In situ deployment of voltammetric, potentiometric. and amperometric microelectrodes from a ROV to determine dissolved O2, Mn, Fe, S(-2), and pH in porewaters

Solid-state microelectrodes have been used in situ in Raritan Bay, NJ to measure pore water profiles of dissolved O-2, Mn, Fe, and sulfide at (sub)millimeter resolution by voltammetric techniques. The voltammetric sensor was positioned with microprofiling instrumentation mounted on a small remote operated vehicle (ROV). This instrumentation and the sensor were controlled and monitored in real time from a research vessel anchored at the study site. The Voltammetric analyzer was connected to the electrodes of the voltammetric cell with a 30 m cable which also bridged receiver-transmitter transducers to ensure signal quality along the cable. Single analyte O-2, pH, and resistivity microsensors were operated alongside the voltammetric sensor. We report on the technology of the system and the concentration changes of redox species observed from 2 to 3 cm above to approximately 4 cm below the sediment-water interface during three deployments. O-2 measurements from both Clark and voltammetric electrodes were in excellent agreement The profiles obtained show that there is no detectable overlap of O-2 and Mn2+ in the sediments which is similar to previous reports from other continental margin sediments which were cored and analyzed in the laboratory. These data indicate that O-2 is not a direct oxidant for Mn2+ when diffusive (rather than advective) processes central the transport of solutes within the sediment. Subsurface Mn2+ peaks were observed at about 2 cm and coincide with a subsurface pH maximum. The data can be explained by organic matter decomposition with alternate electron accepters and by the formation of authigenic phases containing reduced Mn at depth.