2.7 Ga elevated δ98/95Mo and δ186/184W values in deep-marine sediments suggest that the Eh of the ferruginous open-ocean now also increased above −0.25 V. We therefore suggest that the combination of δ98/95Mo and δ186/184W values of shales is a very promising tool to investigate earliest changes in marine redox conditions during the Archean.