Continuous Extraction of Trapped Air from Bubble Ice or Water for On-Line Determination of Isotope Ratios

We describe a new continuous extraction system for trapped air from bubble ice or water for on-line determination of the isotopic composition of the main air components nitrogen and oxygen ((delta)^15N, (delta)^18O, and (delta)^17O). Studies of the composition of air from bubbles trapped in polar ice are providing fundamental information about ancient atmospheric composition and, therefore, are an important tool to learn more about Earthʹs climate. The new system proved to work reliably for standard air admixed and subsequently removed from a water stream. The precision (1 SD) of standard measurements is ~0.04” for (delta)^15N, ~0.1” for (delta)^18O, and ~0.15” for (delta)^17O. Ice measurements with the new on-line system are promising. Continuous measurements of nitrogen as well as oxygen isotope ratios can be performed with a spatial resolution of ~3 cm and nearly the same precision as for the standards. However, the measured (delta)values of ice are generally lower, as compared to ice measured with conventional techniques, as a result of a time-dependent dissolution process of air in water associated with kinetic fractionation, which affects standard and sample differently. By modeling the dynamics of the this dissolution process, we found a reason for the lack of accuracy and propose an improvement of the system that will lead to a better accuracy of the ice measurements.