Speleothem Fluid Inclusions Research Articles

Holocene temperatures follow CO2 in southeastern Alaska

Understanding the temperature evolution during the Holocene is crucial for future climate projections. However, a puzzling discrepancy between climate models simulating a warming trend during the second half of the Holocene parallel to the rise in atmospheric CO2 concentrations and proxy data suggesting cooling during this period – known as the Holocene temperature conundrum – limits our understanding of how climate will evolve in the future. To solve this conundrum, the inclusion of quantitative paleotemperature records from high-latitude sites, which are extremely rare, is essential. Here, we use the hydrogen isotopic composition of fluid inclusions in speleothems from southeastern Alaska to create a quantitative temperature reconstruction of the entire Holocene. We show that the temperature in southeastern Alaska followed CO2, in agreement with modeling results. Because our data show no proxy-model bias, they support a seasonal proxy bias to account for the Holocene temperature conundrum.

Characterization and Correction of Evaporative Artifacts in Speleothem Fluid Inclusion Isotope Analyses as Applied to a Stalagmite From Borneo

AbstractFluid inclusion water isotope measurements in speleothems have great potential for paleoclimate studies as they enable the reconstruction of precipitation dynamics and land temperatures. Several previous observations, however, suggest that inclusion waters do not always reflect the isotopic composition of surface precipitation. In such cases, dripwaters are thought to be modified by evaporation in the cave environment that results in more positive δ2H and δ18O values and shallow δ2H/δ18O slopes. Although evaporation can occur in cave systems, water can also be lost to evaporation during analysis but before water extraction. Here, we examine the likelihood of this possibility with a stalagmite from Borneo. We demonstrate that many samples lose water, and that water loss is controlled by the type and size of inclusions. With multiple replicate measurements of coeval samples, we calculate an evaporative δ2H/δ18O slope of 1.0 ± 0.6 (2SE). This value is consistent with model predictions of evaporative fractionation at high analytical temperature and low humidity. Finally, we propose a method to correct for this effect. We find that fluid–calcite δ18O paleotemperatures calculated with corrected δ18O data show excellent agreement with recent microthermometry temperature estimates for Borneo, supporting the validity of our approach and implying limited stalagmite δ18O disequilibrium variations. Corrected fluid inclusion δ18O and δ2H values follow the expected hydroclimate response of Borneo to periods of reduced Atlantic Ocean meridional overturning circulation. Our results suggest that careful petrographic examination and multiple replicate measurements are necessary for reliable paleoclimate reconstructions with speleothem fluid inclusion water isotopes.

Constraints for precise and accurate fluid inclusion stable isotope analysis using water-vapour saturated CRDS techniques

Hydrogen (δ2H) and oxygen (δ18O) isotopes of water extracted from speleothem fluid inclusions are important proxies used for paleoclimate reconstruction. In our study we use a cavity ring-down laser spectroscopy system for analysis and modified the approach of Affolter et al. (2014) for sample extraction. The method is based on crushing of small sub-gram speleothem samples in a heated and continuously water-vapour purged extraction line. The following points were identified:Injection of reference water shows a precision (1σ) of 0.4–0.5 ‰ for δ18O values and 1.1–1.9 ‰ for δ2H values for water amounts of 0.1–0.5 μl, which improves with increasing water amount to 0.1–0.3 ‰ and 0.2–0.7 ‰, respectively, above 1 μl. The accuracy of measurements of water injections and water-filled glass capillaries crushed in the system is better than 0.08 ‰ for δ18O and 0.3 ‰ for δ2H values. The reproducibility (1σ) based on replicate analysis of speleothem fluid inclusion samples with water amounts >0.2 μl is 0.5 ‰ for δ18O and 1.2 ‰ for δ2H values, respectively. Isotopic differences between the water vapour background of the extraction system and the fluid inclusions have no significant impact on the measured fluid inclusion isotope values if they are within 10 ‰ for δ18O and 50 ‰ for δ2H values of the background. Tests of potential adsorption effects with inclusion free spar calcite confirm that the isotope values are unaffected by adsorption for water contents of about 1 μl (fluid inclusion) water per g of carbonate or above.Fluid inclusion analysis on three different modern to late Holocene speleothems from caves in northwest Germany resulted in δ18O and δ2H values that follow the relationship as defined by the meteoric water line and that correspond to the local drip water. Yet, due to potential isotope exchange reactions for oxygen atoms, hydrogen isotope measurements are preferentially to be used for temperature reconstructions. We demonstrate this in a case study with a Romanian stalagmite, for which we reconstruct the 20th century warming with an amplitude of approximately 1 °C, with a precision for each data point of better than ±0.5 °C.

Variable Response in Alpine Tree-Ring Stable Isotopes Following Volcanic Eruptions in the Tropics and Iceland

The importance of the stable isotopes in tree rings for the study of the climate variations caused by volcanic eruptions is still unclear. We studied δ18O, δD, δ13C stable isotopes of larch and cembran pine cellulose around four major eruptions with annual resolution, along with a superposed epoch analysis of 34 eruptions with 5-year resolution. Initial analysis of the tropical Tambora (1815 CE) and Samalas (1257 CE) eruptions showed a post-eruption decrease in δ18O values attributed to post-volcanic cooling and increased summer precipitation in Southern Europe, as documented by observations and climate simulations. The post-volcanic cooling was captured by the δD of speleothem fluid inclusion. The δ18O decrease was also observed in the analysis of 34 major tropical eruptions over the last 2000 years. In contrast, the eruptions of c. 750, 756, and 764 CE attributed to Icelandic volcanoes left no significant responses in the cellulose isotopes. Further analysis of all major Icelandic eruptions in the last 2000 years showed no consistent isotopic fingerprints, with the exception of lower post-volcanic δ13C values in larch. In summary, the δ18O values of cellulose can provide relevant information on climatic and hydroclimatic variations following major tropical volcanic eruptions, even when using the 5-year resolution wood samples of the Alpine Tree-Ring Isotope Record database.

Last glacial millennial-scale hydro-climate and temperature changes in Puerto Rico constrained by speleothem fluid inclusion <i>δ</i><sup>18</sup>O and <i>δ</i><sup>2</sup>H values

Abstract. We present speleothem fluid inclusion δ18Of and δ2Hf values from Larga Cave, Puerto Rico, that cover the interval between 46.2 and 15.3 ka on the millennial scale, including the Last Glacial Maximum (LGM) and several stadial and interstadial cycles. The data set can be divided in two main clusters of stable isotope compositions of the fluid inclusion water with respect to the global meteoric water line (GMWL), which coincide with strong variations in the water content of the stalagmite. In particular, this clustering is found to be climate related, where one cluster comprises samples from cold and dry periods, such as the Heinrich and Greenland stadials (HSs and GSs), as well as parts of the LGM, which exhibit very high δ18Of and δ2Hf values. We interpret this enrichment as being caused by evaporation inside the cave due to enhanced cave ventilation during these colder and drier times. In contrast, in most samples corresponding to warmer and wetter Greenland interstadials (GIs), but also for some from HS 2 and 3, the δ18Of and δ2Hf values plot on the meteoric water line and modification of fluid inclusion water due to “in-cave” evaporation are found to be negligible. Consequently, variations of recent glacial hydro-climate and temperatures in the western tropical Atlantic can be constrained. In general, δ18Of values from fluid inclusions are up to 3 ‰ higher than those of modern drip water, which is interpreted as a weaker atmospheric convective activity during the last glacial period. In addition, reconstructed temperatures suggest an average cooling of 2–3 ∘C during the LGM compared to modern cave temperatures. Reconstructed cave temperatures yield an average cooling of −1.4 ± 2.8 ∘C for HS 2 and −3.6 ± 2.2 ∘C for HS 3. Higher δ18Of values of these samples further suggest that the drip water was dominated by orographic rainfall and/or cold fronts, along with weak or even absent convective activity. In contrast, during interstadial phases, reconstructed temperatures reached nearly modern values, and convective activity was comparable to or only slightly weaker than today.

Fluid-inclusion petrography in calcite stalagmites: Implications for entrapment processes

ABSTRACT Fluids trapped in speleothems have an enormous potential in frontier fields of paleoclimate and paleohydrological research. This potential is, however, hampered by diverse scientific and technical limitations, among which the lack of a systematic methodology for genetically characterizing fluid inclusions is a major one, as these can have different origins, and thus, the trapped fluid (usually water), different meanings. In this work, we propose a systematic petrological classification of fluid inclusions, based on: 1) the temporal relation between fluid inclusions and the host calcite, 2) the spatial relation between fluid inclusions and the “crystallites” and crystals aggregates, and 3) the phases (water, air) trapped inside fluid inclusions. The first criterion allows dividing fluid inclusions in two main categories: primary and secondary, whose identification is critical in any research based on trapped fluids. The other two criteria allow the definition of eight types of primary and four types of secondary fluid inclusions. Primary fluid inclusions contain the drip water that fed stalagmites at the time of crystal growth, and can be intercrystalline, i.e., located between adjacent crystallites, or intracrystalline, i.e., with the fluid trapped within crystallites. We differentiate six main types among the intercrystalline fluid inclusions (elongate, thorn-shaped, down-arrow, interbranch, macro-elongate, and bucket) and other two among intracrystalline inclusions (pyriform and boudin). In primary inclusions, water is the main phase, while gas is much less abundant. The presence of gas could be related to slow drip rates or degassing in the cave, but also to later leakage due to changes in temperature and humidity often occurring during inadequate handling of speleothem samples. Secondary fluid inclusions were clearly related to younger water inlet through stratigraphic disruptions or unconformities. They are formed after water infiltration, but sealed before the renewed crystal growth. We differentiate four main types of secondary inclusions: interconnected, rounded, triangular, and vertical fluid inclusions. The identification of primary and secondary fluid inclusions in speleothems is a key for interpretation in paleoclimate studies. Integration of petrological results allow establishment of three different genetic scenarios for the formation of fluid inclusions, whose identification can be relevant because of their predictive character.

Challenges in the Direct Determination of 17Oexcess in Microliter Amount of Water Extracted From Speleothem Fluid Inclusions

Water isotopes are powerful proxies able to deliver valuable information about past climate conditions at the precipitation site, along the path of the moisture or at its source. We use a recently developed microliter sampling technique associated with continuous-flow laser spectroscopy in order to investigate its potential for direct determination of17Oexcesson microliter amount of water released from speleothem crushing. Results from Milandre Cave stalagmite (Switzerland) suggest that17Oexcessfrom speleothem fluid inclusion water could likely be used as a paleoclimate proxy and to resolve past hydroclimatic changes mainly depending on (i) the technical capability to produce precise and accurate direct measurements and (ii) our understanding of which factors influence the17Oexcessproxy in speleothem fluid inclusion water. With our setup, the main challenge regarding17Oexcessmeasurements is the very low amount of water released by the crushing of speleothem samples, which also lead to the difficulty of producing sample replications. The precision achieved based on duplicate measurements is ∼30 per meg, which is currently a value too high to retrieve robust paleoclimate information. We suggest sample replications and further improvement of the presented method through the increase of the signal to noise ratio and correction for isotope composition—mixing ratio dependence to reach the required precision of 10 per meg or less.

Measurement of oxygen and hydrogen isotopic ratios of speleothem fluid inclusion water using Picarro

A speleothem fluid inclusion extraction and isotope analysis system was established based on a vacuum extraction device and a Picarro L2140-i wavelength scanned cavity ring-down spectroscopy (WS-CRDS) water isotope analyzer. WS-CRDS allows to measure the stable oxygen and hydrogen isotope composition ( δ 18O and δ D) of small water samples simultaneously, which is faster, simpler, and requires less maintenance compared to currently used continuous-flow isotope ratio mass spectrometry (CF-IRMS). The precision of water isotopic analysis utilizing the WS-CRDS is often superior to that of CF-IRMS. Therefore, the use of Picarro for isotope analysis has rapidly gained popularity. The oxygen isotopic composition of speleothem calcite ( δ 18Oc) is one of the main proxies of speleothem-based paleoclimate reconstructions, but the interpretation of δ 18Oc remains complex as it can be influenced by many factors. Speleothem fluid inclusions contain the characteristics of cave drip water. The δ 18O and δ D values of fluid inclusion water provide direct information about paleoprecipitation, and can be further used to reconstruct paleotemperature when combined with δ 18Oc. The present paper contains a detailed description of the system as we have designed it, and provides an introduction to the system’s performance. The vacuum extraction device, connected to the Picarro analyzer, is designed to release the water trapped in speleothem fluid inclusions by pneumatically-operated crushing of discrete speleothem calcite chips. The whole device is kept at a temperature of 120°C to ensure the total and instantaneous vaporization of the water without isotopic fractionation. The vaporized water is carried by a flow of high-purity nitrogen to a 40 mL volume for homogenization, and is then introduced into the Picarro analyzer for δ 18O and δ D analysis. A syringe injection unit was added to the vacuum extraction device to be able to measure standard water, with δ 18O and δ D values similar to the speleothem samples, before and during measurements. To verify the precision and accuracy of the system, the same standard water samples were measured with an automated Picarro L2140-i set-up that includes an autosampler and a Picarro evaporation unit. The δ 18O and δ D values of standard waters as measured by both systems are comparable. Using the crushing device, speleothem samples from the East Asian Monsoon region yield paired δ 18O/ δ D values within ±0.5‰ of the Global Meteoric Water Line (GMWL), indicating no isotopic fractionation during the measurements. The precision is a function of the water amount released and of the isotopic composition of the sample. The reproducibility of crushed speleothem samples, whose water vapor concentration is between 2500 and 50000 ppm, is 0.5‰ for δ 18O and 2‰ for δ D (1 SD), similar to results obtained from other published measuring systems.

Experimental evaluation of oxygen isotopic exchange between inclusion water and host calcite in speleothems

Abstract. The oxygen and hydrogen isotopic compositions of water in fluid inclusions in speleothems are important hydroclimate proxies because they provide information on the isotopic compositions of rainwater in the past. Moreover, because isotopic differences between fluid inclusion water and the host calcite provide information on the past isotopic fractionation factor, they are also useful for quantitative estimation of past temperature changes. The oxygen isotope ratio of inclusion water (δ18Ofi), however, may be affected by isotopic exchange between the water and the host carbonate. Thus, it is necessary to estimate the bias caused by this postdepositional effect for precise reconstruction of paleotemperatures. Here, we evaluate the isotopic exchange reaction between inclusion water and host calcite based on a laboratory experiment involving a natural stalagmite. Multiple stalagmite samples cut from the same depth interval were heated at 105 ∘C in the laboratory from 0 to 80 h. Then, the isotopic compositions of the inclusion water were measured. In the 105 ∘C heating experiments, the δ18Ofi values increased from the initial value by 0.7 ‰ and then remained stable after ca. 20 h. The hydrogen isotope ratio of water showed no trend in response to the heating experiments, suggesting that the hydrogen isotopic composition of fluid inclusion water effectively reflects the composition of past drip water. We then evaluated the process behind the observed isotopic variations using a partial equilibration model. The experimental results are best explained by the assumption that a thin CaCO3 layer surrounding the inclusion reacted with the water. The amount of CaCO3 that reacted with the water is equivalent to 2 % of the water inclusions in molar terms. These results suggest that the magnitude of the isotopic exchange effect has a minor influence on paleotemperature estimates for Quaternary climate reconstructions.

Enhanced Mediterranean water cycle explains increased humidity during MIS 3 in North Africa

Abstract. We report a new fluid inclusion dataset from northeastern Libyan speleothem SC-06-01, which is the largest speleothem fluid inclusion dataset for North Africa to date. The stalagmite was sampled in Susah Cave, a low-altitude coastal site, in Cyrenaica, on the northern slope of the Jebel Al-Akhdar. Speleothem fluid inclusions from the latest Marine Isotope Stage (MIS) 4 and throughout MIS 3 (∼67 to ∼30 kyr BP) confirm the hypothesis that past humid periods in this region reflect westerly rainfall advected through the Atlantic storm track. However, most of this moisture was sourced from the western Mediterranean, with little direct admixture of water evaporated from the Atlantic. Moreover, we identify a second moisture source likely associated with enhanced convective rainfall within the eastern Mediterranean. The relative importance of the western and eastern moisture sources seems to differ between the humid phases recorded in SC-06-01. During humid phases forced by precession, fluid inclusions record compositions consistent with both sources, but the 52.5–50.5 kyr interval forced by obliquity reveals only a western source. This is a key result, showing that although the amount of atmospheric moisture advections changes, the structure of the atmospheric circulation over the Mediterranean does not fundamentally change during orbital cycles. Consequently, an arid belt must have been retained between the Intertropical Convergence Zone and the midlatitude winter storm corridor during MIS 3 pluvials.

Central Europe temperature constrained by speleothem fluid inclusion water isotopes over the past 14,000 years.

The reasons for the early Holocene temperature discrepancy between northern hemispheric model simulations and paleoclimate reconstructions-known as the Holocene temperature conundrum-remain unclear. Using hydrogen isotopes of fluid inclusion water extracted from stalagmites from the Milandre Cave in Switzerland, we established a mid-latitude European mean annual temperature reconstruction for the past 14,000 years. Our Milandre Cave fluid inclusion temperature record (MC-FIT) resembles Greenland and Mediterranean sea surface temperature trends but differs from recent reconstructions obtained from biogenic proxies and climate models. The water isotopes are further synchronized with tropical precipitation records, stressing the Northern Hemisphere signature. Our results support the existence of a European Holocene Thermal Maximum and data-model temperature discrepancies. Moreover, data-data comparison reveals a significant latitudinal temperature gradient within Europe. Last, the MC-FIT record suggests that seasonal biases in the proxies are not the primary cause of the Holocene temperature conundrum.

Physical weathering of carbonate host-rock by precipitation of soluble salts in caves: A case study in El Orón-Arco Cave (Region of Murcia, SE Spain)

The dissolution of carbonate host-rock by freshwater in phreatic or vadose conditions is the most common mechanism for the formation of caves; however, circulation of saline solutions through carbonate materials and precipitation of soluble salts may also play an important role. We studied the stable isotope composition (δ18O and δ34S of sulfate, δ18O and δD of structurally-bound gypsum hydration water and 87Sr/86Sr) and salinity of fluid inclusions in gypsum speleothems found in El Orón-Arco Cave (Cartagena, SE Spain). We suggest that physical weathering of carbonate host-rock was driven by precipitation of soluble sea-salts (mostly gypsum and halite), and this process controlled the recent geomorphological evolution of the cave. The Triassic carbonate host-rock shows clear evidence for salt weathering, including gypsum/halite infillings in cracks of the bedrock, mechanical spalling of the carbonate, and detachment of rock fragments that lead to the formation cave voids and in-situ accumulations of piles of unsorted rubble. Sulfur and oxygen isotopes of gypsum sulfate (3.0‰ < δ18O < 11.6‰ and 16.7‰ < δ34S < 20.7‰) are generally lower than modern seawater sulfate and suggest contributions from a 34S-depleted source (i.e. oxidation of pyrite). The δ18O and δD of gypsum hydration water are relatively low compared to expected values for the evaporation of pure seawater to gypsum saturation, suggesting that gypsum precipitation involved a secondary calcium-sulfate source or recycling of gypsum from previous stages, along with mixing of seawater and meteoric water seepage to the cave. The 87Sr/86Sr in gypsum shows intermediate values between modern seawater and Triassic carbonate values because of interaction between the solution and the bedrock. The salinities of the speleothem-forming solutions are relatively high (13.2 ± 3.2 wt% eq. NaCl) compared to gypsum formed from evaporated brackish solutions (i.e. ~4–8 wt% eq. NaCl) and indicate dissolution of earlier evaporites before secondary gypsum precipitation. This cave-forming mechanism, which is related to saline water circulation and precipitation of evaporitic minerals, may be common in other coastal caves.

A Newly Designed Analytical Line to Examine Fluid Inclusion Isotopic Compositions in a Variety of Carbonate Samples

Abstractδ18O and δD of fluid inclusions in carbonates provide insights into temperatures and fluid chemical compositions prevailing during the carbonate precipitation, however, various analytical restrictions limit a wider application of this proxy. This paper presents a new fluid inclusions isotopic analytical line coupled to an online cavity ring‐down spectrometer that increased the analytical productivity up to 10 carbonate samples per working day. This efficiency allowed for the first time to assess the reliability a large set of water samples with size ranging from 0.1 to 1 µL. Good reproducibility (±0.5‰ for δ18O and ± 2‰ δD; 1σ) is obtained for water quantity superior or equal to 0.3 μL and no evidence of memory effect is found. The line is further tested using two types of natural carbonates: (1) modern speleothems samples from caves for which δ18O and δD values of drip water were measured and (2) diagenetic carbonates for which the δ18O of the parent water were independently back‐calculated from carbonate clumped isotope Δ47 measurements. Speleothem fluid inclusion values despite falling close to the Global Meteoritic Water Line are not always representative of the isotopic composition of the parent drip water. Results on diagenetic cements show that the δ18Owater values measured in fluid inclusions agree, within 1%, with the δ18Owater independently derived from Δ47 measurements. Overall, this study confirms the reliability and accuracy of the developed analytical line for carbonate fluid inclusion analyses with a good reproducibility obtained for water quantity above 0.3 μL.

High-resolution signatures of oxygenation and microbiological activity in speleothem fluid inclusions

High-resolution signatures of oxygenation and microbiological activity in speleothem fluid inclusions

Triple isotope (δD, δ17O, δ18O) study on precipitation, drip water and speleothem fluid inclusions for a Western Central European cave (NW Switzerland)

Deuterium (δD) and oxygen (δ18O) isotopes are powerful tracers of the hydrological cycle and have been extensively used for paleoclimate reconstructions as they can provide information on past precipitation, temperature and atmospheric circulation. More recently, the use of 17Oexcess derived from precise measurement of δ17O and δ18O gives new and additional insights in tracing the hydrological cycle whereas uncertainties surround this proxy. However, 17Oexcess could provide additional information on the atmospheric conditions at the moisture source as well as about fractionations associated with transport and site processes. In this paper we trace water stable isotopes (δD, δ17O and δ18O) along their path from precipitation to cave drip water and finally to speleothem fluid inclusions for Milandre cave in northwestern Switzerland. A two year-long daily resolved precipitation isotope record close to the cave site is compared to collected cave drip water (3 months average resolution) and fluid inclusions of modern and Holocene stalagmites. Amount weighted mean δD, δ18O and δ17O are −71.0‰, −9.9‰, −5.2‰ for precipitation, −60.3‰, −8.7‰, −4.6‰ for cave drip water and −61.3‰, −8.3‰, −4.7‰ for recent fluid inclusions respectively. Second order parameters have also been derived in precipitation and drip water and present similar values with 18 per meg for 17Oexcess whereas d-excess is 1.5‰ more negative in drip water. Furthermore, the atmospheric signal is shifted towards enriched values in the drip water and fluid inclusions (Δ of ∼ + 10‰ for δD). The isotopic composition of cave drip water exhibits a weak seasonal signal which is shifted by around 8–10 months (groundwater residence time) when compared to the precipitation. Moreover, we carried out the first δ17O measurement in speleothem fluid inclusions, as well as the first comparison of the δ17O behaviour from the meteoric water to the fluid inclusions entrapment in speleothems. This study on precipitation, drip water and fluid inclusions will be used as a speleothem proxy calibration for Milandre cave in order to reconstruct paleotemperatures and moisture source variations for Western Central Europe.

A high-resolution fluid inclusion δ18O record from a stalagmite in SW France: modern calibration and comparison with multiple proxies

The δ18O of speleothem fluid inclusions can provide information about local climate and the atmospheric circulation if the fate of oxygen isotopes from precipitation to the stalagmite is well understood. At Villars cave, SW France, the present-day relationship between the δ18O of precipitation and the δ18O of cave drip water has been determined by previous monitoring studies. In this article, we examine the link between drip water and fluid inclusions in speleothems from the same cave, and investigate whether the fossil water in the stalagmites provides a record of the isotopic composition of precipitation.Replicated fluid inclusion isotope measurements (δD and δ18O) from two modern calcite samples, which had precipitated under the monitored stalactites, are in agreement with the corresponding contemporaneous drip waters. The drip water, in turn, represents average pluri-annual precipitation. This gives strong evidence that the isotopic composition of the infiltrating water is preserved in the pores of speleothem calcite, thus providing a record of average precipitation δ18O at the site.Furthermore, we measured isotope ratios in fluid inclusions in the stalagmite vil-stm1, a 2300 year old sample with a high growth rate (up to 0.6 mm yr−1) and a high water content, which allows a high-resolution reconstruction of the drip water isotopic composition covering the last two millennia. The stalagmite is well dated by 13 U–Th ages and the counting of annual growth layers. The fluid inclusion δ18O varies by 2‰, with high δ18O values in Medieval times, and low values during the Little Ice Age, indicating a temperature influence on precipitation δ18O. However, temperature change alone cannot account for the full range of variability observed.Other proxies measured in the same stalagmite indicate that the cave environment has undergone substantial changes during this period: Stable isotopes and trace element concentrations in calcite show large variations, with amplitudes similar to those found in other Villars speleothems during large climatic shifts such as Dansgaard–Oeschger events or glacial–interglacial transitions. In the context of the last 2000 years, the proxy variability cannot be attributed to climate alone, but is likely influenced by anthropogenic deforestation and the associated changes in carbon dynamics and hydrology.These changes might also have an influence on the isotope ratios in drip water relative to precipitation, e.g. through an altered evaporation–transpiration ratio. Nevertheless, despite some common trends in all proxy records, the marked shifts in the other proxies are not seen in the fluid inclusion stable isotopes, which gives evidence that the relationship between precipitation and drip water δ18O in the past was similar to present-day conditions, and not much affected by the anthropogenic impact. A replication of the fluid inclusion isotope record using stalagmites from the same region will be necessary to determine whether local cave processes or large-scale temperature and atmospheric circulation dominate the fluid inclusion δ18O signal, and in how far the fluid inclusions can provide a direct record of precipitation δ18O.

New online method for water isotope analysis of speleothem fluid inclusions using laser absorption spectroscopy (WS-CRDS)

Abstract. A new online method to analyse water isotopes of speleothem fluid inclusions using a wavelength scanned cavity ring down spectroscopy (WS-CRDS) instrument is presented. This novel technique allows us simultaneously to measure hydrogen and oxygen isotopes for a released aliquot of water. To do so, we designed a new simple line that allows the online water extraction and isotope analysis of speleothem samples. The specificity of the method lies in the fact that fluid inclusions release is made on a standard water background, which mainly improves the δ D robustness. To saturate the line, a peristaltic pump continuously injects standard water into the line that is permanently heated to 140 °C and flushed with dry nitrogen gas. This permits instantaneous and complete vaporisation of the standard water, resulting in an artificial water background with well-known δ D and δ18O values. The speleothem sample is placed in a copper tube, attached to the line, and after system stabilisation it is crushed using a simple hydraulic device to liberate speleothem fluid inclusions water. The released water is carried by the nitrogen/standard water gas stream directly to a Picarro L1102-i for isotope determination. To test the accuracy and reproducibility of the line and to measure standard water during speleothem measurements, a syringe injection unit was added to the line. Peak evaluation is done similarly as in gas chromatography to obtain &amp;amp;delta D; and δ18O isotopic compositions of measured water aliquots. Precision is better than 1.5 ‰ for δ D and 0.4 ‰ for δ18O for water measurements for an extended range (−210 to 0 ‰ for δ D and −27 to 0 ‰ for δ18O) primarily dependent on the amount of water released from speleothem fluid inclusions and secondarily on the isotopic composition of the sample. The results show that WS-CRDS technology is suitable for speleothem fluid inclusion measurements and gives results that are comparable to the isotope ratio mass spectrometry (IRMS) technique.

Daily response of drip water isotopes to precipitation in Liangfeng Cave, Guizhou Province, SW China

Oxygen isotope (δ18O) is one of the most commonly used palaeoclimate proxies, and monitoring their modern evolutionary processes is very significant for palaeoclimate reconstruction. In this study, drip water samples are daily collected from two drip sites in Liangfeng Cave, Guizhou Province, SW China, between June 2008 and June 2010. The stable hydrogen and oxygen isotopes of these samples and the contemporary precipitation samples are measured. The relationships between the isotopes and the local air temperature, precipitation and relative humidity are analysed. The results show that the hydrogen and oxygen isotopic compositions of precipitation have obvious seasonal variations in the study area: lower in the rainy season and higher in the dry season. The local meteoric water line (LMWL) is δD = 8.64 δ18O + 17.44. Precipitation is the only source of cave drip water, and its oxygen isotope signals are reflected in the two drip waters (SD: slower drip rate and previously called 1#, and FD: faster drip rate and previously called 5#). However, the amplitude of the drip water oxygen isotope variation is much smaller than that of the precipitation, i.e., homogenization occurs to some extent. However, there are significantly different responses to precipitation between the two drip sites. The response time of SD to precipitation is much longer than that of FD. The amplitude of oxygen isotope variation in SD, where the isotopic data deviate from the LMWL to a higher degree, is much smaller than that in FD. A comparison of these isotopic differences with previous research in the same cave indicates that the isotopic differences between the two drip waters result from different flow paths. The oxygen isotope signal in cave drip water perhaps mainly reflects summer monsoon information in the study area. The speleothems fed by drip waters (e.g. FD) with shorter response time to precipitation may be more suitable for high-resolution palaeoclimate research. Moreover, the d-excess from speleothem fluid inclusion has the potential to be used as an indicator of relative humidity of local air.

Noble gas concentrations in fluid inclusions as tracer for the origin of coarse-crystalline cryogenic cave carbonates

The chemically inert behaviour of noble gases, their well-known solubility in water and the long-term constancy of atmospheric noble gas mixing ratios make them a unique tool for paleoclimate studies of groundwater. This concept has recently been extended to fluid inclusions in speleothems. Here we use noble gas concentrations of fluid inclusions contained in calcite crystals to constrain the formation history of coarse crystalline cryogenic cave calcites from Heilenbecker Cave (Germany).Cryogenic cave carbonates form under conditions related to the freezing of cave pools and are a new archive for paleo-permafrost timing and extent. The cryogenic origin of these carbonates is typically deduced from their crystal habit and the carbon and oxygen isotope ratios, but this can be ambiguous in some cases. A geochemical method that provides clear-cut proof of the cryogenic origin of individual crystals has been lacking so far.We examined the formation process of cryogenic cave calcites using water and noble gases extracted from inclusions in these crystals. Based on their δ13C and δ18O values, these calcite crystals could have also formed as ‘normal’, i.e. non-cryogenic, speleothems. Noble gas concentrations derived from fluid inclusions in these cryogenic calcites, however, deviate significantly from commonly observed concentrations in atmospheric air, surface water, groundwater, and stalagmites. These concentrations cannot be explained simply by a solubility component nor by the addition of various amounts of excess-air to a solubility component. The data, however, are consistent with the scenario of a slowly freezing water body lacking exchange with the cave atmosphere. The gradual freezing process leads to a partitioning of the noble gases between ice and remaining water and thereby to a pronounced over-abundance of heavy noble gases in the liquid phase recorded in the fluid inclusions of the cryogenic calcite crystals.

Reproducibility and accuracy of noble gas measurements on water samples in the microlitre range

In order to derive meaningful noble gas temperatures (NGTs) from speleothem fluid inclusions, precise and accurate measurements of noble gas concentrations on very small water samples are necessary. To optimise these measurements and their reliability, an investigation of the reproducibility and accuracy of the analytical procedure is essential. Water equilibrated with air under controlled conditions was filled into copper capillaries to produce small (about 1 μL) air-equilibrated water samples (so-called μAEWs). As speleothem samples, the μAEWs were opened in a crusher and the released noble gases analysed with an electron ionisation sector field mass spectrometer run in static mode. For better comparability with speleothem samples, a defined amount of air was added to the noble gases derived from one group of μAEWs. The reproducibility of the experimental procedure was found to be better than 2.2% for all relevant noble gases. Within these uncertainties, the measured noble gas concentrations of μAEWs agree with expectations. The corresponding NGTs reproduce within 0.5°C and deviate by less than 1°C from the equilibration temperature. In the case of air addition, the air to water ratio was determined accurately and, excluding one outlier, the NGTs were determined with a reproducibility and accuracy well below 1°C. The results show that the used measurement procedure is generally suitable to reveal major climatic temperature changes, for which an overall NGT error of less than 1°C is desirable, from very small water samples as obtained from speleothem fluid inclusions.