NGRIP CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; concentration from 120 to 10 kyr before present and its relation to a δ&amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N temperature reconstruction from the same ice core

M Baumgartner,T F Stocker,E Capron,G Floch,P Kindler,A Schilt,J Chappellaz,M Leuenberger,H Fischer,J Schwander,O Eicher,R Spahni

doi:10.5194/cp-10-903-2014

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.

Highlights

Methane (CH4) is a potent greenhouse gas with preindustrial atmospheric concentrations changing between 350 and 750 ppb over the last 800 thousand years (Loulergue et al, 2008)
We further show that the relative interpolar concentration difference 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 new data set provides the most detailed profile obtained by discrete measurements of the evolution of the CH4 concentration during the last glacial cycle, and can be compared with continuous CH4 measurements performed on the North Greenland Eemian Ice Drilling (NEEM) ice core (Chappellaz et al, 2013)

Summary

Introduction

Methane (CH4) is a potent greenhouse gas with preindustrial atmospheric concentrations changing between 350 and 750 ppb over the last 800 thousand years (kyr) (Loulergue et al, 2008). This statement is based on measurements on polar ice cores, a unique archive to reconstruct the past atmospheric composition. The increase rate under the anthropogenic influence peaked around 1981 at 17 ppb yr−1 (Etheridge et al, 1998). Atmospheric CH4 concentrations can be readily measured using polar ice cores, important questions concerning the location and strength of the different sources and sinks, and the sensitivity of the emissions to climate change, are still open

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