Miriam Álvarez-Gallego

Subterranean atmospheres may act as daily methane sinks

In recent years, methane (CH4) has received increasing scientific attention because it is the most abundant non-CO2 atmospheric greenhouse gas (GHG) and controls numerous chemical reactions in the troposphere and stratosphere. However, there is much that is unknown about CH4 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 CH4. 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 CH4 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 CH4 on a daily scale. However, this terrestrial sink has not yet been considered in CH4 budget balances.

Leaching of uranyl–silica complexes from the host metapelite rock favoring high radon activity of subsoil air: case of Castañar cave (Spain)

Castañar cave is a subterranean site with an outstanding natural environmental radioactivity. The maintaining of high radon activity of cave air and the detection of spatially anomalies of this gas in some cave emplacements, suggests that some natural geochemical processes are involved on the mobilization of radioactivity sources to cave environment, other than a simple exhalation of radon from the host-rock. The host rocks are interlaid dolostone–metapelite beds with radioactive nuclides of the three actinium, thorium and uranium decay series. In situ measurements on the spatial distribution of radioactivity, uranyl group’s luminescence and radon gas concentrations inside cave were main focus of this work to model lixiviation and deposition mechanisms of radioactive elements from the host rock to the karstic system. In addition, collected micro-samples were also analyzed by a multi-approach suite of analytical techniques: inductively coupled plasma mass spectrometry, environmental scanning electron microscopy with an attached X-ray energy dispersive system and spectral cathodoluminescence detector, thermoluminescence, Raman spectroscopy, X-ray diffraction, differential-thermal and thermogravimetry analysis, Alpha-spectrometry and Gamma-spectrometry techniques. The host metapelitic beds contain Zr(Hf)–Th(U)–Ti–P–REE phases such as zircon, xenotime-(Y), monazite-(Ce, La) and poly-metallic mineralization veins of hydrothermal origin. Carbonated host beds and speleothems show frequently chemical elements leaked from the upper host rock masses. The weathering leakage processes are favored by the existence of pyrite and limonite in the dolostone masses. The cave exhibits under UV lamps abundant hydrous silica–uranyl coatings covering carbonated speleothems with radionuclides of 238U natural decay series. The long-lived radio-nuclides of the radium radioactive decay chain are responsible of the continuous regeneration of radon gas inside cave. The experimental work was focused to identify origin and remobilization processes of radio-nuclides and their latte settlement into the cave environment associated to mineral phases of speleothems and cave deposits.

Climate-Driven Changes on Storage and Sink of Carbon Dioxide in Subsurface Atmosphere of Karst Terrains

A comprehensive environmental monitoring programme has been recently launched in Ojo Guarena 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 Guarena karst.