A 14-meter-high extinct hydrothermal vent chimney was discovered on Dive 7 at Fina Nagu Caldera A on top of a volcanic dome. This hydrothermal chimney was probably once a high-temperature black smoker, built by hot, chemical-rich fluids created by seawater percolating downward and interacting with hot subsurface rocks above the dome’s magma chamber. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download (mp4, 50.4 MB)
Dive 7 was very exciting. We anticipated that the age of the Fina Nagu craters would decline as we progressed northward through the chain, and this is exactly what we found as we moved from south to north over the last few days. On Dive 7, the remotely operated vehicle touched down at ~2,370 meters and we explored two resurgent domes within the center of the caldera. The slope of the first dome was composed of volcaniclastic sediment, with isolated blocks of manganese-coated basalt. At the top of the first dome, we encountered a spectacular 14-meter tall extinct hydrothermal vent chimney and collected our first geological sample, a sulfide that had fallen off the chimney. The extinct chimney harbored mostly suspension feeders such as brisingid seastars and corals, but there was also one species of gastropods (Desbruyeresia) that was seen at the very top. As with the last two Fina Nagu sites, the fauna was few and far between. We saw snails, a cusk eel (Ophidiidae, Acanthonus), a ctenophore, and several predatory sea squirts (tunicates). We were surprised to see a deep-sea lizardfish (Bathysaurus cf. mollis) swimming in the water column, as they typically dwell on seafloor sediments. As we traversed the slope of the second dome, we saw more volcaniclastics, with outcrops covered in a manganese crust so thick that it was hard to determine their composition. We collected a manganese-crusted basalt sample in this area. At the top of the second dome, we found hydrothermal vent sulfides with several small patches of weak, diffuse hydrothermal flow (3 - 5.5 °C), upon which were dense clusters of small tubeworms.