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Rhodes, Rachael H; Brook, Edward J; Blunier, Thomas; McConnell, Joseph R; Romanini, Daniele (2017): West Antarctic Ice Sheet (WAIS) Divide ice core ultra-high resolution continuous CH4 measurements 67.2-9.8 ka BP. PANGAEA, https://doi.org/10.1594/PANGAEA.875982, Supplement to: Rhodes, Rachael H; Brook, Edward J; Chiang, John C H; Blunier, Thomas; Maselli, Olivia J; McConnell, Joseph R; Romanini, Daniele; Severinghaus, Jeffrey P (2015): Enhanced tropical methane production in response to iceberg discharge in the North Atlantic. Science, 348(6238), 1016-1019, https://doi.org/10.1126/science.1262005

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Abstract:
Rhodes et al. 2015 (doi:10.1126/science.1262005)
The causal mechanisms responsible for the abrupt climate changes of the Last Glacial Period remain unclear. One major difficulty is dating ice rafted debris (IRD) deposits associated with Heinrich events: Extensive icebergs influxes into the North Atlantic Ocean, linked to global impacts on climate and biogeochemistry. In a new ice core record of atmospheric methane with ultra-high temporal resolution, we find abrupt methane increases within Heinrich stadials 1, 2, 4 and 5 that, uniquely, have no counterparts in Greenland temperature proxies. Using a heuristic model of tropical rainfall distribution, we propose that Hudson Strait Heinrich events caused rainfall intensification over Southern Hemisphere land areas, thereby producing excess methane in tropical wetlands. Our findings suggest that the climatic impacts of Heinrich events persisted for 740 to 1520 years.
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Rhodes et al. 2017 (doi:10.1002/2016GB005570)
In order to understand atmospheric methane (CH_4) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh-resolution CH_4 record of the West Antarctic Ice Sheet Divide ice core 67.2-9.8 ka and find novel, atmospheric CH_4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80-50 year range, but we find that age-scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial-scale oscillations in CH_4 concentration-the relative amplitude change is constant. We propose that the centennial CH_4 signal is related to tropical climate variability that influences predominantly low-latitude wetland CH_4 emissions.
Related to:
Rhodes, Rachael H; Brook, Edward J; McConnell, Joseph R; Blunier, Thomas; Sime, Louise C; Faïn, Xavier; Mulvaney, Robert (2017): Atmospheric methane variability: Centennial-scale signals in the Last Glacial Period. Global Biogeochemical Cycles, 31(3), 575-590, https://doi.org/10.1002/2016GB005570
Coverage:
Latitude: -79.462950 * Longitude: -112.125100
Date/Time Start: 2005-12-19T00:00:00 * Date/Time End: 2005-12-19T00:00:00
Comment:
For more information please contact Prof. Edward Brook, Oregon State University (brooke@geo.oregonstate.edu)
In the majority of cases the 2-yearly spline fit will be the most suitable for your application.
-- 2 yearly cubic smoothing spline of CH_4 data from the WAIS Divide ice core: 2 yearly cubic smoothing spline fit to the "Experiment-time-integrated CH4 data from all 3 instruments" (Dataset doi:10.1594/PANGAEA.875980). The 2 yearly cubic smoothing spline fills gaps in the data set and reduces data set size, whilst also reducing noise (that could be noise linked to the wider analytical system e.g., pressure fluctuations, or archival noise that does not reflect the paleo-atmosphere).
-- Experiment-time integrated data: Data integrated to the optimal integration time (n.b. where time refers to experiment time, not age of gas sample). No further smoothing or resampling algorithm has been applied. The optimal integration time was selected by performing an Allan variance test on measurements of a synthetic air standard/water mixture. The optimal integration time is the time at which the Allan deviation is smallest.
"Experiment-time-integrated CH4 from all 3 instruments" file (Dataset doi:10.1594/PANGAEA.875980) includes WAIS Divide discrete data points (measured at Oregon State University (OSU) or Pennsylvania State University (PSU)) inserted into gaps in the continuous data set > 90 cm length.
Analytical setup:
WAIS Divide ice core was analyzed during two laboratory campaigns at the Desert Research Institute, Reno, USA, in early 2012 (1804-2621 m) and 2013 (2709-3404 m). Ice core sticks (3.4 cm**2 x 1 m) were successively melted on a heated melterhead. Ancient air was removed from the sample stream at a gas-permeable membrane (Membrana micromodule) maintained at 497 ± 92 (2 sigma) mbar during the 2012 campaign and 494 ± 57 (2 sigma) mbar during the 2013 campaign. In the 2012 campaign, a wavelength scanned-cavity ringdown spectrometer (WS-CRDS) (Picarro CFADS36 CH4|CO2|H2O) for Center of Ice and Climate (CIC), University of Copenhagen was used to measure methane concentration. In the 2013 campaign, another Picarro instrument (G2401 CH4|CO|CO2|H2O) from Oregon State University and an optical feedback cavity enhanced absorption spectroscopy instrument (SARA, developed at Laboratoire Interdisciplinaire de Physique, Joseph Fourier University, Grenoble, France) from Oregon State Unviersity were used interchangeably. Methane measurements produced by all the laser spectrometers were calibrated to NOAA-certified standard reference gases (CA04382 and CA04332) and are reported on the NOAA2004 scale.
Solubility correction:
Methane is known to dissolve (in preference to nitrogen, argon and oxygen) in the melt water stream during sample transport between the melterhead and the gas extraction apparatus. A different solubility correction factor was calculated and applied to the data from each of the three laser spectrometers used. Depending on the instrument, 5.9-7.1% of CH_4 remained dissolved in the water stream.
Correction for gravitational fractionation:
Gravitational fractionation of methane concentrations results from the mass difference between methane (M = 16.04 g/mol) and dry air (M = 28.96 g/mol) and its magnitude depends on the thickness of the firn column. This can be estimated using the 15N/14N ratio of nitrogen measured in the ice core. The gravitational correction is therefore dM x d15N, where dM = mass difference between methane and dry air. CH_4 concentrations from 2012 campaign are increased by 0.0047% and concentrations from 2013 campaign are increased by 0.0039%.
Discrete data insertion:
Discrete data were inserted into 10 depth ranges: 1843.8-1845.1 m, 2009.0-2015.4 m, 2079.9-2081.3 m, 2214.0-2217.4 m, 2280.6-2281.5 m, 2416.2-2424.4 m, 2607.1-2608.5 m, 2621-2709 m, 3365.8-3368.1 m and 3397.0-3399.0 m. Typically, 2-4 discrete data points were inserted into each depth range. CH_4 concentrations of discrete data points inserted were adjusted to make them consistent with adjacent continuous data according to the median offset between the continuous and discrete data sets for the neighboring 100 m of data (0.55-2.44% range of adjustment). The largest gap in continuous data is 2621-2709 m, which lies between the data sets collected in the 2012 and 2013 analytical campaigns. Discrete CH_4 concentrations measured at PSU have been increased by 2.84%, the median offset between OSU and PSU discrete data for this interval.
For additional information on the continuous measurement of ice core methane concentrations:
- Stowasser, C., Buizert, C., Gkinis, V., Chappellaz, J., Schüpbach, S., Bigler, M., Faïn, X., Sperlich, P., Baumgartner, M., Schilt, A., Blunier, T., 2012. Continuous measurements of methane mixing ratios from ice cores. Atmospheric Meas. Tech. 5, 999-1013. doi:10.5194/amt-5-999-2012.
- Rhodes, R.H., Faïn, X., Stowasser, C., Blunier, T., Chappellaz, J., McConnell, J.R., Romanini, D., Mitchell, L.E., Brook, E.J., 2013. Continuous methane measurements from a late Holocene Greenland ice core: Atmospheric and in-situ signals. Earth Planet. Sci. Lett. 368, 9-19. doi:10.1016/j.epsl.2013.02.034.
For information on the WD2014 timescale:
- Buizert, C., et al., 2015. The WAIS Divide deep ice core WD2014 chronology - Part 1: Methane synchronization (68-31 ka BP) and the gas age-ice age difference. Clim Past 11, 153-173. doi:10.5194/cp-11-153-2015.
- Sigl, M., et al. , 2016. The WAIS Divide deep ice core WD2014 chronology - Part 2: Annual-layer counting (0-31 ka BP). Clim Past 12, 769-786. doi:10.5194/cp-12-769-2016
For information on discrete methane sample analysis and gravitational correction:
- Mitchell, L. E., E. J. Brook, T. Sowers, J. R. McConnell, and K. Taylor, 2011. Multidecadal variability of atmospheric methane, 1000-1800 C.E., J. Geophys. Res., 116, G02007, doi:10.1029/2010JG001441.
- WAIS Divide community members, 2013. Onset of deglacial warming in West Antarctica driven by local orbital forcing. Nature 500, 440-444. doi:10.1038/nature12376
- Marcott, S.A., et al., 2014. Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature 514, 616-619. doi:10.1038/nature13799.
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