Carbon dioxide flux measurements at and adjacent to the Sulphur Bank Mercury Mine, Clear Lake Volcanic Field, California

The 2.1 Ma Clear Lake Volcanic Field (Donnelly-Nolan and others, 1981) and the 174 km2 Clear Lake are located ~150 km north of San Francisco, California. Sulphur Bank Mercury Mine (SBMM) is an abandoned mine located on the eastern shore of Clear Lake that was initially worked in 1865 for sulfur, then for mercury until it closed in 1957 (White and Roberson, 1962). The mine was declared a Superfund site by the U.S. Environmental Protection Agency in 1990 because of the presence of mercury-bearing mine waste and mercury bioaccumulation in fish within Clear Lake. SBMM is underlain by a localized, fault-controlled, liquid-dominated hydrothermal system (Goff and others, 1995). Hydrothermal gases are rich in CO2 with a high contribution of mantle-derived He (Goff and Janik, 1993). We carried out surveys of CO2 flux from ground and water surfaces using the accumulation chamber method to characterize the spatial distribution of and quantify hydrothermal CO2 emissions. In an initial reconnaissance survey on February 1-2, 2023, we made 180 CO2 flux measurements across SBMM and an adjacent area east of the mine. Measured CO2 fluxes ranged from