Climate-driven changes on storage and sink of carbon dioxide in subsurface atmosphere of karst terrains

Abstract

A comprehensive environmental monitoring programme has been recently launched in Ojo Guareña cave system (Burgos, Spain), one of the longest caves in Europe, aimed to assess the magnitude of the spatiotemporal changes of CO2 (g), on daily and synoptic timescales in the cave–soil–atmosphere profile. CO2 concentration of cave air is usually close to atmospheric background but huge daily oscillations of CO2 levels, ranging 680–1,900 ppm/day on average, have registered during periods when exterior air temperature oscillates every day around cave air temperature. These daily variations of CO2 content are hidden once the air temperature outside is continuously below cave temperature and a prevailing advective-renewal of cave air is established, so that daily-averaged concentrations of CO2 reach minimum values close to 500 ppm. The spatiotemporal pattern of CO2(g) provides evidence that the amounts of carbon that might be sequestered and then emitted (CO2) from subsurface air located in the uppermost part of the vadose zone could be noticeable at local or regional scale by considering long subterranean systems as Ojo Guareña karst.