Paper No. 236-8
Presentation Time: 3:45 PM
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 δ18Olake 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 δ13CDIC. The δ13CDIC values in the water column in the open lake are fairly stable at –1 to +1‰. The observed δ13Costracods can be best explained as having formed from mixtures of CO32- 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.