Advances in Understanding and Reporting Oxidation-Reduction Potential (ORP) Anomalies in the Exploration and Characterization of Active Seafloor Chemosynthetic Systems

Oxidation-reduction potential (ORP) of seawater is relatively simple to measure using a Pt electrode paired with a Ag/AgCl reference electrode. While of limited value for quantifying the absolute redox status of the ocean environment, these electrodes are extremely sensitive to nanomolar concentrations of reduced chemical species (i.e., Fe²⁺, HS⁻, and H²) generated by seafloor processes such as hydrothermal vents and methane seeps; sites that support unique chemosynthetic ecosystems with potentially valuable mineral or biological resources. ORP anomalies have been essential for identifying sites where fluids entering the overlying ocean lack particulates, such as low temperature diffuse vents and vent fields where serpentinization (an exothermic water-rock reaction) is the primary heat source (i.e., Lost City Hydrothermal Field); or for mapping the extent of seep areas beyond bubble plumes that can be located acoustically. Analysis and graphic representation of ORP anomalies is complicated by non-equilibrium redox conditions of seawater and the Pt electrode surface, electrode drift, and slow recovery after encountering reduced species, all of which contribute to hysteresis and continually changing “background” values during typical water column survey operations. A simple first-order solution to overcoming these challenges has been to analyze for the time rate of change, dE/dt, and many anomalies can be identified by rapidly decreasing potential (dE/dt < 0) greater than average sample-to-sample variability. However, some significant anomalies do not have dE/dt values in excess of the average variability, but will have a significant overall decrease (ΔE) that may be missed when only dE/dt analyses are used. Laboratory experiments have shown that ΔE is proportional to the concentration of reduced species, so can be considered a quasi-quantitative indicator of relative signal intensity, which is useful for refining seafloor targets for more detailed exploration. By combining dE/dt (where consistently < 0) and ΔE, the duration and magnitude of the signal can be mapped and presented within the context of other plume parameters, geographic location, or seafloor observations. Examples from recent field expeditions will be presented to demonstrate the advantages and disadvantages of these methods.