Gas extraction from continuous flow analysis for dating ice cores with 39Ar and 81Kr

Paleoclimate reconstructions from ice core records can be hampered due to the lack of a reliable chronology, especially in deep ice, when the stratigraphy is disturbed and conventional dating methods cannot be applied. The noble gas radioisotopes ⁸¹Kr and ³⁹Ar can in these cases provide robust constraints as they yield absolute, radiometric ages. ⁸¹Kr (t_1/2=229 ka) covers the time span of 30-1500 ka, which is especially relevant for polar ice cores, whereas ³⁹Ar (t_1/2=268 a) with a dating range of 50-1600 a is suitable for alpine glaciers. The anthropogenic ⁸⁵Kr (t_1/2=10.8 a) is particularly useful to quantify contamination with modern air. Due to advances in the detection of ⁸¹Kr, ⁸⁵Kr and ³⁹Ar with Atom Trap Trace Analysis (ATTA), the sample size has been reduced to ~ 1 kg of polar ice. However, this amount can still be difficult to obtain, for example from the upcoming deepest sections of the “Beyond EPICA – Oldest Ice Core” (BEOI), for which no archive piece will be conserved.

Here, we present ⁸⁵Kr and ⁸¹Kr results for gas samples from an Antarctic ice core extracted at the debubbler waste line of a continuous flow analysis (CFA) system. From the continuous melting of ~3 m long core, discrete ~ 100 mL STP gas samples have been extracted, and subsequently analyzed offline for ⁸⁵Kr and ⁸¹Kr. The ⁸⁵Kr results indicate a minor contamination with modern air of 1-2 %, which can likely be reduced by an earlier bypassing of contaminant air from cracks within a CFA stick and transitions between sequential CFA sticks.

The presented extraction system enables ⁸¹Kr and ³⁹Ar dating of an ice core at numerous depths without additional ice demand, which is particularly relevant for upcoming CFA-melting campaigns of deep polar ice cores.