Leo Richards Blog Legs 2&3

Beds of deep-sea clams burrow into matts of chemosynthetic bacteria at the active seep site at Southern Hydrate Ridge. Credit: UW/NSF-OOI/WHOI; J2-1542.

30 August 2023 Surveying Southern Hydrate Ridge

I have just emerged from the Jason control van after perhaps the most exciting night of my life. The ROV dove to a site of hydrocarbon seepage called Southern Hydrate Ridge (SHR), and I, at last, got to sit in the hot-seat, operating the 4K camera and leading the dive for almost 10 hours. The aim of the dive was to conduct a geological and biological survey across numerous sites of interest along the ridge, recording and observing the life we encountered, while also collecting footage for the film I’ll be creating about the expedition upon my return. This is also part of an annual survey to document sites of venting in case the focused site "Einsteins’ Grotto" shuts down.

Cold seeps like SHR are among the deep’s more peculiar offerings in that they are robust and diverse biotopes reliant not on slow-sinking scraps of marine snow, or leftovers from the highly productive photic zone, but on an entirely different source of energy altogether: methane!

But how did hydrocarbons like methane get here in the first place? As marine life dies and settles on the seafloor, it gets buried, and sub-surface microbes decompose this material, while producing methane as a byproduct. This builds up; at places where tectonic processes squeeze and scrape sediments, these chemical compounds are released into the ocean.

At this particular site, the SHR methane seep formed as the oceanic Juan de Fuca plate is subducted beneath the continental North American plate at the Cascadia subduction zone.

Look closely at the seafloor here, and you’ll find patches of wispy white “fluff” that carpets the sediments. These are dense matts of chemosynthetic bacteria that undergo the crucial process of converting chemical energy from the methane into a food source (instead of converting sunlight energy, as we observe in photosynthetic organisms such as phytoplankton). It’s a process that mirrors photosynthesis in many ways, and similarly supports abundant animal life by forming the base of a complex food web.

Clam beds live in direct association with these microbes by hosting them within their shells, while sea stars, cucumbers, and snails all graze the mats and make the most of the region’s heightened productivity, gathering in far greater abundances than elsewhere on the deep sea floor. Seeps are hotspots of life, but they support deep biodiversity in another way as well.

A field of soft corals thrive atop the carbonate Pinnacle. Credit: UW/NSF-OOI/WHOI; J2-1543; V23.

The submarine landscape here is alien. Rising out of the great silt plain, we find grand rocky structures. Mounds, ridges and overhangs of textured carbonate – reefs of authigenic rock, meaning “precipitated inorganically in situ” – with fascinating origins! These crusts arise as communities of microbes within sediments anaerobically oxidize the methane, producing HCO3− and HS− as byproducts which increase the alkalinity of pore water (within sediments) and lead to the precipitation of the authigenic carbonates.

The carbonate creates complexity through the creation of towering, 3D structures. This is vital, for complexity drives biodiversity. Although the carbonate may “plug” the seepage over time, the hard surfaces continue to allow sessile animals (immobile and fixed in one place) such as corals and sponges to thrive!

Soft corals of the genus Anthomastus cover the carbonate outcrops, west of the active seep site at Southern Hydrate Ridge. Credit: UW/NSF-OOI/WHOI; J2-1543; V23.
A bubblegum coral stands proud atop the carbonate pinnacle. Credit: UW/NSF-OOI/WHOI; J2-1543.

At Pinnacle, a 50 metre tall carbonate mound located at SHR, the walls and overhangs are covered with otherworldly “mushroom” soft-corals (Anthomastus ritteri).

Additionally, the many crevices, nooks, and crannies allow mobile fauna to sequester themselves away and seek shelter from predators.

Squat lobster, hagfish and soft corals at home on the Pinnacle. Credit: UW/NSF-OOI/WHOI; J2-1543; V23.
A hagfish and a rockfish close together in a sedimented area of the Pinnacle. Credit: UW/NSF-OOI/WHOI; J2-1543; V23.

South of the active seep site at SHR, there is another strange world. An odd sight to behold, the landscape here resembles a great, flat shrine of candles laid out in clusters atop a plain of scattered carbonate rocks. This is the Neptunea nursery, and the “candles” are spiral egg stalks for a species of deep-sea snail of the genus Neptunea. During our visit, only the stalks were present, but in previous years the adult snails have oft been seen sitting atop their creations. It is thought that once the eggs hatch, the parent snails fall off and die.

Unattended Neptunea egg stalks on carbonate cobbles cover the seafloor just south of an active seepage site. Credit: UW/NSF-OOI/WHOI; J2-1542; V23.
Neptunea egg stalks, tiny shrimp and a crab make use of the carbonate clasts. Credit: UW/NSF-OOI/WHOI; J2-1542; V23.
Adult Neptunea snails sit atop their egg-stalk structures, taken in 2014. Credit UW/NSF-OOI/CSSF; ROPOS dive 1758; V14

After wrapping up the biology survey, and with still some time to go before the ROV was to be recovered, the pilot Mario invited me to pilot Jason itself for a little while. The opportunity felt tremendously special, and after being shown how to maneuver, change heading, and what to look out for on the many screens to ensure I did not crash this multi-million dollar vehicle into the seabed, I was given free reign to traverse the site. It immediately felt a more intimate experience controlling the ROV itself compared to simply watching from the hot-seat. Seeing the vehicle respond to every button-press and lever-pull, and knowing that the many wonders on-screen were now within reach, the great and mysterious deep-ocean realm which I have adored and revered for so many years felt at last closer than ever!

After a short while, Mario invited me to reach to my right and pull out another controller. This one looked a little different. A piece of jointed metal armature in miniature, this was the controller for Jason’s starboard manipulator. After positioning it to resemble closely the current resting position of the real arm, pressing the large button at the end of the controller unlocked the manipulator and allowed it to be moved through delicate repositioning of its miniature counterpart. This was truly something out of science-fiction. As per Mario’s instructions, I lowered the arm to the seafloor, opened the grabber, and picked up a hefty carbonate rock, before bringing it into view of the cameras.

Mario joked I should bring it back and keep it. Unfortunately, it was twice the size of my head…

The rock I picked up while in control of Jason and its starboard manipulator arm. Credit: UW/NSF-OOI/WHOI; J2-1542; V23.

20 August 2023 – The Great Silt Plain

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 VISIONS’23 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 Margin. 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.

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.

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.