Sources and sinks of methane in sea ice: Insights from stable isotopes

• Published in: Elementa: Science of the Anthropocene
• Main Authors: Jacques, Caroline, Sapart, Celia J., Fripiat, François, Carnat, Gauthier, Zhou, Jiayun, Delille, Bruno, Röckmann, Thomas, van der Veen, Carina, Niemann, Helge, Haskell, Tim, Tison, Jean-Louis
• Format: Article in Journal/Newspaper
• Language: English
• Published: University of California Press 2021
• Subjects: VDP::Mathematics and natural science: 400::Geosciences: 450; VDP::Matematikk og Naturvitenskap: 400::Geofag: 450; Antarctic; Cape Evans; Mount Erebus; Antarc*; Antarctica; Arctic; Barrow; Sea ice; Alaska
• Online Access: https://hdl.handle.net/10037/23138; https://doi.org/10.1525/elementa.2020.00167

We report on methane (CH 4) stable isotope (d 13 C and d 2 H) measurements from landfast sea ice collected near Barrow (Utqiagvik, Alaska) and Cape Evans (Antarctica) over the winter-to-spring transition. These measurements provide novel insights into pathways of CH 4 production and consumption in sea ice. We found substantial differences between the two sites. Sea ice overlying the shallow shelf of Barrow was supersaturated in CH 4 with a clear microbial origin, most likely from methanogenesis in the sediments. We estimated that in situ CH 4 oxidation consumed a substantial fraction of the CH 4 being supplied to the sea ice, partly explaining the large range of isotopic values observed (d 13 C between –68.5 and –48.5 ‰ and d 2 H between –246 and –104 ‰). Sea ice at Cape Evans was also supersaturated in CH 4 but with surprisingly high d 13 C values (between –46.9 and –13.0 ‰), whereas d 2 H values (between –313 and –113 ‰) were in the range of those observed at Barrow.These are the first measurements of CH 4 isotopic composition in Antarctic sea ice. Our data set suggests a potential combination of a hydrothermal source, in the vicinity of the Mount Erebus, with aerobic CH 4 formation in sea ice, although the metabolic pathway for the latter still needs to be elucidated. Our observations show that sea ice needs to be considered as an active biogeochemical interface, contributing to CH 4 production and consumption, which disputes the standing paradigm that sea ice is an inert barrier passively accumulating CH 4 at the ocean-atmosphere boundary.