High-resolution δ¹³C measurements on ancient air extracted from less than 10 cm³ of ice

Abstract

A new method for δ¹³C analysis of very small air amounts of less than 0.5 cm³ STP was developed. This corresponds to less than 10 g of ice. It is based on the needle-crasher technique, which is routinely used for CO₂ concentration measurements by infrared laser absorption. The extracted air is slowly expanded into a large volume through a water trap held at −70 °C where the pressure is measured. This sampled air is then carried by a high helium flux through a preconcentration system to separate CO₂ cryogenically from the air. The small CO₂ amount is then released into a low helium stream which forces the CO₂ via an open split device to a mass spectrometer. The overall precision, based on replicates of standard air without crushing, is significantly better than 0.1‰ for a single analysis, and is further improved by a triplicate measurement of the same sample through a specially designed gas splitter. Performing δ¹³C measurements on ice air through the whole system, we reach a reproducibility of 0.12‰. Additional information is obtained through amplitude vs. pressure ratio determination, which results in a good control of the CO₂ concentration (1 ppm precision for 1σ). The new method allows us to produce highly resolved records of atmospheric δ¹³C from air enclosed in ice, which is required to better understand the evolution and the temporal variability of the global carbon cycle.