δ ¹³ C methane source signatures from tropical wetland and rice field emissions

Abstract

The atmospheric methane (CH 4 ) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH 4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ 13 C CH 4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ 13 C CH 4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH 4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH 4 to the atmosphere during periods of reduced wetland extent. This new data expands the geographical extent and range of measurements of tropical wetland and rice δ 13 C CH 4 sources and hints at significant seasonal variation in tropical wetland δ 13 C CH 4 signatures which may be important to capture in future global and regional models. This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Isotopic signatures of methane emissions from tropical fires, agriculture and wetlands: the MOYA and ZWAMPS flights

“…They are at approximately the same latitude in the outer tropics, and sampling was a few weeks apart during the later part of the rainy season in both places, when wetlands were filling. The Miller-Tans isotopic signature reported here from the outer tropical Upper Congo and Zambezi wetlands is very similar to −59‰ values of large methane fluxes measured in the comparable-latitude Bolivian Llanos de Moxos wetlands [41].…”

“…The Bolivian flights measured very large methane enhancements, from which a δ 13 C CH 4 source signature of −58.7‰ ± 1.9‰ was determined, with similar results from concurrent on-ground in situ sampling [ 41 ]…”

“…Unfortunately, the in situ isotopic determinations from Lukanga gave complex results, suggesting a diverse range of local sources advecting to the low flying aircraft. Similarly, diverse signatures have also been seen from ground-based work in the Okavango, Botswana [ 41 ] and may be related to local burning, variable methanotrophy or locally dominant plant species.…”

“…Bolivian sampling flights were over the Mamore River and Llanos de Moxos of North-East Bolivia [ 39 , 40 ], with simultaneous on-surface sample collection. A parallel paper in this collection [ 41 ] examines this region in more detail in the wider context of global tropical isotopic signatures from wetlands and rice fields.…”

“…Summary of Isotopic Signatures. Senegal regional value from Barker et al [42], Lake Victoria Swamp value from Brownlow et al [7], Kenyan cattle from Cozens et al [47], Zambia (all) from [50] (under review) and Bolivian wetlands from France et al [41] While the magnitude of this impact may differ slightly between models depending on the source and sink assumptions, it represents a shift in δ 13 C CH 4 , similar in magnitude to the shift resulting from uncertainties in the tropospheric chlorine sink [55]. Such a modelled shift is much larger than the observed shift in the global burden since 2007 [56].…”