THE TWIN CRATER LAKES OF NEWBERRY VOLCANO, OR

The Newberry volcano caldera (OR; 43^o41.21’N, 121^o15.18'W) contains two crater lakes separated by a volcanic ridge dated to ~6ka. Both East Lake (EL) and Paulina Lake (PL) are fed by geothermal inputs, consisting of a gas phase in EL (CO₂, H₂S, Hg), and of hot carbonate-rich fluids rich in Fe, Si, P, As, Ca, Na, K, and Mg in PL. The lake ecosystems are largely fed by the geothermal inputs. Water, sediment, and fish in EL are strongly enriched in Hg, whereas PL sediment is rich in Fe, As and P. Vivianite, probably formed diagenetically, is a common mineral in PL sediments. An undergraduate summer research program funded by the Keck Geology Consortium and Wesleyan University, addressed the following topics: the loss of CO₂ from the water/air interface in EL, the chemical mass balance of PL, the ostracod faunal and isotope record from PL cores, the nature of the PL paleo-floods, the chemistry and genesis of vivianite as a potential host mineral for As, and the dissolved methane chemistry of the lakes. The CO₂ flux was studied using a floating accumulation chamber at many points on EL, which were mapped, and a flux pattern was established. The PL paleofloods were a massive paleo-drainage event that created a chaotic sediment apron at the base of the mountain, with many boulders coated with a layer of carbonate (evaporated PL water). The isotope chemistry of these coatings is investigated, and will be compared to carbonate precipitates from modern PL water. Ostracods are being analyzed for stable isotopes to assist in establishing the paleohydrology of PL. We also counted winter eggs of Daphnia preserved as fossils in the sediment. PL water carries 10-15 ppb As, whereas the sediment has up to 250 ppm As and up to 15% Fe₂O₃. An Fe-rich mineral phase is probably the host for the As, and in vivianite the arsenate group may substitute for phosphate. Methane concentrations in both lakes are low, but well-oxygenated surface waters have 10-100x more methane than deeper waters. The methane isotope signatures indicate a fermentation source in surface waters, but a geothermal/ thermogenic source in the bottom waters. The mass balance of PL is approached by equating the Paulina Creek export rate for many elements with inputs of subaqueous hot springs, taking into account conservative versus non-conservative behavior.