Ebbulitive flux of early-diagenetic methane from recent freshwater sediments in lake Nowa Cerekiew (SW Poland)

Mariusz Orion Jędrysek


Strumień ebulicji wczesnodiagenetycznego metanu ze współczesnych osadów słodkowodnych jeziora Nowa Cerekiew (SW Polska)

Observations concerning ebullition of methane from freshwater lake sediments, at depths of 2.8 m and 4.2 m, and occasionally at of 4.5, 5.5 and 11 m have been carried out. A closed type, artificial lake Nowa Cerekiew (SW Poland) about 30 years old, with organic rich sediments 25 cm thick, was selected for this study. Collection of ebulliting gases has been carried out using bottom-situated funnel-shape collectors with connected flasks (exchanged by scubadiving). The observations have been carried out continuously between 1992 to 1996, including winters when the lake was covered by ice.

Concentration of methane in ebullition varied from 9.06% (winter) to 76.82% (early summer). Ebullition (expressed as the mean diurnal production of carbon trapped into the collectors in the form of bubbles of methane naturally released from 1 m3 of the most productive 25 cm uppermost layer of the sediments (C-CH4 x day(-1) x m(-3)) varied from about 100 mg (C-CH4 x day(-1) x m(-3)) to near zero during winter. In contrast to the deeper sampling station (4.2 m), the ebullitive methane from the shallower depth (2.8 m) showed substantial seasonal variation in the δ13C(CH4) value, from -63.12‰ during winter to -52.46‰ during summer. The global freshwater lake ebullitive CH4 flux has been roughly estimated at about 5x10(8)g(C-CH4)xy(-1).

The observed enhanced ebullitive CH4 flux during summer and the higher δ13C(CH4) value during early summer are apparently a result of: (i) efficient decomposition of the fresh organic matter, deposited into the anoxic zone on the sediment surface, and consequently a relatively more enhanced acetic acid pathway and, (ii) increased bacterial activity at higher temperature. The lower δ13C(CH4) value in the deeper sampling station during summer may be a result of: (i) longer time for the organic matter to sink to greater depth and consequently there is greater decomposition of easily degradable compounds which are the main precursors of acetate, (ii) perhaps an increase of bioavailable DIC, due to elevated pressures and lower temperatures at greater depths, enhancing the CO2 pathway, and (iii) limited diffusion of isotopically depleted carbon-bearing compounds from greater depths.

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