White Paper Title: 
In-Situ Chemical Sensors: Their Development, Testing and Future Directions for RIDGE Science

It has become increasingly clear that the evolution of hydrothermal systems at mid-ocean ridges is inherently dynamic where chemical, physical and biologic processes can change dramatically in space and time. As a consequence of this, efforts have proceeded with a range of initiatives to develop and deploy in-situ instrumentation that is capable of measurement and monitoring of dissolved chemicals and biologically active compounds. Instrumentation of this sort is an essential component of any plan involving development of cabled observatories where power will be available for long-term measurements and data communication from sites on the seafloor to facilities throughout the world. If such observatories are to fulfill their intended potential, however, not only must continued sensor development be a priority, but the new sensors must be smaller, and include supporting instrumentation that allows for programmed calibration, such that the effects of signal drift can be accounted for and quality assurance verified.

Past Accomplishments
Investigators at the University of Minnesota have long been concerned with redox and pH controls on the chemistry of hydrothermal vent fluids at mid-ocean ridges [Ding and Seyfried, 2007; Ding, et al., 2005; Ding, et al., 2006; Seyfried, et al., 2010]. Indeed, the first in-situ measurements of pH and dissolved H2 and H2S in high temperature vent fluids were made at the Main Endeavour Field, Juan de Fuca Ridge in 2005 using solid-state sensors constructed of gold sensing elements and corrosion resistant ceramic materials. Subsequently, analogous measurements have been successfully performed at the EPR 9-10°N (2007-2008), and also at Rainbow vent sites (36°N) on the Mid-Atlantic Ridge (2008) [Seyfried, et al., 2010]. Simultaneous in-situ measurements of these species have placed previously unavailable constraints on the compositional evolution of the subseafloor reaction zones from which the fluids are derived. Recent advances have also been made with chemical monitoring instrumentation. For example, in 2008 an auto-calibrating pH monitoring system was deployed for the first time at EPR 9°N in diffuse flow vent fluids. This system included process control hardware and software that permitted intermittent measurement of pH buffer fluids at intervals of approximately six hours, providing effective calibration for the in-situ pH data of the diffuse flow vent fluids that were obtained simultaneously.

Future Directions
We believe it is imperative that in the future efforts be focused on the development of a new generation of sensors that take full advantage of recent advances in micromachining approaches to decrease sensor size, while increasing sensitivity, especially for low temperature applications. The small sensor size will not only expand the range of seafloor applications, but will also mean more frequent calibration cycles for a given volume of standard solution, and thus, permits longer operation. Experimental and field programs should be conducted in parallel such that key cause and effect variables can be isolated and simulated in the lab. Field deployment should occur only after successful performance in the lab at simulated vent conditions. Finally, the ocean science community must be diligent to do all it can to entrain scientists and especially engineers with backgrounds in material science, micromachine-sensor design, and process control systems if the challenges of chemical sensor measurement and monitoring at deep sea vents are to be successfully overcome. Although we emphasize pH and redox sensors, the interdisciplinary approach we advocate can be used as a model for instrumentation development involving an even broader range of chemical sensor systems.