The d13C signature of cave-air CO2 has been investigated in Covadura Cave in the gypsum karst of Sorbas (Almeria, South-East Spain), a semi-arid Mediterranean region characterized by very-dry climate (less than 300 mm of annual precipitation) and by sparse maquis/matorral vegetation over a thin soil cover. The 4.5 km long network of underground cave passages develops within Messinian gypsum strata (5.5 Ma), with up to 7 levels of passages connected by shafts that reach a maximum depth of 120 metres. This cave system has several entrances that connect underground and external atmospheres, thus enabling a strong air flow. Air sampling campaign was carried out at different cave levels in order to obtain representative air samples at different depths and distances from the main cave entrances. The external atmosphere was also collected for comparison. Sampling was carried out by drawing cave air in a sterile syringe and injecting it into 10 mL vials with rubber septum to which the vacuum has been previously made. Air samples were analyzed in the stable isotope laboratory of Boston University using an IRMS Delta V instrument in dual inlet mode. This instrument allows the simultaneous analysis of carbon (d13C) isotopic deviation to an accuracy of 0.1? with respect to the international standard V-PDB. Also the CO2 concentration in each sample was measured. Preliminary data indicate a significant negative correlation between CO2 concentration and d13C. This fact is undoubtedly indicative of the contributions of CO2 from vegetation cover, which typically presents isotopically more depleted values than atmospheric CO2, depending on the type of vegetation (C3 or C4). These results show that although the cave atmosphere receives contributions of CO2 coming from the soil by seeping water, external atmosphere influences the isotopic signature of cave-air CO2. This point validates the importance of the dynamics of air circulation in cave and supports the use of carbonate speleothems from gypsum caves as paleoclimate proxy. Further researches on the carbon dioxide in gypsum cave should be carried out in order to establish transfer functions between the CO2 contribution of the soil and atmosphere, and the response of the cave air to these inputs, then recorded in speleothems.
Isotopic signature of cave-air carbon dioxide in a semi-arid karst region
Laura Sanna
2017
Abstract
The d13C signature of cave-air CO2 has been investigated in Covadura Cave in the gypsum karst of Sorbas (Almeria, South-East Spain), a semi-arid Mediterranean region characterized by very-dry climate (less than 300 mm of annual precipitation) and by sparse maquis/matorral vegetation over a thin soil cover. The 4.5 km long network of underground cave passages develops within Messinian gypsum strata (5.5 Ma), with up to 7 levels of passages connected by shafts that reach a maximum depth of 120 metres. This cave system has several entrances that connect underground and external atmospheres, thus enabling a strong air flow. Air sampling campaign was carried out at different cave levels in order to obtain representative air samples at different depths and distances from the main cave entrances. The external atmosphere was also collected for comparison. Sampling was carried out by drawing cave air in a sterile syringe and injecting it into 10 mL vials with rubber septum to which the vacuum has been previously made. Air samples were analyzed in the stable isotope laboratory of Boston University using an IRMS Delta V instrument in dual inlet mode. This instrument allows the simultaneous analysis of carbon (d13C) isotopic deviation to an accuracy of 0.1? with respect to the international standard V-PDB. Also the CO2 concentration in each sample was measured. Preliminary data indicate a significant negative correlation between CO2 concentration and d13C. This fact is undoubtedly indicative of the contributions of CO2 from vegetation cover, which typically presents isotopically more depleted values than atmospheric CO2, depending on the type of vegetation (C3 or C4). These results show that although the cave atmosphere receives contributions of CO2 coming from the soil by seeping water, external atmosphere influences the isotopic signature of cave-air CO2. This point validates the importance of the dynamics of air circulation in cave and supports the use of carbonate speleothems from gypsum caves as paleoclimate proxy. Further researches on the carbon dioxide in gypsum cave should be carried out in order to establish transfer functions between the CO2 contribution of the soil and atmosphere, and the response of the cave air to these inputs, then recorded in speleothems.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
