Determination of Holocene cave temperatures from Kr and Xe concentrations in stalagmite fluid inclusions
Highlights
► We successfully determine “noble gas temperatures” in fluid inclusions in stalagmites. ► Noble gas temperatures in Holocene samples agree well with modern cave temperatures. ► The calcite structure of a stalagmite is crucial for noble gas temperature determination.
Introduction
Stalagmites, which contain precise, high-resolution δ13C and δ18O records covering timescales of up to 105 years (e.g., Wang et al., 2008, Cheng et al., 2009, Fleitmann et al., 2009), are a major focus of palaeoclimate studies. To explore the full potential of stalagmites as a palaeoclimate archive, direct, independent proxies for cave temperature are needed to constrain and interpret unequivocally the rather complex stable isotope signatures in the calcite, for instance in terms of hydrological variables (Lachniet, 2009). Also, as cave temperatures provide a good measure of the local mean annual air temperature outside the cave (McDermott, 2004), methods to directly determine cave temperatures in stalagmites (e.g. ‘clumped isotope’ thermometry Affek et al., 2008) or the temperature-dependent fractionation of oxygen isotopes between fluid-inclusion water and calcite (Griffiths et al., 2010, Wainer et al., 2011) offer the potential to reconstruct palaeotemperatures over long timescales and in continental regions where alternative high-resolution climate records are generally not available.
During stalagmite growth, minute quantities of drip water and cave air are included in the calcite to form fluid inclusions consisting either of air, water or both (Kluge et al., 2008, Scheidegger et al., 2010). Air inclusions account for up to 3% of stalagmite volume (Badertscher, 2007) and water inclusions for 0.01–0.1% of stalagmite mass (Schwarcz et al., 1976, Kendall and Broughton, 1978, Scheidegger et al., 2010). The concentrations of atmospheric noble gases (He, Ne, Ar, Kr, Xe) dissolved in the water inclusions are expected to reflect directly the temperature and salinity of the water as well as the atmospheric pressure at the time of gas exchange with the cave air, as the equilibrium concentrations of noble gases in water are a function of these variables (Kipfer et al., 2002). Hence, noble gas concentrations in stalagmite water inclusions offer the possibility of determining the noble gas temperature (NGT) if the salinity of the cave water and the atmospheric pressure in the cave are known. The same principle has been widely applied to groundwater and to lake water (Stute et al., 1995, Beyerle et al., 2000, Aeschbach-Hertig et al., 2000, Weyhenmeyer et al., 2000, Kipfer et al., 2002) as well as to pore waters in lake sediments (e.g. Brennwald et al., 2004, Brennwald et al., 2005).
Recent advances in experimental methods now allow noble gas concentrations to be determined even in the small amounts of water (~ 1 mg) typically extracted from a stalagmite sample (Kluge et al., 2008, Scheidegger et al., 2010). However, in these studies, interpretation of the measured noble gas concentrations in terms of NGTs was possible for only a minority of samples. Kluge et al. (2008) were able to convert noble gas concentrations into NGTs solely in stalagmites having an exceptionally low volume ratio of air inclusions to water inclusions. Only when this ratio is sufficiently low can NGTs be determined reliably. This is because noble gases released from air inclusions not only contain no information about the cave temperature, air-derived noble gases also mask the temperature-dependent noble gas signature of the water inclusions. To overcome this limitation, Scheidegger et al. (2010) developed a noble gas extraction technique which efficiently separates air inclusions from water inclusions. This method requires samples to be pre-crushed into grains of a defined diameter to remove air inclusions before the noble gases are extracted from the water inclusions by heating. However, during pre-crushing in air and in N2, the crushed samples strongly adsorbed Ar, Kr and Xe on their freshly created surfaces. This hampered the calculation of NGTs, which could only be determined for a few samples and even then with large absolute errors of 10–30 °C (Scheidegger et al., 2010).
For this study, the analytical protocol for the determination of noble gas concentrations developed by Scheidegger et al. (2010) was modified with the key goal to minimise adsorption of heavy noble gases onto the mineral surfaces of the stalagmite samples during pre-crushing. We show that Kr and Xe concentrations measured in fluid inclusions in stalagmites originating from three different caves with different annual mean air temperatures (8 °C, 12 °C and 27 °C) result in NGT estimates that are consistent with modern cave temperatures.
We analysed noble gas concentrations in stalagmite samples from Dimarshim cave in Socotra Island (Yemen), Sofular cave on the Black Sea coast of western Turkey and Vallorbe cave, Switzerland (details in Table 1). As the caves are located in three different climatic zones, the cave temperatures cover a large part of the temperature range commonly observed in meteoric water systems. The ages of the Dimarshim (D1) and Sofular (SO2, SO3, SO4) cave samples were derived from an age model based on U/Th-series dating (Fleitmann et al., 2007, Fleitmann et al., 2009). No ages are yet available for the Vallorbe cave samples (V1).
Stalagmite samples of 4–6 g were cut along the growth axis of the stalagmite and transferred into a glove box for pre-crushing. The glove box was flushed with 99.9999% pure He which contained no detectable concentrations of Ne, Ar, Kr or Xe (according to our analysis by static noble gas spectrometry). Noble gas analysis of gas samples taken in the glove box showed that after extensive flushing, Ar, Kr and Xe concentrations were reduced to ~ 10% of their respective concentrations in free air. For further purification, we exposed the gas phase in the glove box for several hours to a sorption pump filled with zeolite (5 Å) and held at the temperature of liquid nitrogen (− 196 °C). This additional cleaning step reduced the concentration of Ar to ~ 200 ppm and the concentrations of Kr and Xe to negligible levels (see Table 2).
In the glove box, the samples were pre-crushed into grains of a narrow diameter range to separate air inclusions from water inclusions. To this end, the sample was crushed in a mortar by striking the pestle with a hammer. After each hammer stroke, the crushed sample material was sieved and grains smaller than the upper limit of our preferred diameter range were separated from the rest of the sample. Air inclusions tend to be larger than water inclusions and lie preferentially along crystal boundaries. Hence, the preferred maximum diameter of the crushed grains (350 to 700 μm) was chosen based on the size of individual air and water inclusions in each stalagmite, so that air inclusions were preferentially removed during pre-crushing. Because of the preferential removal of air inclusions, the air content in the pre-crushed samples is 100 to 1000 times lower than in bulk samples (Scheidegger et al., 2010). Still in the glove box, the pre-crushed samples were put into a stainless steel tube, which was closed with a vacuum valve. The stainless steel tube was then connected to the extraction line without exposing the samples to air.
The noble gases were finally extracted from the pre-crushed samples by heating at 300–400 °C for 1 h and were analysed using static noble gas mass spectrometry (see Beyerle et al., 2000). Pre-crushing in a He atmosphere led to high He concentrations in the samples, but did not influence the analysis of the other four noble gases. Each sample analysis was preceded by the analysis of a blank (without heating the stainless steel tube containing the sample) and followed by the measurement of a standard gas aliquot. Noble gas amounts were calculated by peak height comparison of blank-corrected sample signals with the measured standard signal. For the vast majority of the samples, the blank gas intensities accounted for less than 1% of a typical sample signal. Dividing the noble gas amounts by the manometrically determined mass of the extracted water (for details see Scheidegger et al., 2010) then yields the noble gas concentrations. The errors of the calculated noble gas concentrations account for, i) the individual analytical measurement error, ii) the standard deviation of the measured standard signals (1–3%) and iii) the error of the determined water mass (1.5%). The overall analytical 1σ errors of the noble gas concentrations are 2–3% for Ne, Ar and Kr and 3–5% for Xe. The noble gas concentrations determined for the 24 samples analysed are given in Table 3.
Section snippets
Noble gas temperatures (NGTs)
In lakes and groundwater the concentrations of Ne, Ar, Kr and Xe usually originate from binary mixtures of two noble gas components, i.e. air-saturated water (ASW) and atmospheric air. This fact allows NGTs to be determined by least-squares fitting of the measured noble gas concentrations (Aeschbach-Hertig et al., 1999, Ballentine and Hall, 1999). However, this approach cannot be applied directly to stalagmites, as not all noble gas concentrations in stalagmite samples are consistent with
Conclusions and outlook
With our improved analytical protocol we were able to determine NGTs reliably from the measured Kr and Xe concentrations in fluid inclusions in stalagmites. Crushing the samples in pure He reduced the adsorption of Kr and Xe onto the mineral surfaces to a negligible level, allowing the Kr and Xe concentrations to be interpreted using a simple binary mixing model of ASW and atmospheric air. This study hence provides direct experimental evidence that stalagmite fluid inclusions provide a suitable
Acknowledgements
We would like to thank Urs Menet and Heinrich Baur for their assistance in the laboratory, and Henry Schmidt for improving the crushing system and for the noble gas analyses he performed during his master's thesis. We also thank Sebastian Breitenbach for the idea of using He to flush the glove box. Thanks also to David M. Livingstone for proof-reading the manuscript, and to anonymous reviewers who helped to improve an earlier version of the manuscript.
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