Natural Speleothems Research Articles

Patterns and controls of disequilibrium isotope effects in speleothems: Insights from an isotope-enabled diffusion-reaction model and implications for quantitative thermometry

Stable isotope composition of speleothems reflects the physicochemical condition of their formation environment such as the local temperature and the isotopic composition of surface precipitation, making them one of the best archives of terrestrial climate. However, quantitative reconstruction of paleo-temperature from speleothem isotope records is challenging, because most speleothems do not form in isotope equilibrium with the drip water. These disequilibrium isotope effects, often thought to arise from the rapid degassing of CO2 from a thin water film, vary among different speleothems and over time, and hinder the applications of many isotope thermometers in speleothems, including both the conventional carbonate-water oxygen isotope thermometer and the recently developed carbonate clumped isotope thermometer. Here we present an isotope-enabled reaction-diffusion model of speleothem formation (IsoCave), and use it to systematically examine the patterns and controls of the disequilibrium isotope effects in speleothems (δ13C, δ18O, Δ′17O, Δ47, Δ48 and Δ49) and explore their implications for isotope thermometry in speleothems.We show that prior calcite precipitation, cave air pCO2 and δ13CCO2exert the strongest controls on the disequilibrium isotope effects in speleothems, followed by cave temperature, water film thickness and water drip rate. Together, changes in these environment parameters explain the variations of disequilibrium isotope effects in natural speleothems. Our model reproduces the apparent temperature dependence of oxygen isotope compositions of speleothems compiled from multiple caves over a large temperature range, but highlights the challenges in the application of speoleothem-specific calibration of isotope thermometers due to the effects of non-temperature factors. Further, we show the disequilibrium effects in different isotope systems are highly correlated. The slopes of these correlations reflect mainly the kinetic isotope fractionations associated with HCO3− dehydration and dehydroxylation reactions, and remain relatively constant despite changes in environmental conditions especially for speleothems formed during the early evolution period of drip water. Based on these correlations, specifically the correlation between disequilibrium Δ47 and Δ48 effects, we propose a novel approach to quantitatively correct for disequilibrium isotope effects in speleothems and derive more accurate estimates of speleothem formation temperatures. Application of this coupled Δ47-Δ48 approach to synthetic speleothem isotope records reproduces the speleothem formation temperatures, suggesting new opportunities for quantitative paleo-temperature reconstruction based on speleothem isotope records.

Adsorption of actinides within speleothems

AbstractStalagmites and stalactites, as observed within natural caves, may develop inside geological repositories during constructional and post-operational periods. It is therefore important to understand actinide sorption within such materials. Towards this, experimental studies were carried out with233U,238Np (VI),238Np (IV),239Pu and241Am radiotracers using natural speleothem samples collected from the Dharamjali cave of the Kumaon Lesser Himalayas, India. Petrological/mineralogical studies showed that natural speleothems have three general domains: (1) columnar calcite; (2) microcrystalline calcite; and (3) botryoidal aragonite – each with ferruginous materials. Results showed that all domains of speleothems can take up >99% actinides, irrespective of valence state and pH (1–6 range) of the solution. However, distribution coefficients were found to be at a maximum in aragonite for most of the actinides. Such data are very important for long-term performance and safety assessments of the deep geological repositories planned for the disposal of high-level nuclear wastes.

Hydrochemical controls on aragonite versus calcite precipitation in cave dripwaters

Despite the paleoclimatic relevance of primary calcite to aragonite transitions in stalagmites, the relative role of fluid Mg/Ca ratio, supersaturation and CO32− concentration in controlling such transitions is still incompletely understood. Accordingly, we have monitored the hydrochemistry of 50 drips and 8 pools that are currently precipitating calcite and/or aragonite in El Soplao and Torca Ancha Caves (N. Spain), investigating the mineralogy and geochemistry of the CaCO3 precipitates on the corresponding natural speleothem surfaces. The data reveal that, apart from possible substrate effects, dripwater Mg/Ca is the only obvious control on CaCO3 polymorphism in the studied stalagmites and pools, where calcite- and aragonite-precipitating dripwaters are separated by an initial (i.e. at stalactite tips) Mg/Ca threshold at ≈1.1mol/mol. Within the analyzed ranges of pH (8.2–8.6), CO32− concentration (1–6mg/L), supersaturation (SIaragonite: 0.08–1.08; SIcalcite: 0.23–1.24), drip rate (0.2–81drops/min) and dissolved Zn (6–90μg/L), we observe no unequivocal influence of these parameters on CaCO3 mineralogy. Despite the almost complete overlapping supersaturations of calcite- and aragonite-precipitating waters, the latter are on average less supersaturated because the waters having Mg/Ca above ∼1.1 have mostly achieved such high ratios by previously precipitating calcite. Both calcite and aragonite precipitated at or near oxygen isotopic equilibrium, and Mg incorporation into calcite was consistent with literature-based predictions, indicating that in the studied cases CaCO3 precipitation was not significantly influenced by strong kinetic effects. In the studied cases, the calcites that precipitate at ∼11°C from dripwaters with initial Mg/Ca approaching ∼1.1 incorporate ∼5mol% MgCO3, close to the published value above which calcite solubility exceeds aragonite solubility, suggesting that aragonite precipitation in high-relative-humidity caves is favored from a solubility viewpoint. We also show that unaccounted CaCO3 precipitation in intermediate sampling containers and splash effects may in cases result in underestimating dripwater Ca concentration and alkalinity, potentially leading to incorrect conclusions regarding the role of fluid Mg/Ca ratio and supersaturation on CaCO3 mineralogy. A simple way to elude the first effect is by taking water samples directly from stalactites and by titrating alkalinity in the same containers used to collect dripwaters.

Simple and Efficient Separation of Magnetic Minerals From Speleothems and Other Carbonates

Abstract Trace concentrations of iron oxide minerals in carbonate sediments can preserve fine details about Earth processes, from high-resolution recordings of the Earth's ancient magnetic field to microscopic remnants of extraterrestrial impacts. This paper presents a novel flask extraction method which uses a neodymium magnet and an orbital shaker for simple and efficient separation of magnetic minerals from carbonate sediments. A mineral assemblage of magnetic standards (titanomagnetite, magnetite, goethite, and hematite) combined with other mineral standards (kaolinite, quartz, and nanoscale TiO2) was subjected to the extraction procedure and compared to a natural speleothem sample. Exposure of the magnetic standards to a mildly acidic acetate buffer (pH ∼ 4) did not cause physical or chemical alteration. The strongly magnetic minerals were reproducibly extracted, with greater than 90% efficiency (by mass), from mixtures of the mineral standards. XRD and low-temperature magnetic characterization demonstrated phase purity of the extracts. Quantitative comparison with two commonly used literature methods showed that the flask extraction method was more reproducible and efficient. The addition of surfactant (Na(PO3)6) did not significantly improve extraction efficiency. Sequential dissolution and flask extraction of a simulated speleothem containing magnetic particles resulted in consistent extraction efficiencies for samples containing large (> 1 µm) strongly magnetic grains, but a reduction in efficiency was observed for smaller (< 1 µm) grain sizes. No method successfully extracted the weakly magnetic goethite and hematite. However, unprecedented, representative characterization of these minerals was possible through quantitative analysis of the remainder after the collection of the magnetic extract. This approach may facilitate detailed characterization of a wide range of carbonates, such as pelagic limestone, dolostone, unlithified carbonate ooze, speleothems, and freshwater and pedogenic carbonates. Such mineral extractions can lead to new insights into paleoenvironmental processes as well as an improved understanding of the recording of the Earth's magnetic field.

Preservation of NOM-metal complexes in a modern hyperalkaline stalagmite: Implications for speleothem trace element geochemistry

We report the first quantitative study of the capture of colloidal natural organic matter (NOM) and NOM-complexed trace metals (V, Co, Cu, Ni) in speleothems. This study combines published NOM–metal dripwater speciation measurements with high-resolution laser ablation ICPMS (LA-ICPMS) and sub-annual stable isotope ratio (δ18O and δ13C), fluorescence and total organic carbon (TOC) analyses of a fast-growing hyperalkaline stalagmite (pH ∼11) from Poole’s Cavern, Derbyshire UK, which formed between 1997 and 2008 AD. We suggest that the findings reported here elucidate trace element variations arising from colloidal transport and calcite precipitation rate changes observed in multiple, natural speleothems deposited at ca. pH 7–8. We find that NOM–metal(aq) complexes on the boundary between colloidal and dissolved (∼1nm diameter) show an annual cyclicity which is inversely correlated with the alkaline earth metals and is explained by calcite precipitation rate changes (as recorded by kinetically-fractionated stable isotopes). This relates to the strength of the NOM–metal complexation reaction, resulting in very strongly bound metals (Co in this system) essentially recording NOM co-precipitation (ternary complexation). More specifically, empirical partition coefficient (Kd) values between surface-reactive metals (V, Co, Cu, Ni) [expressed as ratio of trace element to Ca ratios in calcite and in solution] arise from variations in the ‘free’ fraction of total metal in aqueous solution (fm). Hence, differences in the preservation of each metal in calcite can be explained quantitatively by their complexation behaviour with aqueous NOM. Differences between inorganic Kd values and field measurements for metal partitioning into calcite occur where [free metal]≪[total metal] due to complexation reactions between metals and organic ligands (and potentially inorganic colloids). It follows that where fm≈0, apparent inorganic Kd app values are also ≈0, but the true partition coefficient (Kd actual) is significantly higher. Importantly, the Kd of NOM–metal complexes [organic carbon–metal ratio) approaches 1 for the most stable aqueous complexes, as is shown here for Co, but has values of 24–150 for V, Ni and Cu. This implies that ternary surface complexation (metal–ligand co-adsorption) can occur (as for NOM–Co), but is the exception rather than the rule. We also demonstrate the potential for trace metals to record information on NOM composition as expressed through changing NOM–metal complexation patterns in dripwaters. Therefore, a suite of trace metals in stalagmites show variations clearly attributable to changes in organic ligand concentration and composition, and which potentially reflect the state of overlying surface ecosystems.

Oxygen isotope variations in rainfall, drip-water and speleothem calcite from a well-ventilated cave in Texas, USA: Assessing a new speleothem temperature proxy

Measurements of the oxygen isotopic composition (δ18O value) of rainfall, drip-water and associated calcite (grown on artificial substrates and in a natural speleothem) at multiple drip sites in a well-ventilated cave in central Texas were conducted to investigate the potential use of speleothem δ18O for quantitative temperature reconstructions. From 2009 to 2011, rainfall δ18O varied by 11.5‰ (−10.5‰ to 1.0‰, V-SMOW), whereas drip-waters had a much narrower range of 0.3‰ (−4.7‰ to −4.4‰, V-SMOW). This contrast indicates that mixing processes along flow paths in the vadose zone above the cave produce a well-homogenized water reservoir that supplies drip-water to the cave. The δ18O values for calcite grown on substrates over the same time period show seasonal variations (summer: ∼−6‰; winter: ∼−3‰) that are strongly correlated with surface air temperatures (ta) at all three monitored sites (r2=0.88-0.96; p<0.001). These results indicate that the dominant control on calcite δ18O is temperature. An empirical relationship was established for one monitored site: ta=−9.1(±0.9)×δ18O×103−20.6(±4.1); r2=0.88, and applied to a δ18O time series of the top 6.7mm of a stalagmite that grew at this drip site. This yields a temperature record that appears to reflect seasonal variations for the period 2005–2009. This speleothem-derived temperature record is offset to lower values (by 0-8°C) compared to the instrumental temperature record. This offset may be a result of differences between substrate and speleothem calcite in terms growth mechanism, extent of non-equilibrium isotopic effects, or temporal shifts in drip-water δ18O values. Despite this offset, the speleothem-derived record reconstructs the amplitudes of seasonal variations and changes in inter-annual summer peak temperatures in the instrumental record. Results of this study have implications for reconstructing past temperatures, and for establishing a speleothem chronology with seasonal resolution, using speleothem calcite δ18O. The results also suggest that relatively seldom-studied speleothem samples near the entrances of caves, where environmental conditions may be similar to surface conditions, could prove valuable as paleo-temperature archives.

An Acetic Acid‐Based Extraction Protocol for the Recovery of U, Th and Pb from Calcium Carbonates for U‐(Th)‐Pb Geochronology

A new method for the simultaneous recovery of U, Th and Pb from ca. 0.5 g calcium carbonate samples for the purpose of U‐(Th)‐Pb geochronometry is presented. The protocol employs ion‐exchange chromatography. Standard anion exchange resin (AG 1‐X8 100–200 mesh) was used as the static phase, and 90% acetic acid was used as the mobile phase to elute the unwanted matrix components; dilute nitric acid was used to elute the U, Th and Pb. Blanks of 1.8 pg Th, 6.4 pg Pb and 8.4 pg U were obtained. The protocol was evaluated by determining the isotopic composition of U‐Th‐Pb separates obtained from an in‐house reference material (prepared from a natural speleothem) by MC‐ICP‐MS. An independently dated speleothem was also reanalysed. Based on these tests, the extraction protocol had an acceptable blank and produced a Pb separate sufficiently free of matrix‐induced instrumental biases to be appropriate for U‐Th‐Pb chronology.

13C 18O clumping in speleothems: Observations from natural caves and precipitation experiments

The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ 18O. Interpreting speleothem δ 18O records in terms of absolute paleotemperatures and δ 18O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ 47 (a geochemical variable reflecting the statistical overabundance of 13C 18O bonds in CO 2 evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ 47 value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation. Here we report Δ 47 values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ 47 and heavier δ 18O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ 47 disequilibrium and that of δ 18O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ 18O and Δ 47 values, probably inherited from prior degassing within the cave system. In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ 47 in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ 47 in inorganic carbonates.

The recovery and isotopic measurement of water from fluid inclusions in speleothems

The isotopic composition of speleothems is a useful palaeoclimatic indicator, but its value would be enhanced if information on the composition of the parent dripwaters could be recovered from fluid inclusions in the speleothem calcite. To develop a robust method for extracting and measuring oxygen and hydrogen isotopic composition of fluid inclusions we have used anhydrous Iceland Spar and microlitre glass capillaries of water as an analogue system. Crushing the capillary and calcite together in a high vacuum cell we have investigated the adsorbtive and isotopic behaviour of water when exposed to clean fracture surfaces. Significant water adsorption occurs at room temperature, accompanied by large negative isotopic shifts of both oxygen and hydrogen in the recovered free water at H 2O/CaCO 3 mass ratios <10 mg g −1. Cryogenic pumping fails to achieve total desorption. The degree of depletion is inversely related to the water/calcite ratio, fractionation of hydrogen isotopes exceeding −20‰, and oxygen isotopes −10‰, at ratios typically observed in natural speleothems. Heating the crushed calcite at 150°C for 60 min. totally desorbs the water and allows retrieval of the correct isotopic composition. Application of these methods to a British Late Holocene speleothem yields δ 18O and δ 2H compositions for the inclusion water which are closely comparable with the modern cave dripwaters and local precipitation. The results show that isotopic compositions can be recovered from inclusion samples of <1μL (equivalent to approximately 1g of calcite) with precisions that are useful for palaeoclimatic research, ±0.4‰ for δ 18O and ±3‰ for δ 2H. Greater precision than this will require replicate analysis for each speleothem growth increment.

Partitioning of Sr 2+ and Mg 2+ into calcite under karst-analogue experimental conditions

There is a paucity of experimental data on calcite precipitation from waters at low ionic strength and low ratios of Mg/Ca and Sr/Ca, using controlled and constant precipitation rates. Such data are particularly needed for studies of speleothem geochemistry in relation to palaeoclimates. We carried out a series of experiments using a karst-analogue set-up in a chamber of constant temperature and 100% humidity. A steady flow of NaHCO 3 and CaCl 2 solutions at PCO 2 around 10 −3.2 were mixed just before passage through a tube (analogous to a soda-straw stalactite) and allowed to drip onto a surface, analogous to a stalagmite. Growth rates were comparable with linear extension rates of natural speleothems. Analytical spots gave reproducible analyses in later analytical cycles after ablation of surface calcite with Na and Mg contamination. Different crystals from the same experiment tended to show positive covariation of Na and Mg with negative covariation with Sr. This may be due to the presence of growth hillocks with vicinal faces with differential partitioning behaviour. The result for the partition coefficient for Mg (D Mg) at 25°C is 0.031 ± 0.004, which is quantitatively in good agreement with the trends of previous workers. At 15°C, the result is 0.019 ± 0.003. The temperature dependency is higher than experimental data on seawater-analogue solutions, but lower than a previous estimate based on a comparison of speleothem chemistry with single water analyses. Data for D Sr are mainly in the range of 0.057 to 0.078, with a possible weak dependency on growth rate, consistent with previous experimental work. Absolute values are higher than studies in Mg-free saline solutions, which is attributed mainly to salinity effects. Values of D Sr are nevertheless somewhat lower than in natural caves, which may relate to crystal growth factors. Mg partition coefficient values should allow robust determination of solution Mg/Ca compositions in enclosed caves, which are at constant temperature on the decadal timescale. The inferred sensitivity of D Sr to growth rate factors implies that Sr values should be interpreted more cautiously. Muted changes could relate entirely to growth rate variations, whereas changes of large magnitude imply a control by solution composition. The absence of local (tens of micron scale) antipathetic variations in Sr and Mg in studied natural speleothems, implies that intracrystalline zoning phenomena, if present, are on a finer scale in those natural materials compared with experimental products.

Spatial distribution profiles of magnesium and strontium in speleothems using laser-induced breakdown spectrometry

Laser-induced breakdown spectrometry (LIBS) has been applied to spatially locate several atomic species in speleothems taken from the Nerja’s Cave (Malaga, Spain). Spatial distribution profiles of Mg at 285.21 nm and Sr at 407.77 nm were obtained while the laser was rastered through different paths along the sample. These elements were selected due to their importance as palaeoclimatic indicators. The 532 nm output of a Nd:YAG laser was used to irradiate the samples and generate the plasma that was spectrally analyzed and detected by using an intensified CCD detector. The signals were normalized to the Ca line to minimize pulse-to-pulse fluctuations in the laser source. Several studies were carried out to check for the point-to-point heterogeneity of the natural speleothem.