ENVIRONMENTAL EVOLUTION OF PAULINA LAKE (NEWBERRY, OR) OVER THE LAST 15,000 YEARS

Paulina Lake (PL) is a volcanic lake in the Newberry volcano caldera, separated by a 7600BP volcanic ridge from neighbouring East Lake (EL). PL is thermally stratified in summer and ice-covered in winter; its waters have a pH of 7.5 to 9. Sediment cores (PL1: 260 cm; PL2: 506 cm) are dated using tephrochronology (including the Mt. Mazama-Crater Lake ash: MAZ-CL) and ¹⁴C dates to provide a record of secular changes in the lake. The section below MAZ-CL may have accumulated in a single rather than 2 separate crater lakes, because the ridge was absent or less pronounced. Ostracod valves vary in abundance in grab samples and cores, with Fabaeformiscandona caudata and Limnocythere paraornata as the most common species present from the lowermost samples on. However, some core sections below MAZ-CL are enriched in Fe, Mn, P and As (High Fe Interval, HFI), and lack ostracods. Ostracod stable isotope values range from –9.8 to –6.2‰ for δ¹⁸O, and from –3.6 to +3.4‰ for δ¹³C. Variations in δ¹⁸O were interpreted in terms of paleo water temperatures, correcting for vital effects. δ¹⁸O data for modern lake water combined with modern ostracod data give the range of observed lake water temperatures: 4-5°C at the bottom and up to 18°C above the thermocline. The δ¹⁸O_{lake water} may have varied over time, with changes in precipitation rate and climate. The δ¹³C values are high in the shallow warm waters, where photosynthesis by submerged aquatic vegetation (including charophytes) may drive up the δ¹³C_DIC. The δ¹³C_DIC values in the water column in the open lake are fairly stable at –1 to +1‰. The observed δ¹³C_ostracods can be best explained as having formed from mixtures of CO₃^2- and HCO₃^- taking into account the δ¹³C variation in ionic species in DIC. Pre- and post-MAZ-CL sequences differ profoundly, with L. paraornata more common in the latter, and with evidence for lateral transport of ostracods from shallow water to deep water: overall abundance, presence of epiphytic species in deep water, and shallow, warm water isotope signatures indicate that the mixing regime in PL must have changed dramatically after the formation of the two separate lakes. The ostracod data show a sensitivity of ostracods during the “high Fe” period for arsenic, and absence of ostracods in PL may be caused by arsenic toxicity.