CARBON CYCLING IN EAST LAKE AND PAULINA LAKE, NEWBERRY VOLCANO, OREGON

East Lake and Paulina Lake are twin crater lakes in the Newberry caldera, near Bend, OR. Despite their proximity, the lakes are chemically different: CO₂ and H₂S inputs in East Lake, and hot carbonate-rich fluids in Paulina Lake. Dissolved carbon in East Lake is isotopically much heavier (up to +5.5‰) than in Paulina Lake (0‰). Both lakes have internal P_CO2 > P_CO2 (atm), leading to diffusive CO₂ loss from the lake surfaces. East Lake has a strong vertical δ¹³C (DIC) gradient due to the diffusional escape of isotopically light CO₂ at the surface, combined with photosynthetic carbon drawdown. Paulina Lake lacks such a gradient, a result of smaller CO₂ losses, higher DIC, and better vertical lake mixing.

Lake CO₂ flux measurements were made in June 2015 and early June 2016 with an accumulation chamber. East Lake had flux rates of 0.03-0.9 moles of CO2 m^-2 day^-1, with most data at 0.2 moles of CO2 m^-2 day^-1. The field data (100 data points) were treated with a sequential Gaussian simulation, and a surface loss of ~45 tonnes of CO₂/day was calculated for East Lake. The flux measurements are done against the high ambient CO₂ background (~400 ppm) and in some cases the chamber was pumped down with an absorbent to ~150 ppm CO₂. The CO₂ flux rate was then measured more precisely and more pure lake gas samples were collected for isotopic analyses. These measurements indicate a flux rate of 0.2-0.5 moles of CO2 m^-2 day^-1, well within the range found during conventional flux measurements. The CO₂ drawdown experiments also provided a novel method for gas flux determinations: the lake CO₂ outflux can be determined from the drawdown flow rate and CO₂ concentration in the chamber at steady state.

Vertical water profiles in both lakes show identical methane abundance patterns and isotopic profiles. The slightly oxygen depleted bottom waters have very low CH₄ concentrations. Well oxygenated shallow waters have 20-50 times higher CH₄ concentrations with much more negative δ¹³C and δD isotopic signatures. These profiles suggest aerobic methanogenesis by fermentation in the surface waters, whereas the deeper water have thermogenic methane.

Ongoing studies need to be completed on the mechanisms and effects of the CO₂ and CH₄ fluxes and gradients in the two lakes in order to understand their carbon cycles. Additional new CO₂ and CH₄ data will be presented at the meeting.