Advancing the use of radiocarbon in studies of global and regional carbon cycling with high precision measurements of carbon-14 in carbon dioxide from the Scripps Carbon Dioxide Program

Measurements of 14C in atmospheric CO2 have served as a powerful geochemical tracer since the first observation programs began over 50 years ago. As the nuclear weapons tests of the 1950s and 60s caused an enormous perturbation to natural atmospheric 14C levels, tracking the response of 14C in CO2 provided a measure of exchange rates between different regions of the atmosphere and between the troposphere and the ocean surface and terrestrial biosphere. Early measurements of 14C/12C, or Delta14 C, in tree rings provided clear evidence that rising CO2 concentrations were due to human activities by revealing the dilution of 14C in the atmosphere by the combustion of million year old fossil carbon, a process termed the "Suess Effect".; This thesis aimed to continue and expand the use of Delta14 C in atmospheric CO2 for investigating carbon cycle dynamics. Since much of the excess 14C derived from nuclear weapons testing has been redistributed into oceanic and biospheric reservoirs, trends and gradients in Delta14C of CO2 have diminished to levels that are nearly commensurate with measurement precision at most laboratories. Development of improved methods for Delta14C analysis by accelerator mass spectrometry at Lawrence Livermore National Laboratory advanced measurement uncertainty to 1.7‰.; Application of the improved analytical procedures to an archive of CO 2 samples from the Scripps CO2 Program produced 2-15 year monthly time series of Delta14C at seven global sampling stations. The high precision observations show variability in the secular trend of Delta14C that could enable new insights to the climatic influences on CO2 exchange. Measurement of a shift in the Delta 14C gradient between the Northern and Southern Hemispheres since the 1980s also places constraints on regional fluxes of carbon, with particular relevance to Southern Ocean dynamics. The measurements presented here contribute significantly to the amount and global coverage of recent Delta14 C observations available to the community.; The thesis also demonstrates the application of Delta14C measurements for identifying fossil fuel-derived CO2 in vertical profiles sampled by aircraft. Similar measurements could be used to distinguish regional sources of industrial or biospheric CO2 or to investigate the mixing of surface CO2 fluxes in the troposphere.