Measurements of hydrogen sulfide (H2S), acidity (pH), hydrogen (H2), and temperature in hydrothermal vents are essential to understanding fluid source and fluid-rock reactions in the subsurface as well as microbial interactions. During seafloor eruptions, high temperature fluid-mineral reactions commonly result in highly elevated H2 concentrations in crustal fluids that then exit the seafloor as megaplumes, diffuse flow out of the seafloor, and as fluids exiting hydrothermal vents.
The temperature-H2S-H2-pH sensor on the Cabled Array is installed in the International District anhydrite-rich chimney called Diva. This small structure has very high gas concentrations, and very low metal concentrations. Anhydrite (CaSO4) is an interesting mineral in that it precipitates out from seawater when seawater is heated above 150°C. Anhydrite chimneys at Axial are commonly very friable and small. There is another such chimney called Virgin that is in the ASHES hydrothermal field.
The cabled temperature-H2S-H2-pH sensor was built by Dr. Bill Seyfried and Dr. Kang Ding at the University of Minnesota. It includes a titanium wand that contains ceramic membrane electrodes and several reference electrodes. The wand is placed in the “throat” of hydrothermal vent orifices, some of the most extreme environments on Earth. In only a few places have such measurements been made, and this is the only place in the oceans where cabled instruments have provided real-time data to shore.