Dynamics of the methane profile through the water column of meromictic Fayetteville Green Lake, N.Y
Abstract
Green Lake in Fayetteville, N.Y. is a meromictic lake with a chemocline approximately 21 meters below the surface where redox chemistry shifts from micro-aerobic to euxinic, and a purple- and green- sulfur bacterial plate is a predominant feature. Historic data, mirrored by our recent (November 2012) high-resolution sampling and analysis, document a monimolimnion methane concentration profile that increases nearly linearly with depth to the bottom sediments. Gas chromatography (GC) and Flame Ionization Detection (FID) analyses reveal concentrations exceeding20 μM CH4 at 30 cm depth in the sediments, lower concentrations ranging from ~5 μM CH4 at 44 m to 1.5 μM CH4 at 21.75 m in the water column, and decreased concentrations with an average of 0.12 μM CH4 from 21 m through the chemocline and the oxic zone, demonstrating a diffusive trend from sediments to the chemocline. However, our findings exhibit a departure from linearity from 21-30 meters in which methane concentrations were higher than expected if the sediments were the sole source of methane. We incubated biomass collected from 24 m (June 2013) to examine the source of this unexpected ';hump' in methane concentrations in the water column. To date, no methane production has been observed. Isotopic analysis for δ 13C in CH4 of seven water samples collected from depths above, below and in the methane ';hump' indicate that methane present in the water column is biogenic. Furthermore, the δ 13C values observed, approximately -100‰, indicate biologically- mediated cycling of methane. δ13C values of dissolved inorganic carbon (DIC) indicate input of oxidized methane. These findings suggest that two sources of methane with similar isotopic compositions exist, one diffusing from lake-bottom sediments and the other laterally injected from seeps at or near the chemocline, with consumption near the base of the chemocline. Coupled geochemical analyses show that sulfide and ammonia exhibit a similar concentration profile to methane, increasing nearly linearly with depth through the water column and exhibiting the ';hump' just below the chemocline. Future work using quantification of coenzyme M, an intermediate necessary for methanogenesis, coupled to molecular analysis of sediment and water column samples will determine if methane-enriched groundwater is the source of biogenic methane. In addition, microcosms and molecular analysis will elucidate potential contributions to the methane cycle by microorganismsm in and below the chemoclime such as, anaerobic methane oxidizers, sulphate- reducing bacteria, and anaerobic ammonia oxidizers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B13E0553M
- Keywords:
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- 0448 BIOGEOSCIENCES Geomicrobiology;
- 1000 GEOCHEMISTRY;
- 1845 HYDROLOGY Limnology;
- 0454 BIOGEOSCIENCES Isotopic composition and chemistry