Stable isotope microsampling of speleothems for palaeoenvironmental studies: A comparison of microdrill, micromill and laser ablation techniques

Three different techniques are currently in use to microsample speleothem (and tufa) specimens at moderate to high spatial resolution, manual microdrilling using a dentist drill, semi automated micromilling, and laser ablation. While a separate handling step is involved in the first two techniques — transferring the sample powder to an on-line carbonate preparation system linked to an isotope ratio mass spectrometer — the laser is directly interfaced with the mass spectrometer via a gas preparation device. In this study we compare stable C and O isotope results obtained by these three preparation techniques using continuous He-flow mass spectrometry on the same speleothem samples. The overall consistency of the results is satisfactorily high, in particular between the micromill and the laser data. Although the precision of the laser data is slightly lower than analyses obtained by the other two methods (in particular with respect to δ18O), the speed of acquisition renders laser ablation a useful reconnaissance technique for rapidly analyzing long sections of speleothems. Micromilling is the method of choice to obtain highly spatially resolved isotope data of high analytical precision, while microdrilling is still an low-cost alternative for sampling at comparably low resolution, e.g. of fast growing stalagmites.