High-precision 14C measurements demonstrate production of in situ cosmogenic 14CH4 and rapid loss of in situ cosmogenic 14CO in shallow Greenland firn

Measurements of radiocarbon (14C) in carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO) from glacial ice are potentially useful for absolute dating of ice cores, studies of the past atmospheric CH4 budget and for reconstructing the past cosmic ray flux and solar activity. Interpretation of 14C signals in ice is complicated by the fact that the two major 14C 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 14CH4 and 14CO content in glacial firn with unprecedented precision to advance understanding of the in situ 14C component. 14CH4 and 14CO were melt-extracted on site at Summit, Greenland from three very large (∼1000 kg each) replicate samples of firn that spanned a depth range of 3.6–5.6 m. Non-cosmogenic 14C contributions were carefully characterized through simulated extractions and a suite of supporting measurements. In situ cosmogenic 14CO was quantified to better than ±0.6 molecules g−1 ice, improving on the precision of the best prior ice 14CO measurements by an order of magnitude. The 14CO measurements indicate that most (>99%) of the in situ cosmogenic 14C is rapidly lost from shallow Summit firn to the atmosphere. Despite this rapid 14C loss, our measurements successfully quantified 14CH4 in the retained fraction of cosmogenic 14C (to ±0.01 molecules g−1 ice or better), and demonstrate for the first time that a significant amount of 14CH4 is produced by cosmic rays in natural ice. This conclusion increases the confidence in the results of an earlier study that used measurements of 14CH4 in glacial ice to show that wetlands were the likely main driver of the large and rapid atmospheric CH4 increase approximately 11.6 kyr ago.

► Three large replicate samples of shallow Greenland firn were analyzed for carbon-14 of methane and carbon monoxide.
► In situ cosmogenic production of 14CH4 in natural glacier ice was definitively confirmed for the first time.
► The measurement technique improved 14CO uncertainties by an order of magnitude as compared to prior work.
► Rapid loss of in situ cosmogenic 14CO from shallow Greenland firn was observed.