Continuous color model as a tool to improve speleothem age model development

Because they can archive a variety of geochemical proxies and be precisely and accurately dated with the U-Th decay series chronometer, stalagmites are widely used for paleoclimate reconstructions. However, limitations in the use of this chronometer arise because U-Th dating is analytically time consuming, expensive, and requires a relatively large sample size. These limitations restrict the number of absolute dates usually obtained, which can result in significant uncertainties in the age model and inhibit the ability to archive high resolution records of environmental variability, particularly in those stalagmites where there are variations in growth rate not constrained by U-Th dates. Here, we explore the relationship between stalagmite color and growth rate. Consequently, we evaluate the use of a simple, practically non-destructive approach to model the age-depth relationship of stalagmites using the sample color to provide a continuous record of growth rate. The method was developed by comparing high-resolution color images with pre-determined U-Th dates along the growth axes of seven stalagmites. The obtained results suggest that prior to dating, a color-derived, continuous growth rate model may be used to identify important changes in growth rate which may aid in the determination of the most efficacious locations for U-Th dating. Further, continuous color-derived interpolations between U-Th derived dates may be superior to traditional linear interpolation methods. Such an approach has the potential to greatly improve a researcher’s ability to efficiently choose sampling locations for more precise, albeit laborious and costly, U-Th dating.

Air pressure propagation through Wind Cave and Jewel Cave: How do pressure waves travel through barometric caves?

In barometric caves, air pressure gradients between the outside atmosphere and the cave induce strong bidirectional compensating currents, which control almost all elements of speleoclimatology, including air temperature, humidity, and CO2 dynamics. Therefore, this study set out to investigate air pressure propagation through Wind Cave and Jewel Cave – two major barometric cave systems in the Black Hills of South Dakota, USA. Based on high-resolution air pressure data from both the surface and several measurement sites inside the caves, four systematic changes of pressure waves during their journey through the caves and their related speleoclimatological processes were identified and discussed: Compared to the outside atmosphere, the pressure signals within Wind Cave and Jewel Cave showed (1) an absolute displacement due to different altitudes of the measuring sites, (2) a delay related to the travel times of the pressure wave to the measuring sites, (3) a smoothing effect, and (4) a damping effect due to long response times of the caves to external pressure changes. The spatial distribution of the changes observed in this study shows that for Wind Cave, the cave opening and the narrow entrance area represent the main obstacle for pressure propagation, while for Jewel Cave, the deep areas have the greatest influence on the development of air pressure gradients. Our analyses provide completely new insights into the processes and mechanisms inside barometric caves, which will significantly contribute to the understanding of pressure-related airflow dynamics and all related elements of speleoclimatology.

Vermiculations in painted caves: New inputs from laboratory experiments and field observations

Vermiculations are aggregates of small particles commonly found on cave walls. They are a major concern for the conservation of painted caves, as they can potentially alter valuable prehistoric cave paintings. A previous rheological study of fine sediment deposits on cave walls revealed that this material can undergo a solid-to-liquid transition triggered by variations in the chemical composition of the water film on the wall. Such a transition could occur at the origin of vermiculations by allowing the sediment to flow under low mechanical stress. In this work, we provide quantitative information on the conditions leading to this transition and show the importance of the chemical composition of the water film on the cave walls. A complete understanding of the phenomenon will, however, require more field information. This includes monitoring of the evolution of vermiculations, for which we have developed a dedicated observation protocol. Based on the combination of photogrammetry and a geographic information system we were able to precisely map the walls of the Hall of Bulls in Lascaux cave from past and future photographs. To better understand the vermiculation process, pictures need to be taken regularly, and the chemical composition of the thin water film covering cave walls needs to be analyzed with a similar time step. The correlation between the evolution of vermiculations, the humidification phases of the walls, temperature changes and the chemical monitoring of the water film should shed new light on conditions triggering vermiculations.

The relative importance of wind-driven and chimney effect cave ventilation: Observations in Postojna Cave (Slovenia)

Density-driven chimney effect airflow is the most common form of cave ventilation, allowing gas exchange between the outside and the karst subsurface. However, cave ventilation can also be driven by other mechanisms, such as barometric changes or pressure differences induced by the outside winds. We discuss the mechanism and dynamics of wind-driven ventilation using observations in Postojna Cave, Slovenia. We show how seasonal airflow patterns driven by the chimney effect are substantially modified by outside winds. Wind flow over irregular topography forms near-surface air pressure variations and thus pressure differences between cave entrances at different locations. These pressure differences depend on wind speed and direction and their relationship to surface topography and the location of cave entrances. Winds can act in the same or opposite direction as the chimney effect and can either enhance, diminish or even reverse the direction of the density-driven airflows. To examine the possibility of wind-driven flow, we used a computational fluid dynamics model to calculate the wind pressure field over Postojna Cave and the pressure differences between selected points for different configurations of wind speed and direction. We compared these values with those obtained from airflow measurements in the cave and from simple theoretical considerations. Despite the simplicity of the approach and the complexity of the cave system, the comparisons showed satisfactory agreement. This allowed a more general assessment of the relative importance of wind pressure for subsurface ventilation. We are certain that this example is not unique and that the wind-driven effect needs to be considered elsewhere to provide better insights into the dynamics of cave climate, air composition or dripwater geochemistry.

Molecular genetic analysis of stygobiotic shrimps of the genus Xiphocaridinella (Crustacea: Decapoda: Atyidae) reveals a connection between distant caves in Central Abkhazia, southwestern Caucasus

Based on the morpho-genetic study of stygobiotic shrimps from the genus Xiphocaridinella Sadowsky, 1930 (Crustacea: Decapoda: Atyidae), a hydrogeological connection of a number of distant caves in Central Abkhazia of the southwestern Caucasus is satisfied, which indicates the possibility of using biospeleological studies in some cases to identify karst hydrosystems together with traditional hydrogeological methods. Moreover, a new stygobiotic atyid shrimp from the genus Xiphocaridinella, X. kelasuri sp. n., is described based on morphology and analysis of mitochondrial cytochrome oxidase I DNA sequences from three distant caves. The new species is genetically divergent from relatives and phylogenetically related to Xiphocaridinella smirnovi Marin, 2020, described from the Besletka (=Tskaro) Cave. Recently, the number of described speciesof the genus Xiphocaridinella from Caucasus has increased to 13 species, while the diversity of Xiphocaridinella found in the Besletka (=Tskaro) Cave is increasing to 3 species, which is higher than in any other known cave where Troglocaris-like shrimps have been discovered.

Geochemistry of phosphatic nodules as a tool for understanding depositional and taphonomical settings in a Palaeolithic cave site (San Teodoro, Sicily)

Interpreting depositional settings of cave sites is generally problematic, especially in absence of palaeontological/archaeological evidence. This is the case of some deposits at San Teodoro Cave (Sicily), a key site for the Mediterranean Palaeolithic. In a stratigraphic level interrupted by a carbonatic concretion, phosphatic nodules are present only in the part enclosed between the concretion and the cave wall. The discovery of these nodules combined with the punctual lack of fossils had initially suggested an erosion phenomenon and subsequent formation of nodules at a vadose level. Here we show the usefulness of an integrated, geochemical-palaeoecological approach in defining stratigraphy and palaeoenvironmental reconstructions. XRD, ICP-OES, ATR-FTIR and EDS analyses allowed the formulation of a new hypothesis regarding the origin of the nodules, the depositional dynamics, and the role played by the guano produced by an extensive colony of bats. The role of barium and rubidium in detecting taphonomical processes has been highlighted.

Geochronological implications of 210Pb and 137Cs mobility in cave guano deposits

Some recent publications on the paleo- and historical environmental interpretation of bat guano sequences have relied on 210Pb and 137Cs distribution to establish age-depth models, even when these are at odds with radiocarbon models in the lower parts of the sequence. Here, we present both field and laboratory evidence for the unpredictable mobility of lead and cesium in decomposing bat guano deposits. We suggest that 210Pb- and 137Cs-based chronologies of bat guano deposits should only be used when independently supported, for example, by a robust radiocarbon age-depth model.

Do different relevance attributes indicate the same conservation priorities? A case study in caves of southeastern Brazil

In the last decade, the scientific community brought to the debate gaps that slow down the advance of knowledge regarding global biodiversity. More recently, this discussion has reached subterranean environments, where these gaps are even more dramatic due to the relict and vulnerable nature of their species. In this context, we tested ecological metrics related to some of these gaps, checking if the biological relevance of the caves would change depending on ecological attributes related to each metric. The study was carried out in caves from southeastern Brazil, located in a region presenting a high richness of troglobitic species restricted to a narrow geographical extent. Thus, we verified: (a) the cave invertebrate communities’ vulnerability with the Vulnerability Index and the Importance Value for Cave Conservation; (b) the distribution and endemicity of the troglobitic species with the Endemicity Index; (c) the phylogenetic diversity of the troglobitic species considering the average taxonomic distinction (∆+), their richness and evenness. We observed a considerable change in the ordering of the caves’ biological relevance according to each tested attribute (index). We discussed how each of these metrics and their attributes indirectly relate to: (a) the preservation and maintenance of the phylogenetic diversity of subterranean communities; (b) the spatial restrictions of different groups, where the greater their restrictions, the greater their vulnerability; (c) the preservation of caves with high biological relevance considering these different attributes together. Thus, we recommend the use of different metrics so that different ecological attributes can be considered, supporting actions that aim to preserve caves in highly altered regions. Finally, we find that the most biologically important cave in the region is not protected (Gruta da Morena cave). We warn that this cave needs to be contemplated by a conservation unit in the region urgently.

Molecular phylogeny of cave dwelling Eremogryllodes crickets (Orthoptera, Myrmecophilidae) across Zagros Mountains and Southern Iran

Recently, several new species and subspecies from the genus Eremogryllodes Chopard, 1929 (Insecta: Orthoptera: Myrmecophilidae) inhabiting caves of Iran, have been described based on morphology. The high variation of genitalia structure along with high similarity of external morphology between populations hamper the precise species identification. Thus, molecular approaches are critical to determine the taxonomic positions of species/subspecies of this genus. Here we provide the molecular phylogeny, based on the 16S rRNA mitochondrial gene, of recently described species of Eremogryllodes along with some unidentified specimens from the same region. The results support the monophyly of the family Myrmecophilidae. The topology of the 16S rRNA tree did not completely support the five morpho-species. The three main recovered clades mainly grouped specimens by their geographical locations. Our study suggests the possibility of more than one species in one cave and the presence of cryptic species among cave dwelling crickets, based on the 16S rRNA marker.

The impact of burning on the structure and mineral composition of bat guano

Here we addressed the question of whether burning of guano produces a characteristic suite of morphological changes and/or unique mineralogical products. The changes observed in our experimental burning of guano (both fresh and decayed) included colour change (blackening), grain size and morphological change (grain size generally reduced, morphology rendered generally less distinct), alteration of minerals by dehydration (e.g., gypsum to anhydrite, brushite to whitlockite), and production of new minerals or compounds (e.g., augelite, bayerite, giniite, graphite, oldhamite, strontium apatite, tridymite). The key morphological feature we found that may be diagnostic of burning was severe damage to crystals from rapid dehydration (cracks and striations, leading to eventual fragmentation). The key mineralogical feature we found was production of graphite. The high temperature exotic minerals that were produced (giniite, augelite, tridymite, oldhamite) were all found not to be high temperature obligate. Evidence gleaned from the literature suggests that a great number of the minerals associated with high temperatures can also be synthesized in low temperature settings such as weathering or microbial action (exemplified in the extremely complex biology and biochemistry of decaying guano). While the presence of any one of these minerals is not diagnostic of fire, it could be argued that the suite taken as a whole is moderately strong evidence for burning. In future studies, the chemistry of carbon aromaticity may prove to be the best diagnostic test for pyrolysis. A survey of the conditions under which documented spontaneous ignition occurs leads us to conclude that spontaneous ignition of guano inside a cave is an extremely unlikely event, and any suggestion/assertion to this effect should be rigorously supported.