DOI: 10.25921/bftn-xa29
title: NOAA/WDS Paleoclimatology - NEEM-2011-S1 Ice Core 1800 Year Continuous Methane Data

description: Ancient air trapped inside bubbles in ice cores can now be analysed for methane concentration utilising a laser spectrometer coupled to a continuous melter system. We present a new ultra-high resolution record of atmospheric methane variability over the last 1800 yr obtained from continuous analysis of a shallow ice core from the North Greenland Eemian project (NEEM-2011-S1) during a 4-week laboratory-based measurement campaign. Our record faithfully replicates the form and amplitudes of multi-decadal oscillations previously observed in other ice cores and demonstrates the detailed depth resolution (5.3 cm), rapid acquisition time (30 m day-1) and good long-term reproducibility (2.6%, 2sigma) of the continuous measurement technique.

In addition, we report the detection of high frequency ice core methane signals of non-atmospheric origin. Firstly, measurements of air from the firn-ice transition region and an interval of ice core dating from 1546-1560 AD (gas age) resolve apparently quasi-annual scale methane oscillations. Traditional gas chromatography measurements on discrete ice samples confirm these signals and indicate peak-to-peak amplitudes of ca. 22 parts per billion (ppb). We hypothesise that these oscillations result from staggered bubble close-off between seasonal layers of contrasting density during time periods of sustained multi-year atmospheric methane change. Secondly, we report the detection of abrupt (20-100 cm depth interval), high amplitude (35-80 ppb excess) methane spikes in the NEEM ice that are reproduced by discrete measurements. We show for the first time that methane spikes present in thin and infrequent layers in polar, glacial ice are accompanied by elevated concentrations of carbon- and nitrogen-based chemical impurities, and suggest that biological in-situ production may be responsible.
description: Methane concentration from the Greenland NEEM-2011-S1 Ice Core from 71 to 408m depth (~270-1961AD). Methane concentrations analysed online by laser spectrometer (SARA, Spectroscopy by Amplified Resonant Absorption, developed at Laboratoire Interdisciplinaire de Physique, Grenoble, France) on gas extracted from an ice core processed using a continuous melter system (Desert Research Institute). Methane data have a 5 second integration time (raw data acquisition rate 0.6 Hz).  Analytical precision, from Allan Variance test, is 0.9 ppb (2 sigma). Long-term reproducibility is 2.6% (2 sigma). Gaps in the record are due to problems during online analysis. Online analysis conducted August-September 2011. Note: Lat-Long provided is for main NEEM borehole. The NEEM-2011-S1 core was drilled 200 m distance away in 2011 to 410 m depth.

Four data series are presented, as follows: 
1: neem2011s1-ch4.txt = entire dataset 
2: neem2011s1-ch4-outliers-removed.txt = Data minus data points exceeding cut-off value. Cut-off value is 2*median absolute deviation (MAD)> 15 yr running median. Different MAD values used for 250-1000 AD and 1100-1835 AD sections of record.
3: neem2011s1-ch4-outliers.txt = Data points removed from dataset according to specified cut-off value. Please refer to Rhodes et al. (2013) for full discussion of origins outlying data points. Briefly, these high frequency features are not artifacts of the continuous method and have been replicated by traditional discrete analyses.  Comparison to chemistry measurements suggests they are related to biological in situ production of methane. 
4: neem2011s1-ch4-5yr.txt = Median CH4 concentrations for 5 year time slices of Data_outliers_removed. This can be utilised as a time series of atmospheric methane concentration because the influence of outliers and other high frequency features (that could not survive firn smoothing processes (see Spahni et al., 2003) is removed. 

Methane concentrations are reported on NOAA2004 scale (instrument calibrated on dry synthetic air standards). A correction factor of 1.079 has been applied to all data to correct for methane dissolution in melted ice core sample prior to gas extraction. Correction factor calculated using empirical data (concentrations not aligned/tied to existing discrete methane measurements).

Additional methods description provided in: Stowasser et al. 2012. Continuous measurements of methane mixing ratios from ice cores. Atmos. Meas. Tech. 5, 999-1013, and Morville et al. 2005. Fast, low-noise, mode bymode, cavity-enhanced absorption spectroscopy by diode-laser self-locking. Appl. Phys. B Lasers Opt. 80, 1027-01038.

NEEM (North Greenland Eemian Ice Drilling) project information http://neem.dk/