A study of carbon-14 of paleoatmospheric methane for the last glacial termination from ancient glacial ice.

摘要

Ice core records from Greenland and Antarctica reveal large and rapid variations in atmospheric methane concentrations ([CH4]) in response to abrupt climate change. Two such events occurred at the Oldest Dryas (OD) - Bolling and Younger Dryas (YD) - Preboreal (PB) climatic transitions during the last glacial termination. A record of 14C of atmospheric CH4 (14CH4) through these transitions can help to identify the sources of the [CH4] increases and constrain the fossil fraction of paleo CH4 budgets.; Very large (∼100 L STP) samples of paleoatmospheric air are needed for such 14CH4 measurements. To obtain these samples, ancient ice outcropping at an ice-margin ablation site in West Greenland was explored. Ice sections dating to the last glacial termination were identified and found to contain high-quality gas records. A new method for large-scale air extraction from glacial ice was developed and twelve large air samples from the YD-PB and the OD-Bolling transitions were obtained. New methods were also developed for processing this air for 14CH4 . 14CH4 procedural blanks were greatly reduced through the construction of a new CH4 conversion line utilizing platinized quartz wool for CH4 combustion and the use of an ultra high purity iron catalyst for graphitization. The overall 14CH 4 processing blank was 0.75 +/- 0.38 pMC.; Measured 14CH4 values were too high by 14--38% as compared to the highest expected values based on paleoatmospheric 14CO2. In-situ production of CH4 and direct cosmogenic production of 14CH4 molecules in ablating ice were identified as the two most likely mechanisms that elevated 14CH4. Sample 14CH4 was then corrected for both mechanisms, however these corrections are speculative and do not allow any definite conclusions to be drawn from the results. Corrected 14CH4 results suggest that there was a 7% increase in the fossil CH4 fraction from the YD to the PB. The corrected 14CH4 results also suggest no large changes in the fossil fraction between the OD and the YD. Finally, the corrected results suggest that up to 80% of the OD - Bolling [CH4]- rise (∼100 ppb) was due to 14C-free CH4.