Pythias Oasis: An Underwater Spring Unlike Any Other

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Scientists, students and the Jason group works together inside the control van during a dive at Pythias Oasis. Credit: M. Elend, University of Washington.

In ancient Greece, the Pythia was the oracle at Delphi who sat in a temple built above a seismic fissure and ‘prophesized’ with the assistance of the mind-altering gases rising from the hot spring below. It seems equally hallucinatory to find a spring of low-salinity, high-temperature, mineral-rich water flowing from the seafloor 1000 meters below the surface off the coast of Oregon, which is why the name “Pythias Oasis” is so apt. This site was discovered in 2015 by Brendan Philip (University of Washington graduate) who was tracking methane bubbles that naturally stream out of the seafloor on the edge of the continental shelf. Thousands of these bubble streams have recently been discovered along the Washington-Oregon margin.

Jason is recovered onboard the R/V Atlantis after completing a dive at Pythias Oasis. Credit: M. Elend, University of Washington.

During the discovery dive, Brendan used the remotely operated vehicle ROPOS to follow the plume down to a bizarre opening on the seafloor gushing sediment and methane-rich fluids. Based on analyses of a few preliminary samples, the chemistry of which astounded the UW team, the National Science Foundation funded the award “Collaborative Research: Pythias Oasis – Access to Deep Subduction Zone Fluids” to the University of Washington (D. Kelley and E. Solomon) and Oregon State University (M. Torres, R. Collier, and R. Harris) to characterize this extraordinary site. Proposed efforts included detailed seafloor and water column surveys, measurement of the heat flow through the surrounding and distal  sedimentary deposits, sampling of the novel fluids issuing from the seafloor, and the collection of sediment cores to examine fluids deeper beneath the seafloor.  The Pythias Oasis expedition (Chief Scientist: Deb Kelley; Co-Chiefs: Evan Solomon & Michael Vardaro) just returned from 3 weeks at sea (September 14-October 5) onboard the R/V Atlantis, hosting the remotely operated vehicle (ROV) Jason and autonomous underwater vehicle (AUV) Sentry.

Lightning on the horizon celebrates Sentry diving at Pythias Oasis during the UW-OSU expedition funded by NSF. Credit: M. Elend, University of Washington.

Pythias Oasis is unlike any other seep yet discovered in the world’s oceans. The site is located about 4 hours steam off Newport, Oregon on a promontory (now) called Pythias Ridge. The main Pythias vent is a “hole in the bottom of the sea” characterized by intense venting of fluid with unexpected chemical properties and temperatures that are three times normal background seawater values (~4°C); it is reminiscent of a mid-ocean ridge hot spring. The released fluids gushing from the seafloor carry suspended particles that form a small plume which splits into a second plume with methane hydrate-encased bubbles that rise > 450 m above the seafloor. A second site to the south includes an extremely rugged collapse zone hosting numerous explosion pits, extensive areas of white and orange bacterial mats, and diffuse flow of methane gas and warm water. A several meter deep “moat” surrounds the collapse zone. The area teems with life including rockfish, eelpout fish, hagfish, anemones, “sea pig” sea cucumbers, crabs, snails, soft corals, and clam beds.

The collapse zone at Pythias Oasis hosts thick yellow and white bacterial mats, here about to be sampled with a push core held in Jason’s manipulator. Credit: D. Kelley, University of Washington/NSF

During the expedition, Jason obtained stunning 4K video and thousands of digital still images to characterize the geology, tectonics, and animal communities that inhabit the seep sites.  Individual detailed photomosaics with the ROV ‘flying” only a few meters above the seafloor collected >4000 images on a single survey. The ROV also collected numerous fluid and gas samples, carbonate rocks, and push core sediment samples. Heat flow measurements made with a “violin” probe (so named because it has a thin string of temperature sensors adjacent to a sturdier 4 m-long wand, making it similar in appearance to a violin bow) led to unparalleled characterization of the thermal structure of Pythias Ridge.

The AUV Sentry conducted high-resolution bathymetric surveys using a multibeam sonar that provided a 1 m resolution map of the ridge. Flying 4 – 5 m off the seafloor, Sentry also collected thousands of overlapping images of the area surrounding the Pythias vent and the collapse zone. The vehicle utilized a multi-frequency system called CHIRP to image the shallow sub-seafloor sedimentary structure underneath Pythias including carbonate- and gas-rich horizons, and perhaps faults that funnel the fluids into channels.

Another significant success of the cruise was an intense sediment coring operation that collected 27 cores up to 3 meters in length (many thanks to P. Walczak and the OSU coring facility). The Pythias Oasis team spent 24-hour days in the ship’s laboratory, from which pore fluids were extracted for follow-on chemical and isotopic analyses. Numerous recovered cores contained an astounding amount of methane hydrate, which is a type of ice that crystallizes around methane molecules under high-pressure, low-temperature conditions. The hydrate degassed at the surface leading to “boiling” mud at the top of the cores and cores that “hissed” on deck as gas and mud spewed from holes in the core liner.

Flaming methane hydrate following recovery of a core from Pythias Oasis. Credit: M. Elend, University of Washington.

The team enjoyed demonstrating how the “ice” could be lit on fire as the gas was released from the methane hydrate compound on the surface. The cruise ended with the discovery of a new extensive methane seep marked by beautiful colonies of sponges, huge rock fish, and bubbling bacterial mats. The team is excited to analyze the wealth of samples and imagery obtained during this cruise, with the anticipation that the results will lead to important discoveries about deep-seated mineralogical, fluid, and deformation processes in this seismically active section of the Cascadia Margin.

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