Sulfur isotopic characterization of bedrock, alkaline lakes, and evaporitic sediment from a closed -drainage basin on the Oregon basalt plateau
Warner Valley is a closed-drainage basin containing numerous alkaline lakes and saline playas in south-central Oregon. Samples of Tertiary basaltic bedrock were collected from outcrops of layered flows surrounding the valley and analyzed for their oxygen (δ18O), hydrogen (δ 2H), and sulfur (δ34S) isotopic compositions. Whole rock δ18O values range from 5.2 to 15.0‰. High δ18O values suggest that rocks underwent low-temperature hydrothermal alteration. Calculated fluid δ18O versus δ 2H values suggest a meteoric origin. Sulfur phases extracted from rock samples include acid-volatile (SAV), Cr-reducible (S Cr), and acid-soluble sulfur (SAS). SAV corresponds to mantle-derived sulfides (δ34SAV = 1.5 to 5.9‰). SCr corresponds to secondary sulfides (δ 34SCr = -0.8 to 10.8‰). SAS corresponds to gypsum (δ34SAS = 3.1 to 9.5‰).
Water samples were collected from springs and lakes in Warner Valley. Values of δ34S for dissolved sulfate (δ 34SDS) in water samples range from 7.5 to 10.8‰ in springs and from 7.0 to 14.8‰ in lakes. Values of δ 34SDS in spring waters largely overlap with values of δ 34SAS in basaltic bedrock, suggesting that gypsum in basalts is the main sulfur source for Warner Valley springs. Higher δ34S DS values in lake waters than in spring waters suggest preferential uptake of 32S from the dissolved sulfate pool during bacterial sulfate reduction (BSR) and sequestration as sedimentary sulfides.
Three sedimentary cores of evaporitic muddy sediment were collected along a transect across a small playa lake. Sulfur phases extracted from the sediments include elemental sulfur (SEI), SCr, and SAS. Isotopic fractionations between SAS and SCr (Δ 34SAS-Cr) range from 5.2 to 21.5‰ and are consistent with BSR. Values of δ34SCr increase downward while values of δ34SAS decrease downward through the sedimentary column. Wet-and-dry seasonal cycles in Warner Valley evaporitic lakes produce cracking of the surface muds, allowing air to penetrate into previously anoxic sediments. Isotopic mass balance indicates that reoxidation of isotopically light sulfide from BSR results in a relative increase in δ 34SCr values. Mixing of primary sulfate with reoxidized sulfide accounts for the relative depletion of δ34S AS values with depth. Results from this study are a contribution to understanding past and present sulfur cycles in non-marine, evaporitic settings.