Abstract
Abstract. Advances in trace gas analysis allow localised, non-atmospheric features to be resolved in ice cores, superimposed on the coherent atmospheric signal. These high-frequency signals could not have survived the low-pass filter effect that gas diffusion in the firn exerts on the atmospheric history and therefore do not result from changes in the atmospheric composition at the ice sheet surface. Using continuous methane (CH4) records obtained from five polar ice cores, we characterise 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 a solubility effect, but we find that an additional in situ process is required to generate the full magnitude of these anomalies. Furthermore, in all the ice cores studied there is evidence of reproducible, decimetre-scale CH4 variability. Through a series of tests, we demonstrate that this is an artifact of layered bubble trapping in a heterogeneous-density firn column; we use the term “trapping signal” for this phenomenon. The peak-to-peak amplitude of the trapping signal is typically 5 ppb, but may exceed 40 ppb. Signal magnitude increases with atmospheric CH4 growth rate and seasonal density contrast, and decreases with accumulation rate. 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. Future analytical campaigns should anticipate high-frequency artifacts at high-melt ice core sites or during time periods with high atmospheric CH4 growth rate in order to avoid misinterpretation of such features as past changes in atmospheric composition.
Highlights
Continuous measurement of ice core methane (CH4) concentrations utilising laser spectroscopy (Stowasser et al, 2012) is rapidly emerging as a powerful tool in palaeoclimatology, producing highly detailed records of atmospheric methane for the Last Glacial Period (Chappellaz et al, 2013; Rhodes et al, 2015) and Late Holocene (Rhodes et al, 2013)
Multi-decadal scale atmospheric CH4 variability, previously observed in Law Dome DSS (MacFarling Meure et al, 2006), West Antarctic Ice Sheet (WAIS) Divide (Mitchell et al, 2011), Greenland Ice Sheet Project 2 (GISP2) (Mitchell et al, 2013) and NEEM-2011-S1 (Rhodes et al, 2013), is faithfully replicated in all the ice cores analysed in this study (Fig. 2)
When the Tunu13, D4 and North Greenland Ice Core Project (NGRIP) data are all plotted on the same axes with the WAIS Divide model simulation (Fig. 5f), the gradient of the modelled linear relationship is within the range of gradients of our empirical data from three different Greenland ice core sites
Summary
Continuous measurement of ice core methane (CH4) concentrations utilising laser spectroscopy (Stowasser et al, 2012) is rapidly emerging as a powerful tool in palaeoclimatology, producing highly detailed records of atmospheric methane for the Last Glacial Period (Chappellaz et al, 2013; Rhodes et al, 2015) and Late Holocene (Rhodes et al, 2013). Rhodes et al.: Local artifacts in ice core methane records a low-pass filter, removing high-frequency atmospheric signals, such as the CH4 seasonal cycle (Schwander et al, 1993; Trudinger et al, 1997). All polar ice cores yield trace gas records that are smoothed versions of the actual atmospheric history, with the degree of smoothing depending on site conditions, temperature and accumulation rate (Schwander et al, 1997). Trace gas signals present at frequencies above those that could be preserved in the face of the natural smoothing cannot represent atmospheric history. If they are present we must assume that they are not related directly to the original atmospheric variation at the surface of the ice sheet
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