Carbon and Hydrogen Isotope Signatures of Dissolved Methane in the Scheldt Estuary

We collected water samples from the Scheldt estuary during December 2015 and November 2016 for methane (CH₄) concentration and isotopic composition (δ¹³ C and δD values) analyses, to investigate the origin of the excess dissolved CH₄, which is a common feature in estuaries. The Scheldt estuary is a eutrophic, heterotrophic tidal estuary, located at the border between Belgium and the Netherlands. The gas chromatography and mass spectrometry analyses revealed (1) variable dissolved CH₄ concentrations reaching up to 302.6 nM in surface waters of the Port of Antwerp, which fits within the higher range of values reported for European estuaries, and (2) the presence of surprisingly high isotopic signatures in the upper estuary. While microbial CH₄ production dominates in the lower part of the estuary, we observe a clear trend towards isotopically heavier CH₄ upstream where isotopic signatures as enriched as - 25.2‰ for carbon and + 101‰ for hydrogen were measured. We conclude that microbial oxidation of most of the CH₄ pool could explain such enrichments, but that the origin of riverine CH₄ enriched isotopic signatures remains to be explained. This study identifies peculiar features associated with CH₄ cycling in the Scheldt estuary, paving the way for a more thorough biogeochemical quantification of various production/removal processes.