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要旨:
Methane is a strong greenhouse gas and large uncertainties
exist concerning the future evolution of its atmospheric
abundance. Analyzing methane atmospheric mixing
and stable isotope ratios in air trapped in polar ice sheets
helps in reconstructing the evolution of its sources and sinks
in the past. This is important to improve predictions of atmospheric
CH4 mixing ratios in the future under the influence
of a changing climate. The aim of this study is to assess
whether past atmospheric 13C(CH4) variations can be
reliably reconstructed from firn air measurements. Isotope
reconstructions obtained with a state of the art firn model
from different individual sites show unexpectedly large discrepancies
and are mutually inconsistent.We show that small
changes in the diffusivity profiles at individual sites lead to
strong differences in the firn fractionation, which can explain
a large part of these discrepancies. Using slightly modified
diffusivities for some sites, and neglecting samples for which
the firn fractionation signals are strongest, a combined multisite
inversion can be performed, which returns an isotope reconstruction
that is consistent with firn data. However, the
isotope trends are lower than what has been concluded from
Southern Hemisphere (SH) archived air samples and highaccumulation
ice core data.We conclude that with the current
datasets and understanding of firn air transport, a high precision
reconstruction of 13C of CH4 from firn air samples is
not possible, because reconstructed atmospheric trends over
the last 50 yr of 0.3–1.5‰ are of the same magnitude as inherent
uncertainties in the method, which are the firn fractionation
correction (up to 2‰ at individual sites), the
Kr isobaric interference (up to 0.8 ‰, system dependent), inter-laboratory calibration offsets (0.2 ‰) and uncertainties in past CH4 levels (0.5 ‰).
