Basinal hydrographic and redox controls on selenium enrichment and isotopic composition in Paleozoic black shales

Abstract

Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.