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

Abstract

Selenium isotopes are becoming an important paleoenvironmental proxy. However, few studies have focused on the behavior of Se isotopes during oxidative weathering. In this paper, a comprehensive set of Se isotopic composition and concentration data were collected from the weathering profiles of Se-rich shales of the Permian Maokou Formation in Yutangba and Shadi, China to investigate Se isotopic fractionation and Se enrichment during weathering processes. The δ82/76Se in fresh shales (148±118mg/kg Se, 1SD, n=40) from Shadi and Yutangba drill cores varies from −1.69‰ to 1.74‰ with an average of 0.40±0.71‰ (1SD, n=40), consistent with the range in other Phanerozoic shales, suggesting that Se isotopes are not strongly fractionated during Se sequestration in the primary sedimentary environment. However, the strongly weathered Se-rich shales from Shadi and Yutangba profiles are isotopically lighter with average δ82/76Se values of −1.96±1.08‰ (1SD, n=5) and −1.08±1.83‰ (1SD, n=23), respectively. These data suggest that Se isotopes can be fractionated during oxidation and reduction processes associated with weathering, with heavier isotopes removed preferentially during oxidative weathering of shales. Such a shift, if found to be a global phenomenon, would have implications for models of the global Se cycle and interpretation of Se isotope data from past biogeochemical regimes.Locally altered shales exposed in a quarry at Yutangba are extremely enriched in Se with 1642±1505mg/kg (1SD, n=45), approximately 10 times greater than that in unaltered drill core samples. These rocks display very strong variation in δ82/76Se over short distances, with a single 60cm transect showing the most strongly negative and positive δ82/76Se values (−14.20‰ to +11.37‰) observed to date in natural samples. This suggests that Se has undergone multiple cycles of oxidation, mobilization, and re-reduction, resulting in a Se-rich redox front that has migrated downward through the organic-rich shales over time. δ82/76Se values vary sharply over distances as small as 10cm, indicating that Se redox conditions change strongly with position and are controlled by fractures and rock layering. Our data and a simple conceptual model suggest that zones of increased permeability that are accessed first by infiltrating waters are isotopically light, whereas less accessible zones are heavy. Repeated redox cycling accentuates this pattern. Furthermore, the average δ82/76Se in Se-rich shales at the Yutangba weathering system is 0.45±5.77‰ (1SD, n=39), identical with that (0.40±0.71‰) in fresh shales from same locality, implying that Se released by weathering accumulates in the redox front with little loss. The strong Se isotopic fractionation occurring in the redox front implies that Se isotopes can be extensively used in tracing geochemical processes of Se in groundwater system, especially related to fractures.