Home of the deepest spot on the planet, the Mariana subduction system serves as a valuable natural laboratory for testing ideas about what governs the distribution of animals at hydrothermal vent systems. The deep trench, shallow to mid-depth volcanic arc, and mid-depth to deep spreading back-arc, provide a wide variety of habitats for research. Of these, more than 600 km of the back-arc has remained relatively unexplored. In December 2016, the science team followed up from a discovery cruise that occurred on Falkor the previous winter. During the 2015 visit, three new hydrothermal vents were discovered; one of them being among the deepest vents in the world. This cruise, the team returned with the 4,500m capable ROV SuBastian to explore the life and activity at the vent sites.
Focusing on the Back-Arc
In November 2015, Principal Investigator Joseph Resing, of the University of Washington, led a team of researchers from the University of Washington, Oregon State University, and NOAA/PMEL, went on a 27 day mission aboard Falkor to shed light on the Mariana Back-arc. The team achieved their goal of finding new hydrothermal sites in the Back-arc spreading center and are now planning to return to better understand the biodiversity that shape these unique chemosynthetic ecosystems.
“Chemosynthesis” refers to organisms that transform carbon dioxide into organic matter in the same manner as plants, but, instead of using the energy from sunlight to do this — as in photosynthesis — they use chemical energy instead, mostly from hydrogen, methane, hydrogen sulfide, and iron. Chemosynthetic ecosystems, like the Mariana Back-arc, exist in locations on the seafloor where chemical energy is present from seafloor hot springs, cold seeps, or other microbiological processes.
The Mariana subduction system offers living organisms a range of conditions unlike any other region on the planet. To the west of the trench in the Mariana Back-arc, is a zone of spreading, where mantle magma wells up and new ocean crust is formed. This tectonic environment creates different geophysical and geochemical conditions for the chemosynthetic organisms, reflecting diversity in biological communities on the seafloor.
Return to the Deep
The research from the 2015 cruise helped test the idea that arc and back-arc sites have distinct ecosystems, controlled by each settings’ geology and unique fluid chemistry. To test this hypothesis, several tasks had to be accomplished. The 2015 science team began by finding and identifying active vent sites along the Mariana Back-arc using AUV Sentry, and characterizing each site by its depth, geologic setting, temperature, chemical composition, and rise height of hydrothermal plumes. This follow-up 2016 expedition allowed scientists to return with Schmidt Ocean Institute’s brand-new 4,500 m capable remotely operated vehicle (ROV) SuBastian to visually explore and sample the newly found vent sites on the seafloor.
The team combined the 2015 model with biodiversity data from this 2016 cruise to better define the relationships between geologic setting, chemical environment, and biological communities. One basic goal was to catalog the life forms found at these vents, their affinities, and their relation to the regional biogeography. Researchers expected surprises, given how little exploration has been done on the Back-arc — recent work conducted at other Back-arc sites led researchers to find huge hairy snails never before seen. The team led by Dr. David Butterfield from JISAO, University of Washington, examined the chemistry and geology of the vents, studied their microbiology, and tested ideas to explain the substantial biological differences between the volcanic arc and back-arc vents.
Written by: Logan Mock-Bunting
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