Differences in sulfur cycling and bacterial composition in two high Arctic catchments

Ruth Hindshaw1, Tim Heaton2, Eric Boyd3
1 Department of Earth and Environmental Sciences, University of St. Andrews
2 NERC Isotope Geosciences Laboratory, , Nottingham, UK
3 Department of Chemistry and Biochemistry, Montana State University, USA

O 11.1 in Weathering and chemical processes as keys to ecosystem functioning

15.07.2014, 11:00-11:20, H20

One of the primary sources of sulfur, an essential plant macronutrient, is the weathering of pyrite. Quantifying the overall release of sulfur from mineral weathering is essential, not only on an ecosystem level, but also for constraining the contribution of coupled sulfide oxidation – carbonate dissolution to the global carbon cycle. This requires knowledge of how oxidation and reduction reactions of sulfur species are modified by external parameters such as temperature, runoff and biological activity.

In order to investigate the effect of local environmental conditions on pyrite weathering we collected stream water samples from two small catchments (each approximately 3 km2) in Svalbard. One catchment is glaciated and the other catchment is un-glaciated but is affected by permafrost and a seasonal snow-pack. The two catchments are situated next to each other with identical bedrock (shale with minor siltstone and sandstone). The proximity of the catchments to each other ensures that meteorological variables such as temperature and precipitation are very similar.  Sampling was conducted early in the 2012 melt-season when there was still significant snow-cover and in mid-summer when most of the seasonal snow-pack had melted. The water samples were analysed for d34S-SO4, d18O-SO4 and d18O-H2O, together with major anions and cations. Two pyrite mineral separates were analysed for d34S. Bacterial 16S rRNA genes were amplified from two sediment samples using PCR techniques and BLAST (basic local alignment search tool) was performed to assign species.

Despite the nominally identical lithology and meteorological parameters, there were significant differences in the stream water chemistry and bacterial composition between the two catchments.  In the glaciated catchment, stream water d34S-SO4 values were higher than pyrite, which, together with a significant positive correlation with d18O-SO4, suggests that sulfate reduction is occurring.  Sulfate reduction occurs under anaerobic conditions and this is corroborated by the detection of 16S rRNA genes associated with the class Deltaproteobacteria, in particular the genera Geobacter, which are known to respire anaerobically. In contrast, no Deltaproteobacteria were detected in the unglaciated stream sediment and stream d34S-SO4 values were identical to pyrite suggesting pyrite oxidation under aerobic conditions and therefore limited sulfate reduction. This difference in sulfur cycling, arguably microbially mediated, results in marked differences in the major ion chemistry of the two streams, particularly in the ratio of SO4:HCO3, which is higher in the unglaciated catchment.

The combination of glaciation and bacteria adapted to living in those environments fundamentally alters weathering processes and the resultant stream water chemistry.

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last modified 2014-06-19