A TALE OF TWO LAKES: POISONOUS FLUIDS THAT FUEL LIFE IN THE NEWBERRY, OR, CRATER LAKES

The twin crater lakes at Newberry Volcano, OR, East Lake and Paulina Lake, may serve as partial analogs of some Precambrian environments. These dimictic, 50-70m deep lakes, separated by a 2 km wide ridge, have sediment largely consisting of diatomaceous silica with up to 8 % organic matter. Subaqueous hydrothermal inputs fuel this diatom productivity with endogene CO₂, P, and Si. Lake Paulina has Fe-rich (14 % Fe₂O₃) and As-rich (280 ppm) sediment with vivianite nodules, whereas East Lake has CO₂-H₂S gas inputs with associated vapor phase Hg (up to 3 ppm Hg in the sediment, high Hg in local fish as well). The Paulina Lake sediments have some traits of BIFs with their high silica and Fe contents. The high organic productivity demands a large fixed nitrogen supply, which may be derived from local N-fixing bacteria. Phase partitioning of a CO₂-rich hydrothermal fluid at depth explains the chemical differences between the two lakes. A high alkalinity carbonate-rich fluid enters Paulina Lake, whereas a gas-dominated fluid phase enters East Lake. The water chemistry of the two lakes is distinct, with East Lake one pH unit lower than Paulina Lake as a result of the CO₂ flux, and high sulfate contents. Paulina Lake has a high alkalinity and a pH near eight, but no sulfate. Vertical water profiles show no chemical stratification but are thermally stratified in late summer from 18 ^oC at the surface to 5 ^oC at the bottom. These neutral bicarbonate lakes have PCO₂ well below hydrostatic pressure at all depths: the potential for explosive degassing is low, but the lakes do emanate probably a substantial flux of CO₂ to the atmosphere. Gas explosions may have occurred in the earlier history of Paulina Lake when lake mixing was less frequent during colder periods. One of the largest recorded floods in central Oregon was the Paulina flood, which potentially may have been caused by an overturn with a Nyos-type explosion event. We present detailed water and sediment chemistry, with isotope data for water, δ¹³C in DIC and sediment, and δ¹⁵N in sediment. A steady state stable isotope model is used to estimate water residence times and hydrological balance. The sediments in both lakes are dated with ²¹⁰Pb-¹³⁷Cs.