High-resolution glacial and deglacial record of atmospheric methane by continuous-flow and laser spectrometer analysis along the NEEM ice core

Abstract

Abstract. The Greenland NEEM (North Greenland Eemian Ice Drilling) operation in 2010 provided the first opportunity to combine trace-gas measurements by laser spectroscopic instruments and continuous-flow analysis along a freshly drilled ice core in a field-based setting. We present the resulting atmospheric methane (CH 4 ) record covering the time period from 107.7 to 9.5 ka b2k (thousand years before 2000 AD). Companion discrete CH 4 measurements are required to transfer the laser spectroscopic data from a relative to an absolute scale. However, even on a relative scale, the high-resolution CH 4 data set significantly improves our knowledge of past atmospheric methane concentration changes. New significant sub-millennial-scale features appear during interstadials and stadials, generally associated with similar changes in water isotopic ratios of the ice, a proxy for local temperature. In addition to the midpoint of Dansgaard-Oeschger (D/O) CH 4 transitions usually used for cross-dating, sharp definition of the start and end of these events brings precise depth markers (with ±20 cm uncertainty) for further cross-dating with other palaeo-or ice core records, e.g. speleothems. The method also provides an estimate of CH 4 rates of change. The onsets of D/O events in the methane signal show a more rapid rate of change than their endings. The rate of CH 4 increase associated with the onsets of D/O events progressively declines from 1.7 to 0.6 ppbv yr −1 in the course of marine isotope stage 3. The largest observed rate of increase takes place at the onset of D/O event #21 and reaches 2.5 ppbv yr −1 .

Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions

“…Online gas measurements of methane were conducted with a SARA laser spectrometer developed at Laboratoire Interdisciplinaire de Physique (Grenoble, France) based on optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS; Morville et al, 2005;Romanini et al, 2006). Such a laser spectrometer has been used before for continuous-flow gas analyses (e.g., Chappellaz et al, 2013;Rhodes et al, 2013Rhodes et al, , 2015Rhodes et al, , 2016Faïn et al, 2014); however, the IGE CFA system was specifically optimized to reduce experimental smoothing by limiting all possible dead and mixing volumes along the sample line. For this study the rate of OF-CEAS spectrum acquisition was 6 Hz.…”

“…Detailed descriptions of this method have been reported before (Stowasser et al, 2012;Chappellaz et al, 2013;Rhodes et al, 2013). Ice core sticks of 34 by 34 mm were melted at IGE at a mean rate of 3.8 cm min −1 using a melt head as described by Bigler et al (2011), and the water and gas bubble mixture was pumped toward a low-volume T-shaped glass debubbler.…”

“…High-resolution methane concentration (combined with carbon monoxide) measurements were performed along a section of the Vostok 4G-2 ice core drilled in the Antarctic plateau. The section studied corresponds to the Dansgaard-Oeschger event number 17 (DO-17, ∼ 60 000 yr BP), a climatic event associated with particularly fast and large atmospheric methane variations (Brook et al, 1996;Chappellaz et al, 2013;Rhodes et al, 2015). This makes this event especially adapted for the quantification of both gas record smoothing and layered trapping.…”

“…From a gas point of view the firn is traditionally divided in three main parts from the surface to the bottom: the convective zone, the diffusive zone, and the trapping zone (e.g., Schwander, 1989;Buizert et al, 2012). The convective zone is characterized by the mixing of air in the snow with atmospheric air through wind action (Colbeck, 1989). In the diffusive zone the dominant gas transport processes are molecular diffusion and gravitational settling.…”

A 156 kyr smoothed history of the atmospheric greenhouse gases CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O and their radiative forcing

“…While there are recent developments towards continuous CH 4 records using gas extraction and measurement systems coupled to continuous-flow analysis systems (Schüp-bach et al, 2009;Chappellaz et al, 2013;Rhodes et al, 2013, 364 P. Köhler et al: 156 kyr smoothed history of CO 2 , CH 4 , and N 2 O -For some of the CO 2 records obtained from different ice cores, there exist significant and as yet unexplained offsets (Ahn et al, 2012;Bereiter et al, 2012;Marcott et al, 2014;Bauska et al, 2015). These offsets need to be addressed in our data compilation.…”