CARBON CYCLING IN TWO VOLCANIC LAKES FED BY GEOTHERMAL FLUIDS: PHOTOSYNTHETIC SEQUESTRATION AND DIFFUSIVE CO2 ESCAPE

The Newberry volcano caldera (OR; 43^o41.21’N, 121^o15.18'W) contains East Lake (EL) and Paulina Lake (PL), twin crater lakes separated by a narrow volcanic ridge formed at ~6ka. EL is a terminal lake (55m max. depth), whereas PL (85m max. depth) overflows through Paulina Creek. Gas bubbles with CO₂, minor H₂S and Hg rise from the EL floor, and dissolve before reaching the surface. Subaqueous hot springs rich in Si, Fe, P, As and abundant carbonate feed PL. The lake ecosystems have diatoms as common primary producers, using volcanic CO₂ and P while photosynthesizing, and the geothermal Si for their frustules; fixed nitrogen is provided by cyanobacteria (Nostoc sp.). Sediments in EL and PL contain resp. 8% and 4% organic carbon that consists of the remains of the unicellular primary producers, with additional 'phytoberg' debris in EL from floating islands of partially submerged aquatic vegetation. The PL sediment also contains carbonate ostracod valves. The EL carbon cycle is determined by the net burial rate of photosynthetic carbon, the removal of fish by fishermen and local raptors, and diffusive CO₂ escape at the water-air interface. The PL carbon cycle is determined by the Paulina Creek efflux, photosynthetic flux and fish catch, whereas the diffusive CO₂ loss is small because of its lower internal pCO₂ (higher pH). The photosynthetic and gas diffusion fluxes remove isotopically light carbon from the water, leading to high δ¹³C values in DIC, especially in EL surface waters (+5‰). Water-depth profiles show strong δ¹³C gradients in EL, with only minor surface enrichment in PL. The EL-DIC shows a small increase with depth while the pH decreases. Our EL data suggest a small increase in DIC concentration over the last 5 years, with slightly increasing δ¹³C values. The diffusive CO₂ loss in EL was measured with a floating accumulation chamber, suggesting a daily loss of ~30 tonnes CO₂. This value will be refined considering seasonal and day-night flux variations. The EL C_org burial rate is derived from the sediment core data with a mean sedimentation rate of 1.5 mm/yr, at a sediment bulk dry density of ~0.12 gr/cm³. We will present multiyear carbon cycle model data for EL for given volcanic CO₂ inputs, measured fluxes and their isotopic compositions, with the vertical water profiles for DIC and δ¹³C as model constraints.