THE OSTRACOD RECORD IN PAULINA LAKE, OREGON: ENVIRONMENTAL HISTORY OF A VOLCANIC CRATER LAKE

Paulina Lake is one of the ~6,500 yr old twin crater lakes of Newberry Volcano (OR). Hydrothermal fluid inputs influence lake water composition, and thus its ecosystem. Study of assemblages and isotopic composition of ostracod valves provides insight into ecosystem resilience with changes in climate and lake chemistry over the last millennia. Ostracods, microscopic crustaceans forming CaCO₃ valves, live as nektobenthos in deep lake waters, epiphytic swimmers in shallow areas. Their O-isotopic composition relates to lake paleohydrology and paleoclimate, their C-isotopic composition to variations in volcanic fluid input, primary productivity, and CO₂ emissions from the lake surface. We studied ostracods in 5 cores, 55 to 500 cm long, from 15-72 m water depth. Preliminary age models are based on ²¹⁰Pb data and tephrochronology, with the longest core representing ~3500 years. The sediment contains up to 14% Fe, 65-70% SiO₂, volcanic ash, and 3-5% organic carbon. Sediments deposited at 2000-2600 BP (recovered in longer cores) have higher concentrations of Fe-P-Mn-As, reflecting elevated hydrothermal inputs (HPP: Hydrothermal Peak Period). Ostracods are generally common, rare in short core CLPA5, and absent in the HPP intervals of two long cores (PL1, PL2). The most common species is Fabisformaecandona caudata, with less common Limnocythere sp. cf. paraornata, and rare epiphytic Cypridopsis vidua. Preliminary C-isotope values in recent F. caudata valves are ~1.5‰ lighter than dissolved inorganic carbon. Older samples have still lighter values, possibly due to decreased loss of CO₂ from the lake at that time. Oxygen isotope values in ostracod valves are ~4 ‰ heavier than lake water, showing considerable variability over time but no clear trend. The bottom water temperature is ~ 5^oC year-round, and we interpret the variable oxygen isotope values as resulting from E-P fluctuations. Ostracods were common before the HPP, disappeared during its early stages, and did not reappear immediately when hydrothermal activity waned. Sudden reappearance after deposition of the 1300 BP volcanic ash layer suggests that dispersal was not limiting, as supported by the cosmopolitan distribution of F. caudata. Further data will enable us to determine under what hydrothermal flux conditions ostracods thrive in this lake.