The effect of sulfate concentration on (sub)millimeter-scale sulfide δ34S in hypersaline cyanobacterial mats over the diurnal cycle

Substantial isotopic fractionations are associated with many microbial sulfur metabolisms and measurements of the bulk delta S-34 isotopic composition of sulfur species (predominantly sulfates and/or sulfides) have been a key component in developing our understanding of both modern and ancient biogeochemical cycling. However, the interpretations of bulk delta S-34 measurements are often non-unique, making reconstructions of paleoenvironmental conditions or microbial ecology challenging. In particular, the link between the mu m-scale microbial activity that generates isotopic signatures and their eventual preservation as a bulk rock value in the geologic record has remained elusive, in large part because of the difficulty of extracting sufficient material at small scales. Here we investigate the potential for small-scale (similar to 100 mu m-1 cm) delta S-34 variability to provide additional constraints for environmental and/or ecological reconstructions. We have investigated the impact of sulfate concentrations (0.2, 1, and 80 mMSO(4)) on the delta S-34 composition of hydrogen sulfide produced over the diurnal (day/night) cycle in cyanobacterial mats from Guerrero Negro, Baja California Sur, Mexico. Sulfide was captured as silver sulfide on the surface of a 2.5 cm metallic silver disk partially submerged beneath the mat surface. Subsequent analyses were conducted on a Cameca 7f-GEO secondary ion mass spectrometer (SIMS) to record spatial delta S-34 variability within the mats under different environmental conditions. Isotope measurements were made in a 2-dimensional grid for each incubation, documenting both lateral and vertical isotopic variation within the mats. Typical grids consisted of similar to 400-800 individual measurements covering a lateral distance of similar to 1 mm and a vertical depth of similar to 5-15 mm. There is a large isotopic enrichment (similar to 10-20 parts per thousand) in the uppermost mm of sulfide in those mats where [SO4] was non-limiting (field and lab incubations at 80 mM). This is attributed to rapid recycling of sulfur (elevated sulfate reduction rates and extensive sulfide oxidation) at and above the chemocline. This isotopic gradient is observed in both day and night enrichments and suggests that, despite the close physical association between cyanobacteria and select sulfate-reducing bacteria, photosynthetic forcing has no substantive impact on delta S-34 in these cyanobacterial mats. Perhaps equally surprising, large, spatially-coherent delta S-34 oscillations (similar to 20-30 parts per thousand over 1 mm) occurred at depths up to similar to 1.5 cm below the mat surface. These gradients must arise in situ from differential microbial metabolic activity and fractionation during sulfide production at depth. Sulfate concentrations were the dominant control on the spatial variability of sulfide delta S-34. Decreased sulfate concentrations diminished both vertical and lateral delta S-34 variability, suggesting that small-scale variations of delta S-34 can be diagnostic for reconstructing past sulfate concentrations, even when original sulfate delta S-34 is unknown. (C) 2009 Elsevier Ltd. All rights reserved.