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 (CH4) record covering the time period from 107.7 to 9.5 ka b2k (thousand years before 2000 AD). Companion discrete CH4 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 CH4 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) CH4 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 CH4 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 CH4 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.
Highlights
The methane (CH4) mixing ratio in air bubbles is one of the components which has been systematically measured along ice cores since the late 1980s
The ice chronology is matched to the Greenland Ice Core Chronology 2005 (GICC05) using 700 match points from electrical conductivity profiles conducted on the NGRIP and North Greenland Eemian Ice Drilling (NEEM) cores
We make use of this to list a full suite of NEEM stratigraphic markers in the gas phase (Table 2), which could be further valorized when more continuous-flow profiles of atmospheric methane will be available from other cores, or, for example, in detailed comparison between the CH4 record and speleothem profiles related to climate/humidity changes in the major source regions of atmospheric methane
Summary
The methane (CH4) mixing ratio in air bubbles is one of the components which has been systematically measured along ice cores since the late 1980s. In the warm laboratory for CFA chemical measurements mixing ratios were installed in series downstream of the (T ≈ 20 ◦C) a peristaltic pump passes the stream of melt CFA gas extraction system: a prototype laser spectrome- water and bubbles of past atmospheric air The NEEM 2010 field season allowed for the first polar field deployment of a prototype OF-CEAS laser spectrometer called SARA (LIPhy, Grenoble, France) It differs from WS-CRDS as the measurement relies on quantifying the light transmission of a high-finesse optical cavity at a sequence of precisely given wavelengths instead of measuring a ringdown time (see Romanini et al, 2006, for technical details). Given the melt rate of 3.2 cm min−1, a periodic CH4 signal with a wavelength of 5.3 cm can still be detected, its amplitude will be significantly dampened
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