Abstract
Sulfur isotope signatures provide key information for the study of microbial activity in modern systems and the evolution of the Earth surface redox system. Microbial sulfate reducers shift sulfur isotope distributions by discriminating against heavier isotopes. This discrimination is strain-specific and often suppressed at sulfate concentrations in the lower micromolar range that are typical to freshwater systems and inferred for ancient oceans. Anaerobic oxidation of methane (AOM) is a sulfate-reducing microbial process with a strong impact on global sulfur cycling in modern habitats and potentially in the geological past, but its impact on sulfur isotope signatures is poorly understood, especially in low sulfate environments. We investigated sulfur cycling and 34S fractionation in a low-sulfate freshwater sediment with biogeochemical conditions analogous to Early Earth environments. The zone of highest AOM activity was associated in situ with a zone of strong 34S depletions in the pool of reduced sulfur species, indicating a coupling of sulfate reduction and AOM at sulfate concentrations < 50 µmol L-1. In slurry incubations of AOM-active sediment, the addition of methane stimulated sulfate reduction and induced a bulk sulfur isotope effect of ~29 ‰. Our results imply that sulfur isotope signatures may be strongly impacted by AOM even at sulfate concentrations two orders of magnitude lower than at present oceanic levels. Therefore, we suggest that sulfur isotope fractionation during AOM must be considered when interpreting 34S signatures in modern and ancient environment.
Highlights
Microbial sulfur transformations play an important role in the biogeochemistry of the modern Earth surface and have done so during most of Earth’s history, while the interactions of the sulfur, oxygen, and carbon cycles have undergone dramatic changes (Canfield and Teske, 1996; Canfield and Raiswell, 1999; Canfield et al, 2006)
We demonstrate that sulfur cycling in the anaerobic oxidation of methane (AOM) zone in Lake Ørn sediment is associated with substantial isotope fractionation and, based on experimental incubations, we infer a direct coupling of sulfur cycling and AOM at sulfate concentrations below 50 μmol L−1
Rates of dissimilatory sulfate reduction (SR) were highest in the depth intervals 2–4 cm and 12–15 cm, and AOM rates increased steeply until 12–15 cm, locating the AOM zone to 8–21 cm (Figure 1C, Table 1), in good agreement with the sulfate-methane-transition zones (SMTZs) (Figure 1A)
Summary
Microbial sulfur transformations play an important role in the biogeochemistry of the modern Earth surface and have done so during most of Earth’s history, while the interactions of the sulfur, oxygen, and carbon cycles have undergone dramatic changes (Canfield and Teske, 1996; Canfield and Raiswell, 1999; Canfield et al, 2006). Stable isotope signatures archived in the geological record have been pivotal for reconstructing milestones of Earth’s biogeochemical history and, the evolution of life, and they are useful for studies of modern sulfur cycling (Canfield and Raiswell, 1999; Canfield et al, 2000; Canfield, 2001). Accurate interpretation of isotope signatures depends on a detailed understanding of the isotopic fractionation associated with different sulfur transformations. A sulfate threshold of ∼200 μmol L−1 was suggested for natural communities of bacterial sulfate reducers (Habicht et al, 2002), but has recently been challenged by the report of substantial isotope fractionation between sulfate and sulfide at sulfate concentrations in the range of 5–10 μmol L−1 in the chemocline of ferruginous Lake
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
