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

Paper No. 11
Presentation Time: 11:00 AM


FINKELSTEIN, David B., Earth and Planetary Sciences, University of Tennessee, 1412 Circle Dr, Knoxville, TN 37996 and PRATT, Lisa M., Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405, dfinkels@utk.edu

Warner Valley, Oregon is an alluvial system containing numerous geothermal springs and evaporative lakes underlain and hosted by basaltic flows and deposits from Pleistocene Lake Warner. Alkaline, fresh to hypersaline lakes in the northern end of the valley are located within an Area of Critical Environmental Concern under management of the BLM, and is free from agricultural and anthropogenic activities, but is seasonally perturbed by large numbers of migrating Arctic Swans. Springs typically originate from fractures within Oligocene basalt, are bicarbonate dominated, have temperatures ranging from 8.9 to 71.2°C and pHs from 6.5 to 8.3. Water chemistries of the lakes exhibit a strong seasonality and range from moderately to highly alkaline to sulfate with a range in pH from 8.3 to 10.5. Lakes are relatively dilute in late fall through early spring as much of the water originates from snow melt with a minor geothermal component. Lake waters undergo significant evaporative concentration from late spring through summer but this can be altered by wetter conditions associated with ENSO and PDO. As a result, individual springs, playas and lakes host a wide variety of chemistries. Conditions and chemistries at any of the sites are dynamic and are controlled in part by location and the seasonality of water. Nearshore sites are buffered by ground water discharge and off-shore sites are controlled by evaporation. Contrasts during a single day of monitoring found pH, temperature, and conductivity to be the most sensitive to diurnal changes in evaporation and photosynthetic activity of algal/bacterial mats. During periods of intense evaporation water levels in the larger lakes are reduced to a few centimeters and exist as isolated puddles. In contrast, during an early spring sampling prior to melting of the snowpack, the off shore sites were desiccated and the nearshore sites were reduced to centimeter-scale puddles whereas post runoff, lake levels changed by as much as one to two meters. These types of seasonally dynamic environments can yield different aqueous paths when considered along chemical divides. Distinct seasonal chemical paths resulting from aqueous signatures are lost when considering the mineralogy of the mud.