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.

Revisiting Selenium Interactions with Pyrite: From Adsorption to Coprecipitation

“…Se has emerged as a promising paleo-redox proxy in marine sediments due to its heightened concentration in anoxic environments, elevated redox potential, and sensitivity to both global and regional perturbations over relatively short time scales. , This study marks a significant advance in our understanding of ancient processes leading to selenium accumulation in pyrite within sulfidic environments. It reveals a pathway involving oxyanion reduction to Se(0) and Se­(−I), shedding light on intricate mechanisms.…”

Selenium Isotope Paleobiogeochemistry

“…In order for sediments to become enriched in heavy Se isotopes (δ 82/78 Se > +0.3), non-quantitative reduction must be operating prior to influx of Se into the basin, such that lighter Se isotopes are sequestered elsewhere and the residual pool of Se oxyanions becomes isotopically enriched. Such scenarios can be encountered in redox-stratified water masses (where Se can be sequestered in shallower sediments; Stüeken et al, 2015a;Kipp et al, 2017) or potentially in restricted basins that receive Se from upwelling (where Se is sequestered nearer to the upwelling source; Kipp et al, 2020).…”

“…In tandem, Se isotopes experience large fluctuations at these times, reflecting a shift in the prevalence of Se oxyanion reduction. Data were compiled in Stüeken et al (2015b), Mitchell et al (2016) and Kipp et al (2017Kipp et al ( , 2020.…”

Oxidation of Dissolved Tetravalent Selenium by Birnessite: Se Isotope Fractionation and the Effects of pH and Birnessite Structure

“…Selenium (Se) is a redox-sensitive element and plays a dual role in the modern environment as an essential micronutrient (Huawei, 2009) and a contaminant detrimental to aquatic wildlife (Ohlendorf et al, 1990;Schuler et al, 1990). It also serves as a paleoredox proxy (Schirmer et al, 2014;Kipp et al, 2020;Stüeken and Kipp, 2020). In nature, it exists in four oxidation states (+6, +4, 0, −2), each displaying different solubility and sorption characteristics.…”

“…Measurements of stable Se isotope ratios (e.g., 82 Se/ 76 Se; other ratios have also been used) have been developed as a means to improve understanding of Se biogeochemical transformations that control the mobility and bioavailability of Se in modern environments (Clark and Johnson, 2010;Zhu et al, 2014;Schilling et al, 2015), and a means to extract information about ancient redox conditions from rocks (e.g., Mitchell et al, 2012;Mitchell et al, 2016;Kipp et al, 2017;Kipp et al, 2020). Laboratory studies have investigated Se isotope fractionation induced by reduction, volatilization, and adsorption of Se, and it has been observed in both theoretical and empirical studies that Se isotopes are highly sensitive to redox reactions (Herbel et al, 2000;Johnson and Bullen, 2004;Li and Lui, 2011;Schilling et al, 2011;Mitchell et al, 2012;Schilling et al, 2020;Xu et al, 2020;Xu et al, 2021).…”