Multiple sulphur and oxygen isotopes reveal microbial sulphur cycling in spring waters in the Lower Engadin, Switzerland

Abstract

Highly mineralized springs in the Scuol-Tarasp area of the Lower Engadin and in the Albula Valley near Alvaneu, Switzerland, display distinct differences with respect to the source and fate of their dissolved sulphur species. High sulphate concentrations and positive sulphur (δ(34)S) and oxygen (δ(18)O) isotopic compositions argue for the subsurface dissolution of Mesozoic evaporitic sulphate. In contrast, low sulphate concentrations and less positive or even negative δ(34)S and δ(18)O values indicate a substantial contribution of sulphate sulphur from the oxidation of sulphides in the crystalline basement rocks or the Jurassic sedimentary cover rocks. Furthermore, multiple sulphur (δ(34)S, Δ(33)S) isotopes support the identification of microbial sulphate reduction and sulphide oxidation in the subsurface, the latter is also evident through the presence of thick aggregates of sulphide-oxidizing Thiothrix bacteria.

Using modern ferruginous habitats to interpret Precambrian banded iron formation deposition

“…However, both lakes are subject of research projects already and contributed to the reconstruction of the ancient microbial sulphur cycle by analyses of the sulphur isotope compositions and contributions of microbial activities to the specific fractionation patterns (Suits & Wilkin 1998;Canfield et al 2010). One example for such a habitat is the Arvadi Spring, located in the higher Engadin window of the eastern Swiss Alps (N46°40′ 14.074″, E9°39′ 52.956″) within the Albula valley in the Canton of Grisons (Strauss et al 2015). Since the holomictic Lake Vechten provides the presence of both, reduced Fe-and S-species at the same time, its investigation as modern model for Proterozoic oceans could improve our understanding of early biogeochemical Fe-and S-turnover and the coupling of these two cycles remarkably.…”

“…Such Feand S-rich habitats do not necessarily need to be stratified lakes but simply have to provide the simultaneous presence of dissolved Fe(II) and sulphide. Identification of the S 0 sulphur isotopic composition revealed a depletion in 34 S with δ 34 S values between −26 and −23‰, suggesting dissolved sulphide to be generated by microbial reduction of sulphate that was quantified at high concentrations up to 10.4 mM (Strauss et al 2015). As one of several sulphur-containing springs of the area, the Arvadi spring stands out from the other springs by the presence of Fe-and S-species at tens of μM concentrations at the same time.…”

“…One example for such a habitat is the Arvadi Spring, located in the higher Engadin window of the eastern Swiss Alps (N46°40′ 14.074″, E9°39′ 52.956″) within the Albula valley in the Canton of Grisons (Strauss et al 2015). Associated with a minor isotope effect, microbial oxidation of sulphide is suggested to occur at the oxic-anoxic redox boundary, also being the likely source of S 0 flocks (Strauss et al 2015). Waters from two separate springs, namely the 3.6-17.9 10 n.a.…”

A case study for late Archean and Proterozoic biogeochemical iron‐ and sulphur cycling in a modern habitat—the Arvadi Spring

“…Collectively, the Arvadi Spring helps us to understand how O 2 consumption by aerobic microorganisms and production of reduced compounds by anaerobic respiring microorganisms (e.g., iron(II) and sulphide) could have affected the abundance, activity and survival of anaerobic members of the upper, oxygenated ancient ocean metabolic network by forming anoxic niches. In particular, for the interpretation of signatures found in the rock record, the Arvadi Spring can help to understand which biotic and abiotic processes resulted in the isotope composition that was preserved in the rock record (Strauss et al., ), to decipher the identity of the primary minerals prior to diagenesis and to know the morphology of the microbial community that exists under envisaged conditions prior to preservation in form of microfossils. To further improve our model and our understanding of the metabolic network of iron‐ and sulphur‐metabolizing microorganisms, we need to know the factors controlling the interrelation and competition of iron‐ and sulphur‐metabolizers with abiotic reactions in the Arvadi Spring.…”

“…Microbial reduction in gypsum-derived sulphate that is present in large amounts in the Alpine orogeny is the major source of sulphide in the Arvadi Spring (Strauss et al, 2016). Compared to iron(II), sulphide is oxidized abiotically by O 2 much more slowly, that is, on the order of hours to days (Luther et al, 2011;Millero, Hubinger, Fernandez, & Garnett, 1987b).…”