Selenium Stable Isotope Investigation into Selenium Biogeochemical Cycling in a Lacustrine Environment: Sweitzer Lake, Colorado

Abstract

We present a comprehensive set of Se concentration and isotope ratio data collected over a 3-yr period from dissolved, sediment-hosted, and organically bound Se in a Se-contaminated lake and littoral wetland. Median isotope ratios of these various pools of Se spanned a narrow isotopic range (delta80/76Se(SRM-3149)) = 1.14-2.40 per thousand). Selenium (VI) reduction in the sediments is an important process in this system, but its isotopic impact is muted by the lack of direct contact between surface waters and reduction sites within sediments. This indicates that using Se isotope data as an indicator of microbial or abiotic Se oxyanion reduction is not effective in this or other similar systems. Isotopic data suggest that most Se(IV) in the lake originates from oxidation of organically bound Se rather than directly through Se(VI) reduction. Mobilization of Se(VI) from bedrock involves only a slight isotopic shift. Temporally constant isotopic differences observed in Se(VI) from two catchment areas suggest the potential for tracing Se(VI) from different source areas. Phytoplankton isotope ratios are close to those of the water, with a small depletion in heavy isotopes (0.56 per thousand). Fish tissues nearly match the phytoplankton, being only slightly depleted in the heavier isotopes. This suggests the potential for Se isotopes as migration indicators. Volatile, presumably methylated Se was isotopically very close to median values for phytoplankton and macrophytes, indicating a lack of isotopic fractionation during methylation.

Selenium isotope ratios, redox changes and biological productivity across the end-Permian mass extinction

“…Stüeken et al, 2015b;Wen et al, 2014, this study) suggest that Se oxyanion reduction can leave a detectable signature in the rock record. Fractionations associated with assimilation into biomass and Se IV adsorption are minor, up to 0.6‰ in favor of the lighter isotopes (Clark and Johnson, 2010;Johnson et al, 1999;Mitchell et al, 2013). Biomass forming in the photic zone (+0.3‰, Mitchell et al, 2012), where Se is quantitatively consumed, may thus record the composition of Se IV and Se VI in seawater, or at least provide a minimum constraint; assuming fractionation of 0.6‰ during assimilation into biomass (Clark and Johnson, 2010), seawater Se oxyanions may be as heavy as +0.9‰.…”

“…Fractionations associated with assimilation into biomass and Se IV adsorption are minor, up to 0.6‰ in favor of the lighter isotopes (Clark and Johnson, 2010;Johnson et al, 1999;Mitchell et al, 2013). Biomass forming in the photic zone (+0.3‰, Mitchell et al, 2012), where Se is quantitatively consumed, may thus record the composition of Se IV and Se VI in seawater, or at least provide a minimum constraint; assuming fractionation of 0.6‰ during assimilation into biomass (Clark and Johnson, 2010), seawater Se oxyanions may be as heavy as +0.9‰. It is plausible that this small isotopic enrichment of dissolved Se oxyanions relative to the Earth's crust (0.0 ± 0.5‰, Rouxel et al, 2002) results from non-quantitative Se oxyanion reduction in suboxic environments, such as in local oxygen minimum zones or in sedimentary pore waters, i.e.…”

“…Negative isotopic composition of less than −1‰, i.e. lighter than the crust (0.0 ± 0.5‰, Rouxel et al, 2002) and potential fractionations by adsorption or assimilation (≤ 0.6‰, Clark and Johnson, 2010;Johnson et al, 1999;Mitchell et al, 2013), should in turn be indicative of oxic to suboxic conditions and low biological productivity, because then organic Se is less abundant and Se oxyanions can be reduced non-quantitatively in pore waters or in oxygen minima in the water column. In other words, negative values should reflect a high Se oxyanion supply relative to the demand.…”

Selenium isotopes trace anoxic and ferruginous seawater conditions in the Early Cambrian

“…Therefore, Se may be enriched in organic-rich black shales formed in anoxic basins with a relatively higher Se/S ratio (10 − 5 to 10 −3 ) in comparison with most clastic sedimentary rocks and evaporates (10 −6 to 10 −5 ) (Hattori et al, 2004). As investigated through experiments and analyses of natural terrestrial samples, uptake or adsorption of Se by algae/plankton (assimilation) induces little isotopic fractionation within the range of 1‰ to 2‰ (Johnson, 2004;Clark and Johnson, 2010;Mitchell et al, 2012).…”

Selenium as paleo-oceanographic proxy: A first assessment

“…After the pre-digestion the sample was transferred to a 23 ml PTFE liner using an additional 0.5 ml of concentrated nitric acid to rinse the PFA beaker (total of 3 ml HNO 3 ). The liner was then placed in a Parr bomb and heated at 165°C for 10 h (Zhu et al, 2008;Clark and Johnson, 2010). After cooling, the sample was moved to a 15 ml conical centrifuge tube and the PTFE liner was rinsed with 1 ml H 2 O; the rinse was also transferred to the centrifuge tube.…”