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.