Deep-Sea Electric and Magnetic Surveys Over Active and Inactive Basalt-Hosted Hydrothermal Sites of the TAG Segment (26 degrees, MAR): An Optimal Combination for Seafloor Massive Sulfide Exploration

A GEOMAR (Kiel, Germany) research team has developed a passive electric field acquisition system for Autonomous Underwater Vehicles (AUVs) to optimize seafloor massive sulfides exploration. This sensor was made of two perpendicular and horizontal pairs of electrodes, and was successfully tested over active basalt-hosted hydrothermal site TAG (26 degrees N, Mid-Atlantic Ridge) and several inactive sites in its vicinity. The resulting data underline the efficiency of combining deep-sea electric and magnetic measurements for searching for active and inactive hydrothermal vent fields. With these datasets, it becomes possible to determine the geological nature of the targets and to constrain the characteristics of fluid circulation at depth without involving costly and invasive underwater tools such as Remotely Operated Vehicles or even manned submersibles to collect samples. Data analysis also revealed that AUV attitude variations induce distortions of the electric signal. These distortions start prevailing for dives at altitudes higher than 90 m above the seafloor, as the distance between the AUV becomes too important to guarantee that the signal produced by the geological target still dominates. To improve the acquisition system and reduce the overall noise, we discuss solutions that limit the impact of such attitude variations. These solutions consist of minor adjustments, such as masts at AUVs stern to tow damping electrodes arrays. In such configurations, we believe that deep-sea passive electric measurements combined with high-resolution magnetic measurements can become a highly efficient seafloor exploration tool, including for sulfide deposits associated with inactive hydrothermal systems. Plain Language Summary Since the 1970s, the scientific community knows that the seafloor in the vicinity of mid-ocean ridges is associated with hydrothermalism. Fluid circulations drain minerals from the deep crust that are precipitated near the seafloor. These minerals may form seafloor massive sulfide deposits that have raised the interest of the scientific community. Whereas active hydrothermal vent fields may be detected from surface vessels thanks to their associated chemical anomaly diffusing, and potentially drifting in the water column, inactive sites are more challenging to identify. Highresolution magnetic data collected by deep- sea exploration vehicles have proven their efficiency to undertake such exploration. Nevertheless, these data have limitations, such as the non-uniqueness of the interpretations, and must be completed by others to optimize the efficiency of deep-sea research. Here, we present a combined dataset made of deep-sea passive electric, magnetic and bathymetric data, as well as water temperature analyzes collected simultaneously over several active and inactive basalt-hosted hydrothermal sites of the Mid-Atlantic Ridge. We emphasize the complementarity nature between highresolution passive electric and magnetic data to identify and study active and inactive hydrothermal sites, and propose engineering adjustments to ensure that this combination becomes the most efficient seafloor exploration tool in the future.