A TALE OF TWO LAKES: NATURAL CONTAMINATION IN THE NEWBERRY CALDERA, OR

East Lake and Lake Paulina are two small, deep (60 m) volcanic lakes within the pristine Newberry Caldera in central Oregon. The lakes are separated by a ~1 mile wide ridge of Holocene age volcanic material. Both lakes have peripheral hot springs, part of the active hydrothermal system underlying the caldera. Lake Paulina has a small outlet in Paulina Creek, whereas East Lake lacks any visible outflows. We studied the chemical and isotopic composition of the lake waters in vertical profiles, and took in both lakes several sediment cores up to ~1 m length. East Lake has the lower alkalinity and pH values (~2 mM; pH ~7.3) compared to Lake Paulina (~6-7 mM; pH ~8.3). The East Lake waters have δ¹⁸O values of -7‰ with high δ¹³C-DIC values of +4‰, whereas Lake Paulina has δ¹⁸O values of -10‰ and lower δ¹³C-DIC values at -0.3 ‰. East Lake waters have up to 60 ppm sulfate and less than 1 ppm chloride, whereas Lake Paulina has low sulfate (3 ppm) and slightly higher chloride (3 ppm) concentrations. The sediments of both lakes consist largely of diatom remains and minor volcanic ash. East Lake sediments show high mercury concentrations (2-3 ppm Hg) with high C_org (6 %) but very little arsenic (20 ppm As). Lake Paulina sediments have natural background mercury concentrations (20-60 ppb Hg), moderate C_org (3%) and high arsenic concentrations (50-250 ppm As). The iron concentrations in Lake Paulina sediment are exceptionally high (up to 14 % Fe₂O₃), whereas those from East Lake have on average 1.5 % Fe₂O₃. These water and sediment data suggest that these two lakes, despite their close proximity, have very different hydrothermal inputs: Lake Paulina has a substantial fluid input with high alkalinity, some chloride and high arsenic, whereas East Lake has a smaller hydrothermal fluid input with an associated gas rich in CO₂, H₂S and Hg. The higher δ¹³C-DIC values in East Lake result from preferential extraction of light carbon by the photosynthetic activity of diatoms. The higher O-H isotopic signature in East Lake results from sustained evaporation in the Oregon high desert climate. Evidence for anthropogenic mercury inputs is absent; the sources of mercury and arsenic contamination in the two lakes are from natural inputs of geothermal fluids and gases.