Subterranean microbial oxidation of atmospheric methane in cavernous tropical karst

Nguyễn-Thùy, Dương; Schimmelmann, Arndt; Nguyễn-Văn, Hướng; Drobniak, Agnieszka; Lennon, Jay T.; Ta, Phuong H.; Nguyen, Nuong Thi Kieu

doi:10.1016/j.chemgeo.2017.06.014

Cited by 21 publications

(19 citation statements)

References 27 publications

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“…Additionally, our data resemble an arctic system characterized by acetoclastic and hydrogenotrophic CH4 sources and methanotrophy (McCalley et al, 2014). Our isotopic evidence for in-situ microbial CH4 oxidation in caves is corroborated by recent results from in-situ mesocosm experiments in Vietnam where cave rocks with live microorganisms were shown to consume CH4 even in cases where surface soils were very thin to non-existent (Lennon et al, 2017;Nguyễn-Thuỳ et al, 2017).…”

Section: Methane Oxidation Mechanismssupporting

confidence: 87%

Subterranean karst environments as a global sink for atmospheric methane

Webster

Drobniak

Etiope

et al. 2018

Earth and Planetary Science Letters

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The air in subterranean karst cavities is often depleted in methane (CH4) relative to the atmosphere. Karst is considered a potential sink for the atmospheric greenhouse gas CH4 because its subsurface drainage networks and solution-enlarged fractures facilitate atmospheric exchange. Karst landscapes cover about 14% of earth's continental surface, but observations of CH4 concentrations in cave air are limited to localized studies in Gibraltar, Spain, Indiana (USA), Vietnam, Australia, and by incomplete isotopic data. To test if karst is acting as a global CH4 sink, we measured the CH4 concentrations, δ 13 CCH 4 , and δ 2 HCH 4 values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO2 concentrations, δ 13 CCO 2 , and radon (Rn) concentrations to support CH4 data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35 exhibited subatmospheric CH4 concentrations in at least one location compared to their local atmospheric backgrounds. CH4 concentrations, δ 13 CCH 4 , and δ 2 HCH 4 values suggest that microbial methanotrophy within caves is the primary CH4 consumption mechanism. Only 5 locations from 3 caves showed elevated CH4 concentrations compared to the atmospheric background and could be ascribed to local CH4 sources from sewage and outgassing swamp water. Several associated δ 13 CCH 4 and δ 2 HCH 4 values point to carbonate reduction and acetate fermentation as biochemical pathways of limited methanogenesis in karst environments and suggest that these pathways occur in the environment over large spatial scales. Our data show that karst environments function as a global CH4 sink.

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Section: Methane Oxidation Mechanismssupporting

confidence: 87%

Subterranean karst environments as a global sink for atmospheric methane

Webster

Drobniak

Etiope

et al. 2018

Earth and Planetary Science Letters

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“…A key reference point in the data interpretation is that the background atmosphere usually has around 1.8 ppm of CH 4 and its carbon and hydrogen isotopic composition (δ 13 C CH4 ≈ −47‰ VPDB, δ 2 H CH4 ≈ −100‰ VSMOW) is a product of inputs from an isotopically wide range of sources. The CH 4 concentration of cave air in epigenetic caves and, in general, in well-ventilated caves independently of their speleogenesis mechanisms are often depleted, confirming that subterranean environments may represent an overlooked sink for atmospheric CH 4 [23,[38][39][40][41][42][43][44] and, further, it is rapidly consumed in caves on time scales ranging from hours to days [23,39]. On the opposite case, underground air of some hypogene caves may contain unusually high levels of methane (up to 3%, e.g., Movile Cave) related to the action of chemoautotrophic bacteria [45], and others have moderate CH 4 concentrations, just above the atmospheric background, related to CH 4 outgassing from spring water in sulphuric acid hypogenic caves (e.g., <4 ppm CH 4 at Cueva Villa Luz [7]).…”

Section: Sources and Sink Processes During Migration Andmentioning

confidence: 99%

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

et al. 2018

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The hydrothermal caves linked to active faulting can potentially harbour subterranean atmospheres with a distinctive gaseous composition with deep endogenous gases, such as carbon dioxide (CO2) and methane (CH4). In this study, we provide insight into the sourcing, mixing, and biogeochemical processes involved in the dynamic of deep endogenous gas formation in an exceptionally dynamic hypogenic karst system (Vapour Cave, southern Spain) associated with active faulting. The cave environment is characterized by a prevailing combination of rising warm air with large CO2 outgassing (>1%) and highly diluted CH4 with an endogenous origin. The δ13CCO2 data, which ranges from −4.5 to −7.5‰, point to a mantle-rooted CO2 that is likely generated by the thermal decarbonation of underlying marine carbonates, combined with degassing from CO2-rich groundwater. A pooled analysis of δ13CCO2 data from exterior, cave, and soil indicates that the upwelling of geogenic CO2 has a clear influence on soil air, which further suggests a potential for the release of CO2 along fractured carbonates. CH4 molar fractions and their δD and δ13C values (ranging from −77 to −48‰ and from −52 to −30‰, respectively) suggest that the methane reaching Vapour Cave is the remnant of a larger source of CH4, which was likely generated by microbial reduction of carbonates. This CH4 has been affected by a postgenetic microbial oxidation, such that the gas samples have changed in both molecular and isotopic composition after formation and during migration through the cave environment. Yet, in the deepest cave locations (i.e., 30 m below the surface), measured concentration values of deep endogenous CH4 are higher than in atmospheric with lighter δ13C values with respect to those found in the local atmosphere, which indicates that Vapour Cave may occasionally act as a net source of CH4 to the open atmosphere.

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“…At the same time, two fragments of pitchblende (containing uraninite as a radiation source) were placed into the terrarium blank experiments with no artificially enhanced radiation to demonstrate the sensitivity of our setup to detect CH 4 -losses. In addition, we conducted (iii) two experiments with moist soils in the absence of added radon isotopes to assess the potential for environmental microorganisms (i.e., MOB) to remove CH 4 as has been demonstrated elsewhere by members of our research team [14,18]. The comparisons among experiments covered a common range of CH 4 concentration and thus only differed in the lengths of their time windows needed to lower the CH 4 concentration from the upper to the lower threshold (i.e.…”

Section: Active Time-series Measurements With Circular Flow At Iumentioning

confidence: 99%

“…A growing number of studies have reported that, throughout the world, concentrations of CH 4 are often depleted in the air of caves suggesting that subterranean environments may represent an overlooked sink for atmospheric CH 4 (e.g., [8][9][10][11][12][13]). Based on ventilation rates and CH 4 pools, it is estimated CH 4 is rapidly consumed in caves on time scales ranging from hours to days [14,15]. Depletion of CH 4 in caves is often attributed to MOB.…”

Section: Introductionmentioning

confidence: 99%

Radiolysis via radioactivity is not responsible for rapid methane oxidation in subterranean air

Schimmelmann¹,

Cortes²,

Cuezva³

et al. 2018

Preprint

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Atmospheric methane is rapidly lost when it enters humid subterranean critical and vadose zones (e.g., air in soils and caves). Because methane is a source of carbon and energy, it can be consumed by methanotrophic methane-oxidizing bacteria. As an additional subterranean sink, it has been hypothesized that methane is oxidized by natural radioactivity-induced radiolysis that produces energetic ions and radicals, which then trigger abiotic oxidation and consumption of methane within a few hours. Using controlled laboratory experiments, we tested whether radiolysis could rapidly oxidize methane in sealed air with different relative humidities while being exposed to elevated levels of radiation (more than 535 kBq m-3) from radon isotopes 222Rn and 220Rn (i.e., thoron). We found no evidence that radiolysis contributed to methane oxidation. In contrast, we observed the rapid loss of methane when moist soil was added to the same apparatus in the absence of elevated radon abundance. Together, our findings are consistent with the view that methane oxidizing bacteria are responsible for the widespread observations of methane depletion in subterranean environments. Further studies are needed on the ability of microbes to consume trace amounts of methane in poorly ventilated caves, even though the trophic and energetic benefits become marginal at very low partial pressures of methane.

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Subterranean microbial oxidation of atmospheric methane in cavernous tropical karst

Cited by 21 publications

References 27 publications

Subterranean karst environments as a global sink for atmospheric methane

Subterranean karst environments as a global sink for atmospheric methane

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

Radiolysis via radioactivity is not responsible for rapid methane oxidation in subterranean air

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