Selenium redox cycling during weathering of Se-rich shales: A selenium isotope study

Isotope Fractionation Processes of Selected Elements

“…Mitchell et al (2012) observed in marine shales with low organic carbon content a small range in δ 82 Se values, whereas in black shales with high Se concentrations larger Se isotope variations occur (Wen and Carigman 2011). In a profile of very Se-rich carbonaceous shales, Zhu et al (2014) observed a range in 82/76 Se-values from −14.2 to +11.4 ‰, suggesting multiple cycles of oxidation and reduction.…”

Selenium Isotopes as a Biogeochemical Proxy in Deep Time

“…Malisa 2001). Unusually high concentrations (up to 26,000 ppm) and large isotopic fractionations of >20‰ were reported from a weathering profile through selenium-rich pyritic black shale (Zhu et al 2014). A slightly smaller isotopic range (up to 7‰) was described from another seleniferous soil with up to 4 ppm selenium (Schilling et al 2015).…”

“…On the other hand, soils with concentrations closer to average crust (up to 0.5 ppm) showed a much smaller range of fractionations (±0.25‰, Schilling et al 2011a). An important factor influencing the isotopic behavior of selenium in soils appears to be the abundance of organic matter and other reductants that re-reduce selenium oxyanions deeper in the soil profile (Zhu et al 2014;Schilling et al 2015). This and other processes further affect selenium uptake into plants (Winkel et al 2015).…”

Selenium Speciation in Framboidal and Euhedral Pyrites in Shales

“…5,9,10 In Se-enriched surface environments, shales are an important source of Se to soils. 5,8,11,12 However, the transfer of Se from shale rocks into soils and groundwaters 5 and ultimately to humans via the food chain 6 is poorly constrained. Furthermore, the role of geological settings or specific geochemical reactions leading to the sequestration of Se in shale components are also poorly understood.…”

“…15 However, neither the importance of OM as a Se host, nor a clear correlation between Se and OM in typical black shales, with average lower Se concentrations, have so far been demonstrated. Conversely, several studies proposed pyrite as the possible Se host mineral in shales, 5,8,16,17 yet a mechanistic or speciation related explanation is still lacking. Experimental studies 18−21 suggested that Se is likely either adsorbed/reduced on preformed pyrite grains or structurally incorporated into the sulfide phase.…”