Publication Date

April 2017

Advisor(s)

Johan Varekamp

Major

Earth & Environmental Sciences

Language

English (United States)

Abstract

East Lake and Paulina Lake are twin crater lakes in the Newberry caldera, near Bend, Oregon. Despite their proximity, the lakes are chemically different: CO2 and H2S inputs in East Lake, and hot carbonate-rich fluids in Paulina Lake. Paulina Lake has four times the concentration of dissolved species as East Lake. Dissolved carbon in East Lake is isotopically much heavier (up to +5.5‰) than in Paulina Lake (0‰). Both lakes have internal PCO2 > PCO2 (atm), leading to diffusive CO2 loss from the lake surfaces. The carbon budgets are different for the two lakes, and this study focuses on the carbon cycling in East Lake, which involves geothermal CO2 input, diffusive CO2 losses from the lake surface, no CO2 uptake from the atmosphere, photosynthesis, respiration, and organic carbon burial. The isotopic composition and C:N of East Lake sediment indicates that buried organic matter is ~45% phytoplankton, 35% subaqueous vegetation, 15% cyanobacteria, and 5% pine needles. East Lake has a strong vertical d13C (DIC) gradient (up to 5‰), because volcanic and biological sources and sinks of carbon in East Lake cause identical trends in DIC concentration and isotopic composition with depth. Isotopically light geothermal CO2 enters the bottom water and evades from the surface waters (-10.5‰ d13C), while isotopically light organic carbon is removed from surface waters and falls to bottom waters through the photosynthetic-respiration loop. Field flux measurements, sequential Gaussian simulations, and models indicate that both Newberry lakes evade CO2 at comparable rates for volcanic lakes with similar surface areas and that CO2 flux rates are highest in the early summer. East Lake evaded an average of ~71 tonnes of CO2 per day in early June 2016, compared to ~45 tonnes of CO2 per day in July 2015. Novel field and lab draw-down build-up experiments were completed to determine the isotopic composition of the evading CO2 gas, and trends in d13C and d18O CO2(aq)-(gas) offsets of up to 7‰ d13C past equilibrium fractionation factors indicate a non-equilibrium fractionation component to lake degassing. Additionally, the well-oxygenated surface waters of both East Lake and Paulina Lake have 20-50 times higher CH4 concentrations with much more negative d13C and dD isotopic signatures. These profiles suggest aerobic methanogenesis by fermentation in the surface waters, whereas the deeper waters have thermogenic methane. All new field and experimental data for this study were collected in May-December of 2016.

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