RESILIENCE OF LIFE IN A VOLCANIC LAKE: THE OSTRACOD RECORD OF PAULINA LAKE, NEWBERRY VOLCANO, OR

Paulina Lake is a ~6500-year-old, hydrothermally-fed volcanic lake in the Newberry Caldera (OR). The carbonate-rich hydrothermal inputs may stimulate primary productivity, but also contain toxic arsenic. We analyzed ostracod assemblages and stable isotopes in 5 sediment cores (lengths 55 - 500 cm; water depths 15-72 m). The most common species, especially in >45 m water depth, is Fabisformaecandona caudata, with Limnocythere sp. cf. paraornata more common at shallower depths, and rare epiphytic Cypridopsis vidua. Core PL2 (70 m water depth) provides a record of ~3600 yrs, according to an age model based on tephrochronology and ²¹⁰Pb data. PL2 sediment contains up to 15.5% Fe₂O₃, 65-70% SiO₂, and is enriched in As, Mn, and P from hydrothermal inputs. The remainder is volcanic ash, and about 3-5 % C_org. Ostracods occur in many samples, but are absent/very rare in sediments deposited during the Hydrothermal Peak Period (HPP, ~2300-3100 yrs BP), indicated by Fe+As-rich sediment. Data from ~40 surface samples (with maximum 600 ppm As) and ~60 core samples indicate that ostracods are sparse or absent at arsenic concentrations above 50 ppm in the sediment, as found during the HPP. Ostracods are present immediately above the HPP in core PL2, but remained absent for approximately 1000 more yrs in core PL1, which has higher As levels. Ostracods apparently survive common volcanic ash falls, but they are rare/absent in the 1300 yr BP Paulina Lake Ash Flow (PLAF) layer, which may have deposited ~40cm of ash and pumice instantaneously. Populations recovered directly above PLAF, but L. sp. cf. paraornata is more abundant above than below PLAF, indicating downslope sediment transport. Oxygen isotopic compositions of F. caudata valves reflect current bottom water temperatures (4-5^oC) assuming modern water compositions, but L. sp. cf. paraornata data give much higher temperatures, as found in shallow waters in summer. C-isotopic values fluctuate, with a broad trend from -3.5‰ at 3500 yrs BP to -1.25‰ in modern samples. This may reflect changes in volcanic inputs, with possibly higher pH values and less CO₂ degassing in the past, increased CO₂ degassing today. The strength of volcanic fluid inputs culminated during the HPP, when ostracods could not survive, and diminished during lake evolution towards the modern environment with abundant ostracods.