UC San Diego

Measurements of \¹\⁴C in atmospheric CO\₂ 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 \¹\⁴C levels, tracking the response of \¹\⁴C in CO\₂ 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 \¹\⁴C/\¹\²C, or [delta]\¹\⁴C, in tree rings provided clear evidence that rising CO\₂ concentrations were due to human activities by revealing the dilution of \¹\⁴C 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 [delta]\¹\⁴C in atmospheric CO\₂ for investigating carbon cycle dynamics. Since much of the excess \¹\⁴C derived from nuclear weapons testing has been redistributed into oceanic and biospheric reservoirs, trends and gradients in [delta]\¹\⁴C of CO\₂ have diminished to levels that are nearly commensurate with measurement precision at most laboratories. Development of improved methods for [delta]\¹\⁴C analysis by accelerator mass spectrometry at Lawrence Livermore National Laboratory advanced measurement uncertainty to 1.7 permil. Application of the improved analytical procedures to an archive of CO\₂ samples from the Scripps CO\₂ Program produced 2-15 year monthly time series of [delta]\¹\⁴C at seven global sampling stations. The high precision observations show variability in the secular trend of [delta]\¹\⁴C that could enable new insights to the climatic influences on CO\₂ exchange. Measurement of a shift in the [delta]\¹\⁴C 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 [delta]\¹\⁴C observations available to the community. The thesis also demonstrates the application of [delta]\¹\⁴C measurements for identifying fossil fuel- derived CO\₂ in vertical profiles sampled by aircraft. Similar measurements could be used to distinguish regional sources of industrial or biospheric CO\₂ or to investigate the mixing of surface CO\₂ fluxes in the troposphere