High-precision ~(14)C measurements demonstrate production of in situ cosmogenic ~(14)CH_4 and rapid loss of in situ cosmogenic ~(14)CO in shallow Greenland firn

摘要

Measurements of radiocarbon (~(14)C) in carbon dioxide (CO_2), methane (CH_4) and carbon monoxide (CO) from glacial ice are potentially useful for absolute dating of ice cores, studies of the past atmospheric CH_4 budget and for reconstructing the past cosmic ray flux and solar activity. Interpretation of ~(14)C signals in ice is complicated by the fact that the two major ~(14)C components-trapped atmospheric and in situ cosmogenic-are present in a combined form, as well as by a very limited understanding of the in situ component. This study measured ~(14)CH_4 and ~(14)CO content in glacial firn with unprecedented precision to advance understanding of the in situ ~(14)C component. ~(14)CH_4 and ~(14)CO were melt-extracted on site at Summit, Greenland from three very large (~1000kg each) replicate samples of firn that spanned a depth range of 3.6-5.6m. Non-cosmogenic ~(14)C contributions were carefully characterized through simulated extractions and a suite of supporting measurements. In situ cosmogenic ~(14)CO was quantified to better than ±0.6moleculesg~(-1) ice, improving on the precision of the best prior ice ~(14)CO measurements by an order of magnitude. The ~(14)CO measurements indicate that most (>99%) of the in situ cosmogenic ~(14)C is rapidly lost from shallow Summit firn to the atmosphere. Despite this rapid ~(14)C loss, our measurements successfully quantified ~(14)CH_4 in the retained fraction of cosmogenic ~(14)C (to ±0.01moleculesg~(-1) ice or better), and demonstrate for the first time that a significant amount of ~(14)CH_4 is produced by cosmic rays in natural ice. This conclusion increases the confidence in the results of an earlier study that used measurements of ~(14)CH_4 in glacial ice to show that wetlands were the likely main driver of the large and rapid atmospheric CH_4 increase approximately 11.6kyr ago.