GEOCHEMISTRY OF TWO CRATER LAKES: THE NEWBERRY CALDERA, OREGON

East and Paulina Lakes are located side-by-side in the Newberry Caldera separated only by a volcanic ridge of rhyolite approximately 2000m wide. Despite their proximity to each other, these two lakes have vastly different water and sediment chemistries, suggesting a disparate hydrologic sourcing. This is highlighted by the concentrations of two very toxic metals in the sediments, mercury and arsenic. Paulina Lake has ~15ppb Hg and up to 250ppm As whilst East Lake has up to 3500ppb Hg and only ~25ppm As. Previous studies propose that these lakes are fed by one volcanic input that branches off to deliver a Hg-rich gaseous phase to East Lake and a As-rich hydrothermal input to Paulina Lake (Lefkowitz 2015). As part of a larger KECK study, this past summer we sampled each lake for water and sediment in addition to the hot springs surrounding each lake to further constrain their geochemical evolution. Early results indicate differences in organic input to each lake, with East Lake having on average 3 times the amount of TOC (~6%) with a carbon isotopic value of -20‰, while Paulina Lake’s organic input is lower (2%) with a more terrestrial isotopic signal (~ -26‰). The sediment’s pore waters also vary from lake to lake: Paulina Lake has ~4.5ppm and ~40ppm Cl and SO_4, respectively, while East Lake has concentrations that are approximately half that. Finally, East Lake has been previously reported to have two isotopically distinct methane pools in its waters: a ¹³C-enriched end member (~-30‰) in its deep water (likely a thermogenic source), and a pool nearer to the surface with δ¹³C of ~-50‰ that may suggest a biogenic component (Varekamp 2015). We are examining the microbial populations in the water column via DNA analysis to further help identify possible sources.