Biogeochemistry of selenium isotopes: processes, cycling and paleoenvironmental applications

Abstract

Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has been suggested as a potentially powerful environmental tracer and paleoenvironmental proxy. Chapter 1 provides a literature review of the Se cycle. While the Se cycle shares some similarities with the S cycle, there are major differences that become apparent from a synthesis of the existing literature, we therefore compare the oceanic cycles of Se and S in Chapter 2. The biogeochemical cycles of S and Se are different because of differences in concentration, distribution, speciation and residence times. The geological evolution of the S cycle and the S isotopic record are used as a framework for discussing the Se isotopic compositions observed in marine sedimentary rocks spanning geological history.Data suggests that assimilatory uptake and nutrient recycling has controlled the Se cycle throughout geologic time. In Chapter 3, the biogeochemical Se cycle model constructed in Chapter 2 is expanded to include isotope compositions and isotope fractionation mechanisms. Because of the short residence time of Se in the oceans, the signatures recorded in marine sediments are most sensitive to changes in the isotopic composition of the Se inputs. The stability of the Se isotope system even when the oceans undergo major changes is consistent with the narrow range of Se isotope compositions that are observed in the sedimentary rock record.In Chapter 4, bulk Se concentrations and Se isotopic compositions are analyzed in a suite of fine-grained marine sedimentary rocks and sediments spanning the Phanerozoic. While the Se concentrations vary greatly (0.22 to 72 ppm), the δ82/76Se values fall in a fairly narrow range from -1 to +1‰. Many samples have Se to total organic carbon ratios (Se/TOC) and δ82/76Se values close to those found in modern marine plankton (1.72±0.15x10-6 mol/mol and 0.42±0.22‰). Overall, our results indicate that to unlock the full proxy potential of marine sedimentary Se records, we need to gain a much more detailed understanding of the sources, chemical speciation, isotopic fractionations and cycling of Se in the marine environment.In Chapter 5, rates of reaction and isotopic fractionations of Se(IV) and Se(VI) during sorption to iron oxides (2-line ferrihydrite, hematite and goethite) and iron sulfides (mackinawite and pyrite) were determined. While XANES spectra revealed no change in Se oxidation state when Se(IV) and Se(VI) sorbed to iron oxides, they showed evidence of reduction in the presence of iron sulfides. Selenium isotopic fractionations were always less than 1‰ in the experiments with iron oxides (mean ε82/76Se: 0.2‰). Fractionations were higher in the experiments with iron sulfides, with ε82/76Se values of up to ~10‰ in the Se(IV)-pyrite system.Finally, Chapter 6 emphasizes that the marine Se cycle is dominated by biological cycling in surface waters. This cycling is so rapid that Se isotopes are homogenized on very short timescales, and marine sediments record changes almost instantaneously. Therefore, most of the recorded changes are due to local or regional variations in Se inputs, rather than global changes in ocean circulation and biogeochemical dynamics.