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Bipolar carbon and hydrogen isotope constraints on the Holocene methane budget
Ist Teil von
Biogeosciences, 2018-11, Vol.15 (23), p.7155-7175
Ort / Verlag
Katlenburg-Lindau: Copernicus GmbH
Erscheinungsjahr
2018
Link zum Volltext
Quelle
EZB Free E-Journals
Beschreibungen/Notizen
Atmospheric methane concentration shows a well-known decrease over the first
half of the Holocene following the Northern Hemisphere summer insolation
before it started to increase again to preindustrial values. There is a
debate about what caused this change in the methane concentration evolution, in
particular, whether an early anthropogenic influence or natural emissions led
to the reversal of the atmospheric CH4 concentration evolution. Here, we
present new methane concentration and stable hydrogen and carbon isotope data
measured on ice core samples from both Greenland and Antarctica over the
Holocene. With the help of a two-box model and the full suite of CH4
parameters, the new data allow us to quantify the total methane emissions in
the Northern Hemisphere and Southern Hemisphere separately as well as their stable isotopic
signatures, while interpretation of isotopic records of only one hemisphere
may lead to erroneous conclusions. For the first half of the Holocene our
results indicate an asynchronous decrease in Northern Hemisphere and Southern Hemisphere
CH4 emissions by more than
30 Tg CH4 yr−1 in total,
accompanied by a drop in the northern carbon isotopic source signature of
about −3 ‰. This cannot be explained by a change in
the source mix alone but requires shifts in the isotopic signature of the
sources themselves caused by changes in the precursor material for the
methane production. In the second half of the Holocene, global CH4
emissions increased by about 30 Tg CH4 yr−1, while
preindustrial isotopic emission signatures remained more or less constant.
However, our results show that this early increase in methane emissions took
place in the Southern Hemisphere, while Northern Hemisphere emissions started
to increase only about 2000 years ago. Accordingly, natural emissions in the
southern tropics appear to be the main cause of the CH4 increase
starting 5000 years before present, not supporting an early anthropogenic influence on the
global methane budget by East Asian land use changes.