Subaerial Speleothems Research Articles

Saint Anthony Cave: Morphology, genesis, and age of one of the oldest religious shrines, Southern Galala Plateau, Eastern Desert, Egypt

Saint Anthony Cave is an important cultural, religious, and historic site; it was home of Saint Anthony, one of the earliest Christian monks (ca. 251–356 AD). The cave is located in a tectonically-complex karst area developed in partially dolomitic, sandy limestones of latest Paleocene age. Saint Anthony Cave had been described as a phreatic cave in previous research, but the cave lacked a detailed map and its speleogenesis was unclear. New data show the cave is a multi-phased uplifted cave formed primarily under phreatic conditions, with later modification by rising water levels. Deep sources of dissolutionally aggressive solutions likely have been involved. Speleogenesis likely began in Oligocene time associated with the Gulf of Suez Rift evolution and has been erosionally dissected. New, detailed mapping using ArcGIS shows the cave has an area of 22.13 m2 with a length and depth of 17.10 m and 5.33 m, respectively. The cave consists of a northwest–southeast oriented, steeply inclined, fracture controlled, upper-level entrance passage connected to an east–west oriented, fracture controlled, lower-level room with a vertical drop of 2.46 m. Dissolution pockets, cupola-like features, ceiling bell holes, notches, and corrosion tables are present in the cave. Subaerial and subaqueous speleothems moderately decorated the cave with likely Egyptian calcite alabaster. Based on field investigation, regional volcanic and tectonic history, paleoclimates, and other background information, we identify four distinct speleogenetic phases: First, during the late Oligocene, volcanic and extensional tectonic activities fractured the rocks, generated deep-seated acids, and enhanced the rising deep thermal water. Dissolution by other mechanisms such as carbonic/sulfuric acid and mixing dissolution were prevalent. In the early Miocene (Burdigalian), the cave was uplifted. Second, in the late Miocene (about 7.5 Ma), subaerial speleothems were being deposited in the cave under humid climate conditions. Third, post-Miocene, the cave enlarged and modified at and above the water level by likely warm, sulfuric water. Fourth, water levels dropped and speleothems were deposited again during later humid climate conditions.

Изотопно-геохимические характеристики минерального заполнения палеополостей в верхнедевонских-нижнекаменноугольных известняках на восточном склоне Среднего Урала (окрестности с. Першино)

Lithology and geochemistry of carbonate filling in ancient karst cavities in the Famennian-Tournaisian limestone sequence of the Rezh subzone at the eastern slope of the Middle Urals are considered. Mineral filling of paleocavities differs from typical modern subaerial speleothems. Caves are mainly filled with white massive coarse-grained calcite (cave spar), red-colored argillaceous-carbonate microgranular deposits (carbonates with an admixture of terra rossa, cave laminites) and collapse breccias, composed of host limestones fragments. The rare earth elements (REE) patterns and the isotopic composition of carbon indicate a very weak effect (or its absence) of soil or meteoric waters on the carbonate system: the speleothem calcite inherited these characteristics from the host limestones. At the same time, the isotopic composition of oxygen indicates the primary meteoric genesis of solutions and/or relatively high temperatures of calcite crystallization. Most likely, the studied speleothems were formed from solutions of surface origin circulating in the sedimentary strata for a long time (hypogene sediments).

Speleothem-based Quaternary research in Croatian karst – A review

Speleothems, as secondary carbonate deposits, precipitate in the cave environment from the water originally sourced from the ocean, transferred via atmosphere through soil and epikarst into the cave system. As such, they archive valuable information on the ongoing changes in the atmosphere, biosphere, hydrosphere and lithosphere which may be used for palaeoenvironmental reconstructions, especially for the Quaternary Period.Speleothem-based Quaternary research in Croatia started in the 1960's with the utilization of the flowstone and speleothems whose radiometric ages (14C and U-Th) provided constrained chronological framework for underlying fossils or archaeological material, mainly without any further palaeoenvironmental interpretations. Since the 1990's, several studies have used speleothems for the reconstruction of particular phases of speleogenesis, while in the early 2000's investigation on submerged speleothems started, with the aim to reconstruct Quaternary relative sea-level changes along the eastern Adriatic coast. Only after that, thorough research on subaerial speleothems from the different regions of the Croatian karst has been initiated, usually associated with monitoring programs.This paper gathers the most important speleothem-based studies, displaying potential of this still understudied, but promising area in the terms of palaeoclimate and palaeoenvironmental changes in a wider European and global context.

Subaerial speleothems and deep karst in central Sweden linked to Hirnantian glaciations

The limestones of the upper Katian Boda mud mounds (Ordovician) of the Siljan district in central Sweden are deeply fractured. The fissures were partly synsedimentary and are often lined with stromatolite-like crusts. These crusts thus far are the only known subaerial Ordovician speleothems. They reach depths of up to 30 m below the former mound top. Macroscopically the crusts form decimetre-sized, cone-shaped domal aggregates, stalactites and stalagmites. Microfabric and morphology identify them as microbially mediated speleothems in a dark environment. Combined Sr and C isotope values indicate a formation of the speleothems from meteoric waters without influence of a significant soil horizon. For the first time the age of the speleothems can be precisely constrained by δ 13 C whole-rock and brachiopod shell isotope data to the mid-Hirnantian. Repeated and/or prolonged subaerial exposure of the Boda mud mounds during the Hirnantian is evident from karst surfaces and early cements in the mound capping carbonates. The speleothems and the karst surfaces record an estimated sea-level fall in the range of 80–130 m within the time window of the Hirnantian Isotopic Carbon Excursion. This massive regression coincides with maximum ice sheet extent inferred from sections in West Gondwana. Supplementary material: 87 Sr/ 86 Sr isotope ratio of selected brachiopod shells and results of Energy-dispersive-X-ray spectroscopy are available from http://www.geolsoc.org.uk/SUP18809

Stable isotope data as constraints on models for the origin of coralloid and massive speleothems: The interplay of substrate, water supply, degassing, and evaporation

Many speleothems can be assigned to one of two morphological groups: massive speleothems, which consist of compact bulks of material, and coralloids, which are domal to digitate in form. Faster growth on protrusions of the substrate occurs in the typical growth layers of coralloids (where those layers are termed “coralloid accretions”), but it is not observed in the typical layers of massive speleothems, which in contrast tend to smoothen the speleothem surface (and can therefore be defined as “smoothing accretions”). The different growth rates on different areas of the substrate are explainable by various mechanisms of CaCO3 deposition (e.g., differential aerosol deposition, differential CO2 and/or H2O loss from a capillary film of solution, deposition in subaqueous environments). To identify the causes of formation of coralloids rather than massive speleothems, this article provides data about δ13C and δ18O at coeval points of both smoothing and coralloid accretions, examining the relationship between isotopic composition and the substrate morphology. In subaerial speleothems, data show enrichment in heavy isotopes both along the direction of water flow and toward the protrusions. The first effect is due to H2O evaporation and CO2 degassing during a gravity-driven flow of water (gravity stage) and is observed in smoothing accretions; the second effect is due to evaporation and degassing during water movement by capillary action from recesses to prominences (capillary stage) and is observed in subaerial coralloids. Both effects coexist in smoothing accretions interspersed among coralloid ones (intermediate stage). Thus this study supports the origin of subaerial coralloids from dominantly capillary water and disproves their origin by deposition of aerosol from the cave air. On the other hand, subaqueous coralloids seem to form by a differential mass-transfer from a still bulk of water toward different zones of the substrate along diffusion flux vectors of nutrients perpendicular to the iso-depleted surfaces. Finally, this isotopic method has proved useful to investigate the controls on speleothem morphology and to obtain additional insights on the evolution of aqueous solutions inside caves.

Karst hydrology of Grand Canyon, Arizona, USA

Caves in Grand Canyon, Arizona, USA fall into two main categories: those formed under unconfined conditions and those formed under confined conditions. This study focuses on the hydrology and paleohydrology of the confined caves in the Redwall–Muav aquifer, where the aquifer is overlain by rocks of the Supai Group and underlain by the Bright Angel Shale. Unconfined caves are discussed only in their relation to confined caves. Discharge for confined groundwater was, as it is today, primarily from the Redwall Limestone where it has been incised by the main canyon or its tributaries and where it has converged along a structural low or fault. Descent of the potentiometric surface (or water table) over time is recorded by one ore episode and six cave episodes: (1) emplacement of Cu–U ore, (2) precipitation of iron oxide in cavities, (3) dissolution of cave passages, (4) precipitation of calcite-spar linings over cave passage walls, (5) precipitation of cave mammillary coatings, (6) minor replacement of cave wall and ceiling limestone by gypsum, and (7) deposition of subaerial speleothems. The mammillary episode records the approximate position of the water table when the incision of the canyon was at that level. Discharge toward spring points has reorganized and adjusted with respect to ongoing canyon and side-canyon incision. The dissolution of Grand Canyon confined caves was the result of the mixing of epigene waters with hypogene waters so that undersaturation with respect to calcite was achieved. The karst hydrology of Grand Canyon may be unique compared to other hypogene cave areas of the world.

On the origin of fiber calcite crystals in moonmilk deposits

In this study, we show that moonmilk subaerial speleothems in Altamira Cave (Spain) consist of a network of fiber calcite crystals and active microbial structures. In Altamira moonmilks, the study of the typology and distribution of fiber crystals, extracellular polymeric substances, and microorganisms allowed us to define the initial stages of fiber crystal formation in recent samples as well as the variations in the microstructural arrangement in more evolved stages. Thus, we have been able to show the existence of a relationship among the different types of fiber crystals and their origins. This allowed us to outline a model that illustrates the different stages of formation of the moonmilk, developed on different substrata, concluding that microbes influence physicochemical precipitation, resulting in a variety of fiber crystal morphologies and sizes.

Origen bacteriano de espelotemas tipo moonmilk en ambiente karstico (Cueva de Altamira, Cantabria, España)

Moonmilk subaerial speleothems of Altamira Cave are constituted by a network of needle-fiber calcite crystals (NFC) and active microbial filaments. These recent deposits allow us the observation of the first evolution stages, as well as defining microclimatic and hydrochemical conditions of mineral precipitation. In previous works, the origin of the NFC has been determined being associated to fast mineral precipitation related with evaporation and criodesiccation processes. From our data, an evolutionary model has been proposed for this type of deposits. The stable microclimatic conditions of the cave, with air humidity close to saturation, lead us to conclude that their origin doesn't have to be related with these processes. Also it has been proven that the NFC of monocrystallyne rods type is generated by means of calcitization of microbial filaments. On the other hand, epitaxial cements and polycristalline chains are related with organic activity and inorganic processes in a confined aqueous environment developed inside the biofilm.

Syn-rift geofluids in fractures related to the early-middle Miocene evolution of the Vallès-Penedès half-graben (NE Spain)

The Vallès-Penedès half-graben, located in the central part of the Catalan Coastal Ranges, is NE–SW oriented, 10–14km wide and up to 100km long, and is filled mainly with continental to marine siliciclastic deposits ranging from late Oligocene(?)–early Miocene to late Miocene. The syn-rift Burdigalian continental deposits are well stratified (S0 approx. 090/15N) red beds, affected by NNW–SSE-trending, nearly vertical, extensive fractures, middle Burdigalian–early Langhian in age. These extensive fractures, which cut quasi-perpendicularly the stratification, are filled with calcite veins presenting two filling stages. The first stage is formed by millimetric to centrimetric thick bands of ‘crystalline’ calcite, consisting of palisade calcite cement, anhedral spar texture and rhombic calcite cement, non to dull-orange luminescent, with δ18O values ranging from −9.0 to −5.2‰ PDB, δ13C values varying from −9.3 to −6.7‰ PDB, 87Sr/86Sr ratios ranging from 0.71088 to 0.71361, relatively high values of Mg, and variable values of Mn, Sr and Fe. This stage is interpreted as subaerial speleothems (rimstones) which developed in meteoric vadose conditions.The second stage is formed by euhedral spar–megaspar cement and ‘coconut-meat’ cement with zoned bright yellow to orange luminescence, with δ18O values ranging from −16.2 to −9.9‰ PDB, δ13C values varying from −7.4 to −1.9‰ PDB, 87Sr/86Sr ratios ranging from 0.71608 to 0.71800, and relatively low values of Mg, Mn, Sr and Fe. This stage is interpreted as subaquatic cements developed in meteoric phreatic conditions.The two stages of calcite fillings reflect successive circulation of two meteoric fluids with very different geochemical compositions and represent the change from vadose to phreatic environment during the evolution of the syn-rift stage (middle-late Burdigalian to late Langhian). The meteoric fluid producing the first filling stage was a superficial fluid proceeding directly from the surface influenced by soil-CO2; the meteoric fluid generating the second stage was a more evolved fluid which had previously undergone interaction with sediments and reservoir effect. The beginning of the thermal subsidence coupled with a change from a more arid to a hotter and perhaps wetter climate could had been the ultimate reasons for the observed evolution.

Late Pleistocene Paleoclimates and sea-level change in the Mediterranean as inferred from stable isotope and U-series studies of overgrowths on speleothems, Mallorca, Spain

Phreatic overgrowths on speleothems form at the surface of brackish pools which are present in many coastal caves of Mallorca (Spain). Unlike common vadose speleothems formed in subaerial environments, their occurrence directly identifies the height of the sea level at the time of the carbonate deposition, because such pools are physically connected with the sea. These deposits from the Late Pleistocene have recorded the same global variations in oxygen isotopes as planktonic and benthonic foraminifera, corals and cave deposits. A number of phreatic overgrowths on speleothems have been dated using the Th/U method in order to determine the position of the sea level during Late Quaternary. The same samples, as well as some subaerial speleothems, have been analysed for 18 O/ 16 O and 13 C/ 12 C ratios. The former samples allow the analysis of variations in sea water component in the brackish pools associated with glacio-eustatic Mediterranean fluctuations, and the latter permit inferences concerning past continental climate changes. Some high sea-stands have been recognized in correspondence with Oxygen Isotope Stages (OIS): 9 or older, 7, 5e, 5c and 5a. Phreatic overgrowths on speleothems have also been formed during a colder phase (OIS 4), demonstrating that also in that period there were environmental conditions favourable for this kind of carbonate precipitation. Vadose crystallizations have recorded a cold period at 36 ka, corresponding with the end of OIS 3.

Speleogenetic history of the Hungarian hydrothermal karst

The hydrothermal karst of Hungary displays at least two principal stages of development in two differing environments. Caves of an early stage were formed within a deep zone of low thermal gradient. These caves (vugs) are small (tens of centimeters) and lined with scalenohedral crystals of calcite that are often in association with barite. Calcite yields fluid inclusion temperatures of 55–95°C and is depleted in18O (−11.2 to −17.6 per mil PDB). The caves were formed by ascending thermal waters charged with CO2. Solubility of CaCO3 in such a system gradually increases with the ascent of the fluid (solutional zone) but drops sharply at a depth of −250 m to −500 m below the water surface (depositional zone). Caves formed in the solutional zone may be shifted into the depositional zone due to tectonic uplift, and calcite lines their walls. Large caves (tens to thousands of cubic meters) of a late stage were formed within a shallow zone of high thermal gradient immediately below and above the thermal water table. The calcite of the phreatic crusts has a rhombohedral habit, displays lower fluid inclusion temperatures (35–55°C and less), and a depletion in18O of −9.5 to −14.6 per mil PDB. Several powerful cave-forming processes may operate there including convection, mixing/cooling corrosion, and condensation corrosion. Due to differences in the rate of tectonic uplift, rate of hydrothermal system decay, and hydrogeologic pattern, these caves were either filled with water for a long period of time (phreatic calcite crusts are formed) or partly dewatered early in their history (waterline and subaerial speleothems are formed). The zones of thermal cave formation recognized in Hungary may have a universal character. Very similar features are found in other hydrothermal karst areas of the world (Kirghizia, Algeria, South Dakota).

Maximum sea levels for the last glacial period from U-series ages of submerged speleothems

PLEISTOCENE sea levels reflect changes in the surface distribution of land, sea and ice, which in turn significantly influence the Earth's climate1. The most complete record of sea levels for the last glacial cycle has come from raised coral reef terraces of the Huon Peninsula, Papua New Guinea2–4, but interpretation of this record is hampered by relatively large age uncertainties and the possibility of variation in the rate of tectonic uplift. Subaerial speleothems (stalagmites, stalactites and flowstones) from submerged caves in tetonically stable areas provide an alternative source of data5–8, as their growth stops when rising sea levels flood the caves. Previous studies of speleothems have been limited by dating precision5,6 and sample availability5–7. Here we present high-precision thermal- ionization mass-spectrometric 230Th ages9–11 for a comprehensive suite of speleothem samples from the Bahamas. Our results provide a record of maximum elevation of sea levels for the period from 80 to 10 kyr before the present, for much of which sea levels are poorly known. We also find that regional palaeoclimatic change can be an important factor in the termination of speleothem growth.