DOI: 10.25921/0fhn-ns06
title: NOAA/WDS Paleoclimatology - Antarctic Ice Cores 160KYr Methane Isotope Data

description: During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. Here we assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition d13C of methane in Antarctic ice cores from Dronning Maud Land and Vostok. We find that variations in the d13C of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO2 concentrations. We interpret this to reflect a climatic and CO2-related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable d13C values occurred during intervals of large changes in the atmospheric loading of methane. We suggest that most methane sources - most notably tropical wetlands - must have responded simultaneously to climate changes across these periods.
description: We reconstructed atmospheric d13CH4 records using wet extraction techniques and continuous flow gas
chromatography combustion isotope ratio mass spectrometry (GC/C/IRMS) measurements performed on ice core
material from both the European Project for Ice Coring in Antarctica (EPICA) core from Dronning Maud Land
(EDML) and the Vostok cores. At the Alfred Wegener Institute (AWI) we analyzed 151 samples from the EDML core
(including 32 replicates, reproducibility of 0.2 , 1s) to construct a high resolution record between 20 kyr and 75 kyr BP
(1.000 years before present, present referring to 1950 AD) with an average temporal resolution of better than one
sample every 500 years. The period covering the two most pronounced climatic excursions in the Greenland
temperature record during MIS 3 (DO events 7 and 8) were sampled with a resolution of ~200 years. A
complementary record of 79 samples of Vostok ice core material was measured at the Pennsylvania State University
(PSU) covering a time period from 50 kyr to 160 kyr BP with an analytical uncertainty of 0.3 (1s), equivalent to an
average temporal resolution of 1660 years.
The two records were corrected for an instrumental interference from atmospheric krypton, for gravitational settling in
the firn, and a minor inter-laboratory offset of 0.14 .
All d13C values are reported versus Vienna PeeDee Belemnite (VPDB).
We further analyzed 20 samples of EDML ice for dD(CH4) with an external precision of about 2.5 (1s). The
measurements were performed at the University of Bern using a purge and trap extraction coupled to a gas
chromatography pyrolysis isotope ratio mass spectrometer (GC/P/IRMS) and cover the MIS 5/4 transition with an
average resolution of 1.5 kyr in between 54 and 85 kyr BP. The dD(CH4) values are reported with respect to
the international Vienna Standard Mean Ocean Water (VSMOW) scale.