DOI: 10.25921/f28c-1r24
title: NOAA/WDS Paleoclimatology - Atmospheric CH4, Interpolar Difference and Excesss Methane Data from the WAIS Divide, GISP2, and NEEM Ice Cores, Last Glacial Maximum and Deglaciation

description: Constraining the causes of past atmospheric methane variability is important for understanding links between methane and climate. Abrupt methane changes during the last deglaciation have been intensely studied for this purpose, but the relative importance of high-latitude and tropical sources remains poorly constrained. The methane interpolar concentration difference reflects past geographic emission variability, but existing records suffered from subtle but considerable methane production during analysis. Here, we report an ice-core-derived interpolar difference record covering the Last Glacial Maximum and deglaciation, with substantially improved temporal resolution, chronology and a critical correction for methane production in samples from Greenland. Using box models to infer latitudinal source changes, we show that tropical sources dominated abrupt methane variability of the deglaciation, highlighting their sensitivity to abrupt climate change and rapidly shifting tropical rainfall patterns. Northern extratropical emissions began increasing ~16,000 years ago, probably through wetland expansion and/or permafrost degradation induced by high-latitude warming, and contributed at most 25 Tg yr−1 (45% of the total emission increase) to the abrupt methane rise that coincided with rapid northern warming at the onset of the Bølling–Allerød interval. These constraints on deglacial climate–methane cycle interactions can improve the understanding of possible present and future feedbacks.
description: Site locations - WAIS Divide: -79.468 S, -112.086 W, 1797m; GISP2: 72.583333 N, -38.466667 W, 3200m; NEEM: 77.45 N, -51.066667 W, 2450m. This study includes measurements of atmospheric methane from samples from the NEEM, GISP2 and WAIS Divide ice cores. 
All measurements were made at the Oregon State University Ice Core and Quaternary Geochemistry Laboratory (Corvallis, OR) using an established analytical system. 433 samples from the NEEM ice core were measured between 1420 and 1560m depths. A measurement uncertainty ranging from 2.8 to 4.2 ppb, depending on the measurement year, was determined from replicate samples. 340 samples from the GISP2 ice core were measured between 1740 and 2060m depths. A measurement uncertainty ranging from 3.1 to 3.4 ppb, depending on the measurement year, was determined from replicate samples. All GISP2 and NEEM data were corrected for excess methane contamination using the established relationship between excess methane and Ca2+ (Lee et al., 2020). Both corrected and uncorrected data are included in the publication. 340 samples from the GISP2 ice core were measured between 1957 and 3081m depths. A measurement uncertainty ranging from 3.1 to 3.4 ppb, depending on the measurement year, was determined from replicate samples. Depths reflect the mid-points of the depth range of each samples, which is typically ~8cm (Lee et al.(2020) DOI: 10.1016/j.gca.2019.11.020).
 This study includes estimates of the atmospheric methane relative interpolar difference (rIPD) across the Last Glacial Maximum and Deglaciation. The rIPD was calculated using discrete, high-resolution methane measurements from the WAIS Divide, NEEM and GISP2 ice cores. Two independent IPD records were determined: One using NEEM and WAIS and one using GISP2 and WAIS. The dataset includes rIPD values calculated using both Greenland methane data both corrected and uncorrected for excess methane (Lee et al., 2020). The rIPD was calculated by smoothing each methane record and synchronizing them to the WD2014 gas age scale. 1-sigma rIPD uncertainties are included. This dataset also includes the output of the four-box troposphere model used to interpret the rIPD. For both excess methane-corrected records, the model output and 1-sigma uncertainty is provided for northern extratropical (30N - 90N) and total tropical (30S - 30N) sources in Tg yr-1. For the NEEM-derived rIPD, sections of the rIPD where atmospheric methane changed rapidly were deemed untrustworthy were removed from the dataset (Lee et al.(2020) DOI: 10.1016/j.gca.2019.11.020).
 This study includes new measurements of in-situ, non-atmospheric methane release during the melt-refreeze analysis of ice samples for atmospheric methane measurement, also known as excess methane (CH4xs). All measurements were made at the Oregon State University Ice Core Laboratory and followed the methods of Lee et al., 2020 and Mitchell et al., 2013. Measurements were made using ice from the GISP2 and NEEM ice cores in 2018, 2019, and 2021. In total, 70 new measurements were made on NEEM ice and 406 measurements on GISP2 ice. The Ca2+ content of the ice was calculated using the published Ca2+ concentration records, and the mass and depth ranges of each sample. For GISP2, the year the measurement was made is included. Provided Keywords: Methane, Last Glacial Maximum, Deglaciation, Greenhouse gases, terrestrial biogeochemistry