The ribbed drapery of the Puerto Princesa Underground River (Palawan, Philippines): morphology and genesis

During the 2011 speleological expedition to the Puerto Princesa Underground River (Palawan, Philippines) a drapery characterized by several close-to-horizontal ribs has been noticed. Even without sampling and analyzing its internal growth layers, a detailed morphological study allowed to present a possible genetic model. The presented model helps to explain its evolution, which is mainly controlled by variation in water flow as a consequence of the Palawan climate. When validated by further analyses, the same genetic mechanism could define also the evolution of the very common but still unexplained complex flowstones, which exhibit several close-to-horizontal steps, widenings and narrowings along their growth axis

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.This work was supported by the Spanish Ministry of Economy and Competitiveness [projects: GEIs-SUB (CGL2016-78318-C2-1R and CGL2016-78318-C2-2R AEI/FEDER/UE) and CGL2017-83931-C3-2-P]

Monitoring and characterisation of greenhouse gases in active-hypogenic subterranean environments: case of ‘Sima del Vapor’ (Alhama de Murcia)

Los mecanismos hidrogeoquímicos implicados en los procesos espeleogenéticos activos de carácter hipogénico son especialmente efectivos en las cavidades hidrotermales vinculadas a las fallas con actividad sísmica. Estas cavidades pueden albergar atmósferas subterráneas con una particular composición de gases endógenos. Por ejemplo, con dióxido de carbono (CO2) y metano (CH4) relacionados con la actividad sismo-tectónica actual, o bien con signos residuales de la desgasificación de las aguas subterráneas enriquecidas con estos gases o con un origen de fuentes profundas geotermales. En este estudio abordamos algunos aspectos clave referentes a la generación, mezcla y procesos biogeoquímicos implicados en la dinámica de gases endógenos en un sistema kárstico hipogénico excepcionalmente activo (Sima del Vapor, Alhama de Murcia). Este estudio se basa en la monitorización de las condiciones micro-meteorológicas y de la composición gaseosa del aire subterráneo, combinado con un seguimiento de la geoquímica isotópica del dióxido de carbono y el metano (fracción molar y δ13C).The hydro-geochemical mechanisms involve on the active hypogene karstification are specially effectives in caves associated with faulting and geothermal activity. These caves can potentially harbour subterranean atmospheres with distinctive composition of endogenous gases. For instance, carbon dioxide (CO2) and methane (CH4) influenced by the current seismotectonic activity, or with residual gas coming from degassing of groundwater or deep-sourced geothermal gases. In this study, we provide key insight into the sourcing, mixing, and biogeochemical processes involved in the dynamic of deep endogenous gas formation in an exceptionally dynamic hypogenic karst system (Sima del Vapor, Alhama de Murcia). Monitoring of microclimate conditions and gas composition of underground air were used in combination with an isotopic tracking of carbon dioxide and methane (molar fractions and their δ13C values).Este estudio forma parte de los proyectos del Ministerio de Economía y Competitividad: GEIs-SUB (CGL2016-78318-C2-1R y CGL2016-78318-C2-2R AEI/FEDER/UE) y 3GEO (CGL2017-83931-C3-2-P)

Karst: a very diverse concept

La gran diversidad intrínseca del karst y la variedad de campos científicos desde los que ha sido estudiado este medio ha generado un espectacular aumento de su conocimiento. Ese avance ha venido acompañado también de nuevas incógnitas y nuevos planteamientos. El presente trabajo trata de recoger una pequeña muestra de esa amplia diversidad, asociada a un elevado grado de especialización, que de forma integrada permite completar el puzzle del karst. Inicialmente, se presenta una revisión sintética del concepto de karst, del proceso de karstificación y de las morfologías asociadas al mismo. Seguidamente, y planteado a partir de una serie de interrogantes, se muestran algunos aspectos y curiosidades relacionadas con el karst. Las características microclimáticas subterráneas, el karst no tradicional o los depósitos y mineralizaciones asociados al karst, son algunos de los temas que se incluyen en este trabajo y que puede interesar a los docentes que imparten docencia sobre el karst.The great intrinsic diversity of karst and the variety of scientific fields in which it has been studied has generated a dramatic increase in our knowledge about it. This advance has also been accompanied by new questions and issues that open the door to new approaches. Our work intends to show a small sample of the wide diversity, often associated with a high degree of specialization, which allows completing the karst puzzle. In the first place, this paper provides a synthetic review of the concept of karst, the karstification process and the morphologies associated with it. Next, starting with a few questions, it shows some aspects and curiosities related to karst. The features of the underground microclimate, the non-traditional karst or deposits and mineralization associated with karst, are some of the topics included in this work, which can be interesting for the teachers dealing with the topic of karst in their classes

Some examples of gypsum karsts and the more important gypsum caves in Spain

Spain possesses some of the most important examples of gypsum karst in Europe, in terms of the extent and variety of the gypsiferous outcrops. These are divided into gypsum belonging to the Triassic, Palaeogene and Neogene epochs, each of which displays different lithological and structural aspects. Some of Spain’s most significant gypsum karsts, from the speleological standpoint, are described, and these share a common characteristic of all supporting the development of large caves. Reference is made to the geomorphology, hydrogeology and hydrochemistry of the gypsum karsts of Sorbas, Vallada and Gobantes-Meliones, which provide significant examples of intrastratal karst, speleogenesis by saline groundwater mixing and the influence of carbonate strata, respectively. Finally, brief geomorphological and speleogenetic descriptions of the more significant gypsum caves in Spain are given, together with a list of the longest and deepest gypsum caves in Spain

Black Mn-Fe Crusts as Markers of Abrupt Palaeoenvironmental Changes in El Soplao Cave (Cantabria, Spain)

Peculiar iron and manganese deposits coating walls, floors and ceilings of many galleries are one of the special features of the El Soplao Cave (Cantabria, Spain). These speleothems appear to have been deposited over wall clay deposits, as well as forming part of flowstones. Structure of crusts is essentially amorphous but several manganese and iron oxides were identified like goethite and birnessite, though all occur with a low degree of crystallinity. In the outer layer of the crusts, alteration iron minerals appear that derive from previous minerals in a process probably mediated by microorganisms. EDX microanalyses report fairly high values of Fe and Mn in the crusts, though the Mn/Fe ratio varies considerably as a function of distance from the substrate/bedrock. The present study proposes a genetic model for crust speleothems in El Soplao, based on oscillations of the phreatic level. The origin of these deposits is related to mobilization, under phreatic conditions, of polymetallic sulfides in the host rock. Metal ions (including Fe²⁺ and Mn²⁺) released into the cave under reducing conditions, are oxidized and fixed in a process mediated by bacteria, giving rise to oxides and hydroxides of low crystallinity. The presence of various black intercalated layers in aragonite flowstones indicate periods when cave conditions suddenly changed from vadose, when aragonite is precipitated, to phreatic and epiphreatic conditions, when the Mn-Fe deposits are precipitated. Subsequently, vadose conditions were re-established, leading to the final stages of precipitation of aragonite recorded in the flowstone and recent aragonite helictites on the surface of the Mn-Fe crusts

Hydrogeochemical processes as environmental indicators in drip water: study of the Cueva del Agua (Southern Spain)

Karst caves exhibit a wide range of hydrological and hydrochemical responses to infiltration events, due to their physical heterogeneity in space and dynamic variability over time, and due to non-Gaussian inputs (rain) and outputs (discharge). This paper reviews different approaches of studying seepage water in caves, in order to understand the infiltration regimen in the non-saturated zone of karst areas. As an illustration, we describe a four-year study of the active carbonate-water system in the Cueva del Agua (Granada, southern Spain) that automatically logs the discharge from a stalactite. The results indicate that: (1) the drip water regime is not seasonal, but is linked instead to slow infiltration. Sudden changes in drip water regime occur due to infiltration along preferential flow paths and the draining of water of supersaturated water from reserves in the microfissure and pore system; (2) the drip rate is not linear over time. When dripping is constant, barometric oscillation of the air is the principal factor causing a chaotic a drip flow regime. Over a short period of two to three days, a mean variation in air pressure inside the cave of 10 (±3.7) mbar causes a mean oscillation in the drip rate of 0.5 (±0.2) mm/h. The increase in air pressure translates into an increase in the relative thickness of the gaseous phase of the drip water at the cost of the aqueous phase, so leading to a reduction in the drip rate from the stalactite

The ribbed drapery of the Puerto Princesa Underground River (Palawan, Philippines): morphology and genesis

During the 2011 speleological expedition to the Puerto Princesa Underground River (Palawan, Philippines) a drapery characterized by several close-to-horizontal ribs has been noticed. Even without sampling and analyzing its internal growth layers, a detailed morphological study allowed to present a possible genetic model. The presented model helps to explain its evolution, which is mainly controlled by variation in water flow as a consequence of the Palawan climate. When validated by further analyses, the same genetic mechanism could define also the evolution of the very common but still unexplained complex flowstones, which exhibit several close-to-horizontal steps, widenings and narrowings along their growth axis

Using stable isotopes (δ¹⁷O, δ¹⁸O and δD) of gypsum hydration water to ascertain the role of water condensation in the formation of subaerial gypsum speleothems

We analyzed the stable isotopes (δ17O, δ18O and δD) of gypsum hydration water (GHW) in a variety of speleothems, as well as condensation and infiltration waters in five caves of the semiarid gypsum karst of Sorbas basin (Almeria, SE Spain). Microclimate parameters (air temperature, relative humidity and effective condensation rate) were also monitored over an annual cycle. We found that the mother solution from which the majority of gypsum speleothems grow is composed of a mixture of condensation (~ 60%) and infiltration water (~ 40%) that undergoes evaporation. Although evaporation of infiltration water alone was thought to be responsible for secondary gypsum precipitation in vadose caves, our results suggest that condensation can be a major source of water for the formation of gypsum speleothems. The modelled d-excess and Δ17O trajectories of water during the evaporative process confirm that the majority of speleothems precipitate from a mixture of condensation and infiltration water under relative humidity of 75–85%, similar to that measured in the cave atmosphere during winter. These findings have important implications for future studies of gypsum speleothems as paleoenvironmental archives

Organic matter of fossil origin in the amberine speleothems from El Soplao Cave (Cantabria, Northern Spain)

Unusual amberine-coloured speleothems were recently found in El Soplao Cave (Cantabria, Spain). Chromophore elements such as Fe, Mn, Cd, Co or Ti were not present in significant quantities. Rather, our data show that their colour comes from leachates of fossilized organic material hosted in the carbonaceous Urgonian facies of the host rock. These leachates are related to the Cretaceous amber deposit that has been recently discovered in the vicinity of El Soplao Cave. The presence of humic and fulvic acids of fossil origin were confirmed by IR and Raman spectroscopic analysis of the carbonaceous strata and the speleothems. In addition, the mineralogy of the amberine speleothems was studied. Alternating bands made of calcite and aragonite reveal that periods of humidity and aridity occurred within the cave during the speleothem genesis