Development and application of a new mobile pH calibrator for real-time monitoring of pH in diffuse flow hydrothermal vent fluids

In situ measurement of pH in diffuse flow hydrothermal vent fluids is necessary to investigate the feedback between geochemical and biochemical processes. Accurate pH determination has been unusually challenging owing to temperature and pressure effects that place severe constraints on the performance of a wide variety of pH sensor systems. In this paper, we describe a newly developed mobile pH calibrator (MpHC), whichmakes use of in situ calibration protocols that enhance the accuracy of pH measurement and monitoring on the ocean floor at deep-sea hydrothermal vents. The MpHC combines the physically robust and highly sensitive iridium solid-state pH electrode with a flow control system to perform 2-point calibration with on-board pH buffer solutions. The small size and novel design of the sensor probe allow more effective access to seafloor hydrothermal vent fluids and their associated sulfide structures and biological communities. The MpHC is capable of in situ deployment by submersible via ICL (inductively couple link) communication around hydrothermal vents at pressures and temperatures up to 45 MPa and 100°C, respectively. In this paper, we also present results of in situ calibration methods used to correct the standard potential and slope (mV/pH) of the solid-state electrode for temperature effects. The MpHC has been deployed most recently using the submersible Alvin during cruise AT26-17 to Axial Seamount and Main Endeavour Field, Juan De Fuca Ridge in the NE Pacific. With in situ calibration functionality, the MpHC offers the prospect of more successful longer-termmeasurements in keeping with power availability provided by cabled seafloor observatories coming online in the NE Pacific.