Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions
Abstract. We investigate for the first time the loss and alteration of past atmospheric information from air trapping mechanisms under low-accumulation conditions through continuous CH4 (and CO) measurements. Methane concentration changes were measured over the Dansgaard–Oeschger event 17 (DO-17, ∼ 60 000 yr BP) in the Antarctic Vostok 4G-2 ice core. Measurements were performed using continuous-flow analysis combined with laser spectroscopy. The results highlight many anomalous layers at the centimeter scale that are unevenly distributed along the ice core. The anomalous methane mixing ratios differ from those in the immediate surrounding layers by up to 50 ppbv. This phenomenon can be theoretically reproduced by a simple layered trapping model, creating very localized gas age scale inversions. We propose a method for cleaning the record of anomalous values that aims at minimizing the bias in the overall signal. Once the layered-trapping-induced anomalies are removed from the record, DO-17 appears to be smoother than its equivalent record from the high-accumulation WAIS Divide ice core. This is expected due to the slower sinking and densification speeds of firn layers at lower accumulation. However, the degree of smoothing appears surprisingly similar between modern and DO-17 conditions at Vostok. This suggests that glacial records of trace gases from low-accumulation sites in the East Antarctic plateau can provide a better time resolution of past atmospheric composition changes than previously expected. We also developed a numerical method to extract the gas age distributions in ice layers after the removal of the anomalous layers based on comparison with a weakly smoothed record. It is particularly adapted for the conditions of the East Antarctic plateau, as it helps to characterize smoothing for a large range of very low-temperature and low-accumulation conditions.
