Pingo-like Features (PLFs) and Mud Volcanoes on the Eastern Mackenzie Shelf
Over the last few days, we have conducted three remotely operated vehicle (ROV) dives and two autonomous underwater vehicle (AUV) surveys at areas of geologic interest known as pingo-like features (PLFs) and mud volcanoes on the eastern Mackenzie Shelf.
About 1,350 pingos are found on the adjacent land of the Tuktoyaktuk Peninsula. A pingo, which is an Inuit word, is a mound-shaped, ice-cored hill that forms on land when fresh water enters the near-surface sediments in summer and then freezes in winter. As the ice forms, physical expansion occurs and pushes up the sediment layers above it, creating the pingo.
Pingo-like features found on the seafloor are circular mounds that rise up like haystacks from the seafloor and superficially resemble pingos found on land. The underwater PLFs were first discovered in this area in 1969 and investigated as a potential hazard to navigation. Since then, thousands of PLFs have been identified along the continental shelf/slope, but only a handful have been studied in detail to understand how they form. One of the goals of this trip is to try to understand whether the marine PLFs and the terrestrial pingos are actually similar features and formed under similar processes.
Photo 1: Pingo-like features in bathymetry images.
In multibeam bathymetry images, underwater PLFs look like rows of circular mounds or, when they collapse, donuts. We chose four PLFs, 10 metres high and 70 metres across, to investigate with the MiniROV’s HD video camera. While surveying, we find abrupt transitions between the smooth and flat seafloor, which surrounds the PLFs, to 30–40 degree slopes of the PLF flanks. The flanks contain small boulders, cobble and gravel. As these materials are not seen on the surrounding seafloor, we speculate that they have been pushed up with the freshwater from deeper in the seabed.
We also encounter communities of suspension-feeding organisms, animals that feed on the small planktonic organisms that drift past them in the currents. We often find these animals — which include soft corals, basket stars and crinoids — on sloped terrain, like the flanks and summits of the PLFs, where current speeds are higher and more food flows past them. Other organisms include a variety of benthic fishes, worms, sponges, brittle stars and anemones.
Photo 2: Gersemia sp. coral, sponges and brittle stars on a pingo-like feature.
Mud volcanoes are also circular seafloor features, both conical and flat-topped, but unlike pingos do not contain ice cores. These larger, pancake-like features, greater than 1 km across and 10–30 m in height, are areas that have subtle changes in seafloor topography. Mud volcanoes form when methane gas, seawater and mud bubble up on the seafloor from approximately 1 km below the seafloor.
The first mud volcano we surveyed was at 420 m below the surface. We can see the form of the mud flows in the multibeam bathymetry, and we can detect what we believe are the various ages in both the sidescan sonar and with the ROV, which shows abrupt color changes from light grey to dark grey between very young and very old flows. We also see differences in the chemistry of the layers below the seafloor and, on the oldest of flows, dense populations of chemosynthetic siboglinid polychaete worms, which thrive in the chemical-rich sediments. Other organisms found at the 420m mud volcano include benthic fishes, shrimp and several species of anemones.
At the 740 m mud volcano we see evidence of mud flows in the bathymetry and, using repeat mapping techniques, we identify annual changes in the surface topography. During the ROV dive, we find a vent area where fluidized mud was actively moving, like the top of boiling water roiling the surface of the seafloor. Small gas bubbles were coming out of the fluidized mud almost continuously, and large bursts of gas carried sediment up into the water column. Surrounding the vent area were trails of what appear to be very fresh mud flows running down the gentle slope on the side of the mud volcano.
These observations provide a glimpse of how mud volcanos build up. Sea life was less abundant here but included a few scattered anemones, fishes, beautiful skates and the ever-present krill, which are incredibly abundant on nearly all of our ROV surveys.
Photo 3: Contact showing juxtaposition of young and old mud flows, with dense siboglinids on old flow in background on 420m mud volcano.
The seas have been calm and winds have been light. But Arctic sea ice was approaching the ship during the ROV survey, which meant that we had to abort the ROV dive very quickly and move to a new survey location. Our second survey site was also not accessible due to sea ice. We took advantage of this opportunity to enter the sea ice edge, if briefly, to see first-hand the incredible beauty of the drifting sea ice. It was truly amazing. Tomorrow we transit back to the west Mackenzie Shelf for our final two days of surveying.
Photo 4: Sea ice off the bow of the Araon.
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