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  • Leo R

The Great Silt Plain | Visions23 Cruise Log 2

Updated: Sep 4, 2023

Nothing prepares you for the enormity of the ocean. The sheer vastness of it. Standing on the ship’s bow, view unobstructed, gazing out at nothing but water - more water than I have ever beheld before - I find there’s something curious about how it’s able to appear at once both barren and lively. It is desolate, yet majestic. There’s simply no parallel. A great roaring, rolling, foaming desert, ever-changing and reshaping from moment to moment. I will never grow tired of gazing. Nothing unlocks the mind more than looking out upon its surface and pondering on the secrets held within. Out here in particular, at times adrift over waters up to 3 kilometres deep, tuning in to that awareness and thinking on the stories playing out on the bottom or in the vast space between is not something I’ve experienced before. It feels a little like stargazing. Thankfully, with ROV Jason’s help, these stories can be revealed to us.

I must admit, I’ve been remiss in my duty of keeping up with this blog. The excitement of being here so easily swallows up time, so I tend not to notice as it slips away. Leg 2 of the Visions23 cruise has already commenced, and since my last entry we’ve conducted many more dives with Jason and carried out key maintenance work on the RCA’s numerous moorings and instruments. Of all the dives, one stood out, at a site called Slope Base (water depth of 2,900 metres) during leg 1 where we witnessed some of the most peculiar sights so far!

Following a 2 hour descent through miles of water speckled with jellies and small, silvering fish, we at last descended upon a great desert of silt and mud. Over millions of years, marine snow has accumulated here to create a layer of organic detritus at the very bottom of the Cascadia Slope. It seems a barren stretch at first, but the seafloor here is far from uniform. Mounds, bumps, and tracks in the sediment map the activity of life in the benthic environment (the term for the ecological zone comprising the subsea sediment surface and sub-surface layers), from bottom-dwelling fish to urchins and holothurians (sea cucumbers). These are the bioturbators, which play a key role in the deep benthos by disturbing and reworking sediments through burrowing, ingestion, and defecation. Bioturbation is a crucial ecosystem service in any sedimented habitat, whether it be terrestrial or aquatic, but its importance is notable in the deep ocean in particular since ecosystems here depend almost entirely on nutrients and organic inputs from the productive photic zone above. Without marine snow, much of the deep ocean would starve. The action of bioturbators recycles nutrients and aerates the sediments, making them habitable to burrowers and other infauna - the animals that live within the sediment itself.

(Left) Mounds, created by bioturbators, rise up out of the silt plain at Slope Base, 2.900 metres down off the coast of Oregon. Credit NSF-OOI/UW/WHOI.

The fish that live down here are obscurities, well-adapted to survive the depths through interactions with the world of mud and ooze below.

The cusk eels - slim, eel-like fish of the family Ophidiidae - have rearranged the position of their fins. The dorsal, anal, and tail fins are fused into one long structure. Below the chin, their pelvic fins are thin and resemble feelers. These act as sensory organs that dangle down and search for food on the plain below as the fish swims just above the bottom.

A cusk eel at Slope Base. Credit NSF-OOI/UW/WHOI.

In a similar manner, rattail fish (also called grenadiers) thrive in this dark world thanks to their heightened senses. In a world without sunlight, you might expect deep-sea denizens to have lost their eyesight - but for many, this is not the case at all. Large, blue eyes allow rattails to glimpse the faint flashes of bioluminescence produced by potential prey.

Deep-sea rattail. Credit NSF-OOI/UW/WHOI.

During ascent, we glimpsed (for all of 3 seconds) one of the deep sea's most elusive inhabitants. A giant phantom jelly, Stygiomedusa gigantea, one of the largest jellyfish known to science!

Giant phantom jelly, sighted in the midwater during ascent at around 2,400 metres down. Credit NSF-OOI/UW/WHOI.

- Leo Richards

To find out more about Visions23 and the Regional Cabled Array, click here.

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