Documentation for Methyl Chloride Measurements in the Taylor Dome M3C1 Ice Core, Version 1

Back to Top

Collapse All / Open All

Detailed Data Description

This data set presents measurements of CH3CI in air extracted from the Taylor Dome ice core. The data provide a record of CH3CI covering the Holocene (11-0 thousand years before 1950, kyr B.P.), with more limited coverage of the last glacial period between 50-30 kyr B.P. The Taylor Dome record provides new information about the range of natural CH3CI variability over glacial/interglacial timescales. The CH3CI data from Taylor Dome also exhibit a relationship to atmospheric CH4, which has not been previously observed. The ice core samples were about 15 cm long on average. The depths represent the mid-depth for each sample. Methyl chloride levels are reported as dry-air molar mixing-ratios (Verhulst 2013).

Format

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

Background color on
File and Directory Structure

Data are available on the FTP site in the ftp://sidads.colorado.edu/pub/DATASETS/AGDC/nsidc0600_saltzman/ directory. Within this directory, there is one Microsoft Excel file: UCI methyl chloride.xls.

Background color on
File Size

20 KB

Background color on
Spatial Coverage

Taylor Dome, East Antarctica: 77.48° S, 158.43° E

Spatial Resolution

113.1 meters to 446.2 meters

Background color on
Temporal Coverage

Data were collected from January 2008 to December 2012.

Background color on
Parameter or Variable

Depth (m)
Methyl Chloride (CH3CI) in parts per trillion (ppt)

Sample Data Record

Figure 1 is sample data from the UCI methyl chloride.xls data file.

sample data record
Figure 1. Methyl Chloride Measurements
Background color on

Software and Tools

Background color on

Data Acquisition and Processing

Air was extracted from bubbles in ice core samples by mechanical shredding at -50°C under vacuum in an evacuated stainless steel chamber. The extracted air was collected in a stainless steel tube cooled with liquid helium (4K). Sample air was analyzed by gas chromatography with high-resolution mass spectrometry using an internal isotope-labeled gas standard calibrated against ppm-level primary gas standards. The apparatus and procedures used to extract and analyze gases from the ice core samples were similar to those used in earlier studies (Saltzman et al. 2004Aydin et al. 2007Saltzman et al. 2009). A correction of approximately -1 percent was applied to the measured CH3CI mixing ratios to account for gravitational enrichment in firn air, based on a bubble lock-in depth of 66 m and a mean annual surface air temperature of -42.5°C (Craig et al. 1988). Gas ages for the Holocene samples used in this study are based on stratigraphic correlation of the CO2 record from Taylor Dome ice core to the European Project for Ice Coring in Antarctica (EPICA) Dome C and Dronning Maud Land ice cores (Monnin et al. 2004). Gas ages for the last glacial and transition samples are based on correlation of CO2 and CH4 records from Taylor Dome to the Greenland Ice Sheet Project 2 ice core (Brook et al. 2000Ahn and Brook 2008; Verhulst 2013).

Methyl chloride analysis requires 400-600 g of high-quality ice, without fractures, a clean sampling, extraction, and analytical procedures. This data set measures CH3CI in Taylor Dome ice core samples with equally spaced gas ages throughout the length of the core. However, ice samples of sufficient size/quality were limited for the last glacial period. As a result, the sample age distribution was heavily biased towards the Holocene. We analyzed 41 Taylor Dome ice core samples at 250-300 year intervals over the last 11 kyr B.P., and four additional samples from 15-11 kyr B.P. There were sampling gaps around 8 and 4 kyr B.P. due to lack of ice availability. In total, 94 ice core samples were analyzed. Data from only 62 samples were of sufficient quality for use in this data set. Two samples were below the age-depth scale, and the remainder were considered poor quality due to:

  • drill fluid contamination (n-butyl acetate)
  • modern air contamination (CFC-12>1pmol mol-1)
  • contamination of the extraction vessels (Aydin et al. 20072010). Refer to Figure 2.

Methyl chloride levels in the samples ranged from 412 to 588 pmol mol-1. The average analytical uncertainty was 6±2.6% (1δ) of the measured levels (Verhulst 2013).

In Figure 2, open squares in both panels represent samples with calcium at or above 0.25 µM. Panel A shows methyl chloride measurements in all 94 ice core samples analyzed from Taylor Dome. Samples with gas ages older than 50 ky BP are not discussed elsewhere in this data set due to the overall poor quality of these samples. Panel B show a zoom in view of methyl chloride measurements from the last 15 ky BP (Verhulst 2013).

gas age image
Figure 2. Methyl Chloride Measurements
Background color on

References and Related Publications

Contacts and Acknowledgments

Eric S. Saltzman
University of California, Irvine
Deptartment of Earth System Science
Croul Hall 1212
Irvine, CA 92697

Murat Aydin
University of California, Irvine
Deptartment of Earth System Science
Croul Hall 1212
Irvine, CA 92697

Acknowledgments: 

This research was supported by NSF Division of Polar Programs (PLR) Grant Number 0636953.

Document Information

Document Creation Date

Septembert 2014