The paper reports the results of SiO
2 analyses in the Aonda Cave system, located on the Auyán-tepui, one of the widest table-mountains of the Gran Sabana (South Venezuela), characterised by karst ...landforms developed in siliceous rock. Chemical analyses underline the very low concentration of SiO
2 of the surface water. Percolation and cave drip waters have a SiO
2 concentration of about 1 mg/l. The mean silica load of the cave stream is 184 mg/s, mainly derived from surface solution removal in the allogenic recharge area. In the Aonda Cave system, the mean SiO
2 dissolved load is 40 mg/s, in part from surface solution (15%) and mainly from underground processes (85%). The low solubility of SiO
2 in slightly acidic water implies the importance of the time factor in the formation of cave systems. With the present dissolution rate, about 10 Ma would be necessary to form the known karst system. This estimation can be significant only if we assume that climate has been stable in the last few tens of millions of years. Furthermore, this age can be taken as a minimum estimate, while, according to the geomorphic evolution of the area, the origin of the Aonda Cave system could be reasonably dated back to at least 20–30 Ma, that is, to the Oligocene.
•160 Water chemical analyses were performed on the tepui table mountains.•Chemical composition of waters is coupled with host-rock chemical analyses.•The SiO2 content is related mainly to dissolution ...of quartz and not to hydrolysis.•These chemical data suggest that the arenisation weathering process is active.•Quartz dissolution plays a key role in long-term geomorphic evolution of tepuis.
In situ measurements of discharge, pH, electric conductivity (EC), temperature, and SiO2 content have been carried out during five expeditions in the last 20years on the summit plateaus, inside caves and along the rivers of the surrounding lowlands of three tepui massifs in Venezuela (Auyan, Roraima, and Chimanta). Additionally, detailed chemical analyses were performed on waters sampled in a newly discovered extensive quartz-sandstone cave system on the Auyan Tepui. Rock samples of the quartz-sandstone bedrock from different locations have been analysed to obtain their chemical composition with a wavelength dispersive X-ray fluorescence spectrometer. These data show that the majority of silica present in surface and subsurface water comes from dissolution of quartz and only in minor amount from hydrolysis of other silicate minerals. Probably the presence of a hardened crust of iron hydroxides limits the dissolution of silica on the top surface of tepuis. Dissolution in the subsurface, instead, is more significant and causes, in the long term, the “arenisation” of the quartz-sandstone and its subsequent removal by mechanical erosion. On the other hand, waters flowing on the arkosic rock outcropping on the lowland below the tepuis obtain their high dissolved silica content mainly from hydrolysis of silicates.
The morphological evolution of these table mountains appears thus to be controlled mainly by the underground weathering of the quartz-sandstone, with the opening of deep fractures (grietas) and the collapse of large underground horizontal cave systems. Scarp retreat, instead, seems to be related to the higher weathering rate of the more arkosic formations underlying the quartz-sandstones.
•A hybrid model is developed to estimate fracture karstification in quartz sandstone.•Chemical potential allows the removal of silica from the rock pores.•Diffusion-dissolution increases porosity to ...a point where disaggregation occurs.•The Grain Release Threshold marks an increase in the fracture enlargement rate.•Model outcomes are in agreement with field hydrogeochemisty data.
A hybrid numerical, finite differences and semi-analytical/empirical model has been developed to evaluate the spatial and time scale of chemical weathering along a fracture in quartz sandstones. The model is based on the diffusion transport occurring from the bedrock mass toward the fracture walls driven by the SiO2 concentration gradient between water in the bedrock pores (high silica content) and water flowing in the fracture (low silica content). Because of molecular diffusion, intergranular water becomes undersaturated with respect to silica. This promotes dissolution of quartz at grain boundaries and results in a porosity profile decreasing from the fracture walls toward the rock interior. Bedrock individual grains are released and gradually removed by the water flowing into the fracture when a critical value of porosity is reached (Grain Release Threshold, GRT): this process drastically increases the fracture enlargement rate.
The model outcomes establish a minimum time of 90 ka to reach the GRT. This time is independent of length, aperture and dip of the fracture, but is controlled by parameters such as the initial fracture aperture, water temperature, quartz grain size, and initial bedrock porosity. It is attested that the water flowing in the fracture remains undersaturated with respect to silica over very long timescales (in the order of 105 years) and over very long flow paths. In turn, this suggests that the extremely slow reaction between quartz and water is the key-factor for the formation of subterranean karst-like features in quartz sandstone, otherwise silica saturation would be reached after short distances and deep weathering in this lithology would be prevented.
Finally, the model outcomes were compared to field data from the Gran Sabana caves (Venezuela) and other quartz sandstone weathering landforms elsewhere, showing that dissolution/diffusion in the rock matrix is a reliable mechanism for the formation of these peculiar karst-like features.
Quartz sandstone of the Sarisariñama massif in Venezuela hosts the world biggest collapse dolines in quartz-rich lithologies, with volumes up to some millions of cubic meters. Due to extremely ...complex logistics required to reach the massif, the genesis of these depressions and of the underlying caves has never been studied in detail. The lack of field campaigns and extended data has fostered a decade-long scientific debate on whether their origin was due to epigenic or hypogenic processes. This study integrates petrological, structural and hydrochemical observations, including analyses of silica concentration, pH, conductivity of surface and cave waters (EC), to investigate the speleogenetic processes acting underground.
Petrographic and compositional analyses of the host rock (Matauí Formation) show that in the Sarisariñama region quartz sandstones are regularly characterized by clay interlayers with significant content of pyrophyllite and kaolinite and minor amount of iron hydroxides. Compared to surface waters, subsurface infiltration along vertical fractures and fault planes show enrichment in silica, higher pH and lower EC, confirming that chemical weathering is effective underground provoking intergranular silica dissolution along structural discontinuities. The weathering of the clay and iron hydroxide interlayers guides the speleogenesis, weakening specific stratigraphic levels and causing the collapse and fragmentation of the more resistant quartz sandstone strata. The initial void, created by piping of the loose sand released by quartz sandstone weathering, can migrate upwards by means of roof and wall breakdown; this chain of events eventually triggers a collapse at the surface, which generates a circular or squared sinkhole. The weathering acts mainly along the dominant fracture networks, showing a clear guidance by regional tectonics. These speleogenetic controls rule out the hypothesis of a hypogenic origin of the simas, suggesting a primary role of long-term epigenic chemical weathering and mechanical erosion guided by joints, weak clay and iron hydroxide interlayers, followed by subsequent massive collapses.
Meghalaya (NW India) is one of the regions in the world with highest recorded rainfalls. Because of these peculiar climatic conditions and intense solutional weathering, karstic caves are widely ...reported in the numerous limestone areas of this part of India. Likewise, the extremely high rainfall and the tropical monsoon climate have fostered the formation of several caves in silicate-rich lithologies. The most important of these caves is Krem Puri, over 25.5 km long, characterized by a fracture-controlled labyrinth forming a bi-dimensional maze with mostly phreatic morphologies and some recent, still active, vadose entrenchments. A detailed petrographic study of the host rocks has shown this cave to be formed along a set of quartzarenite, hybrid arenite and calcarenite layers in a mostly siliciclastic sedimentary succession. XRF elemental analysis of the different rock strata guiding the speleogenesis confirms that the early stage of cave development is confined mainly in the calcium carbonate-rich layers. Geochemical analyses carried out on water flowing in the cave (drips, streams) have shown that carbonate dissolution is the dominant chemical weathering process contributing to Krem Puri water solutes, whereas the dissolution of quartz, feldspars and clays play only a secondary role in rock weathering.
The morphology of the cave points to a lithology-guided dissolution of the carbonate-rich layers and piping of the residual quartz sand in phreatic conditions, when the surface river network was not yet evolved and the water table was at the level of or higher than the cave. This phreatic speleogenetic phase predates the start of river network entrenchment (that started not earlier 5 Ma ago, and probably accelerated around 3.5 Ma), which deactivated the karst maze. With a low denudation rate of 0.15 mm/yr, this active phreatic cave level, which currently lies beneath a more or less 120 m overburden, was originally covered by a sedimentary sequence at least 1–2 km thick. After its decoupling from the underground drainage system, denudation slowly brought the surface close to the maze system gradually introducing infiltration of vadose waters into the conduits. This caused the formation of the vadose entrenchments probably <100 ka ago, and carbonate speleothem deposition in the cave.
•Krem Puri developed in arenites forming a fracture-controlled bi-dimensional maze.•Cave passages have phreatic morphologies and few active vadose entrenchments.•Petrographic analyses show the cave to have formed in few carbonate-rich arenites.•Cave waters analysis demonstrates that carbonate dissolution dominates speleogenesis.•The cave formed 5–3.5 Ma ago and deactivated due to uplift and valley entrenchment.
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Titanium dioxide is produced or imported into the EU for over one million tons/year. The International Agency for Research on Cancer (IARC) classification is 2B, a possible inhalation carcinogen for ...humans. This study evaluates urinary biomarkers of oxidative stress in workers of a plant producing TiO
pigment powder, having 0.25 µm average particle size and an ultrafine fraction, compared to unexposed subjects. Urine samples were collected from forty workers before and after the shift, from six employees of the same company and eighteen volunteers from the same geographical area. Titanium and other metals concentrations were measured by ICP-MS, while DNA, RNA, and protein oxidation products by HPLC/MS-MS. A statistically significant increase was found for the urinary concentration of Al, Cd, Cr, Cu, Fe, Mn, Pb, Ti, and Zr, and for all biomarkers of oxidative stress in post-shift workers' urine samples. Urinary concentrations after the working shift were higher than for employees and volunteers pooled together for Cd, Mn, and Zr, and for the oxidative stress biomarkers 8-oxoGuo, 8-oxodGuo, and 3NO
Tyr. Biomonitoring studies on dose and effect biomarkers for TiO
occupational exposure provide information useful for protecting workers' health even in conditions that comply with health and safety standards, highlighting reversible effects of chronic exposure at very low doses.
The Cuatro Ciénegas area is renown worldwide for its thermal springs, which feed a unique ecosystem consisting of many pools, lakes and marshes. The pools also represent a very important water ...resource in a region characterized by scarce rainfalls. Field investigation has emphasized the role of karst in the hydrogeology of the area. Only few and restricted forms of surface karst are represented; caves are mainly relics of old speleogenetic phases of thermal and bathyphreatic water flow.
In the recent work of Aubrecht et al. (2011) the presence of “unlithified or poorly-lithified beds” of sands in the quartz-sandstone stratigraphic succession is proposed as a key factor for ...speleogenesis in the Venezuelan tepuis. In this comment we observe that in the cited work the geologic history of the region, in terms of sedimentation environment, diagenesis and low grade burial metamorphism, has not been considered. Furthermore, the peculiar “pillar flow” columns that Aubrecht et al. describe as a proof of the unlithification are lacking in many other different cave systems in the same area.
Four critical points are discussed: the burial metamorphism of the Mataui Formation, the significance of the Schmidt Hammer measurements, the cave morphologies and the role of SiO2 dissolution. Finally we suggest that weathering, in its wider significance, is probably the triggering process in speleogenesis, and there is no need to invoke a differential diagenesis of the sandstone beds.
► “Unlithified beds” model is unlikely in sandstones affected by buried metamorphism. ► Schmidt hammer measures are related more to weathering than to diagenesis grade. ► Cave morphologies don't show evidence of the “unlithified beds” speleogenesis. ► Drip water SiO2 content shows that silica dissolution is an active process. ► Quartz sandstone cave development depends on a wide range of weathering processes.
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