A thesis for the degree of Doctor of Philosophy defended November 2018.
The PhD School of Science, Faculty of Science, Physics of Ice Climate and Earth, Niels Bohr Institute, University of Copenhagen
Prof. Thomas Blunier
Dr. Alexey Ekaykin
High-resolution methane record of the RECAP ice core (Eastern Greenland) over the last climatic cycle
Methane (CH4) is among the three most powerful greenhouse gases. Since the beginning of the atmospheric monitoring, its concentration has risen abruptly for both natural and anthropogenic reasons. Such abrupt variations may also have occurred in the past, before the start of atmospheric monitoring. The only known direct measure of prehistoric gas content is to recover the air composition trapped within the ice sheets. The current study is focused on the CH4 concentration variability mainly over three timespans: Last Glacial period, modern interglacial Holocene and at present days.
Here we present a high-resolution CH4 record, drilled in 2015 through the Renland Ice Cap (RECAP) in eastern Greenland – 71.3N 26.6W 2300 m a.s.l. , a record that covers the last climatic cycle (120 000 years). This record represents the first continuous high-resolution Holocene CH4 record of the Northern Hemisphere. The pattern of the centennial scale variability is coherent with earlier published Southern Hemisphere ice core CH4 records, on the long- and midterm variability (600-700, 200-500 yr periodicity) as well as a short-term (down to 70 yr) in the Late Holocene. A possible explanation for this observation could be in the global teleconnection of the intertropical convergence zone and its influence on the monsoon activity, and consequently the CH4 emission from tropical wetlands.
CH4 concentrations exhibit variable amplitudes during the succession of the stadial-interstadial (GS/GI) transitions of the Last Glacial. These amplitudes differ from the corresponding temperature variabilities. A comparison between CH4 concentrations in the two hemispheres, and a paleoclimatic reconstruction at the transitions GS/GI-19-21 (67-87 000 years ago), reveals a presence of an additional CH4 source in Northern Eurasia, active in 78 000 – 83 000 years ago. The identified elevated relative interpolar difference in the CH4 concentration possibly occurred due to exposed wetlands, after the retreat of the sheet and the following drainage of ice-dammed lakes in the area.
A CH4 signal was also measured on site in the ice surface air layer and the upper firn column in the vicinity of the East Greenland Ice-core Project (EGRIP), 75.6268N 35.9915W, during the summer season 2017. The presence of the diurnal variability is exhibited, it is persistent during days with low wind speed (3-5 knots) and without surface melting; the upper firn signal matches the amplitude and the average value of the atmospheric signal. The presence of the diurnal cycle in the convective firn zone demonstrates an earlier unrevealed feature of the atmospheric gas signal propagation.