Abstract. Methane (CH4) seepage (i.e., steady or episodic flow
of gaseous hydrocarbons from subsurface reservoirs) has been identified as a
significant source of atmospheric CH4. However, radiocarbon data from
polar ice cores have recently brought into question the magnitude of fossil
CH4 seepage naturally occurring. In northern high latitudes, seepage of
subsurface CH4 is impeded by permafrost and glaciers, which are under
an increasing risk of thawing and melting in a globally warming world,
implying the potential release of large stores of CH4 in the future.
Resolution of these important questions requires a better constraint and
monitoring of actual emissions from seepage areas. The measurement of these
seeps is challenging, particularly in aquatic environments, because they
involve large and irregular gas flow rates, unevenly distributed both
spatially and temporally. Large macroseeps are particularly difficult to
measure due to a lack of lightweight, inexpensive methods that can be
deployed in remote Arctic environments. Here, we report the use of a mobile
chamber for measuring emissions at the surface of ice-free lakes subject to
intense CH4 macroseepage. Tested in a remote Alaskan lake, the method
was validated for the measurement of fossil CH4 emissions of up to 1.08 × 104 g CH4 m−2 d−1 (13.0 L m−2 min−1
of 83.4 % CH4 bubbles), which is within the range of global fossil
methane seepage and several orders of magnitude above standard ecological
emissions from lakes. In addition, this method allows for low diffusive flux
measurements. Thus, the mobile chamber approach presented here covers the
entire magnitude range of CH4 emissions currently identified, from
those standardly observed in lakes to intense macroseeps, with a single
apparatus of moderate cost.
Technical note: Mobile open dynamic chamber measurement of methane macroseeps in lakes
