EVOLUTION OF PAULINA LAKE (NEWBERRY, OR) OVER THE LAST FEW THOUSAND YEARS: UPDATED TEPHROCHRONOLOGY WITH NEW MICROPROBE ANALYTICAL RESULTS

Paulina Lake (PL) is a volcanic lake inside the Newberry caldera, separated by an ~7600 BP volcanic ridge from neighbouring East Lake (EL). PL is fed by subaqueous carbonate-rich hot springs with fluids rich in Na, K, Ca, Mg, Si, Fe, Mn, P, and As. The sediment consists largely of silica and poorly crystalline ferrous oxides, with arsenic concentrations up to 620.8 ppm and up to 17.6 wt. % iron measured in the lake sediments. PL contains a possible large hydrothermal mound called Dante’s Peak, which rises almost 60 m from the lake bottom and has abundant hydrothermal deposits at its summit. Sediment cores provide a chemical record of secular changes in the lake constrained by tephrochronology and ¹⁴C dating. Recently updated using microprobe analysis of volcanic glasses, the amended age model indicates that the oldest sediments were deposited within a more extensive caldera lake that contained the modern footprint of both EL and PL. The Paulina Lake Ash Flow deposit (PLAF, ~1300 BP) and the Mt Mazama Crater Lake ash deposits (MAZ-CL; 7600 BP) form prominent ash layers in the top 150 cm of the two core records. The longer PL2 record carries an additional three distinct ash layers that have not yet been identified. The ¹⁴C dates were done on bulk organic matter and carry a significant ‘reservoir effect’ because of the hydrothermal carbon inputs. Core sections below MAZ-CL are strongly enriched in Fe, Mn, P, and As, possibly reflecting an increase in the hydrothermal input rate. The section below the MAZ-CL layer reflects the characteristics of a larger, caldera-wide lake. Climatic changes may have influenced the sediment record as well: the Fe-rich layer could be related to the Younger Dryas period (11.7 – 12.9 kyr cal), during which the lake may have been frozen over for extended periods, preventing meteoric water inputs. A combination of sediment accumulation rates and Fe contents provides an input function for the hydrothermal fluids over the last 7000 years, assuming that all sedimentary Fe is hydrothermally derived. Overall, the sedimentary deposits of PL show similarities to Precambrian Iron formations, which in PL sediment formed from the precipitation of ferric oxides and hydrothermal silica when the reducing hot fluids mixed with cold (~4 ^oC), oxygenated lake waters.