Focusing on the Back-Arc

Last November, scientists aboard Falkor aimed to shed light on the Mariana Back-arc, expecting it to be teeming with activity and life. 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, over the course of the 27 day mission at sea. Their goal was to explore the back-arc spreading center to find new sites of hydrothermal activity and to better understand the physical, chemical, and geological forces that shape biodiversity in these unique ecosystems.

Life in the Shadowy Depths

These researchers expected that the Mariana back-arc would host many sites of hydrothermal activity with 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 exist in locations on the seafloor where chemical energy is present from seafloor hot springs, cold seeps, or other microbiological processes.

Shaping Earth, Shaping Life

The Mariana subduction system offers living organisms a range of conditions unlike any other region on the planet. During subduction, two of Earth’s tectonic plates collide. However, instead of crashing into each other and crumpling like a car wreck, one of the plates dives beneath the other, producing a deep trench. The down-going plate causes melting in the overlying mantle, creating magma that feeds the active volcanoes of the overlying arc. Further to the west, the back-arc is a zone of spreading in the overriding plate, where mantle magmas well up and new ocean crust is formed. For the chemosynthetic ecosystems, each of these tectonic environments create different geophysical and geochemical conditions. These differences seem to be reflected in the diversity of biological communities on the seafloor.

The Right Tools

Searching for hydrothermal vents is not an easy task due to the technical challenges of working in the deep ocean. The team used the Sentry Autonomous Underwater Vehicle (AUV), as well as instrument packages consisting of optical and chemical sensors to survey systematically for hydrothermally active areas. These sites could hold the answers to fundamental questions about the relationships between ecosystems, geologic settings, and fluid geochemistry.

Identifying vent sites

The research from this cruise helped test the idea that arc and back-arc sites have distinct ecosystems, controlled by each settings’ geology and unique fluid chemistry. In order to test this hypothesis, several tasks had to be accomplished. The science team began by finding and identifying active vent sites along the Mariana back-arc, and characterizing each site by its depth, geologic setting, temperature, chemical composition, and rise heights of hydrothermal plumes. During a follow-up expedition in 2016, the scientists will return with a remotely operated vehicle (ROV) to visually explore and sample the new vent sites on the seafloor.

The science team found some surprises given how little exploration had been done on the back-arc, including several new sites. We consider the Mariana system as a natural laboratory where hypotheses about the interrelations between tectonics, biodiversity, and biogeographic patterns can be tested. After constructing a geological and geochemical model of the back-arc vent settings, the team plans on combining the model with biodiversity data, to better define the relationships between geologic setting, chemical environment, and biological communities.

Partial support for this expedition comes from the NOAA Ocean Exploration and Research Program and the NOAA Pacific Marine Environmental Laboratory.