Carbonate Speleothems Research Articles

LA-ICPMS for In Situ U-Th Dating of Holocene Stalagmites.

Here, we present an in situ U-Th dating approach of carbonate speleothems using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) with a detection efficiency of 1-2%. By online addition of a 229Th-233U-236U isotope triple spike to the laser-generated aerosol, instrumental mass discrimination and U/Th elemental fractionation could be monitored and corrected. With this approach, the 234U/238U and 230Th/238U activity ratios of a flowstone sample in secular equilibrium could be accurately reproduced as unity with two-sigma uncertainties ±0.053 and ±0.050, respectively. The method was used for the determination of the formation ages of individual layers in natural stalagmites ranging between 210 and 1 thousand years ago (ka). The determined ages corresponded well with those obtained using conventional solution multi collector-ICPMS techniques after isotope separation. Particularly, Holocene stalagmites, as young as 1 ka, could be accurately dated with 2 standard error of ±76 years. This developed microdomain U-Th dating approach thus can be applicable for diverse research areas, such as paleoclimatology, oceanography, geomagnetism, and archeology.

Growth rates of Brazilian carbonate speleothems by the 210Pb chronological method

Growth rates of Brazilian carbonate speleothems by the 210Pb chronological method

Speleothems uncover Late Holocene environmental changes across the Nuragic period in Sardinia (Italy): A possible human influence on land use during bronze to post-Iron Age cultural shifts

During the Bronze and Iron Age, Sardinia was home of one of the most technologically advanced Mediterranean societies (the Nuragic culture). Given its key geographical location, the island was also the fulcrum of deep cultural exchanges. Toward the end of the Iron Age, Phoenicians, and especially Carthaginians and Romans, massively frequented Sardinia for different purposes. This marks an important cultural transition for the region, as the ancient Nuragic-related society terminated. At the same time, this impacted the subsistence and land use practices. Together with middle to late Holocene climate changes, the novel anthropic activities had a pivotal role in shaping the landscape around the island. However, high resolution climate and environmental records for these culturally important phases are still lacking in Sardinia. Thus, this paper explores palaeoenvironmental changes from the Bronze Age to post Iron Ages times by using carbonate speleothems from Suttaterra de Sarpis Cave (Urzulei, central east Sardinia), strategically located nearby the Or Murales Nuragic Village. U–Th ages (n = 20) indicate that five stalagmites comprehensively span the last ∼7000 years. Peculiarly, they all show an evident stratigraphic discontinuity. Age models attest that hiatuses can be at times associated with the discontinuities, spanning periods of ∼1200 to ∼200 years. Importantly, the discontinuities occurred from the Late Iron Age to the Roman period. Based on fabric observations, trace elements and δ13C- δ18O analyses, the discontinuities are primarily attributed to a progressive change of land use above the cave. We suppose that deforestation aimed to clearance for agriculture and livestock practices probably was the most impacting factor for infiltration dynamics and soil state, thus affecting the studied speleothems, although archaeological and historical data are absent for the specific study area. Instead, this is in line with the cultural transition occurring in Sardinia toward the end of the Iron Age, with novel agricultural practices imported by the overseas populations. The anthropic disturbance to the millennial-long karst equilibrium possibly overprinted the response of speleothem proxies to climate oscillations, although future higher resolution analyses are necessary to better investigate the evolution of climate during the Holocene as well as its role in the development of ancient civilizations. Indeed, this is the first speleothem-based Holocene palaeoenvironmental reconstruction accomplished in Sardinia. Considering the paucity of natural lakes in this key location, speleothems here demonstrate their potential in exploring human-driven palaeoenvironmental changes during times of cultural transitions within the Mediterranean context.

Subaqueous carbonate speleothems as paleotemperature archives – clumped isotope thermometry and stable isotope compositions of inclusion-hosted water

Clumped isotope measurements of carbonates and stable isotope analyses of water trapped in fluid inclusions are both promising techniques to determine carbonate formation temperatures. Cave-hosted carbonate deposits (speleothems) are excellent targets for such studies, but kinetic fractionations and diagenetic influences frequently deteriorate the temperature data obtained from these methods. However, subaqueous carbonate deposits may provide reliable data, as kinetic fractionations are less significant in underwater environments. In this study, subaqueous speleothems, whose formation temperatures were directly measured in the water, were investigated. Additionally, temperatures calculated from the oxygen isotope fractionations between calcite and fluid inclusion-hosted water were compared with clumped isotope temperatures obtained for subaqueous carbonate formations in cave-hosted lakes. The clumped isotope temperatures fit the measured and calculated fluid inclusion temperatures within the analytical precisions. Carbonate deposits formed at elevated temperatures (~50°C or above) may undergo post-formational calcite-water oxygen isotope exchange, altering the composition of the inclusion-hosted water. In contrast, subaqueous speleothems formed at about 20-25°C appear to preserve the primary isotopic compositions. Our study shows that subaqueous carbonate speleothems are useful targets for clumped isotope and inclusion water analyses, making them valuable paleotemperature archives.

The incorporation of chlorine and cosmogenic 36Cl into speleothem carbonate

AbstractCave carbonate mineral deposits (speleothems) contain trace elements that are intensively investigated for their significance as palaeoclimate and environmental proxies. However, chlorine, which is abundant in marine and meteoric waters, has been overlooked as a potential palaeo-proxy, while cosmogenic 36Cl could, in principle, provide a solar irradiance proxy. Here, total Cl concentrations analysed from various speleothems were low (3–14 mg/kg), with variations linked to crystal fabrics. High-resolution synchrotron radiation micro X-ray fluorescence (μ-XRF) trace element mapping showed Cl often associated with Na, Si, and Al. We propose that speleothems incorporate Cl in two fractions: (1) water soluble (e.g., fluid inclusions) and (2) water insoluble and strongly bound (e.g., associated with detrital particulates). However, disparities indicated that alternate unidentified mechanisms for Cl incorporation were present, raising important questions regarding incorporation of many trace elements into speleothems. Our first measurements of 36Cl/Cl ratios in speleothems required large samples due to low Cl concentrations, limiting the potential of 36Cl as a solar irradiance proxy. Critically, our findings highlight a knowledge gap into how Cl and other trace elements are incorporated into speleothems, how the incorporation mechanisms and final elemental concentrations are related to speleothem fabrics, and the significance this may have for how trace elements in speleothems are interpreted as palaeoclimate proxies.

Biogenic calcium carbonate as evidence for life

Abstract. The history of the Earth is a story of the co-evolution of minerals and microbes: not only have numerous rocks arisen from life but also life itself may have formed from rocks. To understand the strong association between microbes and inorganic substrates, we investigated the moonmilk, a calcium carbonate deposit of possible microbial origin, occurring in the Iron Age Etruscan necropolis of Tarquinia, in Italy. These tombs provide a unique environment where the hypogeal walls of the tombs are covered by this speleothem. To study moonmilk formation, we investigated the bacterial community in the rock in which the tombs were carved: calcarenite and hybrid sandstone. We present the first evidence that moonmilk precipitation is driven by microbes within the rocks and not only on the rock surfaces. We also describe how the moonmilk produced within the rocks contributes to rock formation and evolution. The microbial communities of the calcarenite and hybrid sandstone displayed, at the phylum level, the same microbial pattern of the moonmilk sampled from the walls of the hypogeal tombs, suggesting that the moonmilk originates from the metabolism of an endolytic bacterial community. The calcite moonmilk is the only known carbonate speleothem on Earth with undoubted biogenic origin, thus representing a robust and credible biosignature of life. Its presence in the inner parts of rocks adds to its characteristics as a biosignature.

Formula omitted] in speleothem carbonates in Soreq Cave as an archive for hydro-climatic conditions

δ18O in speleothem carbonates is a common archive for paleoclimate on land. Recently, it has been shown that triple oxygen isotopes in CaCO3 (given as Oexcess17=106[ln⁡(10−3δ17O+1)−0.528(ln⁡(10−3δ18O+1)]) record Oexcess17 of its parent water, thus providing additional paleo-hydrology information, primarily about relative humidity. The O17 fractionation between CaCO3 and water has been determined in biogenic and synthetic carbonates. In speleothems, however, this fractionation is expected to be modified by kinetic isotope effects associated with CO2 degassing from cave drip water. Here, we used Soreq Cave as a case study to examine this fractionation and the use of Oexcess17 in speleothems as an archive for paleo rainfall composition. We first characterized Oexcess17 in rainfall collected above the cave. The Oexcess17 value in amount weighted integrated rainfall was 49 per meg, consistent with the low relative humidity at the moisture source of the Eastern Mediterranean Sea. The measured cave water was slightly lower in Oexcess17 and higher in δ18O relative to rainfall, due to modifications of the infiltrating water by epikarst processes such as evaporation and mixing. Oexcess17 in CO2 extracted from CaCO3 of modern speleothems was compared to that in modern cave water, to calculate the carbonate-water fractionation slope (θ=ln17⁡α/ln18⁡α). The resulting θ in Soreq Cave was 0.5230±0.0002, consistent with that observed in other carbonates, indicating the lack of significant dis-equilibrium effects in speleothems Oexcess17. Hence, the common fractionation slope can be used together with a speleothem-specific α18 to reconstruct rainfall Oexcess17 from speleothem carbonates. Whereas speleothems from a variety of caves can be used through this approach to reconstruct Oexcess17 in paleo rainfall, the atmospheric processes that govern rainfall Oexcess17 are expected to be region-specific. Oexcess17 measurements in ancient Soreq speleothems are expected to reveal potential glacial-interglacial changes in moisture source conditions as well as possible changes in epikarst processes over time.

Corrosion of carbonate speleothems by bat guano

Interactions between bat guano and speleothems were studied in caves of Slovakia (Domica, Drienovská, and Jasovská caves) and Poland (Nietoperzowa Cave). This contribution focuses on the estimation of the rate of guano-induced corrosion of carbonates and on the transformations of speleothems caused by the corrosion. The intense interaction between bat guano accumulations and cave carbonates is mainly controlled by the chemical composition of guano. Corrosion of carbonates by guano leachates, suggested by negative calcite saturation index, was confirmed by weight loss of experimental carbonate tablets exposed to interaction with guano in situ in caves, and under laboratory conditions. The calculated corrosion rate of the tablets was up to 0.111 mm/y. However, measurements of selected corrosion forms in the studied caves, compared to the guano 14C age, suggest rates of the corrosion up to 0.65 mm/y. Transport of aggressive guano leachates is possible in the presence of water; increased drip water supply is considered as contributing to the higher dissolution rates. Crystallization of phosphates as byproducts of the interaction of guano with carbonates can also alter the dissolution process. Phosphates, represented by ardealite, brushite, and hydroxylapatite, were observed on the surfaces of experimental tablets and speleothems alike. Interestingly, hydroxylapatite was the only mineral associated with guano-related hiatuses within speleothem sections. Phosphates are usually accompanied by various dissolution features, such as corrosion pits and considerably dissolved calcite crystals. Dissolution features currently forming on speleothem surfaces are analogous to those identified in the fossil record. U-series dating of guano corrosion episodes within stalagmites from Domica and Drienovská caves indicates their Pleistocene age, while within a flowstone from Nietoperzowa Cave, a Holocene age. Considerable guano accumulations are common in low-latitude caves, whereas in temperate climatic zones guano accumulates irregularly in space and time. The guano accumulations are usually restricted to cave sections distant no more than few hundred meters from the nearest entrance. Considering the widespread distribution of bats across the globe, this process is expected to be common in caves. Nevertheless, the random distribution of guano in caves and its temporal deposition, but also the climate-dependent availability of water, suggest that guano-induced corrosion can be regarded as a site specific process.

Lunaemycins, New Cyclic Hexapeptide Antibiotics from the Cave Moonmilk-Dweller Streptomyces lunaelactis MM109T

Streptomyces lunaelactis strains have been isolated from moonmilk deposits, which are calcium carbonate speleothems used for centuries in traditional medicine for their antimicrobial properties. Genome mining revealed that these strains are a remarkable example of a Streptomyces species with huge heterogeneity regarding their content in biosynthetic gene clusters (BGCs) for specialized metabolite production. BGC 28a is one of the cryptic BGCs that is only carried by a subgroup of S. lunaelactis strains for which in silico analysis predicted the production of nonribosomal peptide antibiotics containing the non-proteogenic amino acid piperazic acid (Piz). Comparative metabolomics of culture extracts of S. lunaelactis strains either holding or not holding BGC 28a combined with MS/MS-guided peptidogenomics and 1H/13C NMR allowed us to identify the cyclic hexapeptide with the amino acid sequence (D-Phe)-(L-HO-Ile)-(D-Piz)-(L-Piz)-(D-Piz)-(L-Piz), called lunaemycin A, as the main compound synthesized by BGC 28a. Molecular networking further identified 18 additional lunaemycins, with 14 of them having their structure elucidated by HRMS/MS. Antimicrobial assays demonstrated a significant bactericidal activity of lunaemycins against Gram-positive bacteria, including multi-drug resistant clinical isolates. Our work demonstrates how an accurate in silico analysis of a cryptic BGC can highly facilitate the identification, the structural elucidation, and the bioactivity of its associated specialized metabolites.

Corrosion of carbonate speleothems by an allogenic river inferred from petrography and a weight loss experiment: a case study from the Demänová Cave System, Slovakia

Corrosion of carbonate speleothems by an allogenic river inferred from petrography and a weight loss experiment: a case study from the Demänová Cave System, Slovakia

Features of carbonate speleothems from the Belaya Cave (Zabaykalsky Krai)

The research gives the first data on mineralogical studies of carbonate speleothems in the karst cave named Belaya. Dolomite-calcite, calcite-magnesite and hydromagnesite compositions of secondary carbonate sediments in various grottoes of the cave were determined by thermal analysis (TG-DSC) and X-ray diffraction (XRD). Creating a mineralogical reserve or park with the Belaya Cave as its core is suggested.

Environmental changes in Sorbas arid region (Southern Spain) during MIS 5a inferred from a carbonate flowstone from a gypsum cave

Present climate conditions impede the formation of calcite speleothems in the gypsum caves of the semi-arid region of Sorbas (Southern Spain). However, U-Th dating reveals the uninterrupted deposition of a 46 mm-thick carbonate flowstone in the Sorbas caves from ∼78 to ∼71 ka, during Marine Isotope Stage (MIS) 5a. This indicates that the area was vegetated during this period, producing biogenic CO2 in soils necessary for the underground deposition of the speleothem, which contrasts with the current unvegetated landscape. During MIS 5a, rainfall recharged the karst aquifer, leading to cave seepage. The development of soils and persistent water infiltration suggests a higher precipitation/evapotranspiration ratio with respect to the present, which was likely favored by lower regional temperatures and a different annual rainfall regime. δ18O and δ13C analyses testify that the speleothem recorded climate pulses synchronous, and structurally similar, with Greenland interstadial and stadial periods, with relatively wetter conditions recorded during interstadials 21.1, 20 and 19.2, and drier periods during stadials 21.1 and 20. This demonstrates that the climate in the desert-like environment of Sorbas is sensitive to inter- and intra- hemispherical triggers, such as the extent of Northern ice sheets and the Atlantic meridional overturning circulation. Interstadials (stadials) were also characterized by a higher (lower) Atlantic/Mediterranean moisture source ratio. For the first time, we show that carbonate speleothems formed in arid gypsum caves can record reliable palaeoenvironmental information spanning multiple millennia, procuring proxies for detailed palaeoclimate reconstructions. Such speleothems are thus excellent candidates for understanding past climate dynamics and patterns in desert-like gypsum terrains.

Investigating stable oxygen and carbon isotopic variability in speleothem records over the last millennium using multiple isotope-enabled climate models

Abstract. The incorporation of water isotopologues into the hydrology of general circulation models (GCMs) facilitates the comparison between modeled and measured proxy data in paleoclimate archives. However, the variability and drivers of measured and modeled water isotopologues, as well as the diversity of their representation in different models, are not well constrained. Improving our understanding of this variability in past and present climates will help to better constrain future climate change projections and decrease their range of uncertainty. Speleothems are a precisely datable terrestrial paleoclimate archives and provide well-preserved (semi-)continuous multivariate isotope time series in the lower latitudes and mid-latitudes and are therefore well suited to assess climate and isotope variability on decadal and longer timescales. However, the relationships of speleothem oxygen and carbon isotopes to climate variables are influenced by site-specific parameters, and their comparison to GCMs is not always straightforward. Here we compare speleothem oxygen and carbon isotopic signatures from the Speleothem Isotopes Synthesis and Analysis database version 2 (SISALv2) to the output of five different water-isotope-enabled GCMs (ECHAM5-wiso, GISS-E2-R, iCESM, iHadCM3, and isoGSM) over the last millennium (850–1850 CE). We systematically evaluate differences and commonalities between the standardized model simulation outputs. The goal is to distinguish climatic drivers of variability for modeled isotopes and compare them to those of measured isotopes. We find strong regional differences in the oxygen isotope signatures between models that can partly be attributed to differences in modeled surface temperature. At low latitudes, precipitation amount is the dominant driver for stable water isotope variability; however, at cave locations the agreement between modeled temperature variability is higher than for precipitation variability. While modeled isotopic signatures at cave locations exhibited extreme events coinciding with changes in volcanic and solar forcing, such fingerprints are not apparent in the speleothem isotopes. This may be attributed to the lower temporal resolution of speleothem records compared to the events that are to be detected. Using spectral analysis, we can show that all models underestimate decadal and longer variability compared to speleothems (albeit to varying extents). We found that no model excels in all analyzed comparisons, although some perform better than the others in either mean or variability. Therefore, we advise a multi-model approach whenever comparing proxy data to modeled data. Considering karst and cave internal processes, e.g., through isotope-enabled karst models, may alter the variability in speleothem isotopes and play an important role in determining the most appropriate model. By exploring new ways of analyzing the relationship between the oxygen and carbon isotopes, their variability, and co-variability across timescales, we provide methods that may serve as a baseline for future studies with different models using, e.g., different isotopes, different climate archives, or different time periods.

Bromate incorporation in calcite and aragonite

The presence of bromine as a trace-element in calcium carbonate speleothems constitutes a useful proxy of past volcanic activity, thus helping to provide input parameters for climate model simulations and risk assessment. However, the chemical nature of bromine-containing impurities in the calcium carbonate phases forming speleothems is not understood, which limits interpretation of experimental measurements on speleothems. We present here a computer simulation study, based on quantum mechanical calculations, of the incorporation of bromine as BrO3− oxyanions in CaCO3 polymorphs calcite and aragonite. We discuss how the relative distributions of bromate oxyanions and charge-compensating alkali-metal cations are determined by the interplay between an impurity binding effect (which is stronger for aragonite than for calcite, and changes in the order Li < Na < K) and a configurational entropic effect that tends to disassociate the impurities. For concentrations above parts-per-million, bromate impurities can be expected to be paired, in nearest-neighbour configurations, with the compensating cations. Bromate substitution, compensated by sodium or potassium cations, is predicted to be metastable with respect to phase separation of the impurities as solid NaBrO3 or KBrO3 phases, respectively, but the solubility limits of BrO3− in calcite and aragonite are still higher than those calculated for tetrahedral oxyanions (SO4)2− and (MoO4)2− that are used as alternative volcanic records in speleothems.

Calcium Carbonate Precipitating Cultivable Bacteria from Different Speleothems of Karst Caves

The aim of the study was to reveal and compare the microbiota of different carbonate speleothems from two different epigenic karst regions (Baradla and Csodabogyós caves) in Hungary. Stalagmites, stalactites, soda straws, moonmilks, cave wall surfaces and dripping waters were sampled and plated on subsurface-environment imitating culture media containing low amount of nutrients, minerals and extract of cave sediments. Diverse oligo-heterotrophic bacterial community composition (192 different species in the phyla Actinobacteria, Proteobacteria, Firmicutes and Bacteroidetes) – with hitherto unknown cave bacteria – varied in the samples. The CaCO3 precipitating capacity of 126 bacterial strains from different taxonomical positions was tested on solid B4 medium. 75% of the strains precipitated crystals with various morphology in the colonies and metabolic tests showed that higher alkalizing activity led to higher rate of precipitation in the mucoid colonies. Scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction analysis indicated that the precipitates were dominated by calcite, and nanosize vaterite was identified for Paeniglutamicibacter kerguelensis (strain BaSD-225). Our study suggests that CaCO3 precipitated by proteolytic bacteria are widespread in the biofilms in karst caves and these bacteria can actively contribute to CaCO3 formation through alkalizing their microenvironment.

(U,Th)-He dating of Pleistocene carbonates: Analytical methods and age calculations

The uranium-thorium-helium ((U,Th)-He) dating method applied to calcium carbonate speleothems holds much promise for constraining the timeline of hominin evolution, as well as for palaeoclimatology research beyond the range of U/Th disequilibrium dating. Technical problems are posed by often low U concentrations and the requirement that samples need to be individually removed from the ultrahigh vacuum (UHV) system after helium is extracted from them, to be then analyzed for U, Th content and U series disequilibrium. We describe a low-cost furnace with this capability, constructed from standard UHV components, as well as the methods for subsequent U/Th disequilibrium analysis using multicollector ICP mass spectrometry. We present analytical and numerical solutions to determine (U,Th)-He ages for four conditions that have been encountered: (1) for an age range in which the residual activity ratio (234U/238U) can still be resolved but not that of (230Th/234U) (up to about 3000 ka), (2) for an age range where neither can be resolved (unlimited), (3) for ages up to 1000 ka where both activity ratios may be resolvable, and (4) for cases where (234U/238U) and (230Th/234U) indicate ages < 200 ka but (U,Th)-He systematics point to much older ages.•Helium extraction is carried out using an in-house built vacuum furnace that allows for large sample sizes (20 to 100 mg) of powdered carbonate material wrapped in Cu foil.•U and Th are separated together using Eichrom UTEVAⓇ resin and their isotope abundances are measured together in a single 2-cycle dynamic run using the multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS).•(U,Th)-He ages are calculated using four methods that take into account the U/Th disequilibria.

A framework for triple oxygen isotopes in speleothem paleoclimatology

Speleothem oxygen isotope (δ18O) records provide key insight into the rate and timing of terrestrial paleoclimate changes during the late Quaternary. However, it can be difficult to deconvolve the δ18O signal into individual components, which include processes related to moisture source, moisture transport, temperature, precipitation amount, infiltration, and the cave environment. We developed a framework that uses triple oxygen isotope distributions in speleothems to refine interpretations of δ18O speleothem records. This framework identifies the influence of dominant processes on δ18O values through time by their characteristic (although not necessarily unique) trends in Δ′17O vs. δ′18O space, where Δ′17O = δ′17O – 0.528δ′18O and δ′xO = ln(δxO + 1). Following Guo and Zhou (2019a), we expect that ‘cave kinetic’ processes (e.g., fast degassing at the drip site, prior calcite precipitation) will drive positive trends between δ′18O and Δ′17O. In contrast, we can identify hydrologic processes from near-horizontal trends that reflect Rayleigh-type meteoric water processes and negative trends driven by changes in evaporation processes at the moisture source region or at the cave site, mineralization temperature, and seasonality in precipitation/infiltration amount. We applied this framework to four western USA speleothems from Cave of the Bells (Arizona), Leviathan Cave (Nevada), and Lehman Caves (Nevada). The Cave of the Bells and Leviathan data have near-horizontal to negative trends indicating δ18O variability was driven largely by changes in Rayleigh distillation of atmospheric moisture and moisture source conditions, supporting prior interpretations. We analyzed two Lehman Caves records because they were likely influenced by non-equilibrium processes and the data show weak to moderate negative trends. For sample LMC-12b, chosen for its extreme 7.5‰ δ18O range, the trend is statistically distinct from the near-horizontal Rayleigh-process trend and most consistent with changes in local evaporation intensity and infiltration seasonality as primary drivers. None of these records displays a positive covariation slope between δ′18O and Δ′17O, suggesting limited variability in cave kinetic processes through time or unknown limitations to the kinetic model of Guo and Zhou (2019a). Additionally, reconstructed formation waters for all sites fall near the Δ′17O vs. δ′18O Local Meteoric Water Line, a correlation we suggest as a novel test of the absolute magnitude of isotopic offset due to cave kinetic processes. More broadly, our framework adds context to the only other study of carbonate speleothem triple oxygen isotope composition (Sha et al., 2020). We find that positive to negative Δ′17O vs. δ′18O trends likely exist in speleothem data that may reasonably be expected from regional climate processes and that, combined with other proxy data, triple oxygen isotope data will be useful in constraining interpretations of δ18Ospeleothem records.

Stalagmite carbon isotopes suggest deglacial increase in soil respiration in western Europe driven by temperature change

Abstract. The temperate region of western Europe underwent significant climatic and environmental change during the last deglaciation. Much of what is known about the terrestrial ecosystem response to deglacial warming stems from pollen preserved in sediment sequences, providing information on vegetation composition. Other ecosystem processes, such as soil respiration, remain poorly constrained over past climatic transitions but are critical for understanding the global carbon cycle and its response to ongoing anthropogenic warming. Here we show that speleothem carbon isotope (δ13Cspel) records may retain information on soil respiration and allow its reconstruction over time. While this notion has been proposed in the past, our study is the first to rigorously test it, using a combination of multi-proxy geochemical analysis (δ13C, Ca isotopes, and radiocarbon) on three speleothems from the NW Iberian Peninsula and quantitative forward modelling of processes in soil, karst, and cave. Our study is the first to quantify and remove the effects of prior calcite precipitation (PCP, using Ca isotopes) and bedrock dissolution (using the radiocarbon reservoir effect) from the δ13Cspel signal to derive changes in respired δ13C. The coupling of soil gas pCO2 and δ13C via a mixing line describing diffusive gas transport between an atmospheric and a respired end-member allows the modelling of changes in soil respiration in response to temperature. Using this coupling and a range of other parameters describing carbonate dissolution and cave atmospheric conditions, we generate large simulation ensembles from which the results most closely matching the measured speleothem data are selected. Our results robustly show that an increase in soil gas pCO2 (and thus respiration) is needed to explain the observed deglacial trend in δ13Cspel. However, the Q10 (temperature sensitivity) derived from the model results is higher than current measurements, suggesting that part of the signal may be related to a change in the composition of the soil respired δ13C, likely from changing substrate through increasing contribution from vegetation biomass with the onset of the Holocene.

A reactive transport approach to modeling cave seepage water chemistry I: Carbon isotope transformations

The majority of Critical Zone research has emphasized silicate lithologies, which are typified by relatively slow rates of reactivity and incongruent weathering. However, the relatively simpler weathering of carbonate-dominated lithology can result in secondary mineral deposits, such as speleothems, which provide a long-term archive for Critical Zone processes. In particular, carbon isotopic variability in speleothems has the potential to provide records of changes in vegetation, soil respiration, carbon stabilization in deep soils, and/or chemical weathering in the host rock. Despite this opportunity to reconstruct many Critical Zone processes, multiple influences can also make interpretion of these speleothem carbon isotope records challenging. The integration of observational data and simulations specific to karst systems offers an interpretive framework for these unique time-averaged records accumulated through the evolution of carbonate landscapes. Here, we present a forward and process-based reactive transport simulation based on a multi-year monitoring study of Blue Spring Cave in central Tennessee, USA. The simulations describe the fluid-driven weathering of limestone including explicit tracking of dissolved calcium, stable carbon, and radiocarbon isotope ratios based on reaction rates calibrated through laboratory batch reaction data. We find that calcium concentrations and radiocarbon isotope ratios are strongly influenced by the combination of fluid flow rate and soil CO2 content, and require rapid gas phase communication between the overlying soil boundary condition and interior karst to sustain both elevated limestone weathering rates and relatively modern radiocarbon signatures. Stable carbon isotopes are largely dictated by temperature-dependent equilibrium fractionation among contemporaneous species. These simulations are extended to a wide range of parameter space to demonstrate the environmental factors that these isotope proxies record.

Effects of glaciation on karst hydrology and sedimentology during the Last Glacial Cycle: The case of Granito cave, Central Pyrenees (Spain)

In Alpine regions, speleothem development on karst systems largely occurs during warm interglacial or interstadial phases due to their limited growth during cold stages. Still, recent attention has been given to the role of clastic sediments in caves, less dependent on temperature conditions. Yet, only a small number of caves worldwide preserve both speleothems and detrital deposits. Here we present an outstanding record of fine-grain laminated sediments and carbonate speleothems from the Granito cave (South Central Pyrenees, Spain) associated with seasonal to annual hydroclimatic pulses through the Last Glacial Cycle (LGC). Analysis of cave clastic facies together with new absolute dates on glacial deposits along the valley has provided new insights into the karst-glacial interactions in relation with long-term landscape evolution, with support of geomorphological, sedimentological, mineralogical, palynological, geochemical and geochronological data (U/Th series, OSL and IRSL).The Granito cave was formed before 153.6±1.2 ky (Marine Isotopic Stage, MIS 6), as indicated by the oldest stalagmites dated in the cave, although speleothem formation occurred preferentially during MIS 5 and the Holocene interglacial stages. A 60 m thick clastic deposit was IRSL dated from 71.8±5.6 ky to after 26.5±2.5 ky corresponding with the maximum ice extent (MIE) during the LGC and subsequent glacial stabilization at 30.3±1.7 – 36.2±2.2 ky. The sediment infill was produced by water ponded in the cave coeval with the presence of glacial ice blocking the cave entrance. The cave clastic sequence includes channel, slackwater and backswamp facies, interpreted to result from pulses of sediment-laden water flow under vadose and phreatic flow regimes. The cave sediment’s mineralogical composition points to an exogenous source associated with glacial till sediments overlying the hillslope above the cave and along the Ara River valley. The palynological results obtained from the cave sediments show the dominance of an open landscape composed of steppe vegetation indicating cold glacial climatic conditions. The opening of the cave occurred before 14.9±1.5 ky, the stabilization age of the innermost Ara glacier moraine, giving rise to scouring and cut-and-fill sequences within the detrital infill. Speleothem growth during the Holocene indicates the return to warm climatic conditions with the development of a soil and vegetation cover above the cave. This study shows how a combined interpretation of clastic and speleothem lithofacies is critical for elucidating landscape evolution and surface-groundwater palaeohydrological changes in northern Iberia during the LGC.