Origin and evolution of hydrogen-rich gas discharges from a hot spring in the eastern coastal area of China

Unlike the typical low-temperature ( < 150 degrees C) continental geothermal systems usually characterized by high N-2, CH4 and CO2 concentrations but a trace H-2 concentration, the sandstone-dominated Jimo hot spring on China's eastern coast exhibits: (1) abnormally high H-2 concentrations (2.4-12.5 vol%) and H-2/CH4 (up to 46.5); (2) depleted delta D-H-2(-822 to -709 parts per thousand), comparable to the Kansas hot springs near the Mid-Continent rift system with the most depleted delta D-H-2(-836 to -740 parts per thousand) recorded in nature; and (3) dramatic gas concentration and isotope ratio variations within an area of 0.2 km(2). Gas chemistry and H-C-He-Ne isotope ratios are studied with reference to published H-2 isotope data from various systems. The origin of the gas is most likely attributed to: (a) allochthonous abiotic H-2 generated by the reduction of water and oxidation of Fe-II-rich pyroxene and olivine (serpentinization) in the basalt located 2 km away under near-surface conditions and migration to the deep sandstone reservoir; (b) primary thermogenic CH4 produced in the sandstone; (c) mixing with a considerable amount of microbial H-2 from shallow fresh and marine sediments; and (d) biotic CH4 with typical abiotic signatures resulting from isotope exchanges with fluids high in H-2/CH(4 )and CO2/CH4 ratios. Allochthonous abiotic H-2 in a sandstone-dominated continental geothermal system and massive microbial fermentation-based H-2 production in shallow fresh and residual marine sediments with insignificant but differential consumption activity are highlighted. The published hydrogen isotope ratios for H-2 produced under various natural geological environmental and experimental conditions have been collected systematically to provide a fundamental framework and an initial tool for restricting the dominant origin of H-2.