Assessment of CO2 dynamics in subsurface atmospheres using the wavelet approach: from cavity–atmosphere exchange to anthropogenic impacts in Rull cave (Vall d′Ebo, Spain)

Subsurface environments are non-negligible contributors to the net carbon balance because they act as natural sinks of CO2 and are responsible for the efflux to the Earth’s atmosphere during their ventilation states. In this way, the characterization of the CO2 dynamics in these underground environments is essential to determine the gas exchange between both atmospheres. A complete microclimatic analysis and trace gas (CO2 and 222Rn) monitoring of Rull cave (southeast Spain) were conducted to characterize the natural dynamics and anthropogenic influence on the cavity. The analysis was accomplished by implementing wavelet analysis and resemblance techniques. This study enhances wavelet analysis as an efficient tool to analyse microclimatic time series, as it allows for the detection of the main periodicities of signals located in the time domain and the prevailing relationships between them. The analysis indicates that the low-frequency components of the signals were close to the identified annual natural cycles. For a 1-year cycle, the ventilation of the cavity causes the CO2 concentration to decrease from 3569 to 932 ppm in nearly 1 month, highlighting the existence of an output efflux from the cavity. On the contrary, the high-frequency components are linked to human perturbations caused by visitors in the cavity.This research was funded by the Spanish Ministry of Economy and Competitiveness Projects CGL2011-25162 and CGL2013-43324-R, and its programme Torres Quevedo (PTQ 13-06296 and PTQ 12-05601). A pre-doctoral research fellowship (BES-2012-053468) was awarded to C. Pla for the Project CGL2011-25162. Funding was also provided by the People Programme (Marie Curie Actions—Intra-European Fellowships, call 2013) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA Grant Agreement No. 624204

Assessment of CO2 dynamics in subsurface atmospheres using the wavelet approach: from cavity–atmosphere exchange to anthropogenic impacts in Rull cave (Vall d′Ebo, Spain)

Subsurface environments are non-negligible contributors to the net carbon balance because they act as natural sinks of CO2 and are responsible for the efflux to the Earth’s atmosphere during their ventilation states. In this way, the characterization of the CO2 dynamics in these underground environments is essential to determine the gas exchange between both atmospheres. A complete microclimatic analysis and trace gas (CO2 and 222Rn) monitoring of Rull cave (southeast Spain) were conducted to characterize the natural dynamics and anthropogenic influence on the cavity. The analysis was accomplished by implementing wavelet analysis and resemblance techniques. This study enhances wavelet analysis as an efficient tool to analyse microclimatic time series, as it allows for the detection of the main periodicities of signals located in the time domain and the prevailing relationships between them. The analysis indicates that the low-frequency components of the signals were close to the identified annual natural cycles. For a 1-year cycle, the ventilation of the cavity causes the CO2 concentration to decrease from 3569 to 932 ppm in nearly 1 month, highlighting the existence of an output efflux from the cavity. On the contrary, the high-frequency components are linked to human perturbations caused by visitors in the cavity.This research was funded by the Spanish Ministry of Economy and Competitiveness Projects CGL2011-25162 and CGL2013-43324-R, and its programme Torres Quevedo (PTQ 13-06296 and PTQ 12-05601). A pre-doctoral research fellowship (BES-2012-053468) was awarded to C. Pla for the Project CGL2011-25162. Funding was also provided by the People Programme (Marie Curie Actions—Intra-European Fellowships, call 2013) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA Grant Agreement No. 624204.Peer Reviewe