Journal cover Journal topic
Climate of the Past An interactive open-access journal of the European Geosciences Union
Clim. Past, 12, 1061-1077, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
26 Apr 2016
Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation
Rachael H. Rhodes1,a, Xavier Faïn2, Edward J. Brook1, Joseph R. McConnell3, Olivia J. Maselli3,b, Michael Sigl3,4, Jon Edwards1, Christo Buizert1, Thomas Blunier5, Jérôme Chappellaz2, and Johannes Freitag6 1College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis OR, USA
2Université Grenoble Alpes/CNRS, Laboratoire de Glaciologie et Géophysique de l'Environnement, Grenoble, France
3Division of Hydrologic Sciences, Desert Research Institute, Reno NV, USA
4Laboratory for Radiochemistry and Environmental Chemistry, Paul Scherrer Institut, Villigen, Switzerland
5Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark
6Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
anow at: Department of Earth Sciences, University of Cambridge, Cambridge, UK
bnow at: Department of Chemistry, University of Adelaide, South Australia, Australia
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.

Citation: Rhodes, R. H., Faïn, X., Brook, E. J., McConnell, J. R., Maselli, O. J., Sigl, M., Edwards, J., Buizert, C., Blunier, T., Chappellaz, J., and Freitag, J.: Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation, Clim. Past, 12, 1061-1077,, 2016.
Publications Copernicus
Short summary
Local artifacts in ice core methane data are superimposed on consistent records of past atmospheric variability. These artifacts are not related to past atmospheric history and care should be taken to avoid interpreting them as such. By investigating five polar ice cores from sites with different conditions, we relate isolated methane spikes to melt layers and decimetre-scale variations as "trapping signal" associated with a difference in timing of air bubble closure in adjacent firn layers.
Local artifacts in ice core methane data are superimposed on consistent records of past...