Brazilian cave heritage under siege

info:eu-repo/semantics/publishe

Dissolution of Gypsum from field observations

The paper reports the results of field measurements of gypsum dissolution in various countries (Ukraine, Spain, Italy and others) and in different environments (river waters, precipitation, vadose zone, unconfined aquifer, perched cave lakes, ephemeral streams in caves, confined aquifer, cave air)

Allogenic contact caves in Central East Sardinia (Italy): their speleogenesis and evolution

Several important cave systems in Central East Sardinia develop, at least partially, along the contact between the granite or phyllite basement rocks and the covering fractured Middle Jurassic sediments. The latter are composed of layers of clays, marls, conglomerates and sands of variable thickness (generally some metres) followed by sandy dolostones, becoming increasingly carbonatic moving upwards in the sequence. These karst systems are fed by allogenic streams with flow rates ranging from a couple to some thousand of L s-1. Parts of these vadose passages are sometimes developed in the insoluble basement rocks for some metres. In the summer of 2010 cavers of the Centro Speleologico Cagliaritano have discovered a new cave, Gutturu \u2018e Murgulav\uf2 (Baunei), that hosts some passages entirely excavated in granitic rocks. The typical morphology of all these caves is characterised by very large and rather low chambers, which floor is masked by important graviclastic deposits. Locally the Palaeozoic bedrock is exposed along the underground streams. Speleogenesis of these cave systems is very much relied upon weathering of the basement rocks and their successive erosion by flood waters, rather than on dissolution of the carbonate rocks. Mineralogical analyses of the weathering products at the contact between Palaeozoic rocks and Mesozoic sediments have revealed the presence of montmorillonite, kaolinite, chlorite and illite, typical weathering products of granitic rocks. Dissolution, however, plays an important role in the first stages of speleogenesis, dissolving the carbonate cement in the basal sandy dolostone beds, thus leaving loose sands that can readily be eroded by running waters. Once a critical dimension in void is reached, cave formation occurs very rapidly, leading to huge chambers in some thousands of years

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

The hydrothermal caves linked to active faulting can potentially harbour subterranean atmospheres with a distinctive gaseous composition with deep endogenous gases, such as carbon dioxide (CO2) and methane (CH4). In this study, we provide insight into the sourcing, mixing, and biogeochemical processes involved in the dynamic of deep endogenous gas formation in an exceptionally dynamic hypogenic karst system (Vapour Cave, southern Spain) associated with active faulting. The cave environment is characterized by a prevailing combination of rising warm air with large CO2 outgassing (>1%) and highly diluted CH4 with an endogenous origin. The δ13CCO2 data, which ranges from −4.5 to −7.5‰, point to a mantle-rooted CO2 that is likely generated by the thermal decarbonation of underlying marine carbonates, combined with degassing from CO2-rich groundwater. A pooled analysis of δ13CCO2 data from exterior, cave, and soil indicates that the upwelling of geogenic CO2 has a clear influence on soil air, which further suggests a potential for the release of CO2 along fractured carbonates. CH4 molar fractions and their δD and δ13C values (ranging from −77 to −48‰ and from −52 to −30‰, respectively) suggest that the methane reaching Vapour Cave is the remnant of a larger source of CH4, which was likely generated by microbial reduction of carbonates. This CH4 has been affected by a postgenetic microbial oxidation, such that the gas samples have changed in both molecular and isotopic composition after formation and during migration through the cave environment. Yet, in the deepest cave locations (i.e., 30 m below the surface), measured concentration values of deep endogenous CH4 are higher than in atmospheric with lighter δ13C values with respect to those found in the local atmosphere, which indicates that Vapour Cave may occasionally act as a net source of CH4 to the open atmosphere