The First Northern Hemisphere High-Resolution Holocene Methane Record
Reveals a Centennial Variability
Abstract
We present the first Northern Hemisphere high-resolution Holocene
methane (CH4) record obtained from the Renland Ice Cap (ReCAP) – 71.3N
26.6W 2300 m a.s.l. in 2015. The total length of the core is 584 m
containing 532 m of the Holocene ice without brittle ice zone thus
allowing to obtain an uninterrupted CH4 record over the Holocene. An ice
core analytical technique developed in 2010 – continuous flow analysis
(CFA) provides a unique opportunity of revealing high-resolution
greenhouse gas record of the past. Considering a mean annual layer
thickness the depth resolution translates to a nominal temporal
resolution of 35 data points 100 yr-1 over the Meghalayan (back to 4.2
ka BP) Holocene section. Note that due to the average residence time of
CH4 in the atmosphere and the gas age distribution width in the firn,
the maximum temporal effective resolution is lower by a factor of 10.
The pattern of the centennial scale variability seen in the ReCAP CH4
record is coherent with the earlier published Southern Hemisphere record
based on the West Antarctic Ice Divide (WAIS) CH4 record. The wavelet
coherence analysis identifies a high correlation on the long- and
midterm variability (600-700, 200-500 yr periodicity) as expected; the
Meghalayan Holocene section reveals a common variability down to 70 yr.
Inter-laboratory offsets of the absolute CH4 values are likely constant
as the entire cores were running over continuing measurement campaigns.
The elevated ReCAP CH4 level due to the local dust presence is disproved
for at least the Last Glacial section. Gas trapping uncertainties should
not matter on a decadal scale besides probable layered bubble trapping.
Melt layers are untraceable prior to 2 ka BP due to the annual layers
thinning. The analysis was performed on the ReCAP record cleaned from
the possible melting- and the CFA technique-related spikes. This study
leads, however, to a more detailed evaluation of the interpolar
difference in the future work as the absolute value remains unresolved
though we are confident that there were no big variations. We argue that
the centennial variability in the CH4 is explained by the intertropical
convergence zone global teleconnection and its influence on the monsoon
activity and thus the CH4 production by tropical wetlands. The duration
of the periods in CH4 concentration evolves through the time, which
could potentially suggest a change in the atmospheric residence time of
CH4.