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Dynamics and drivers of ventilation in caves are of growing interest for different f ields of science. Accumulated CO2 in caves can be exchanged with the atmosphere, modifying the internal CO2 content, affecting stalagmite growth rates, deteriorating rupestrian paintings, or creating new minerals. Current estimates of cave ventilation neglect the role of high CO2 concentrations in determining air density – approximated via the virtual temperature (Tv) –, affecting buoyancy and therefore the release or storage of CO2. Here we try to improve knowledge and understanding of cave ventilation through the use of Tv in CO2-rich air to explain buoyancy for different values of temperature (T) and CO2 content. Also, we show differences between T and Tv for 14 different experimental sites in the vadose zone, demonstrating the importance of using the correct def inition of Tv to determine air buoyancy in caves. The calculation of Tv (including CO2 effects) is currently available via internet using an excel template, requiring the input of CO2 (%), air temperature (oC) and relative humidity (%).

Subterranean ventilation is a non‐diffusive transport process that provokes the abrupt transfer of CO2‐rich air (previously stored) through water‐free soil pores and cracks from the vadose zone to the atmosphere, under high‐turbulence conditions. In dryland ecosystems, whose biological carbon exchanges are poorly characterized, it can strongly determine eddy‐covariance CO2 fluxes that are used to validate remote sensing products and constrain models of gross primary productivity. Although subterranean ventilation episodes (VE) may occur in arid and semi‐arid regions, which are unsung players in the global carbon cycle, little research has focused on the role of VE CO2 emissions in land–atmosphere CO2 exchange. This study shows clear empirical evidence of globally occurring VE. To identify VE, we used in situ quality‐controlled eddy‐covariance open data of carbon fluxes and ancillary variables from 145 sites in different open land covers (grassland, cropland, shrubland, savanna, and barren) across the globe. We selected the analyzed database from the FLUXNET2015, AmeriFlux, OzFlux, and AsiaFlux networks. To standardize the analysis, we designed an algorithm to detect CO2 emissions produced by VE at all sites considered in this study. Its main requirement is the presence of considerable and non‐spurious correlation between the friction velocity (i.e., turbulence) and CO2 emissions. Of the sites analyzed, 34% exhibited the occurrence of VE. This vented CO2 emerged mainly from arid ecosystems (84%) and sites with hot and dry periods. Despite some limitations in data availability, this research demonstrates that VE‐driven CO2 emissions occur globally. Future research should seek a better understanding of its drivers and the improvement of partitioning models, to reduce uncertainties in estimated biological CO2 exchanges and infer their contribution to the global net ecosystem carbon balance. [ABSTRACT FROM AUTHOR]

Highlights: [•] An empirical approach aimed to disentangle biological and degassing processes involved in eddy fluxes is proposed. [•] The time trend in CO2 emissions was largely explained by subterranean CO2 ventilation. [•] Improved decomposition of the net flux into credible biological component processes was achieved. [Copyright &y& Elsevier]

Dynamics and drivers of ventilation in caves are of growing interest for different fields of science. Accumulated CO2 in caves can be exchanged with the atmosphere, modifying the internal CO2 content, affecting stalagmite growth rates, deteriorating rupestrian paintings, or creating new minerals. Current estimates of cave ventilation neglect the role of high CO2 concentrations in determining air density -- approximated via the virtual temperature (Tv) --, affecting buoyancy and therefore the release or storage of CO2. Here we try to improve knowledge and understanding of cave ventilation through the use of Tv in CO2-rich air to explain buoyancy for different values of temperature (T) and CO2 content. Also, we show differences between T and Tv for 14 different experimental sites in the vadose zone, demonstrating the importance of using the correct definition of Tv to determine air buoyancy in caves. The calculation of Tv (including CO2 effects) is currently available via internet using an excel template, requiring the input of CO2 (%), air temperature (•C) and relative humidity (%). [ABSTRACT FROM AUTHOR]

Abstract: This review article analyzes different abiotic processes that could contribute to the global carbon cycle on short time scales, beginning with high rates of net CO2 release or uptake measured over ecosystems by the FLUXNET community. The two main abiotic interpretations for these “anomalous” measurements are weathering processes and subterranean cavity ventilation. After analyzing their mechanisms and drivers, we evaluate their possible relevance and contributions in the studies mentioned above. Analyzing weathering (calcite dissolution and precipitation) chemistry and using a geochemical model, we conclude that CO2 dissolution processes could explain the measured CO2 release following dry season rain events, but their contribution is far from sufficient to explain large magnitudes of daytime CO2 emissions or annual CO2 uptake measured in some desert ecosystems. In this context, we hypothesize and evaluate a further abiotic mechanism: the role of subterranean cavities as a temporal depot of CO2, along with their seasonal ventilation. A first approximation estimates that the subterranean CO2 pool (and its potential ventilation) could represent more than half of the total CO2 content of the atmosphere. Therefore, the non-negligible potential contribution to the net ecosystem carbon balance requires further investigation towards a better understanding of its drivers. [Copyright &y& Elsevier]