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Noble gases in fluid inclusions of stalagmites as a new palaeoclimate proxy

Final Report Summary - NOGAFLUIS (Noble gases in fluid inclusions of stalagmites as a new palaeoclimate proxy)

In the first stage of the project we adapted and developed a measurement procedure for mass spectrometric analysis of noble gases dissolved in large samples of water (20-40 ml). The achieved precision allows us to calculate solubility temperatures by an uncertainty of 0.3-0.4 °C (1s). In case of standard water samples prepared at well known atmospheric pressure and temperature, the calculated temperatures are very close to the temperature at which the water samples have been equilibrated with air. The differences are 0.1-0.6 °C which can be compared to the uncertainty of the individual temperature calculations. These results proves that our measurement process is able to provide accurate noble gas concentrations for scientific and research purposes. As the ability of noble gas measurements from large water samples is a prerequisite for fluid inclusion nobel gas measurement, we could proceed to adapt this measurement technique for very small amounts of water, i.e. fluid inclusions of speleothems.

In geoscience, fluid inclusions of, for example, speleothems are considered in order to determine palaeotemperatures from noble gas concentrations of such tiny amounts of water. In this project we have successfully adapted the noble gas measurement procedure for very small samples of water in the range of microlitres. When the water has been extracted from the sample, the amounts of both water and noble gases have to be determined to obtain noble gas concentrations at the end. Thus, two independent measurements play an important role in this measurement procedure: 1. water determination; 2. determination of noble gas abundances.

The water content of the sample is frozen in a cold finger. After collecting the gases in a cryogenic cold system (cryo), the determination of the water amount can be started. The water is melted in the cold finger as it is heated up to room temperature. The volume of the cold finger is large enough so that the water turns completely to vapour phase, and then the water vapour pressure is measured by an active capacitance vacuum gauge. To increase the stability of the pressure measurement the cold finger is kept at 24 ± 0.3 °C, while the whole laboratory is conditioned to 24 ± 0.5 °C. The water determination process is calibrated by means of well known water amounts. Distilled water aliquots were sucked into glass capillaries, and then both ends of the capillary were flame-sealed. Determination of the water amounts was performed by precise weight measurements of the empty glass capillary and the filled one. A balance with a precision of 0.002 mg was used for this purpose. The difference is thought to be the water weight, which has an uncertainty of 0.004 mg. The capillary filled with distillate water was then put into a crusher and evacuated for a few hours. Afterwards, the glass capillary was broken by a magnetic ball, and the released water was frozen into the cold finger at -78 °C for 20 minutes. The water was melted at 24 °C, and the vapour pressure was then measured by the vacuum gauge. Plotting the measured vapour pressure values against the known water weights, we obtain the calibration curves for all volumes involved. The calibration curve of the pressure gauge was fitted on nine measurement data between the water masses of 0.466 mg and 3.143 mg. Figure 1 shows the calibration line for the largest volume.

The measurements of noble gas abundances extracted from tiny water amounts are calibrated with well-known air aliquots in the expected range. A reservoir of 6370 (± 0.1‰) cm3 have been filled to a pressure of 2.0009 (± 3‰) Pa. This reservoir called 'diluted air' is used to prepare well-known air aliquots in the range of 2.5·10-5 to 1.0·10-4 ccSTP by means of a gas pipette of 1.259 (± 1‰) cm3. In this range the calibration air aliquots are admitted into the inlet system and handled in the same way as for a sample. The reproducibility of the calibration measurements is of major importance.