2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 6
Presentation Time: 1:30 PM-5:30 PM

USING δ34S AND δ 18O OF SULFATE TO RESOLVE PROCESSES AFFECTING SULFUR IN ALLUVIUM BENEATH A RIPARIAN AREA, OKLAHOMA


BREIT, George N., N/a, U.S. Geological Survey, MS 964, Denver Federal Center, Denver, CO 80225, TUTTLE, Michele L.W., U.S. Geological Survey, MS 964, Box 25046, Denver Federal Center, Denver, CO 80225 and COZZARELLI, Isabelle M., U.S. Geological Survey, National Research Program, Eastern Branch, Reston, VA 20192, gbreit@usgs.gov

The δ34S and δ18O of sulfate in ground water within alluvium of the Canadian River (CRA) contribute to understanding the spatially variable processes affecting sulfur in a 1 km2 riparian area of central Oklahoma. The principal supply of sulfate is from dissolution of bedrock gypsum resulting in 5mM sulfate in Canadian river water (δ34SSO4 12 ‰ VCDT and δ18OSO4 10 ‰ VSMOW). Smaller amounts of isotopically similar sulfate are supplied by surface and ground water flowing from the valley sides toward the river. Natural conditions at much of the study site are modified by a reducing (<0.1mM sulfate) leachate plume from a closed municipal landfill. Processes that modify the concentration and isotopic composition of sulfate within the CRA include bacterial sulfate reduction (BSR), sulfide oxidation, and organic matter degradation. BSR produces residual sulfate with δ34SSO4 and δ18OSO4 as high as 80 and 24 ‰, respectively, and iron sulfide minerals that are depleted in 34S (δ34Spyrite = -10 to -24 ‰). Low water levels favor oxidation of iron sulfide to produce sulfate with δ34SSO4 of -10 ‰ and δ18OSO4 of -7 ‰. Sulfate is also removed from ground water by plant uptake, which retains the δ34S of the sulfate in proximal ground water. Mineralization of plant litter and DOC within the leachate plume releases sulfate. Sulfate in the leachate plume has a δ34SSO4 of 8 ‰, which is consistent with sulfur in bedrock from the source of yard wastes in the landfill. δ18OSO4 of leachate sulfate and sulfate from modern litter are near 6 ‰, presumably a result of ester sulfate hydrolysis. Progressive, microbial reduction of sulfate from river water, sulfide oxidation and mineralization of organic matter define three distinct subparallel trends of oxygen and sulfur isotopic evolution. The slope of all lines is consistent with the 4:1 ratio (δ34SSO418OSO4) previously reported for bacterial sulfate reduction. Where most of the dissolved sulfate has been reduced, the oxygen isotopic composition of sulfate diverges from the linear trends and approaches isotopic equilibrium with oxygen in H2O. The ability to measure the magnitude and distribution of these processes has implications for improved geochemical descriptions of riparian areas.