Selenium as paleo-oceanographic proxy: A first assessment

Abstract

Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has been suggested as a potentially powerful paleoenvironmental proxy. In this study, bulk Se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine-grained marine sedimentary rocks and sediments spanning the entire Phanerozoic. While the Se concentrations vary greatly (0.22-72ppm), the δ 82/76Se values fall in a fairly narrow range from -1 to +1‰ (relative to NIST SRM3149), with the exception of laminated black shales from the New Albany Shale formation (Late Devonian), which have δ 82/76Se values of up to +2.20‰. Black Sea sediments (Holocene) and sedimentary rocks from the Alum Shale formation (Late Cambrian) have Se to total organic carbon ratios (Se/TOC) and δ 82/76Se values close to those found in modern marine plankton (1.72±0.15×10 -6mol/mol and 0.42±0.22‰). For the other sedimentary sequences and sediments, the Se/TOC ratios show Se enrichment relative to modern marine plankton. Additional input of isotopically light terrigenous Se may explain the Se/TOC and δ 82/76Se data measured in recent Arabian Sea sediments (Pleistocene). The very high Se concentrations in sedimentary sequences that include the Cenomanian-Turonian Oceanic Anoxic Event (OAE) 2 may reflect an enhanced input of volcanogenic Se to the oceans. As the latter has an isotopic composition not greatly different from marine plankton, the volcanogenic source does not impart a distinct signature to the sedimentary Se isotope record. The lowest average δ 82/76Se values are observed in the OAE2 samples from Demerara Rise and Cape Verde Basin cores (δ 82/76Se=-0.14±0.45‰) and could reflect fractionation associated with microbial or chemical reduction of Se oxyanions in the euxinic water column. In contrast, a limiting availability of seawater Se during periods of increased organic matter production and burial may be responsible for the elevated δ 82/76Se values and low Se/TOC ratios in the black shales of the New Albany Shale formation. 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.