GEOCHEMICAL MODELING OF SERPENTINIZATION REACTIONS THAT INFLUENCE THE CHEMISTRY AND MICROBIAL ECOSYSTEMS OF THE HYPERALKALINE AQUA DE NEY SPRING, NORTHERN CALIFORNIA, USA

The Aqua de Ney Spring is one of a number of hyperalkaline cold springs associated with the serpentinization of ultramafic rocks in the California Coast Range. These springs, and others like them found in ophiolite settings around the world, are of interest for several reasons. They represent an extreme environment with a geochemical energy source that, through hydration and oxidation of olivine and pyroxene during serpentinization, can support chemotrophic microbial life similar to that observed in the Lost City submarine hydrothermal field. And, they release reduced gas species such as hydrogen and methane that may provide a source for green hydrogen.

The Ney Spring consists of a sodium bicarbonate water (TDS ~40,000 ppm) with significant amounts of chloride (~8000 ppm) and silica (~4000 ppm). The Ney waters are very distinct in their oxygen and hydrogen stable isotope geochemistry (δ¹⁸O = 5.5‰, δD = -14‰) in comparison with local meteoric waters (δ¹⁸O = -12.1‰, δD = -85‰), suggesting a potential marine component to the fluids. One distinguishing feature of the Ney Spring is that, in addition to the high pH (>12) fluids, it also emits a large fraction (>80 mole % of the noncondensable gas) of methane. Two distinct processes have been identified as potential mechanisms to generate methane. The first is via an abiotic process, where hydrogen generated by the oxidation of Fe²⁺ is then reacted with CO₂ to form methane. An alternative process would involve microbes that consume hydrogen as an energy source, producing methane as a byproduct.

The Ney Spring has seen renewed interest recently for its potential value as an analog for the briny, subsurface oceans of icy moons in our outer solar system, which have the potential to support chemosynthetic microbial life. The primary focus of our research project is the microbial communities and their metabolisms that persist in the Ney Spring waters. However, for this presentation, we will summarize our geochemical modeling to evaluate how abiotic processes have led to the generation of this unusual spring chemistry.