THE GEOCHEMISTRY OF PAULINA LAKE, NEWBERRY, OR

Paulina Lake, within Newberry Volcano crater in Oregon, is located ~2km in elevation with its adjacent sister, East Lake (EL), and experiences inflow constraints by Newberry’s precipitation, seasonal freezing, and basement material. PL sediments are rich in As, Fe, vivianite (in diagenetic phase, Fe₃(PO₄)_{2 ･} 8H₂O) and silica (a mixture of diatom frustules, possibly hydrothermal Si, and glass from volcanic ash). Waters are carbonate-rich, of pH 8-8.5, with elevated major element concentrations and high conductivity (~0.6 mƱ/cm). Paulina Creek (PC) drains PL to the south-west, with artificial controls. 

We assume that for conservative elements, PC export rate is equivalent to hot fluid venting at the lake bottom. Water samples from several depth profiles were analyzed for alkalinity by titration, major and trace elements (by IC, ICP-AES and ICP-MS) and for stable isotopes (C, O and H in resp. DIC, and water by Picarro and MS). We hypothesize that dissolved Fe ions precipitate as HFO upon entering the lake, adsorbing both As and P. During diagenesis in the organic rich sediment (4% C_org), HFO dissolves and precipitates vivianite, possibly with arsenate ions substituting for phosphate in the mineral structure.

Our goal is to characterize the flux and composition of the hydrothermal input fluids. For this, we developed a steady state model for major elements and used stable isotope compositions for the water balance model, constraining evaporation, precipitation, seepage, and groundwater flow rates. With negligible groundwater inflow from EL, a considerable volume of thermally altered water must be injected into PL to explain water compositions. While PL exhibits thermal and conductive stratification, most element concentrations do not adhere to any discernible relationship with respect to depth; hence, we believe that PL is a well mixed, though thermally stratified, body of water with elemental inputs from its hotsprings circulating through the water column. Furthermore, we believe the lake ecosystem is largely driven by the hydrothermal inputs (carbonate/CO2, P, Si, Ca) with cyanobacteria providing fixed nitrogen.