Subterranean atmospheres may act as daily methane sinks

Abstract

In recent years, methane (CH 4 ) has received increasing scientific attention because it is the most abundant non-CO 2 atmospheric greenhouse gas (GHG) and controls numerous chemical reactions in the troposphere and stratosphere. However, there is much that is unknown about CH 4 sources and sinks and their evolution over time. Here we show that near-surface cavities in the uppermost vadose zone are now actively removing atmospheric CH 4 . Through seasonal geochemical tracing of air in the atmosphere, soil and underground at diverse geographic and climatic locations in Spain, our results show that complete consumption of CH 4 is favoured in the subsurface atmosphere under near vapour-saturation conditions and without significant intervention of methanotrophic bacteria. Overall, our results indicate that subterranean atmospheres may be acting as sinks for atmospheric CH 4 on a daily scale. However, this terrestrial sink has not yet been considered in CH 4 budget balances. M ethane (CH 4 ) is currently the most abundant non-CO 2 greenhouse gas (GHG) in the atmosphere, reaching a global average concentration of B1,800 p.p.b. at midnorthern latitudes 1 . Despite significant research progress in recent years, large uncertainties remain about the CH 4 budget and its evolution over time because there is a lack of knowledge about CH 4 sinks and sources [2][3][4][5] . Superimposed on the long-term trend of increasing atmospheric CH 4 , there is significant interannual variability 1,6,7 , and the sources of variation remain controversial 8 .Current CH 4 estimates account for B22% of the total forcing potential of all long-lived GHGs 9 . By weight, CH 4 is 28 times more effective at trapping heat in the atmosphere than CO 2 over a 100-year period 10 . Sensible mitigation strategies also require a quantitative understanding of the CH 4 budget regarding emissions and sinks 11 . The total global emissions of CH 4 are constrained reasonably well by atmospheric observations and estimates of its lifetime, based on multiple atmospheric CH 4 inversion models (top-down studies). However, the uncertainties concerning emissions/consumption from individual sources/ sinks 12 are greater, and they are poorly constrained by the current atmospheric observation network 6 . Some unaccounted sinks (or sources) could contribute the global CH 4 budget and its long-term variations.The Earth's surface exerts its influence on the free atmosphere through the atmospheric boundary layer. This lowest portion of the atmosphere ranges from a few tens of metres to 1-2-km deep 13 . The subsurface atmosphere is usually overlooked as an important part of this boundary layer. At the top of the subsurface layer in the vadose zone (below the subsoil and above the groundwater table) highly specific biogeochemical processes occur, which may act as regulators of gas exchanges between the surface and the free atmosphere.The uppermost part of the vadose zone may contain large amounts of underground air, that is, a CO 2 -rich air reservoir permeating the unsaturated...

Abiotic and seasonal control of soil-produced CO2 efflux in karstic ecosystems located in Oceanic and Mediterranean climates

“…Further details about the methodological procedures and quality results can be found in Fernandez-Cortes et al (2015b). In addition, both in-house standards and air samples were also subjected to quality control by comparing the results obtained with the Picarro G2101-i analyser with duplicated bags collected, in situ (on field) and from cylinders, and subsequently analyzed independently in the greenhouse gas laboratory at Royal Holloway University of London (RHUL).…”

Changes in the CO2 dynamics in near-surface cavities under a future warming scenario: Factors and evidence from the field and experimental findings

“…The analyser measures the isotopologues of the carbon dioxide ( 12 CO2 and 13 CO2) and automatically calculates the isotopic value, δ 13 CO2. Further technical specifications and details about the quality control of measurements are described by Garcia-Anton et al (2014) and Fernandez-Cortes et al (2015a). The number of analysed samples was 197 (96 from the cavity, 58 from soil air and 46 from exterior atmosphere).…”

“…Shallow caves are distributed widely across continental ecosystems and contain significant volume of underground air with high concentrations of soil-derived CO2 (Wood, 1985;Bourges et al, 2001Bourges et al, , 2014Faimon et al, 2006;Cuezva et al, 2011;Fernandez-Cortes et al, 2011, 2015aGarcia-Anton et al, 2014). In particular, karst ecosystems are characterized by an inherent high capacity for gas exchange with the atmosphere.…”

Atmospheric characterization through fused mobile airborne and surface in situ surveys: methane emissions quantification from a producing oil field

“…Rigby et al (2017) suggest a decline in OH is likely to have contributed to increasing CH 4 since 2007. The recent discovery of a new significant CH 4 loss mechanism, terrestrial uptake (Fernandez-Cortes et al, 2015), illustrates the need to understand loss mechanisms better (Allen, 2016).…”