UMaine CCI Stable Isotope Laboratory Procedures

• Main Author: Introne, Douglas S.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Zenodo 2021
• Subjects: Stable Isotopes; Water, Snow, Ice, Precipitation; Laser Ring Down Spectroscopy; Antarctic; Greenland; Antarc*; GISP; Ice Sheet
• Online Access: https://dx.doi.org/10.5281/zenodo.4721044; https://zenodo.org/record/4721044

Measuring the isotopic composition of high latitude and high altitude snow and ice is one of the routine analyses used in paleoclimate reconstructions. Measuring water isotope signals in modern precipitation and other natural waters allows us to better understand the meteoric processes by which the isotopic signature is imparted to those historical records. Measuring both signals allows us to calibrate and interpret better those historical climate records contained in various ice cores and snow pits. The Climate Change Institute at The University of Maine, Orono, ME. uses a Picarro L-2130-I Isotopic H2O Laser Ring Down Spectrometer to measure the isotopes of current and ancient waters by laser adsorption spectrophotometry. Snow, ice and water is recovered by Climate Change Institute (CCI) researchers from all over the globe and is stored at the CCI, as either frozen snow and/or ice in a -20C freezer facility, or it is melted and then stored at room temperature in glass vials with special polyvinyl conical inserts in the caps. These caps prevent evaporation and fractionation of the samples during storage, analysis and sample archiving. Our tests have shown that there is no measureable fractionation in samples that have been archived in these vials for at least 20 years. The Picarro instrument is fitted with an autosampler and vaporizer peripheral. 1.6 ml of sample water is aliquoted from the storage vials, to a 2 ml vial with a septum and placed in the autosampler. A high tolerance syringe then delivers 5 ul of water to the vaporizer and sequentially measures both the delO18 and delH/D as water vapor in a heated ring down chamber until there is no significant sample memory from the previously injected sample. Samples are reported to the universally accepted international standard VSMOW (Vienna Standard Mean Ocean Water) and expressed in delta notation normalized to the VSMOW-SLAP scale. 1 Three internal lab standards with widely varying isotope signatures have been calibrated to VSMOW, NIST-SLAP (Standard Light Antarctic Precipitation) and NIST-GISP (Greenland Ice Sheet Precipitation). 2 We have also calibrated these internal standards and the instrument to the consensus values of IAEA-OH-1, IAEA-OH-2, IAEA-OH-3 and IAEA-OH-4. 3 (see Table 1) The lab standards are used on a daily basis for daily calibration purposes and instrument monitoring. The standard deviation of measurement is reported as +/- 1 per mille for delta D/H and +/- 0.1 per mille for delta 18O. Although individual analytical runs are often better than reported we report the more conservative error. We base this determination on instrument factory specifications but also because several of the listed secondary standards are only known to .1 per mille for 18O and 1 per mille for D/H. (see Table 1) UMaine’s participation in 2 IAEA interlaboratory comparisons using our calibration, have shown that the error associated with the calibration is on the order of what we report. 2, 3 The natural variations in both modern and historic water isotope signals exceed these reported errors and obviate underestimating the standard uncertainty.