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Data Set ID: 
NSIDC-0586

Late Holocene Methane Concentrations from WAIS Divide and GISP2, Version 1

This data set measures methane concentrations in ancient air trapped in the West Antarctic Ice Sheet (WAIS) Divide and Greenland Ice Sheet Project (GISP2) ice cores; presenting two, high-resolution ice core methane records of the past 2500 years, one from each pole. These measurements were used to reconstruct the methane Inter-Polar Difference (IPD) during the late Holocene. Also included are model results of methane emissions that were presented in the manuscript describing this data set.

Geographic Coverage

Parameter(s):
  • Ice Core Records > Ice Core Air Bubbles
  • Ice Core Records > Methane
Spatial Coverage:
  • N: -79.4676, S: -79.4676, E: -112.0865, W: -112.0865

  • N: 72.6, S: 72.6, E: -38.5, W: -38.5

Spatial Resolution: Not Specified
Temporal Coverage:
  • 1 August 2008 to 2 August 2012
Temporal Resolution: 10 year
Data Format(s):
  • Microsoft Excel
Platform(s) LABORATORY
Sensor(s): GAS CHROMATOGRAPHS
Version: V1
Data Contributor(s): Logan Mitchell, Edward Brook, James Lee, Christo Buizert, Todd Sowers
Please contact the data provider for the correct Data Citation for this data set.

Literature Citation

As a condition of using these data, we request that you acknowledge the author(s) of this data set by referencing the following peer-reviewed publication.

  • Mitchell, L. E., E. J. Brook, J. E. Lee, C. Buizert, and T. A. Sowers. 2013. Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget, Science. 342. 964-966. http://dx.doi.org/10.1126/science.1238920

  • Mitchell, L. E., E. J. Brook, J. E. Lee, C. Buizert, and T. A. Sowers. 2013. Supplementary Materials for Constraints on the Late Holocene Anthropogenic Contribution to the Atmospheric Methane Budget, Science. 342. 964-966. http://dx.doi.org/10.1126/science.1238920

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Detailed Data Description

This data set consists of two high-resolution, high-precision decadally resolved ice core methane concentration records from the WAIS Divide and the GISP2 ice cores, which were used to reconstruct the Inter-Polar Difference (IPD) from 800 B.C.E. to 1800 C.E.; thus, providing data-driven constraints on the early anthropogenic hypothesis. The IPD record constrains the latitudinal distribution of emissions and shows that Late Preindustrial Holocene (LPIH) emissions increased primarily in the tropics, with secondary increases in the subtropical Northern Hemisphere. Anthropogenic and natural sources have different latitudinal characteristics, which are exploited to demonstrate that both anthropogenic and natural sources are needed to explain LPIH changes in methane concentration (Mitchell 2013).

In Figure 1, CH4 data points show the mean concentration from replicate samples measured at that depth. The thin IPD line shows the IPD obtained by linear interpolation between ice core measurements at an annual spacing, and the thick line was computed using a 20-year low-pass filter. The IPD 1-sigma error bands were obtained using a Monte Carlo procedure accounting for analytical uncertainty of the measurements and chronologic uncertainty of the tie points (Mitchell 2013).

IPD from 800 B.C.E. to 1800 C.E.
Figure 1. IPD (top), Ice Core CH4 Records (middle), and Calculated Emissions from Scenario L3 (bottom)
Format

Data are provided in Microsoft Excel format (.xlsx).

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File and Directory Structure

Data are available on the FTP site in the ftp://sidads.colorado.edu/pub/DATASETS/AGDC/nsidc0581_mitchell/ directory. Within this directory, there is one Microsoft Excel (.xlsx) file (Methane_Concentration_IPD_WAIS_GISP2.xlsx) containing 19 worksheets. The first five worksheets contain data on the methane concentration results from the WAIS Divide (WDC05A and WDC06A) and GISP2. The sixth worksheet contains the calculated IPD. The remaining worksheets contain the methane emissions and atmospheric burden of methane from the modeled scenarios. Units of methane concentration are in parts per billion (ppb), modeled methane emissions are in terragrams of methane per year (Tg CH4/yr), and modeled isotopic ratios are in per mill. Refer to Table 1 for an explanation of the content for each worksheet.

Table 1: Worksheet Content Description
Worksheet Title
Description of Content
WDC06A Contains mean depth (m), gas age (Year C E.), mean concnetration (ppb), and four core samples (ppb) for the WDC06A core.
WDC05A Contains mean depth (m), gas age (Year C.E.), mean concnetration (ppb), and four core samples (ppb) for the WDC05A core.
GISP2D Contains mean depth (m), gas age (Year C.E.), mean concnetration (ppb), and four core samples (ppb) for the GISP2D core.
GISP2D Contaminated Contains samples that are suspected of containing in situ contamination of mean depth (m), gas age (Year C.E.), mean concnetration (ppb), and four core samples (ppb) for the GISP2D core.
Rejected Contains rejected measurements of mean depth (m), gas age (Year C.E.), mean concnetration (ppb), and two core samples (ppb) for the WDC06A and GISP2D cores.
IPD Contains data on the year, IPD annually interpolated, IPD smoothed, upper bound, lower bound, and mean IPD for the methane Inter-Polar Difference (IPD).
Latitudinal Scenario 1 (L1) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
Latitudinal Scenario 2 (L2) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
Latitudinal Scenario 3 (L3) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
Natural Emisions Scenario (N1) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
Natural Emisions Scenario (N2) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
Anthropogenic Emissions Scenario (A1) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
Anthropogenic Emissions Scenario (A2) Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
N2 + A1 Scenario Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
N2 + A1 w -5% Scenario Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
N2 + A1 w -75% Scenario Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
N2 + A2 Scenario Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
N2 + A2 w -50% Scenario Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
N2 + A2 w -75% Scenario Contains data on modeled methane emissions (Tg CH4/yr) and modeled atmospheric methane budget.
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File Size

5.5 MB

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Spatial Coverage

GISP2
72.6° N, 38.5° W

WAIS Divide
79.4676° S, 112.0865° W

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Temporal Coverage

Data Set Temporal Coverage
Data were collected from 01 August 2008 to 02 August 2012.

Paleo Temporal Coverage
2604 BCE to 1909 CE

Temporal Resolution

10 years

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Parameter or Variable

Methane Concentration (ppb) parts per billion
Modeled Methane Emissions (Tg CH4/yr) terragrams of methane per year
Modeled Isotopic Ratios (per mill)

Sample Data Record

data record

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Software and Tools

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Data Acquisition and Processing

Air was extracted from ice core samples using a wet extraction technique, and the methane concentration was measured with a Gas Chromatograph (GC). The high-precision CH4 measurements [pooled standard deviation (SD ± 2.4 ppb)] reproduce multidecadal-scale variability observed in a shallow core (WAIS Divide core WDC05A) (Mitchell et al. 2011) and in the Law Dome ice core (MacFarling Meure et al. 2006). The WAIS Divide layer-counted ice chronology and a dynamic firn densification model was used to construct a gas-age chronology. A Monte Carlo correlation technique using the multidecadal variations was then used to create a GISP2 gas-age chronology synchronized with the WAIS Divide chronology. When the synchronized GISP2 chronology was compared to one constructed independently with a firn densification model and a layer-counted ice chronology, a difference of 0 ± 11 years was found, demonstrating that the chronology is robust. The IPD was calculated by subtracting the WAIS Divide from the GISP2 CH4 concentration after linear interpolation to annual spacing. Uncertainty bands (1 sigma) were computed with a Monte Carlo technique incorporating measurement precision and timescale uncertainties (Mitchell 2013).

The IPD remains essentially constant (781 B.C.E. to 1803 C.E. mean, 41.6 ppb; trend 0.9 ± 0.3 ppb/ka) throughout the LPIH despite a 115 ppb (17 percent) increase in the global concentration, which is broadly consistent with previous low-resolution estimates. The IPD record shows small (~5 ppb) centennial-scale variations, with a minimum around 250 B.C.E. and maximum around 1100 C.E (Mitchell 2013).

An Eight Box Atmospheric CH4 Model (EBAMM) was used to examine hypothesized CH4 emission scenarios and to compare modeled concentrations with the ice core records. The model has six tropospheric boxes each covering a 30° latitude and one stratospheric box per hemisphere. These boxes are refered to as the tropical (0° to 30°), mid-latitude (30° to 60°), and high-latitude (60° to 90°) boxes. The distribution of CH4 sources is fundamentally underconstrained by concentration data from just the two poles. However, the modern source distribution provides additional constraints on the relatively small emissions from the 30° to 90° S and 60° to 90° N regions. With these constraints, the data can be used in conjunction with EBAMM to solve for the source strength of two latitudinal bands at a time (Mitchell 2013). Refer to Figure 2.

Three latitudinal emission scenarios were constructed (L1, L2, and L3) that balance the global budget and represent the range of realistic emissions. While keeping emissions outside the zonal bands of interest constant, we solved for emissions in the Southern Hemisphere (SH) versus NH tropics (L1), tropical (30°S to 30°N) versus mid-latitude NH (L2), and tropical versus mid- to high-latitude NH (L3). Refer to Figure 2. Whenever two EBAMM boxes fell within a latitudinal band, a fixed emission ratio was assumed between them. L3 is equivalent to a simpler three-box model (Mitchell 2013). Refer to Figure 3.

fig2
Figure 2. Eight Box Atmospheric CH4 Model Showing Latitudinal Scenarios 1 - 3 (L 1-3)

Each of the three scenarios obtains the same concentration and IPD results. The colors correspond to EBAMM boxes while the pattern of the line indicates scenarios L1 - 3. These scenarios all assume a methane lifetime of 10 years.

fig3
Figure 3. Three Box Model Results


The three box model is described in Chappellaz J., et al. 1997. The light solid lines are the model results solved for the annually interpolated data, the dark solid lines are the model results solved for the 20-year low pass filtered concentration data, and the dotted lines are the results of the EBAMM scenario L3 where the boxes have been combined to yield the same zonal regions as the three box model. There is a slight divergence in the tropical emissions over time as a result of a difference in the sink parameterization in the EBAMM.

Next, the net change in emissions between 800 B.C.E. and 1400 C.E. in each latitudinal band was calculated, using linear regression (Mitchell 2013). Refer to Table 2.

table2

By focusing on the time period from 800 B.C.E. to 1400 C.E., this avoided the exponential population increase after 1500 C.E. and potential natural emission reductions related to the Little Ice Age. Assuming an atmospheric lifetime of 10 years, the latitudinal scenarios show that global sources increased by ~24 Tg/year between 800 B.C.E. and 1400 C.E., with the majority of that increase coming from tropical sources. Refer to Figure 1. Varying the CH4 lifetime from 8 to 12 years caused the increase in global emissions to change by ± 5 Tg/year but did not affect the latitudinal distribution of CH4 emissions over the LPIH (Mitchell 2013).

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References and Related Publications

Contacts and Acknowledgments

Logan Mitchell
Oregon State University
College of Earth Ocean and Atmospheric Sciences
104 CEOAS Administration Building
Corvallis, OR 97331

Acknowledgments: 

This research was supported by NSF OPP Grant Numbers 0538578, 0520523, 0944584 and 0538538 and by NASA/Oregon Space Grant Consortium grant NNG05GJ85H and the NOAA Climate and Global Change Fellowship Program, administered by the University Corporation for Atmospheric Research.

Document Information

DOCUMENT CREATION DATE

January 2014

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