A CHEMO-TOPOGRAPHIC TRANSECT ACROSS THE COAST RANGE AND CASCADES, CENTRAL OREGON: MODERN INSIGHTS INTO THE CONTROL OF LANDSCAPE ON LAKE WATER COMPOSITION IN DEEP TIME

Natural lakes and reservoirs were sampled along a longitudinal gradient from the Pacific Ocean up and over the Coast and Cascade Ranges of central Oregon to the High Lava Plains in 2013 and 2015. This transects spans lakes of different origins, six geomorphic regions and an elevation range from 2 to 1942 meters above sea level. Almost all Coast Range lakes are sand hosted whereas the remaining are bedrock (volcanic and sedimentary) hosted. These lakes are hydrologically open and dominated by meteoric recharge. The residence time of water ranges from months to decades. Twenty-two lakes and reservoirs were analyzed in the field for temperature, pH, total dissolved solids (TDS), and alkalinity. Samples were also analyzed for major cations and anions and stable isotopes of D and O. The pH ranges from 7 to 9.8 and shows no systematic variation based on substrate type or elevation. All the lakes are dilute (avg. TDS = 35.8 ppm) except for those in the High Lava Plains (avg. TDS = 337 ppm). Alkalinities average 18.9 mg/l CaCO₃ except for the High Lava Plains (291.2 mg/L CaCO₃). In the Coast Range, sodium is the major cation on an equivalent basis reflecting proximity to the ocean. Loon and Triangle Lakes, the easternmost lakes within the Coast Range, are dominated by calcium, reflecting different drainage basins and substrate type. Lakes in the Western and High Cascades are dominated by calcium. Both the dominant cation and stable isotopic analyses clearly differentiate waters from different geomorphic regions. The δ¹⁸O ranges from -5.7 to -9.3 ‰ (VSMOW), and δD ranges from -37.8 to -63.6 ‰ (VSMOW) in the Coast Range whereas the δ¹⁸O ranges from -9.7 to -12.1 ‰ (VSMOW) and δD ranges from -71.5 to -86.5‰ (VSMOW) in the Cascades. Stable isotopic differences between ranges reflect distance from the ocean and increasing elevation from west to east. The stable isotopes of water show no correlation with air or lake water temperatures. Average annual precipitation and bedrock across this topographic gradient controls the major ions and stable isotopic composition of these lakes. Reconstructing stable isotopic compositions of lake water typically requires an assumption of paleotemperature. Hydrologically open lakes preserved in deep time maybe recording paleoprecipitation along topographic gradients that are independent of lake water temperature.