Skip to main content
Open Access Publications from the University of California

UC San Diego

UC San Diego Electronic Theses and Dissertations bannerUC San Diego

The natural geochemistry of tetrafluoromethane and sulfur hexafluoride : : studies of ancient Mojave Desert groundwaters, North Pacific seawaters and the summit emissions of Kilauea Volcano


Tetrafluoromethane (CF₄) and sulfur hexafluoride (SF₆) are potent, long-lived greenhouse gases whose natural atmospheric sources and sinks are poorly understood. CF₄ and SF₆ concentrations were measured in groundwater, deep and surface seawater, and volcanic gas samples to provide a better constraint on their lithospheric sources to the atmosphere. Groundwaters collected from the Mojave Desert and nearby Big Bear Lake Watershed contain CF₄ and SF₆ concentrations well in excess of air-saturated water concentrations for the conditions of recharge, providing in situ evidence for a crustal degassing of CF₄ and SF₆. Excess CF₄ and SF₆ concentrations can be attributed to release during weathering of the surrounding granitic alluvium and to a deeper crustal flux of CF₄ and SF₆ entering the study aquifers through the crystalline basement. The crustal flux of CF₄, but not SF₆, is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic crustal fracturing driven by tectonic activity. When the crustal degassing rate of CF₄ and SF₆ into studied groundwaters is extrapolated to a global scale, it is consistent with the lithospheric flux required to sustain their preindustrial atmospheric abundances using best-estimate atmospheric lifetimes. CF₄ and SF₆ in volcanic emissions from Kilauea summit originate from air entrained into rising volcanic gases and from gases exsolved from Kilauea's hydrothermal system. An upper limit to a hypothetical volcanic flux of CF₄ and SF₆ is negligible when compared to the continental flux, indicating that the upper mantle is not a significant source of either gas to the atmosphere. Surface seawaters collected off of Scripps Pier during calm weather are in equilibrium with expected air- saturated seawater CF₄ concentrations. Deep Pacific seawater samples are oversaturated by roughly 4%, consistent with a predicted 5% oversaturation for these waters. The oceanic crust is therefore not a significant source of lithospheric CF₄. This suggests that CF₄ is conservative in seawater, and, combined with its rapid accumulation in the atmosphere, indicates that dissolved anthropogenic CF₄ concentrations are an effective time- dependent tracer of ocean circulation and mixing processes

Main Content
For improved accessibility of PDF content, download the file to your device.
Current View