Fike, David A. and Finke, Niko and Zha, Jessica and Blake, Garrett and Hoehler, Tori M. and Orphan, Victoria J. (2009) The effect of sulfate concentration on (sub)millimeter-scale sulfide δ^(34)S in hypersaline cyanobacterial mats over the diurnal cycle. Geochimica et Cosmochimica Acta, 73 (20). pp. 6187-6204. ISSN 0016-7037 http://resolver.caltech.edu/CaltechAUTHORS:20100127-153113402
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Substantial isotopic fractionations are associated with many microbial sulfur metabolisms and measurements of the bulk δ^(34)S 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 δ^(34)S measurements are often non-unique, making reconstructions of paleoenvironmental conditions or microbial ecology challenging. In particular, the link between the μm-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 (~100 μm–1 cm) δ^(34)S variability to provide additional constraints for environmental and/or ecological reconstructions. We have investigated the impact of sulfate concentrations (0.2, 1, and 80 mM SO_4) on the δ^(34)S 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 δ^(34)S 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 ~400–800 individual measurements covering a lateral distance of ~1 mm and a vertical depth of ~5–15 mm. There is a large isotopic enrichment (10–20‰) in the uppermost mm of sulfide in those mats where [SO_4] 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 δ^(34)S in these cyanobacterial mats. Perhaps equally surprising, large, spatially-coherent δ^(34)S oscillations (~20–30‰ over 1 mm) occurred at depths up 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 δ^(34)S. Decreased sulfate concentrations diminished both vertical and lateral δ^(34)S variability, suggesting that small-scale variations of δ^(34)S can be diagnostic for reconstructing past sulfate concentrations, even when original sulfate δ^(34)S is unknown.
|Additional Information:||© 2009 Elsevier Ltd. Received 9 January 2009; accepted 6 July 2009. Associate editor: Juske Horita. Available online 12 July 2009. We would like to acknowledge the Gordon and Betty Moore Foundation and the Caltech Center for Geochemical and Cosmochemical Microanalysis for funding (to V.J.O.) as well as support from National Aeronautics and Space Administration grant NAI02-003-0001 issued through the Astrobiology Program and NASA grant 07-EXOB07-0093 issued through the Exobiology Program. D.A.F. was additionally supported by the Caltech O.K. Earl Postdoctoral Fellowship. N.F. was supported by an EU-Marie Curie Postdoctoral Fellowship Contract MOIF-CT-2005-22154. We would like to thank Y. Guan and J. Eiler for analytical assistance and invaluable discussions, and T. Lyons for conventional d34S analysis of aqueous sulfate. We are also indebted to the NASA Ames group (K. Turk, M. Kubo, L. Jahnke and D. Des Marais) for support and assistance with sample collections.|
|Official Citation:||David A. Fike, Niko Finke, Jessica Zha, Garrett Blake, Tori M. Hoehler, Victoria J. Orphan, The effect of sulfate concentration on (sub)millimeter-scale sulfide [delta]34S in hypersaline cyanobacterial mats over the diurnal cycle, Geochimica et Cosmochimica Acta, Volume 73, Issue 20, 15 October 2009, Pages 6187-6204, ISSN 0016-7037, DOI: 10.1016/j.gca.2009.07.006. (http://www.sciencedirect.com/science/article/B6V66-4WRM6K5-2/2/8b40090318e4684bedd1eaa35b656053)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||29 Jan 2010 00:39|
|Last Modified:||26 Dec 2012 11:43|
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