Assessing the Utility of Selenium Isotopes as a Deep-Time Redox Proxy

Abstract:

Non-traditional isotopic systems, in particular molybdenum, chromium or iron, have become traditional tools in biogeochemical studies of ancient environments, raising hopes that selenium may join the group. Like sulfur, selenium is found in sulfide minerals and organic matter, it forms oxyanions under oxic conditions, and its isotopes are fractionated during dissimilatory reduction, albeit at relatively higher redox potential. One would therefore expect that redox changes over Earth’s history led to a response in selenium isotopes. For a first assessment, we analyzed selenium isotopes and abundances in black shales from marine and non-marine environments spanning the last 3.2 billion years (Stüeken et al. 2015). Combined with published data from the Phanerozoic eon, our results show that isotopic fractionations in bulk rocks are mostly between -2‰ and +2‰ in δ^82/78Se (relative to NIST SRM 3149, bulk Earth ~ 0‰) and thus significantly smaller than what has been reported from biological experiments (up to 25‰, Johnson & Bullen 2004). In the Precambrian before 1.0 Ga, marine data are mostly positive (+0.4 ± 0.4‰); negative values occur in non-marine settings in the late Archean. In the Phanerozoic, the mean of open marine samples drops to -0.2 ± 0.6 ‰ whereas restricted anoxic basins are on average positive (+0.2 ± 0.4 ‰). The range of fractionations is generally larger in samples with a molar Se_total:C_org ratio greater than that of marine biomass (up to ~5·10^-6, Mitchell et al. 2012). We conclude that selenium isotopes are sensitive to atmospheric and marine oxygenation events, but due to the scarcity and nutrient-type behavior of selenium in seawater, organic-binding likely dominates the selenium cycle, which leads to smaller net fractionations in black shales and a strong response to changes in biological productivity. The largest isotopic fractionations are probably observed where the selenium supply exceeds the biological demand due to a combination of enhanced oxidative selenium weathering, greater stability of selenium oxyanions in oxic waters, or declining biological assimilation. Selenium isotopes are therefore most powerful in combination with other biogeochemical proxies.