Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation

• Published in: Climate of the Past
• Main Authors: Rhodes, Rachael, Faïn, Xavier, Brook, Edward, Mcconnell, Joseph, Maselli, Olivia, Sigl, Michael, Edwards, Jon, Buizert, Christo, Blunier, Thomas, Chappellaz, Jérôme, Freitag, Johannes
• Other Authors: College of Earth, Ocean and Atmospheric Sciences Corvallis (CEOAS), Oregon State University (OSU), Laboratoire de glaciologie et géophysique de l'environnement (LGGE), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Observatoire des Sciences de l'Univers de Grenoble (OSUG ), Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 )-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 ), Division of Hydrologic Sciences, Desert Research Institute (DRI), Paul Scherrer Institute (PSI), Centre for Ice and Climate Copenhagen, Niels Bohr Institute Copenhagen (NBI), Faculty of Science Copenhagen, University of Copenhagen = Københavns Universitet (UCPH)-University of Copenhagen = Københavns Universitet (UCPH)-Faculty of Science Copenhagen, University of Copenhagen = Københavns Universitet (UCPH)-University of Copenhagen = Københavns Universitet (UCPH), Université Grenoble Alpes 2016-2019 (UGA 2016-2019 ), Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung (AWI)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology; Greenland; Greenland ice cores; ice core; Ice Sheet
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-01387742; https://hal-insu.archives-ouvertes.fr/insu-01387742/document; https://hal-insu.archives-ouvertes.fr/insu-01387742/file/CLIMATE%20OF%20THE%20PAST%20-%20Local%20artifacts%20in%20ice%20core%20methane%20records%20caused%20by%20layered%20bubble%20trapping%20and%20in%20situ%20production%20a%20multi-site%20investigation.pdf; https://doi.org/10.5194/cp-12-1061-2016

International audience 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 (CH 4) records obtained from five polar ice cores, we characterise these non-atmospheric signals and explore their origin. Isolated samples, enriched in CH 4 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 CH 4 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 CH 4 growth rate and seasonal density contrast, and decreases with accumulation rate. Significant annual periodicity is present in the CH 4 variability of two Greenland ice cores, suggesting that lay-ered 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 arti-facts at high-melt ice core sites or during time periods with high atmospheric CH 4 growth rate in order to avoid misinterpretation of such features as past changes in atmospheric composition.