Subterranean karst environments as a global sink for atmospheric methane

Abstract

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.

Diversity, Distribution and Co-occurrence Patterns of Bacterial Communities in a Karst Cave System

“…Chemoautotrophic bacteria in a Romanian cave were found capable of fixing inorganic carbon and using hydrogen sulfide as an energy source, and their chemoautotrophic production also supported cave-adapted invertebrates (Sarbu et al, 1996). Geobiological evidence in recent years suggests that subterranean karst environments might be a global sink for atmospheric methane due to processes involving microbes (Nguyễn-Thuý et al, 2017; Webster et al, 2018; Zhao et al, 2018). A better understanding of the microbial community of cave ecosystems will not only expand our knowledge of global microbial diversity, but it also provides valuable information about energy dynamics of novel community assemblages.…”

Microbial Activity in Subterranean Ecosystems: Recent Advances

“…In fact, one of the important •OH sources in cave air may be from radioactive 222 Rn decay [34]. However, further research verified that the mechanism of CH 4 consumption was seasonally changing and methane-oxidizing bacteria were primarily responsible for the widespread observations of CH 4 depletion in subterranean environments, discarding any evidence of radiolysis contribution [37][38][39].…”

A Vulnerable Environment Study in Karst Regions between 1991 and 2017: A Bibliometric Analysis

“…In the past few years, a lot of researches about vulnerable karst environment have been undertaken. Some of the researches covered areas of characteristics of vulnerable karst environment, cause of formation, and ecological restoration and rehabilitation [11,20,[23][24][25][26], while others focused on the landscape of karst rocky desertification [5,10,18,19,27], the soil erosion in karst region [9,[28][29][30][31], cave environment [32][33][34], aquifer and spring based water environment [35][36][37][38][39][40][41][42][43][44][45], carbon source and sink [46][47][48], eco-hydrology [49,50], and protection and management of vulnerable karst environment [13,51,52]. Those researches addressed the major issues, yet there is still a lack of a general review that may reveal the trend of studies in this field.…”