Spatial variability of cave-air carbon dioxide and methane concentrations and isotopic compositions in a semi-arid karst environment

McDonough, Liza; Iverach, Charlotte P.; Beckmann, Sabrina; Manefield, Mike; Rau, Gabriel C.; Baker, Andy; Kelly, Bryce F. J.

doi:10.1007/s12665-016-5497-5

Cited by 38 publications

(35 citation statements)

References 52 publications

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“…This difference may be due to the lack of atmospheric connectivity at the site in Movile Cave, which is mostly waterfilled. We did not observe CH 4 concentrations below 1.5 ppmv, as have been observed in many epigenic, non-sulfidic caves in Gibraltar, Australia, the United States, and Spain (Mattey et al, 2013;Fernandez-Cortes et al, 2015;McDonough et al, 2016;Webster et al, 2016;Lennon et al, 2016). Our results demonstrate that if in-situ CH 4 oxidation processes were operating in CVL, they were not strong enough to react all of the CH 4 in the collected samples.…”

Section: Discussion Hydrogen Sulfide Methane and Carbon Dioxide Entsupporting

confidence: 60%

Isotopic evidence for the migration of thermogenic methane into a sulfidic cave, Cueva de Villa Luz, Tabasco, Mexico

Webster¹,

Lagarde²,

Sauer³

et al. 2017

JCKS

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Methane (CH 4 ) is an economic resource and a greenhouse gas, but its migration through rocks is not immediately associated with speleogenesis. Sulfuric-acid speleogenesis is a cave-forming mechanism that has produced a variety of economically important oil fields and aquifers, and is theorized to be related to the oxidation of CH 4 and hydrocarbons. Despite hypotheses that the oxidation of CH 4 may provide a basis for the generation of sulfides during sulfuric-acid speleogenesis, evidence from active systems has not yet been obtained.

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Section: Discussion Hydrogen Sulfide Methane and Carbon Dioxide Entsupporting

confidence: 60%

Isotopic evidence for the migration of thermogenic methane into a sulfidic cave, Cueva de Villa Luz, Tabasco, Mexico

Webster¹,

Lagarde²,

Sauer³

et al. 2017

JCKS

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“…Nevertheless, researchers tracked 13 C‐labeled CH 4 into the DNA of bacteria that were closely related to known methanotrophs such as Methylomonas , Methylococcus , and Methylocystis / Methylosinus (Hutchens et al., ). In a recent study of the semiarid Wellington Caves in Australia, up to 16% of the 16S rRNA gene sequences recovered from cave soils belonged to groups of known methanotrophs (McDonough et al., ). The high relative abundance of methanotrophs in these systems suggests that microbially mediated CH 4 oxidation should be important in at least some caves.…”

Section: Resultsmentioning

confidence: 99%

Microbial contributions to subterranean methane sinks

et al. 2016

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Sources and sinks of methane (CH ) are critical for understanding global biogeochemical cycles and their role in climate change. A growing number of studies have reported that CH concentrations in cave ecosystems are depleted, leading to the notion that these subterranean environments may act as sinks for atmospheric CH . Recently, it was hypothesized that this CH depletion may be caused by radiolysis, an abiotic process whereby CH is oxidized via interactions with ionizing radiation derived from radioactive decay. An alternate explanation is that the depletion of CH concentrations in caves could be due to biological processes, specifically oxidation by methanotrophic bacteria. We theoretically explored the radiolysis hypothesis and conclude that it is a kinetically constrained process that is unlikely to lead to the rapid loss of CH in subterranean environments. We present results from a controlled laboratory experiment to support this claim. We then tested the microbial oxidation hypothesis with a set of mesocosm experiments that were conducted in two Vietnamese caves. Our results reveal that methanotrophic bacteria associated with cave rocks consume CH at a rate of 1.3-2.7 mg CH · m · d . These CH oxidation rates equal or exceed what has been reported in other habitats, including agricultural systems, grasslands, deciduous forests, and Arctic tundra. Together, our results suggest that depleted concentrations of CH in caves are most likely due to microbial activity, not radiolysis as has been recently claimed. Microbial methanotrophy has the potential to oxidize CH not only in caves, but also in smaller-size open subterranean spaces, such as cracks, fissures, and other pores that are connected to and rapidly exchange with the atmosphere. Future studies are needed to understand how subterranean CH oxidation scales up to affect local, regional, and global CH cycling.

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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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Spatial variability of cave-air carbon dioxide and methane concentrations and isotopic compositions in a semi-arid karst environment

Cited by 38 publications

References 52 publications

Isotopic evidence for the migration of thermogenic methane into a sulfidic cave, Cueva de Villa Luz, Tabasco, Mexico

Isotopic evidence for the migration of thermogenic methane into a sulfidic cave, Cueva de Villa Luz, Tabasco, Mexico

Microbial contributions to subterranean methane sinks

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

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