ALBERT

Description: Superimposed on the coherent and major atmospheric changes in trace gases revealed by ice core records, local high frequency, non-atmospheric features can now be resolved due to improvement s in resolution and precision of analytical techniques. These are signals that could not have survived the low-pass filter effect that firn diffusion exerts on the atmospheric history and therefore do not result from changes in the composition of the atmosphere at the surface of the ice sheet. Using continuous methane (CH4) records obtained from five polar ice cores, we characterize these non-atmospheric signals and explore their origin. Isolated samples, enriched in CH4 in the Tunu13 (Greenland) record are linked to the presence of melt layers. Melting can enrich the methane concentration due to preferential dissolution of methane relative to nitrogen, but we find that an additional in-situ process is required to generate the full magnitude of these anomalies. Furthermore, in the all ice cores studied there is evidence of reproducible, decimetre-scale CH4 variability. Through a series of tests, we demonstrate that this sign al is an artifact of layered bubble trapping in a heterogeneous-density firn column; we term this phenomenon ‘trapping noise’. The magnitude of CH4 trapping noise increases with atmospheric CH4 growth rate and seasonality of density contrasts, and decreases with accumulation rate. Firn air transport model simulations, accounting for layered bubble trapping, are in agreement with our empirical data. Significant annual periodicity is present in the CH4 variability of two Greenland ice cores, suggesting that layered gas trapping at these sites is controlled by regular, seasonal variations in the physical properties of the firn.