STRATIGRAPHY OF THE NEWBERRY VOLCANO CRATER LAKE SEDIMENTS: TEPHRA AND OSTRACOD DATA

The Newberry (NB) Volcano crater lakes in Oregon, USA (Paulina Lake, PL and East Lake, EL) occur inside the ~75ky caldera, and are separated by a narrow volcanic ridge made of a pumice cone and obsidian flows emplaced ~7500BP. East Lake has inputs of CO₂-rich gases with minor hot fluids, whereas PL has carbonate-rich hydrothermal fluid inputs. We took sediment cores and grabs in both lakes, with two piston cores of 2.5 m and 5 m length in PL. The PL sediments are mainly hydrothermal precipitates with diatoms, C-organic, and tephra, whereas EL sediment consists of diatoms with tephra and minor amounts of hydrothermal precipitates. Both lakes contain a discrete ash layer (up to ~ 50 cm) of ~1300 BP Newberry Pumice/PL ash flow, associated with the 'Big Obsidian Flow'. One core also contains the ~7600BP climactic Mazama tephra layer (~ 50cm) and three older tephra layers, probably of NB origin. Bulk tephra XRF and ICP-MS analyses and microprobe glass analyses are used for tephra identification and tracing compositional fractionation during transport and deposition. Bulk analyses of the NB Pumice and Mazama tephra show a higher Y content in the NB tephra (44 ppm vs 26 ppm Y), and higher Sr content in the Mazama tephra (312 ppm vs 54 ppm Sr). Enrichment of Sr in some tephra samples may be due to in-lake plagioclase sorting processes. Microprobe analyses show higher MgO, CaO and TiO₂ in Mazama glasses, and higher K₂O in NB pumice glasses. The basal tephra layer in an EL core identifies as NB pumice, but dispersed ash in that core shows a wider compositional range. The bottom of the Mazama tephra carries a debris horizon of milfoil water plants, suggesting strong wave action upon arrival of the tephra. 14-C analysis of that plant material provides an age that is too old (~24,000 BP vs 7600BP true age), showing a strong influence of ‘dead’ hydrothermal carbon inputs. We use this age offset to estimate paleo lake water DIC or paleo-salinity. Ostracods are abundant in most PL grab and core sediment, but two Fe-As rich core intervals poor in tephra lack ostracods. PL sediment grab samples show a wide range of chemical compositions and ostracod abundances, suggesting that ephemeral local hot water seeps are the main source of hydrothermal inputs. The sudden deposition of ~50cm thick tephra units must have impacted the hydrothermal discharge and benthic ecosystem.