| Summary: | Thesis (Ph. D.)--University of Rochester. Department of Earth and Environmental Sciences, 2020. This body of work expands the understanding of in situ cosmogenic 14C production and retention in the upper layer of accumulating ice sheets and presents new measurements of 14CH4 that improve our understanding of the fossil component of the CH4 budget. Samples were collected at Summit, Greenland from the firn air open porosity, the firn matrix and from ice below the depth of bubble closure. Large volume (~100L STP) air samples requiring ~1000kg ice/sample were collected for measurements of 14CH4 and 14CO via on site melt-extraction. Air for 14CO2 analysis was extracted via sublimation of ~1 kg ice samples using a new technique developed as part of this thesis. A model of firn gas transport and in situ cosmogenic 14C production was used to interpret the 14CO results, finding that only ~0.5% of in situ cosmogenic 14C produced in the firn is retained by the accumulating ice crystal lattice. Further, production rates of 14C in ice from deeply-penetrating muons are found to be overestimated by a factor of 3-4. The in situ cosmogenic 14CO2 component in accumulating ice is demonstrated be smaller in magnitude than the combined uncertainty from measurement and model characterization of paleoatmospheric 14CO2 bubble trapping. This study also used the ice core and firn air measurements in combination with an inverse model to reconstruct the atmospheric history of 14CH4 back to ~1750 CE. The samples collected show that natural fossil CH4 emissions during the preindustrial were ~1.6 Tg CH4/yr, with a maximum of 5.4 Tg CH4/yr (95% confidence limit), an order of magnitude smaller than indicated by bottom-up inventories. Using this constraint to reassess the contemporary CH4 budget with an atmospheric box model of 13CH4 shows that anthropogenic CH4 emissions from the fossil fuel industry are currently underestimated by ~25-40%. This result provides additional clarity with respect to the global CH4 budget and will help to inform strategies for targeted emission reductions aiming to limit future global warming.
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