iMAGE-CREATE trainee Jessie Kehew reports from the RV Sonne SO294 expedition
After four weeks at sea, the R/V Sonne SO294 expedition has just passed its half-way point. The vessel departed Vancouver on September 13th and has been travelling along the western coast of Vancouver Island over the Cascadia subduction zone. Subduction zones are regions with an increased risk of high magnitude earthquakes, so research in this field is incredibly important for the safety of communities within areas of potential danger. Limited research has been done in the Cascadia subduction zone to understand its behaviour and structure, so the goal of this cruise is to seismically image the subduction zone to identify the extent of the seismogenic zone.
Another smaller project on this expedition focuses on the potential storage of CO2 in the seafloor. This is a contribution to a set of pre-site surveys in an area being considered as a test site for the injection of CO2 into basalt in the Cascadia Basin. In this CO2 sequestration method, CO2 is removed from the atmosphere and injected into basalt formations, where it is fixed as a solid carbonate. The seismic imaging completed on the SO294 cruise will help to determine lateral changes in porosity in the basalt of the proposed test site.
Throughout this cruise, two datasets are being collected almost continuously: multibeam and Parasound. Multibeam data measures the bathymetry, as well as backscatter and water column data. Many of the regions visited during this cruise have never been mapped with multibeam, and the high resolution bathymetry has revealed many interesting features. Parasound is a sub-bottom profiler which images the top layers of sediment, penetrating up to 200 m deep. From this data, we can interpret the recent sedimentation history.
My role for the duration of the expedition is to monitor the multibeam and Parasound data in the hydroacoustics lab to ensure the continuous collection of data (Figure 1). Someone must always be present to turn off the echo sounders when they cause interference with other acoustic devices on board and when mammals are found in the exclusion zone by the Marine Mammal Observers (MMOs). It is fascinating to watch the bathymetry appear on screen as it is recorded, and to see the map grow each day, revealing ridges, channels, landslides and canyons that have never been mapped before.
During the first few days of transit from Vancouver to the first station, we passed over an area called Swiftsure Bank, a known site for methane gas flares. By looking at the water column data collected by the multibeam system, we identified several gas flares. One of my responsibilities on the cruise is to process the water column data and identify the locations of all the flare sites for future research done by the Geological Survey of Canada.
One of the main scientific activities on deck is the deployment and recovery of Ocean Bottom Seismometers (OBS) for seismic refraction surveys which produce an image of the crust. On this cruise, we are deploying long-term and short-term OBS. Long-term OBS stay on the seafloor for 1 year and use seismic waves created by earthquakes to image the crust. Short-term OBS are deployed for the duration of the survey (several days to 1 month) and rely on the seismic waves created by airguns that are being towed by the ship (Figure 2). Before deployment, a lot of preparation is involved to assemble the OBS, build the steel anchors that help them sink them to the seafloor, and test the systems that release the OBS from the anchor to allow them to float to the surface for recovery. During OBS recovery, we drop a hydrophone into the water which sends a signal to the release system, releasing the buoyant OBS from its anchor. Many of the OBS are at depths greater than 1000 m, and it can take up to 45 minutes for it to float through the water column. We must diligently watch the water to spot the flag and flashing light of the OBS once it reaches the surface, so that it can be brought back on-board where the data is offloaded and processed.
The scientific activities don’t stop there! We have collected several gravity cores at landslides to identify turbidites, whose deposition is thought to be triggered by seismic events. By dating each turbidite, we can calculate earthquake frequency in the area. The core is attached to a 1 ton weight which is lowered to the seafloor, and the gravitational force of the weight pushes the core into the top several meters of sediment (Figure 3). When the core is brought back on the ship, it is cut into meter-long sections, the pore water is sampled, and the core is refrigerated. No core analysis is to be conducted during this cruise but will be completed at the Pacific Geoscience Centre in Victoria.
The mammal mitigation measures on the R/V Sonne are extensive, and the team of MMOs is thorough. Since the frequencies and amplitudes of the acoustic sounders used on the ship have the potential to harm the mammals and interfere with their acoustic communication, we must be prepared to turn the sounders off as soon as any mammals are identified in the range of potential harm. There are five MMOs who continuously scan the ocean around us in search of whales, dolphins, sealions and other marine mammals. An infrared camera is also installed on board, which can identify whale blows. At night when the MMOs can no longer visually observe the surroundings, a team of three technicians deploy and monitor the Passive Acoustic Monitoring (PAM) system, which is a towed system that records the songs and clicks of any marine mammals within range. By listening to the recorded sounds, they can identify specific species and calculate their distance from the ship, allowing the PAM operators to identify when they are close enough to the ship for the echo sounders and airguns to be switched off.
30 scientists make up the science crew onboard the R/V Sonne (Figure 4). The majority come from GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, with members ranging from undergraduate and graduate students to technicians, post-docs, senior scientists and the Chief Scientist, Michael Riedel. A group of scientists from JAMSTEC in Japan, two representatives from the Geological Survey of Canada as well as a scientist from Ocean Networks Canada are on board. Finally, there are five MMO from LGL and three PAM operators from JASCO, both Canadian companies. With the influence from geophysicists, geologists, geographers and marine biologists, this expedition has a wide range of expertise.
After several weeks at sea, we have come up with lots of ways to entertain ourselves during our time off. Ping-pong tournaments, trivia nights, movies, and socials happen regularly to keep spirits high. The wildlife help with morale too! We have seen countless humpback whales, fin whales, orcas, dolphins, and sunfish (Figure 5). One day there was a spectacular super pod of dolphins (Figure 6) that swam beside the ship for over an hour, diving right next to the main deck. Although we weren’t able to collect any data during this time, we got to enjoy a free dolphin show.
On this cruise, I have had the opportunity to help out with the OBS preparation and recovery, and gravity core deployment and recovery. With a trip up to the observation deck, the PAM technicians are happy to show you humpback whale songs, and the MMOs will give you tips for spotting and identifying orcas and dolphins. Being surrounded by geologists and geophysicists, there is always an interesting conversation to join, and always something new to learn. Having not been involved with the collection of the data for my master’s project, seeing all the effort, logistics, and collaboration involved with a project like this has given me a deeper understanding and appreciation for marine geoscience.