NGRIP CH<sub>4</sub> concentration from 120 to 10 kyr before present and its relation to a δ<sup>15</sup>N temperature reconstruction from the same ice core

Abstract

Abstract. During the last glacial cycle, Greenland temperature showed many rapid temperature variations, the so-called Dansgaard–Oeschger (DO) events. The past atmospheric methane concentration closely followed these temperature variations, which implies that the warmings recorded in Greenland were probably hemispheric in extent. Here we substantially extend and complete the North Greenland Ice Core Project (NGRIP) methane record from the Preboreal Holocene (PB) back to the end of the last interglacial period with a mean time resolution of 54 yr. We relate the amplitudes of the methane increases associated with DO events to the amplitudes of the local Greenland NGRIP temperature increases derived from stable nitrogen isotope (δ15N) measurements, which have been performed along the same ice core (Kindler et al., 2014). We find the ratio to oscillate between 5 parts per billion (ppb) per °C and 18 ppb °C−1 with the approximate frequency of the precessional cycle. A remarkably high ratio of 25.5 ppb °C−1 is reached during the transition from the Younger Dryas (YD) to the PB. Analysis of the timing of the fast methane and temperature increases reveals significant lags of the methane increases relative to NGRIP temperature for DO events 5, 9, 10, 11, 13, 15, 19, and 20. These events generally have small methane increase rates and we hypothesize that the lag is caused by pronounced northward displacement of the source regions from stadial to interstadial. We further show that the relative interpolar concentration difference (rIPD) of methane is about 4.5% for the stadials between DO events 18 and 20, which is in the same order as in the stadials before and after DO event 2 around the Last Glacial Maximum. The rIPD of methane remains relatively stable throughout the full last glacial, with a tendency for elevated values during interstadial compared to stadial periods.

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

“…Northern hemispheric (NH) data are shown for comparison but are not included in the spline, since for such efforts chronologies of ice cores from both hemispheres have to match perfectly during abrupt climate changes of the D/O events. However, as has been shown (Baumgartner et al, 2014), there remains some mismatch in the timing of the NH and the SH CH 4 records in the most recent chronology AICC2012. NH CH 4 , and consequently global CH 4 concentrations, should, according to the estimates of the interpolar difference, be larger than our SH CH 4 values.…”

“…However, even with the recent efforts on ice core age scale development, there remain issues with this north-south synchronisation. For example, inconsistencies in the timing of abrupt changes in CH 4 concentration in the North Greenland Ice Core Project (NGRIP), EPICA Dronning Maud Land (EDML), and Talos Dome (TALDICE) ice cores have been identified for several D/O event transitions (Baumgartner et al, 2014) if based on AICC2012, the Antarctic Ice Core Chronology of four major Antarctic ice cores . Furthermore, when comparing data from the West Antarctic Ice Sheet Divide ice core (WDC) on its most recent age scale, WD2014, with data from Greenlandic ice cores, the chronology of the latter (GICC05) has been stretched by 0.63 % in order to find the best match to the absolute U/Th-dated paleo record of Hulu Cave (WAIS Divide Project Members, 2015).…”

Climatic and insolation control on the high-resolution total air content in the NGRIP ice core

“…On average, the TAC minimum is synchronous with the δ 18 O ice maximum within the error (12 ± 290 years) on the AICC2012 age scale. However, as stated by Baumgartner et al (2014), the AICC2012 gas age scale suffers from inconsistency with the AICC2012 ice age scale for several DO events as gas and ice have been synchronized between different ice cores to some extent independently. This leads to a dephasing of CH 4 and δ 18 O ice records in some cores on the AICC2012 age scale, which is absent in the original age scales, where gas age scale has been determined by adding the gas-age-ice-age difference to the ice age scale.…”

“…We infer that a considerable part of the scattering in neighboring samples seems to result from the small-scale variability of the TAC signal in the ice itself. One option to verify this is to check whether the scattering diminishes with depth and therefore decreasing annual layer thickness, as described in Baumgartner et al (2014). If the scattering is a signal in the ice itself, the standard deviation of the values in the five adjacent reproducibility samples should get smaller with increasing depth, since high-frequency variations will be smoothed by the increasing time interval per sample due to layer thinning with depth.…”

“…Using this vacuum-melt technique 62 NGRIP samples of 160 g were also measured and are presented in this study. The 1626 other data points were measured as a by-product of CH 4 and N 2 O concentration measurements performed in Bern over many years (Flückiger et al, 2004;Schilt et al, 2010;Baumgartner et al, 2012;Schilt et al, 2013;Baumgartner et al, 2014) with a different instrument. The meltwater was refrozen during the gas extraction process; therefore, we refer to this data as melt-refreeze data.…”