Speleothem Calcite Research Articles

Impact of precipitation intermittency on NAO-temperature signals in proxy records

Abstract. In mid and high latitudes, the stable isotope ratio in precipitation is driven by changes in temperature, which control atmospheric distillation. This relationship forms the basis for many continental paleoclimatic reconstructions using direct (e.g. ice cores) or indirect (e.g. tree ring cellulose, speleothem calcite) archives of past precipitation. However, the archiving process is inherently biased by intermittency of precipitation. Here, we use two sets of atmospheric reanalyses (NCEP (National Centers for Environmental Prediction) and ERA-interim) to quantify this precipitation intermittency bias, by comparing seasonal (winter and summer) temperatures estimated with and without precipitation weighting. We show that this bias reaches up to 10 °C and has large interannual variability. We then assess the impact of precipitation intermittency on the strength and stability of temporal correlations between seasonal temperatures and the North Atlantic Oscillation (NAO). Precipitation weighting reduces the correlation between winter NAO and temperature in some areas (e.g. Québec, South-East USA, East Greenland, East Siberia, Mediterranean sector) but does not alter the main patterns of correlation. The correlations between NAO, δ18O in precipitation, temperature and precipitation weighted temperature are investigated using outputs of an atmospheric general circulation model enabled with stable isotopes and nudged using reanalyses (LMDZiso (Laboratoire de Météorologie Dynamique Zoom)). In winter, LMDZiso shows similar correlation values between the NAO and both the precipitation weighted temperature and δ18O in precipitation, thus suggesting limited impacts of moisture origin. Correlations of comparable magnitude are obtained for the available observational evidence (GNIP (Global Network of Isotopes in Precipitation) and Greenland ice core data). Our findings support the use of archives of past δ18O for NAO reconstructions.

Global Speleothem Oxygen Isotope Measurements Since the Last Glacial Maximum

This synthesis of thirty-six sites (sixty cores with over 27 000 measurements) located around the world facilitates scientific research on the climate of the last 21 000 years ago obtained from oxygen isotope ( or delta-O-18) measurements. Oxygen isotopes in speleothem calcite record the influence of ambient temperature and the isotopic composition of the source water, the latter providing evidence of hydrologic variability and change. Compared to paleoclimate proxies from sedimentary archives, the age uncertainty is unusually small, around +/−100 years for the last 21 000-year interval. Using data contributed to the World Data Center (WDC) for Paleoclimatology, we have created consistently formatted data files for individual sites as well as composite dataset of annual to millennial resolution. These individual files also contain the chronology information about the sites. The data are useful in understanding hydrologic variability at local and regional scales, such as the Asian summer monsoon and the Intertropical Convergence Zone (as discussed in the underlying source publications), and should also be useful in understanding large-scale aspects of hydrologic change since the Last Glacial Maximum (LGM).

Stable isotopes in caves over altitudinal gradients: fractionation behaviour and inferences for speleothem sensitivity to climate change

Abstract. The interpretation of stable isotope ratios in speleothem calcite is complex, and only in a few cases, unequivocal relationships with palaeoclimate parameters have been attained. A major issue is temperature, which has an effect on both the isotope incorporation into calcite and on environmental processes. Here, a field approach is taken, by studying the isotopic composition of calcites from monitored caves located in steep altitudinal topography in the northern Italian Alps. These create a thermal gradient (3–12 °C) apt to study the effects of temperature on the speleothem isotope record. Our data indicate that the magnitude of oxygen isotope disequilibrium effects, calculated as an offset from the experimentally determined equilibrium, decreases with increased elevation (cooler temperatures) and faster drip rate. Carbon isotope values exhibit 13C enrichment at high altitudes (colder temperatures) and slow drip rates. The results obtained support modelling and laboratory cave analogue experiments that indicate temperature, drip rate, pCO2 and supersaturation are important factors controlling stable isotope fractionation, but also stress the significance of ventilation and evaporation in the cave environment. It is proposed that the effects on stable isotope ratios observed along the altitudinal gradient can be analogues for glacial to interglacial temperature changes in regions which were extensively glaciated in the past.

Simulated European stalagmite record and its relation to a quasi-decadal climate mode

Abstract. A synthetic stalagmite δ18O record for the Bunker Cave (51° N, 7° E) is constructed using a combined climate–stalagmite modelling approach where we combine an atmospheric circulation model equipped with water isotopes and a model simulating stalagmite calcite δ18O values. Mixing processes in the soil and karst above the cave represent a natural low-pass filter of the speleothem climate archive. Stalagmite δ18O values at Bunker Cave lag the regional surface climate by 3–4 yr. The power spectrum of the simulated speleothem calcite δ18O record has a pronounced peak at quasi-decadal time scale, which is associated with a large-scale climate variability pattern in the North Atlantic. Our modelling study suggests that stalagmite records from Bunker Cave are representative for large-scale teleconnections and can be used to obtain information about the North Atlantic and its decadal variability.

Luminescence of Speleothems: A Comparison of Sources and Environments

Calcite speleothems are typically coarsely crystalline, colored various shades from brown through orange and white, and strongly luminescent. For most speleothems, the color is due to higher molecular weight humic substances incorporated in the calcite crystal structure, while the luminescence is mainly due to lower molecular weight fulvic acids. The fine scale banding of luminescence intensity has importance as a climatic indicator. A suite of eighteen speleothems spanning a range of geologic and climatic settings and a range of colors from deep brown to nearly white were selected for detailed characterization and spectroscopic measurements. Spectra were measured on solid samples and on solutions prepared by dissolving the speleothems in dilute HCl. The luminescent emission appears as a single broad band with peak wavelength varying from 390 to 450 nm. The excitation spectra are typically more complicated, with several maxima, and show more locality-to-locality variation. The emission bands shift to longer wavelengths as the excitation bands move to longer wavelengths, indicating that a mixture of molecular species is being selectively excited. The spectra of the solutions are similar but not identical to the spectra of the crystalline solids. The decay time of the luminescence (phosphorescence) is in the range of 0.5 to 0.7 second. Comparison of speleothem spectra from caves in different climatic settings and of speleothem spectra from the same cave indicate that each speleothem produces spectra characteristic of specific overlying soils and pathways through the epikarst and the vadose zone. No features were discovered that characterize regional scale geologic or climatic settings.

Peculiar calcite speleothems filling fissures in calcareous sandstones and their palaeohydrological and palaeoclimatic significance: an example from the Polish Carpathians

Peculiar calcite speleothems developed in fissures in the Cergowa Sandstones were found in the Klęczany Quarry (Polish Western Carpathians). They represent flowstone and stalactites, rafts and various sparry crusts. Such speleothems, especially phreatic ones, are uncommon in the Outer Carpathians that are composed mainly of siliciclastic rocks of flysch type, with only limited calcium carbonate content. The speleothems analysed grew in vadose and phreatic conditions as well as at the air-water interface. Phreatic speleothems and thin rafts comprise calcite crystals of eccentric morphology. Based on their stable isotope composition the majority of the speleothems form two clusters. The first is characterized by d 18 O values between –9.8 and –8.5‰ and of d 13 C values between –5.7 and –0.6‰ whereas the second cluster of samples yields d 18 O values between –9.4 and –7.3‰ and d 13 C values from –11.5 to –9.7‰. Speleothems grew between 230 +14 –13     ka and Holocene time. Phreatic speleothems, including massive rafts, precipitated from ascending water of deep circulation whereas vadose and water table speleothems crystallized from local infiltration water charged with soil CO 2 . Mixing of both waters in the shallow phreatic zone is plausible.

Chalicogloea cavernicola gen. nov., sp. nov. (Chroococcales, Cyanobacteria ), from low-light aerophytic environments: combined molecular, phenotypic and ecological criteria

This work characterizes a unicellular cyanobacterium with nearly spherical cells and thin-outlined sheaths that divide irregularly, forming small packets immersed in a diffluent mucilaginous layer. It was isolated growing on calcite speleothems and walls in a show cave in Collbató (Barcelona, Spain). Spectral confocal laser and transmission electron microscopy were used to describe the morphology, fine structure and thylakoid arrangement. The pigments identified were phycoerythrin, phycocyanin, allophycocyanin and chlorophyll a. Three-dimensional reconstructions, generated from natural fluorescence z-stacks, revealed a large surface area of nearly flat, arm-like thylakoidal membranes connected to each other and forming a unified structure in a way that, to our knowledge, has never been described before. Phylogenetic analyses using the 16S rRNA gene sequence showed 95% similarity to strain Chroococcus sp. JJCM (GenBank accession no. AM710384). The diacritical phenotypic features do not correspond to any species currently described, and the genetic traits support the strain being classified as the first member of an independent genus in the order Chroococcales and the family Chroococcaceae. Hence, we propose the name Chalicogloea cavernicola gen. nov., sp. nov. under the provisions of the International Code of Nomenclature for Algae, Fungi and Plants. The type strain of Chalicogloea cavernicola is COLL 3(T) (=CCALA 975(T) =CCAP 1424/1(T)).

The magnesium isotope record of cave carbonate archives

Abstract. Here we explore the potential of magnesium (δ26Mg) isotope time-series data as continental climate proxies in speleothem calcite archives. For this purpose, a total of six Pleistocene and Holocene stalagmites from caves in Germany, Morocco and Peru and two flowstones from a cave in Austria were investigated. These caves represent the semi-arid to arid (Morocco), the warm-temperate (Germany), the equatorial-humid (Peru) and the cold-humid (Austria) climate zones. Changes in the calcite magnesium isotope signature with time are compared against carbon and oxygen isotope records from these speleothems. Similar to other proxies, the non-trivial interaction of a number of environmental, equilibrium and disequilibrium processes governs the δ26Mg fractionation in continental settings. These include the different sources of magnesium isotopes such as rainwater or snow as well as soil and host rock, soil zone biogenic activity, shifts in silicate versus carbonate weathering ratios and residence time of water in the soil and karst zone. Pleistocene stalagmites from Morocco show the lowest mean δ26Mg values (GDA: −4.26 ± 0.07‰ and HK3: −4.17 ± 0.15‰), and the data are well explained in terms of changes in aridity over time. The Pleistocene to Holocene stalagmites from Peru show the highest mean value of all stalagmites (NC-A and NC-B δ26Mg: −3.96 ± 0.04‰) but only minor variations in Mg-isotope composition, which is consistent with the rather stable equatorial climate at this site. Holocene stalagmites from Germany (AH-1 mean δ26Mg: −4.01 ± 0.07‰; BU 4 mean δ26Mg: −4.20 ± 0.10‰) suggest changes in outside air temperature was the principal driver rather than rainfall amount. The alpine Pleistocene flowstones from Austria (SPA 52: −3.00 ± 0.73‰; SPA 59: −3.70 ± 0.43‰) are affected by glacial versus interglacial climate change with outside air temperature affecting soil zone activity and weathering balance. Several δ26Mg values of the Austrian and two δ26Mg values of the German speleothems are shifted to higher values due to sampling in detrital layers (Mg-bearing clay minerals) of the speleothems. The data and their interpretation shown here highlight the potential but also the limitations of the magnesium isotope proxy applied in continental climate research. An obvious potential lies in its sensitivity for even subtle changes in soil-zone parameters, a hitherto rather poorly understood but extremely important component in cave archive research. Limitations are most obvious in the low resolution and high sample amount needed for analysis. Future research should focus on experimental and conceptual aspects including quantitative and well-calibrated leaching and precipitation experiments.

Simulated oxygen isotopes in cave drip water and speleothem calcite in European caves

Abstract. Interpreting stable oxygen isotope (δ18O) records from stalagmites is still one of the complex tasks in speleothem research. Here, we present a novel model-based approach, where we force a model describing the processes and modifications of δ18O from rain water to speleothem calcite (Oxygen isotope Drip water and Stalagmite Model – ODSM) with the results of a state-of-the-art atmospheric general circulation model enhanced by explicit isotope diagnostics (ECHAM5-wiso). The approach is neither climate nor cave-specific and allows an integrated assessment of the influence of different varying climate variables, e.g. temperature and precipitation amount, on the isotopic composition of drip water and speleothem calcite. First, we apply and evaluate this new approach under present-day climate conditions using observational data from seven caves from different geographical regions in Europe. Each of these caves provides measured δ18O values of drip water and speleothem calcite to which we compare our simulated isotope values. For six of the seven caves modeled δ18O values of drip water and speleothem calcite are in good agreement with observed values. The mismatch of the remaining caves might be caused by the complexity of the cave system, beyond the parameterizations included in our cave model. We then examine the response of the cave system to mid-Holocene (6000 yr before present, 6 ka) climate conditions by forcing the ODSM with ECHAM5-wiso results from 6 ka simulations. For a set of twelve European caves, we compare the modeled mid-Holocene-to-modern difference in speleothem calcite δ18O to available measurements. We show that the general European changes are simulated well. However, local discrepancies are found, and might be explained either by a too low model resolution, complex local soil-atmosphere interactions affecting evapotranspiration or by cave specific factors such as non-equilibrium fractionation processes. The mid-Holocene experiment pronounces the potential of the presented approach to analyse δ18O variations on a spatially large (regional to global) scale. Modelled as well as measured European δ18O values of stalagmite samples suggest the presence of a strong, positive mode of the North Atlantic Oscillation at 6 ka before present, which is supported by the respective modelled climate parameters.

Climatic backdrop to the terminal Pleistocene extinction of North American mammals

Research Article| November 01, 2012 Climatic backdrop to the terminal Pleistocene extinction of North American mammals Victor J. Polyak; Victor J. Polyak 1Earth & Planetary Sciences, University of New Mexico, 200 Yale Boulevard, Northrop Hall, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar Yemane Asmerom; Yemane Asmerom 1Earth & Planetary Sciences, University of New Mexico, 200 Yale Boulevard, Northrop Hall, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar Stephen J. Burns; Stephen J. Burns 2Department of Geosciences, 233 Morill Science Center, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA Search for other works by this author on: GSW Google Scholar Matthew S. Lachniet Matthew S. Lachniet 3Department of Geosciences, University of Nevada, Las Vegas, Nevada 89154, USA Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (11): 1023–1026. https://doi.org/10.1130/G33226.1 Article history received: 23 Jan 2012 rev-recd: 05 May 2012 accepted: 08 May 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Victor J. Polyak, Yemane Asmerom, Stephen J. Burns, Matthew S. Lachniet; Climatic backdrop to the terminal Pleistocene extinction of North American mammals. Geology 2012;; 40 (11): 1023–1026. doi: https://doi.org/10.1130/G33226.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract North American terminal Pleistocene mammal extinctions are the subject of a long-running scientific debate. Although the role of climate has figured centrally, we lack clear knowledge of the timing and nature of terminal Pleistocene climate variability. Herein we document lengthy terminal Pleistocene drought in the southwestern United States (USA) using δ13C and δ234U effective moisture proxy data in speleothem calcite (stalagmite FS2) from Fort Stanton Cave, New Mexico, supplemented with age data from pool basin shelfstone speleothems from the Big Room in Carlsbad Cavern. This terminal Pleistocene drought, defined by a sharp rise in both δ13C and δ234U values, began just before 14.5 k.y. ago and lasted at least until 12.9 k.y. ago, when it was briefly and only mildly interrupted by the Younger Dryas. The timing and length of this drought (∼1500 yr) match the Northern Hemisphere Bølling-Allerød oscillation preserved in Greenland ice cores and exhibited in the δ18O record of stalagmite FS2. Rapid transition from cool moist Late Glacial to warm dry Holocene-like climatic conditions was likely unfavorable to many species of Pleistocene mammals in the southwestern USA. A climate-induced extinction implies that this last glacial cycle and its termination were more extreme than previous glacial cycles and/or glacial terminations. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The sources and sinks of CO2 in caves under mixed woodland and grassland vegetation

We measured concentrations and stable carbon isotope compositions of carbon dioxide in the atmospheres of three caves in central Texas and one cave in southern Arizona in order to identify CO2 sources and sinks. The vegetation above the caves studied is either savannah (two caves, above which vegetation has been minimally disturbed) or discrete patches of grassland and woodland (two caves, above which vegetation has been highly disturbed). We tested two hypotheses concerning CO2 in the cave atmospheres: (1) cave ventilation by tropospheric air is the primary sink for CO2 and (2) CO2 is primarily derived from the deepest rooting plants growing above the caves. Within caves, we monitored CO2 at individual locations on monthly and daily time-scales and measured CO2 along transects with increasing distance from the cave entrances. We also measured CO2 in the pore spaces of soils under grasses and trees above each of the caves. We calculated δ13C values of respired CO2 (δ13Cr) for all gas samples using measured δ13C values and CO2 concentrations. We then identified the sources of cave CO2 by comparing cave-air and soil CO2δ13Cr values. At all locations in each Texas cave, CO2 concentrations were highest (lowest) and δ13C values were lowest (highest) during the summer (winter). Cave-air CO2 concentrations consistently increased and δ13C values consistently decreased with distance from the cave entrances. Similar but smaller magnitude seasonal variations in CO2 concentrations occurred in the Arizona cave and no seasonal or spatial variation in the δ13C of cave-air CO2 was observed. The mean δ13Cr values of CO2 in soils under grass were 3.5–4.5‰ higher than the δ13Cr values of CO2 in soils under trees. In the caves under savannah, mean δ13Cr values of cave-air CO2 (−24‰ in both caves) were within 1‰ of the mean δ13Cr values of CO2 in soils under trees. In caves covered by large, contiguous areas of grassland, the δ13Cr values of cave-air CO2 were similar to grassland soil values during the summer and were intermediate between grassland and woodland soil values during the winter. The observed spatial and temporal variations in cave-air CO2 are consistent with density-driven ventilation controlled by seasonal surface temperature changes as the primary sink for CO2 in the Texas caves. The consistent agreement between soil and cave δ13Cr values indicate that the same mixing and diffusion equations that are used to calculate δ13Cr values of soil CO2 also apply to cave-air CO2. Our results suggest that the majority of CO2 advects or diffuses into these caves from soils as a gas rather than being transported in aqueous solution. Measured δ13Cr values and numerical production-diffusion modeling supports our hypothesis that the majority of gaseous CO2 in these caves is derived from deeply rooted vegetation. The carbon isotope composition of groundwater and speleothem calcite used for paleoclimate records are therefore likely biased toward deeply rooted plants, even if sparsely present.

Quantifying kinetic fractionation in Bunker Cave speleothems using Δ47

Isotopic signals in speleothems are used for investigating paleoclimate variability on land and are useful to constrain the dating of prominent climate events. A quantitative use, however, is limited by an incomplete understanding of parameters contributing to the carbon and oxygen isotope signals. These include external and environmental parameters such as δ18O of cave drip waters as well as internal parameters associated with speleothem formation, such as the presence of non-equilibrium effects and especially the magnitude of their isotopic shifts.We explore the use of clumped isotopes as a new tool for investigating the kinetic isotope effect in speleothems. Holocene and modern speleothems from Bunker Cave (Germany) as well as modern material from the adjacent Dechen Cave are all offset from the equilibrium relationship due to kinetic fractionation. This kinetic offset in clumped isotopes is observed in a stalagmite despite mostly negative Hendy tests, providing a sensitive indicator for kinetic fractionation in cave carbonates. The temperature dependence of the clumped isotope values (0.005‰ per °C) is low compared to the observed magnitude of kinetic offsets (between −0.021 and −0.075‰), so that the mean offsets in apparent temperatures due to kinetic isotope effects are on the order of 10 °C. As a result clumped isotopes are useful in identifying temporal variations in the kinetic fractionation in a stalagmite, when the temperatures during the speleothem growth period are either relatively constant (variations <2 °C) or can be independently constrained.The variations in the kinetic isotope fractionation in Bunker Cave are associated with changing drip water super saturation with periods of stronger prior calcite precipitation associated with lower kinetic offsets in the speleothem calcite. In contrast, stalagmite growth rates show no direct correlation with the degree of kinetic fractionation in the investigated range (13–1500 μm/a).

Response of a modern cave system to large seasonal precipitation variability

Speleothems are capable of providing information on the response of middle and low-latitude terrestrial environments to global climate change during the Pleistocene and Holocene. Multiproxy speleothem studies, however, have demonstrated that complex interactions can occur in cave settings between processes that are directly and indirectly related to climate change. Thorough and extended monitoring of modern cave environments is necessary in order to fully understand how each cave responds to these processes on seasonal and interannual timescales, and how environmental signals are preserved in speleothem carbonate.Regular environmental monitoring began at Black Chasm Cavern in the Sierra Nevada foothills, California, during the winter of 2006–2007. Monthly measurements of cave air temperature, humidity, and pCO2 in Black Chasm demonstrate that the cave is ventilated in the winter months, when cold, dense surface air sinks into the cave. Cave drip-water flow nearly ceases during the late summer and autumn, increases substantially during the winter and spring, and responds within hours to storm events during the height of the rainy season. Rainwater and drip water δ18O and δ2H are controlled by variations in surface air temperature and moisture source. While rainfall source influences rainwater isotopes through individual storm events, it has less influence on drip water isotopic composition due to mixing of recharge waters delivered by different rainfall events in the epikarst. Variations in drip water chemistry (δ13C, Mg/Ca, and Sr/Ca) indicate that the greatest level of calcite precipitation upflow from the drip water collection site (prior calcite precipitation) occurs during the autumn (October–November) when drip rates are slow and cave air pCO2 is low. The least prior calcite precipitation occurs during the summer (July–August) when drip rates are slow but cave air pCO2 is at a maximum. While pCO2 is a primary control on prior calcite precipitation during all seasons, the predominant influence of drip rate variability on prior calcite precipitation is evident when considering only those seasons (winter, spring, and autumn) characterized by low cave air pCO2. Thus, drip rate variability, and in turn rainfall amount, should provide the primary control on trace element variations ultimately captured in speleothem calcite. The isotopic and chemical variability observed in Black Chasm drip waters supports previous interpretations of speleothem paleoclimate proxy records from a nearby cave where such monitoring is not feasible. Observations of the modern cave environment at Black Chasm provide a reference point for the interpretation of stalagmite proxy records from similar seasonal (Mediterranean) climates.

Factors Influencing Observed Variations in the Structure of Bacterial Communities On Calcite Formations in Kartchner Caverns, AZ, USA

Kartchner Caverns is an oligotrophic subterranean environment that hosts a wide diversity of actively growing calcite speleothems (secondary mineral deposits). In a previous study, we demonstrated that bacterial communities extracted from these surfaces are quite complex and vary between formations. In the current study, we evaluated the influence of several environmental variables on the superficial bacterial community structure of 10 active formations located in close proximity to one another in a small room of Kartchner Caverns State Park, Arizona, USA. Physical (color, dimensions) and chemical (elemental profile and organic carbon concentration) properties, as well as the DGGE-based bacterial community structure of the formations were analyzed. While elemental concentration was found to vary among the formations, the differences in the community structure could not be correlated with concentrations of either organic carbon or any of the elements evaluated. In contrast, the locations of formations within a distinct region of the cave as well as the relative location of specific formations within a single room were found to have a significant influence on the bacterial community structure of the formations evaluated. Interestingly, Canonical Correspondence Analysis suggests an association between the observed drip pathways (drip lines) feeding the formations (as determined by the patterns of soda straws and small stalactites that reveal water flow patterns) and the bacterial community structure of the respective formations. The results presented here indicate that a broad range of formations fed by a diversity of drip sources must be sampled to fully characterize the community composition of bacteria present on the surfaces of calcite formations in carbonate caves.

U and Pb variability in older speleothems and strategies for their chronology

Speleothem proxy records, coupled with U–Th chronologies, are widely used in studies of palaeoclimate, landscape and faunal evolution. The prospect of extending such studies beyond ∼500 ka has fuelled considerable interest in the U–Pb dating method. While theoretical mechanisms for the transport and incorporation of both U and Pb in speleothem calcite remains a matter of debate, it is clear that natural samples span a broad spectrum of compositional variation, and likely contain Pb components with multiple origins and/or transport pathways. Based upon these observations, optimal methods for sample selection are proposed and a variety of techniques for age estimation are explored. Laser ablation ICPMS and autoradiography are ideal rapid screening methods for reconnaissance studies. Common Pb isotopic variability is a key parameter in the choice of analytical strategy for age determination. In large U–Pb studies where invariant common Pb can be established, common Pb corrected ages can be produced for individual sample aliquots promoting rapid throughput and permitting the establishment of detailed age-depth models. Even with these technologies, however, age resolution becomes an issue beyond a few million years, at which time alternative strategies for developing internal speleothem chronologies, such as lamina counting, must be considered in unison with U–Pb dating.

Oxygen isotopic fractionation between drip water and speleothem calcite: A 10-year monitoring study, central Texas, USA

Speleothem δ18O values can serve as a paleoclimate proxy, yielding information about past temperature and rainfall. An accurate determination of the equilibrium calcite–water oxygen isotopic fractionation factor and an assessment of kinetic isotopic fractionation are required for interpretation of speleothem δ18O data. In this study, δ18O values were measured for calcite deposited on artificial substrates at four sites in two central Texas caves monitored for over 10years. The results are used to evaluate the equilibrium isotopic fractionation factor and the impact of climatic and hydrologic conditions on kinetic isotopic fractionation.The δ18O values of calcite from the four sites ranged from 24.8‰ to 26.7‰ (V-SMOW), and associated drip water values ranged from −4.6‰ to −3.9‰. A comparison of predicted equilibrium calcite δ18O values, calculated using a commonly-used isotopic fractionation factor, with measured values indicates that 94% of the calcite samples are not in oxygen isotopic equilibrium with respect to their associated drip water. The departure from oxygen isotopic equilibrium (expressed as Δ18Occ-e) ranges from −0.7‰ to 1.4‰. Three of the four drip sites yield similar linear relationships between Δ18Occ-e and water temperature (tw), and can be collectively expressed as: Δ18Occ-e=0.3tw−4.7; r2=0.56 (n=93). Therefore, calcite deposited during time periods of lower tw, lower cave-air CO2 concentration and faster calcite deposition rates, have δ18O values closer to equilibrium. This contradicts conceptual models, which predict that a faster calcite deposition rate leads to a larger departure of calcite δ18O from equilibrium.If slower calcite deposition indeed facilitates equilibrium fractionation (i.e., Δ18Occ-e→0), then the results of this study support a larger than commonly accepted value for the equilibrium calcite–water oxygen isotopic fractionation factor. Adopting a larger published value for the fractionation factor yields negative Δ18Occ-e values of up to −2.4‰. These negative values cannot be explained by existing kinetic fractionation models. Alternatively, they may reflect the trapping of a calcite “surface layer” with a lower δ18O value than that of calcite in isotopic equilibrium with ambient water. The relationship between deposition rates and Δ18Occ-e for this study is consistent with the same relationship using data for synthetic calcite from the literature. This relationship indicates a −0.8‰ shift of calcite δ18O for every ten-fold increase in deposition rate at 5 to 25°C and a pH of 8.3. The significant kinetic fractionation observed in this study warrant consideration in applying measured speleothem calcite δ18O values to interpret past climate conditions.

Hydrogeochemistry and fractionation pathways of Mg isotopes in a continental weathering system: Lessons from field experiments

The potential of magnesium isotope records from cave carbonate archives (speleothems) has been documented but remains underexplored. This is due to the limited knowledge regarding the complex suite of physico-chemical and biological disequilibrium fractionation processes affecting meteoric fluids in the soil zone, the carbonate hostrock and calcite precipitation in the cave. This study presents δ26Mg data from a monitored cave in Germany (Bunker Cave) including rain water (δ26Mg: −0.70±0.14‰), soil water (δ26Mg: −0.51±0.10‰) and drip waters (δ26Mg: −1.65±0.08‰) sampled between November 2009 and May 2011. Field precipitation experiments, i.e., calcite precipitated on watch glasses (δ26Mg: −3.56±0.26‰; May 2006 to June 2010), were found to be of limited use. This is because of experimental, crystallographic and sampling artefacts. Conversely, variations in soil and drip water δ26Mg over time are predominantly related to seasonal variations in water availability and air temperature affecting the subtle weathering ratio between Mg-bearing clay minerals in the soil, here mainly chlorite and montmorillonite, and the low-Mg calcite hostrock. Bunker Cave δ26Mgdrip water values display a significant dependency on the air temperature outside the cave. This is because air temperature influences CO2 levels in the soil and hence rock-water interaction. For fast drip sites, the direct correlation of δ26Mgsoil water and δ26Mgdrip water documents a relative short residence time of the fluid in the carbonate aquifer and thus limited isotope equilibration and mixing of different reservoirs. This result is encouraging and adds new evidence to the poorly understood hydro-geochemistry of carbonate aquifers. Slow (seepage flow) drip sites display an annual δ26Mgdrip water pattern that is geochemically unrelated to that of the soil water. Further research, including laboratory experiments, must focus on the complex fractionation between drip water and speleothem calcite Mg isotope record.

Les spéléothèmes de la grotte Chauvet-Pont-d'Arc : apports chronologiques et paléoclimatiques. Synthèse des travaux publiés

The study of the Chauvet Cave (Ardèche, South-France) speleothems, initiated in 1999, brought results about the chronology of the history of the cave and allows paleoclimatic reconstructions of the last climatic cycle. We present here a synthesis of already published results that were obtained with the help of numerous collaborators. Fiveteen speleothems were analysed : 11 stalagmites, 3 flowstones and one stalactite. Thanks to 90 uranium-thorium ages, three main phases of speleothem growing have been defined : (i) 124.2-99.8 ka coinciding with M1S5 ; (ii) 32.8-22.3 ka that covers MIS3 and 2 and (iii) 15.2-0 ka covering the Lateglacial and Holocene periods and in which most speleothems developed. A growth hiatus that coincides with the climatic deterioration of the last glacial maximum was precisely dated on a single stalagmite between 22.1 ka ± 0.4 and 15.3 ka ±0.25. These U-Th datings also permitted to constrain the age of a collapse that happened far inside the cave (Salle Hillaire) after 7.7 ka ± 1. 6. Ln the same area, the first calcite layers that grew on the archaeological soil were dated at ~ 27.8 ka±1.9 confirming the old 14C ages made on prehistoric charcoals found on this soil (33. 7 to 35.3 cal. ka). Finally, the oldest ages found on the first calcite layers of several stalagmites that grew on the top part of the entrance collapse demonstrate that the Chauvet Cave entrance was closed before 13.1 ka ±0.1, which is in agreement with other dating methods that dated this collapse at about 22 ka. Parallel to the U-Th dating, more than 600 isotopic analyses, performed on the speleothem calcite (δ18O δ13C), permitted to reconstruct detailed paleoclimatic changes since 15.3 ka. These results highlight similarities but also differences with Greenland ice core records and gave precise ages to climatic events that occurred during the Bølling-Allerød and Younger-Dryas periods.

A high-resolution record of atmospheric 14C based on Hulu Cave speleothem H82

The development of a calibration of atmospheric radiocarbon (∆14C) is a significant scientific goal because it provides the means to link the numerous 14C dated paleoclimate records to a common timescale with absolutely dated records, and thereby improve our understanding the relationships between the carbon cycle and climate change. Currently, few calibration datasets that directly sample the atmospheric 14C reservoir are available beyond the end of the dendro-dated Holocene tree ring record at 12.6kyr BP (Before 1950 AD). In the absence of suitable true atmospheric records, 14C calibrations beyond this age limit are based largely on marine data, that are complicated by the marine reservoir effect, which may have varied over the glacial cycle. In this paper, we present a high-resolution record of U–Th series and 14C measurements from Hulu Cave speleothem H82, spanning 10.6–26.8kyr BP. Corrections for detrital 230Th are negligible, and the contribution of 14C-free geologic carbon to the speleothem calcite is small (5–6%) and is stable across major climate shifts. The time series provides a 16kyr record of atmospheric ∆14C as well as an updated age model for the existing Hulu Cave δ18O record. The 14C data are in good overall agreement with existing marine and terrestrial 14C records, but comparisons with the Cariaco Basin marine ∆14C record through the deglacial interval reveal that the Cariaco reservoir age appears to have varied during parts of the Younger Dryas and Heinrich Stadial 1 cold events. This highlights the importance of developing extended high-resolution marine and terrestrial 14C records as a means of detecting changes in ocean circulation over the glacial cycle.

Speleothem magnetism

The magnetic recordings preserved in calcite speleothems and the nature of their constituent magnetic minerals hold enormous potential for paleomagnetic and paleoenvironmental reconstructions. Speleothems lock magnetization in instantly, are not affected by post-depositional effects, and can be dated with high precision. The natural remanence in speleothems is carried mainly by magnetite, and the main remanence acquisition mechanism is depositional, through physical alignment of detrital magnetic grains parallel to the Earth’s magnetic field. Detrital particles are deposited on speleothem surfaces either from receding flood waters, or directly from drip water percolating through overlying rock and soil. Chemical remanence, acquired in situ by particles growing through a critical volume, is of secondary importance. Previous studies of speleothem magnetism have shown that geomagnetic features are successfully recorded by speleothems, and can be reproduced at a local and regional scale. Future studies that benefit from increasingly sensitive magnetometers, operating at high spatial resolution, should be able to resolve short-term geomagnetic variability, and characterize events such as geomagnetic excursions at an unprecedented scale. The environmental magnetism of speleothems is still an untapped reservoir, but holds the promise of bridging the gap between mineral-magnetic records from other sedimentary archives and established speleothem environmental proxies (e.g., oxygen and carbon isotopes).