ISOTOPIC CHARACTERIZATION OF SULFUR CYCLING IN BASALT-HOSTED ALKALINE LAKES, SOUTH-CENTRAL OREGON

Warner Valley is a closed basin within the tectonic province of the Oregon-Nevada Great Basin. Low hills and gentle slopes on the western margin contrast with cliffs and steep slopes on the eastern margin of this half-graben. A chain of saline to hypersaline alkaline lakes and evaporitic ponds extends N-S along the depositional axis of the valley. About 400 meters of continental flood basalts (CFB), genetically related to the Columbia River Group, are exposed along the eastern boundary fault. Cool springs flow from the basalts and alluvial fans, hydrothermal springs discharge predominantly in the southern part of the valley, and one major stream enters the basin from the west. Characterization of sulfur cycling in Warner Valley provides insight into geochemical processes potentially influencing basalt-hosted lakes on early Earth and paleolakes on Mars.

The isotopic composition of sulfur (d³⁴S) in CFB reflects Earth’s mantle and generally is in the range of -0.5 to 1.0 per mil. Basalt samples from Warner Valley contain 200-1000 ppm sulfur. Sequential chemical extraction was used to isolate and purify sulfur species for determination of d³⁴S. Disulfide minerals were volatilized with Cr reduction, yielding d³⁴S values in the range of 0.7 to 1.8 per mil relative to V-CDT for fresh basalt. Sulfate with a d³⁴S of 0.7 per mil was recovered from one fresh basalt sample, reflecting oxidation and retention of S with little isotope fractionation. In contrast, sulfate in weathered basalts ranges from 9.3 to 5.9 per mil.

Dissolved sulfate in cool springs has d³⁴S values ranging from 7.5 to 10.2 per mil. Dissolved sulfate in hydrothermal springs has d³⁴S values ranging from 8.6 to 9.4 per mil. Dissolved sulfate in lakes and evaporitic ponds has d³⁴S values ranging from 9.4 and 14.8 per mil. Values of d³⁴S for sulfate in cool springs and hydrothermal springs are indistinguishable from sulfate in weathered basalts. A marked positive shift in d³⁴S values along the flow path for the Warner lakes probably reflects bacterial sulfate reduction and associated depletion of ³²S in residual sulfate. Hydrothermal and biological processes pervasively overprint original igneous signatures in these basalt-hosted alkaline lakes.