In situ deployment of voltammetric, potentiometric, and amperometric microelectrodes from a ROV to determine dissolved O{sub 2}, Mn, Fe, S({minus}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{sub 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{sub 2}, pH, and resistivity microsensors were operated alongside the voltammetric sensor. The authors 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{sub 2} measurements from both Clark and voltammetric electrodes were in excellent agreement. The profiles obtained show that there is no detectable overlap of O{sub 2} and Mn{sup 2+} 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{sub 2} is not a direct oxidant for Mn{sup 2+} when diffusive (rather than advective) processes control the transport of solutes within the sediment. Subsurface Mn{sup 2+} 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 acceptors and by the formation of authigenic phases containing reduced Mn at depth.