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The First Northern Hemisphere High-Resolution Holocene Methane Record Reveals a Centennial Variability
  • Diana Vladimirova
Diana Vladimirova
Centre for Ice and Climate, University of Copenhagen

Corresponding Author:diana.vladimirova@nbi.ku.dk

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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.