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

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Abstract

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 H2O/CaCO3 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 δ18O and δ2H 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 δ18O and ±3‰ for δ2H. Greater precision than this will require replicate analysis for each speleothem growth increment.

Introduction

In contrast to the high resolution marine record, the isotopic record of climate change in terrestrial deposits is less well documented and understood. Of the potential continental isotopic indicators of climate, speleothems (cave deposits) show great promise. They are widely distributed and speleothem deposition in low to mid-latitude regions occurred over long periods of the Quaternary. Through combining isotopic analysis with high precision uranium-series dating, reconstruction of long time series and time slices of regional climate variation at key stages in the recent geologic past becomes a realistic possibility.

Speleothems are unique in containing a high resolution record of both palaeotemperature and isotopic composition of palaeogroundwater. They contain up to 0.1 wt.% of relict drip water in the form of fluid inclusions that were trapped in the host calcite at the time of growth (Kendall and Broughton, 1978). Modern cave seepage water is closely related to the mean annual meteoric precipitation at the cave site Harmon et al 1978, Yonge et al 1985, while cave temperatures closely reflect the mean annual air temperature at the surface above the cave Wigley and Brown 1971, Gascoyne 1992, Moore and Sullivan 1978. Thus, provided that a speleothem grew in thermal isotopic equilibrium with its parent drip water and that there has been no subsequent re-equilibration, isotopic measurements of the inclusion and host calcite can be used to define the contemporaneous palaeoprecipitation isotope composition and cave palaeotemperature. Both are important quantitative climatic parameters that can be modelled by, and therefore used to validate, GCM climate models.

The importance of cave deposits as palaeoclimatic indicators has been recognised for nearly three decades (Hendy and Wilson, 1968) though, in the absence of drip water data, the climate-oxygen isotope link can be complex Hendy 1971, Gascoyne et al 1981, Goede et al 1986, Frumkin et al 1999. Interpretation of temporal and areal trends in speleothem calcite composition are normally heavily reliant on other, independent, climatic evidence. A positive correlation exists between δ18O in rainfall and condensation temperature (Dansgaard, 1964), but δ18O in secondary calcite is negatively correlated with temperature of deposition (Kim and O’Neil, 1997). Quantitative interpretation requires calibration of the speleothem data against some sensitive indicator of local climate to establish which of these effects is dominant. Although many studies have adopted this approach Harmon et al 1979, Gascoyne et al 1981, Goede et al 1990, Dorale et al 1992, Dorale et al 1998, Lauritzen 1995, Bar-Matthews et al 1997, Frumkin et al 1999, McDermott et al 1999, it is one that is not always possible.

Complete characterisation of the palaeoclimate signal in speleothems requires data for the isotopic composition of the relict parent drip water that is preserved in fluid inclusions. Measurement of both the calcite and inclusion water leads to a direct deciphering of the climate signal, precluding the necessity for calibration against other proxy data.

Pioneering investigations of inclusions in North American speleothems by Schwarcz et al 1976, Harmon et al 1978 and Harmon et al. (1979) were encouraging since they succeeded in detecting significant isotopic differences between inclusion waters of glacial and interglacial age. Similar studies by Rozanski and Dulinski (1987) succeeded in recovering the hydrogen isotopic composition of palaeodripwaters of interglacial ages from Poland. In these and other studies only the hydrogen isotope compositions of inclusion waters were measured. There has been a widely held view that fluid inclusions may have exchanged oxygen isotopes with the host calcite through geological time and thus δ18O measurements may be of little value Schwarcz et al 1976, Rozanski and Dulinski 1987. Whilst this proposition has not been systematically tested we have published preliminary data suggesting that a robust δ18O signal can be recovered from some speleothem fluid inclusions (Dennis et al., 1998). In the absence of measurements, estimates of fluid inclusion oxygen isotope compositions have been made using present-day local meteoric water line relationships and the measured δ2H composition (e.g., Goede et al., 1990).

In addition to the lack of oxygen isotope data, later studies highlighted unresolved technical difficulties in the recovery and measurement of the hydrogen isotope composition of inclusion water with large deviations of analysed compositions from those expected. For example, Yonge 1982, Goede et al 1986 and Goede et al. (1990) consistently found depletions of 20‰ to 30‰ in the hydrogen isotope composition of recovered modern inclusion waters relative to their associated cave dripwaters. Our own experiments, reported here, suggest that the measured depletion is an artefact of the analytical procedure, occurring as a result of isotopic fractionation during water adsorption onto crushed or calcined calcite surfaces. A deuterium and oxygen-18 depleted fraction is collected when recovery of the water is incomplete.

These observations have tended to reinforce an underlying scepticism concerning the reliability of isotopic results derived from fluid inclusions, and since considerable accuracy (better than ±0.5‰ for δ18O and ±5‰ for δ2H) is required for useful climatic reconstruction, this must be demonstrated before inclusion isotopic measurements can gain wide acceptance. An additional constraint for inclusion studies is the desirability of high temporal resolution. Previous studies have generally used several grams of calcite for inclusion analyses, allowing only a very limited resolution. To resolve time scales of <103 years requires development of methods for the recovery and measurement of inclusion waters with total volumes of less than 1 μL from subgram samples.

In this paper we present the results of experiments with analog inclusion systems designed to help develop a robust technique for recovering the true isotopic composition of inclusion water from speleothems. The results highlight the importance of surface adsorption phenomena during liberation of inclusion water and the need for controlled thermal desorption to ensure 100% recovery without fractionation of either the hydrogen or oxygen isotopes. We have used the method to recover inclusion water from a British Late Holocene speleothem and present new results for the hydrogen and oxygen isotope composition that show high accuracy and precision with a temporal resolution of less than 200 yr.

Section snippets

Crushing cell, vacuum preparation line and water extraction procedure

To minimise the risk of oxygen isotope exchange between inclusions and the host carbonate, we have developed methods to crush rather than to thermally decrepitate or dissociate samples. Thermal decrepitation of inclusions in calcite at 400 to 500°C is accompanied by the release of large volumes of CO2 Norman and Sawkins 1987, Harmon et al 1979, Schwarcz and Yonge 1983, Yonge 1982, Goede et al 1986, Goede et al 1990, Lecuyer 1994. This hinders the efficient separation and isolation of the

Analogue system experiments

Results are available for recovered yields and isotopic compositions of water extracted in 3 types of experiment: (1) single glass capillaries crushed in the absence of calcite at room temperature; (2) single glass capillaries and calcite crushed at room temperature; and (3) single glass capillaries and calcite crushed at room temperature followed by heating to 150°C to thermally desorb any water bound to crushed calcite surfaces.

Measurement of fluid inclusions in a speleothem

The stalagmite GB40 (Fig. 5) was collected from its growth position in GB Cave in the Mendip Hills near Bristol, England (lat. 51°18.2′N, long. 2°45.1′W; UK National Grid Ref. ST 47595623) in 1980. It has been U/Th dated and the part analysed here is known to have grown over the interval from c.5000 BP to almost the present day. This is a period in which variability in the palaeoclimate of Britain is small and difficult to detect from proxy records (Briffa and Atkinson 1997). Therefore, it may

Conclusions

Simple synthetic inclusion experiments indicate that when fluid inclusions are ruptured at room temperature in the presence of freshly crushed calcite, there is preferential adsorption of isotopically heavier molecules onto active grain surfaces. At water concentrations of < 10 mg · g−1 of calcite, fractionation of the recovered water becomes significant. The degree of fractionation increases as the H2O/CaCO3 ratio decreases and it appears that complete desorption cannot be effected by

Acknowledgements

We gratefully acknowledge the support of the University of East Anglia Research Promotion Fund in providing an initial grant to start this work and the continued support of the Natural Environment Research Council under grants NERC GR3/9014 and NERC GR3/10473. Dan Moseley carried out the BET surface area measurements. The data on water isotopic compositions were obtained in collaboration with Peter Smart under grant NERC GR3/3177. We thank the perceptive reviews of Stephen Burns and two

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