Time‐series analysis of high‐resolution ebullition fluxes from a stratified, freshwater lake

Abstract

.[1] Freshwater lakes can emit significant quantities of methane to the atmosphere by bubbling. The high spatial and temporal heterogeneity of ebullition, combined with a lack of high-resolution field measurements, has made it difficult to accurately estimate methane fluxes or determine the underlying mechanisms for bubble release. We use a high-temporal resolution data set of ebullitive fluxes from the eutrophic Upper Mystic Lake, Massachusetts to understand the triggers that lead to bubbling from submerged sediments. A wavelet approach is introduced to detect ebullition events for multiple time-scales, and is complemented with traditional statistical methods for data analyses. We show that bubble release from lake sediments occurred synchronously at several sites, and was closely associated with small, aperiodic drops in total hydrostatic pressure. Such results are essential to constrain mechanistic models and to design future measurement schemes, particularly with respect to the temporal scales that are needed to accurately observe and quantify ebullition in aquatic ecosystems.Citation: Varadharajan, C., and H. F. Hemond (2012), Time-series analysis of high-resolution ebullition fluxes from a stratified, freshwater lake,

The geochemistry of methane isotopologues

“…2.2), due to strong D/H disequilibria between methane and water (Sherwood Lollar et al, 2008) and low ∆ 13 CH 3 D temperature signals of 56-90°C that are consistent with other temperature estimates for these groundwaters (Sherwood Lollar et al, 2008). Although the specific mechanisms by which the proposed abiotic hydrocarbons at Kidd Creek are generated remain under investigation (Sherwood Lollar et al, 2002;Sherwood Lollar et al, 2014) enables the differentiation of methane that has been formed at extremely low rates in the subsurface (Pohlman et al, 2009;Bates et al, 2011;Holler et al, 2011) from methane formed in cattle and surface environments in which methanogenesis proceeds at comparatively high rates (Johnson and Johnson, 1995;Varadharajan and Hemond, 2012).…”

“…Next, we used this framework to place constraints on the origins of methane at two sites of present-day serpentinization in Phanerozoic ophiolites [The Cedars (Morrill et al, 2013) enables the differentiation of methane that has been formed at extremely low rates in the subsurface (Pohlman et al, 2009;Bates et al, 2011;Holler et al, 2011) from methane formed in cattle and surface environments in which methanogenesis proceeds at comparatively high rates (Johnson and Johnson, 1995;Varadharajan and Hemond, 2012).…”

“…Ebullition of methane from this lake has been previously documented (Scandella et al, 2011;Varadharajan and Hemond, 2012). We collected gas bubbles using inverted funnel-shaped bubble traps [modified from an inverted-funnel design described previously (Varadharajan et al, 2010;Varadharajan and Hemond, 2012)] deployed~2 m above the lake floor (~18 m water depth) using a custom rope and buoy structure.2 The deep deployment depth was chosen to minimize dissolution and/or oxidation of bubbles during their transit from the sediment to the lake surface. The collected gases were transferred via syringes to serum vials (either pre-evacuated or pre-filled with deionized water that was displaced to make room for the gas sample) sealed with blue butyl septa, fixed with either saturated NaCl solution or 1 M NaOH, and stored at either 4°C or room temperature until analysis.…”

“…Upper Mystic Lake is a freshwater lake in the Boston metropolitan area. Ebullition of methane from this lake has been previously documented (Scandella et al, 2011;Varadharajan and Hemond, 2012). We collected gas bubbles using inverted funnel-shaped bubble traps [modified from an inverted-funnel design described previously (Varadharajan et al, 2010;Varadharajan and Hemond, 2012)] deployed~2 m above the lake floor (~18 m water depth) using a custom rope and buoy structure.…”

Pumping methane out of aquatic sediments – ebullition forcing mechanisms in an impounded river

“…Most studies suggest that ebullition occurs episodically (Coulthard et al, 2009;Goodrich et al, 2011;Varadharajan and Hemond, 2012;Wik et al, 2013). The episodic pattern may be related to a complex interplay between bubble buoyancy and sediment mechanics.…”