Abstract
Abstract. For atmospheric CO2 reconstructions using ice cores, the technique to release the trapped air from the ice samples is essential for the precision and accuracy of the measurements. We present here a new dry extraction technique in combination with a new gas analytical system that together show significant improvements with respect to current systems. Ice samples (3–15 g) are pulverised using a novel centrifugal ice microtome (CIM) by shaving the ice in a cooled vacuum chamber (−27 °C) in which no friction occurs due to the use of magnetic bearings. Both, the shaving principle of the CIM and the use of magnetic bearings have not been applied so far in this field. Shaving the ice samples produces finer ice powder and releases a minimum of 90% of the trapped air compared to 50%–70% when needle crushing is employed. In addition, the friction-free motion with an optimized design to reduce contaminations of the inner surfaces of the device result in a reduced system offset of about 2.0 ppmv compared to 4.9 ppmv. The gas analytical part shows a higher precision than the corresponding part of our previous system by a factor of two, and all processes except the loading and cleaning of the CIM now run automatically. Compared to our previous system, the complete system shows a 3 times better measurement reproducibility of about 1.1 ppmv (1 σ) which is similar to the best reproducibility of other systems applied in this field. With this high reproducibility, no replicate measurements are required anymore for most future measurement campaigns resulting in a possible output of 12–20 measurements per day compared to a maximum of 6 with other systems.
Highlights
Ahn and Brook, 2008; BeGreieteor estcali.e, 2n0t1if2i)cenclosed in air bubbles in the ice
Produced gas-free single-crystal ice was pulverised in the device. This showed no visual difference to powder from real polar ice and the produced powder was filled into the dewar
This is in line with the finer pulverisation of the ice sample of the centrifugal ice microtome (CIM) compared to the cracker (Fig. 3)
Summary
The diameter of air clathrates in polar ice cores lies between 20–200 microns (Uchida et al, 2011). To open all enclosures and, to reach maximum extraction efficiency with dry extraction, the ice sample must be pulverised to grain sizes smaller than the diameter of the enclosures. In the ice powder produced by our needle cracker, more than 90 % of the grains are larger, reaching up to several mm in diameter (see Fig. 3) explaining the low extraction efficiency of only 50 %–70 % (Luthi et al, 2010). By rotating the sample holder anti-clockwise (arrows) with a frequency of up to 2000 rotations per minute, the ice sample gets pushed towards the inner mantle surface due to the centrifugal force and a layer of ice gets shaved off once per revolution when passing over the knife. The pulverisation principle of this new dry extraction device – called centrifugal ice microtome (CIM) – has not yet been applied in ice core research.
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