Sulfur isotopic characterization of bedrock, alkaline lakes, and evaporitic sediment from a closed-drainage basin on the Oregon basalt plateau.

摘要

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 (delta18O), hydrogen (delta 2H), and sulfur (delta34S) isotopic compositions. Whole rock delta18O values range from 5.2 to 15.0‰. High delta18O values suggest that rocks underwent low-temperature hydrothermal alteration. Calculated fluid delta18O versus delta 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 (delta34SAV = 1.5 to 5.9‰). SCr corresponds to secondary sulfides (delta 34SCr = -0.8 to 10.8‰). SAS corresponds to gypsum (delta34SAS = 3.1 to 9.5‰).; Water samples were collected from springs and lakes in Warner Valley. Values of delta34S for dissolved sulfate (delta 34SDS) in water samples range from 7.5 to 10.8‰ in springs and from 7.0 to 14.8‰ in lakes. Values of delta 34SDS in spring waters largely overlap with values of delta 34SAS in basaltic bedrock, suggesting that gypsum in basalts is the main sulfur source for Warner Valley springs. Higher delta34S 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 (Delta 34SAS-Cr) range from 5.2 to 21.5‰ and are consistent with BSR. Values of delta34SCr increase downward while values of delta34SAS 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 delta 34SCr values. Mixing of primary sulfate with reoxidized sulfide accounts for the relative depletion of delta34S AS values with depth. Results from this study are a contribution to understanding past and present sulfur cycles in non-marine, evaporitic settings.