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The trend in atmospheric methane δ 13 C and implications for isotopic constraints on the global methane budget
Author(s) -
Lassey Keith R.,
Lowe David C.,
Manning Martin R.
Publication year - 2000
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/1999gb900094
Subject(s) - disequilibrium , methane , atmospheric methane , environmental science , mixing ratio , atmospheric sciences , atmospheric chemistry , isotopic signature , trace gas , atmosphere (unit) , isotope , geology , chemistry , meteorology , physics , nuclear physics , medicine , organic chemistry , ozone , ophthalmology
A recent paper by Tans [1997] has drawn attention to the isotopic disequilibrium that inevitably prevails when atmospheric methane is not in steady state with its sources, noting in particular the very slow adjustment of the isotopic signature δ 13 C toward its steady state. Our aim in this paper is to clarify the nature of disequilibrium effects on δ 13 C(CH 4 ) and to assess their likely magnitudes in the global atmosphere over recent decades. We use a simple model simulation incorporating a plausible scenario of the global methane source history over 1700–2010, which includes an unchanged source since 1990. The simulation of both mixing ratio and δ 13 C compare favorably with the secular features of a 10‐year data set (1988–1998) from Baring Head, New Zealand, and of a 17‐year data set (1978–1995) in air archived from Cape Grim, Australia. This corroborates a recent analysis of those data sets and their compatibility with stabilized sources. We show that the slow adjustment of δ 13 C toward steady state arises from the effect of isotope fractionation on the cancellation of contributing terms to δ 13 C. We explore the implications of disequilibrium for the usual practice of relating δ 13 C values in the atmosphere to those in the aggregate source through a shift induced by fractionation and quantify the flaws in this practice. Finally, we examine the sensitivity of the atmospheric secular response, in both mixing ratio and δ 13 C, to sustained changes in source and sink and show that δ 13 C is a potentially powerful diagnostic of such changes.