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Rhodes, Rachael H.
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Faïn, Xavier
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Brook, Edward J.
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McConnell, Joseph R.
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Maselli, Olivia J.
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Sigl, Michael
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Edwards, Jon
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Buizert, Christo
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Blunier, Thomas
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Chappellaz, Jérôme
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Freitag, Johannes
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| 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.
- This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Copernicus Publications on behalf of the European Geosciences Union. The published article can be found at: http://www.climate-of-the-past.net/
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- Rhodes, R. H., Faïn, X., Brook, E. J., McConnell, J. R., Maselli, O. J...& Freitag, J. (2016). Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation. Climate of the Past, 12(4), 1061-1077. doi:10.5194/op-12-1061-2016
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| Funding Statement (additional comments about funding) |
- This work was supported by US National Science Foundation (NSF) grants 1204172, 0944552, 1204176 and 0909541 and NSF Partnerships in International Research and Education (PIRE) Grant 0968391. This work was additionally supported by the French ANR program RPD COCLICO (ANR-10-RPDOC-002-01) and received funding from the European Research Council under the European Community's Seventh Framework Program FP7/2007-2013 Grant Agreement #291062 (project ICE&LASERS). It is supported by funding agencies and institutions in Belgium (FNRS-CFB and FWO), Canada (NRCan/GSC), China (CAS), Denmark (FIST), France (IPEV, CNRS/INSU, CEA and ANR), Germany (AWI), Iceland (RannIs), Japan (NIPR), Korea (KOPRI), The Netherlands (NWO/ALW), Sweden (VR), Switzerland (SNF), United Kingdom (NERC) and the USA (US NSF, OPP).
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