UC 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
- Author(s): Deeds, Daniel A.
- et al.
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