Polar ice core records reveal atmospheric methane mixing ratios ([CH4]) changing slowly over time scales the length of glacial-interglacial cycles, and also rapidly over a few decades. Measurement of the δ13CH4 value of gases entrained in glacial ice can help identify the sources of the observed [CH4] changes.
To facilitate these measurements, an improved on-line extraction and continuous flow isotope ratio mass spectrometer (CF-IRMS) method was developed. Samples of outcropping ablation-zone ice from Påkitsoq, Greenland were measured for δ13CH4 over the Last Glacial Maximum (LGM) to the Preboreal period (PB).
CF-IRMS measurement of the Påkitsoq samples revealed an irregular, spot contamination consisting of elevated [CH4] in the interstitial air, likely due to in-situ methanogenesis. All samples were then filtered to reject contaminated samples by comparison against contemporaneous [CH4] from the GISP2 ice core. The filtered samples show more 13C-enriched δ13CH4 values during cold climatic periods, as well as a potential shift to more 13C-enriched δ13CH4 values across the densely sampled Younger Dryas termination. Interpretation of the stable time periods of the filtered record is aided by a data-constrained 4-box steady-state atmospheric CH4 model run in Monte Carlo mode. From the box model results, tropical wetlands show relatively consistent CH4 flux across all time periods except the YD. The cold, dry climates of the LGM and YD decreased wetland CH4 flux, however the LGM flux is likely compensated (increased) by the additional wetland area available on the exposed continental shelves. Boreal wetlands are an important source of 13C-depleted CH4 during warm periods, and their flux is likely predominantly from thermokarst lakes. Biomass burning CH4 flux increases throughout the deglaciation with fluxes in the Preboreal comparable to present-day. Gas hydrate releases indicate terrestrial hydrates are potentially more important than marine hydrates during the deglaciation.
The Påkitsoq δ13CH4 record of the abrupt YD termination suggests that the primary sources responsible for the initial [CH4] increase were a mixture of biomass burning (~40‰) and a boreal wetlands source (~60‰), most likely thermokarst lakes. Perhaps surprisingly, this analysis found no important role for biogenic gas hydrates, or tropical wetlands, in the YD termination [CH4] increase.
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/2327 |
| Date | 05 March 2010 |
| Creators | Melton, Joe R. |
| Contributors | Whiticar, Michael J. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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