After a first week of dodging a typhoon, hanging on during rough seas, and ironing out minor equipment problems, we are now reaching our stride and have made our first hydrothermal discovery in the Mariana Back-Arc.
After leaving Guam, we have been proceeding roughly from south to north. To organize our search, we have divided the Mariana Back-Arc between 12.7°N and ~18.5°N latitude into 10 segments, each about 50-75 km long. So far, we have completed partial surveying in the southern five segments with a combination of three Sentry AUV dives and four CTD tow-yo’s (a tow-yo is when the crew drops the CTD rosette while moving, pulling it up and down, so it moves like a yo-yo). In addition, we have made four trace-metal CTD casts both near and away from the back-arc. We will be alternating between these three tools throughout the cruise to spend our time most efficiently.
Cloudy Water and Chemical Signals
From previous work in the back-arc, we knew that Segment 1 (the southern-most), parts of Segment 2, and Segment 10 (the northern-most of our planned survey) had active hydrothermal vents, but no information was available for the segments in-between. We had our first hydrothermal “hit” during the Sentry dive that was recovered yesterday evening. Plume sensors on the AUV detected cloudy water and chemical signals over a horizontal distance of ~1 km of the back-arc axis and up to hundreds of meters above the seafloor in Segment 4: clear evidence of a hydrothermal plume. We plan to return to the site tomorrow to conduct a CTD cast and take water samples, and to deploy Sentry for another dive to map the seafloor and further localize the vent source. We are currently conducting a CTD tow over an adjacent back-arc segment, and have several off-axis trace-metal casts scheduled after that before we return to investigate.
Place Your Bets!
To have some fun during our #HydrothermalHunt we’ve started an investment opportunity (a $1 betting pool) among the science party and the crew to see who can guess which segment of the back-arc (and even the exact location within the segment) will have the largest plume signals discovered on our cruise. After the pool was announced, the crew immediately started pressing the science party for “inside information” about what factors to weigh in making a good wager. Is a shallow volcanic ridge a good sign, or are the deep basins more attractive? How can you tell younger volcanic terrain from older faulted seafloor? What parts of a depth profile are vents likely to be located along? Suddenly the crew seem much more interested in the science and goals of our expedition since they are now “invested” in the outcome! We have a lot more of the back-arc to explore, so hopefully there will be many more discoveries to come.