23 June 2021

The fragile beauty of the deep sea

The deep oceans, once thought to be lifeless, are home to many extraordinary creatures. But now a mining boom threatens their – and our – existence

By Tim Flannery

In the still and frozen blackness of the ocean deep swims a worm that resembles the buttocks of a pig. When disturbed, it glows bright blue, then squirts a green material from the crack between its buttocks (more correctly known as its middorsal ciliated groove). The pig buttock worm is emblematic of the mirror-world that is the ocean deep, a world so alien and paradoxical it’s often hard to compute. The worm’s startling display was first seen in 2009, and is part of an expansion of knowledge about the deep sea so voluminous it’s hard to keep up. An excellent way to do so is to read Helen Scales’s The Brilliant Abyss. Written by a highly articulate expert in the field, it’s so comprehensive and insightful that it will be a long time before it’s surpassed.

Just 150 years ago, it was scientific dogma that the ocean lying below 550 metres was devoid of life. This “azoic” theory was proposed by the British naturalist Edward Forbes, who in 1841 undertook a biological survey of the Aegean Sea. The deeper he sampled, the less life he found, leading him to extrapolate a theoretical point at around 550 metres deep where life, he hypothesised, was extinguished. The idea caught on quickly and Forbes, who coined the term “abyss” for water lying deeper than 100 fathoms, became famous.

But the Aegean Sea, later research revealed, is not typical of the oceans as a whole. Its surface waters are starved of nutrients, so it’s unusually devoid of life at depth. Yet the azoic theory was tenacious, surviving even after the recovery of cold-water corals from 400 fathoms in Antarctic waters, and the discovery off Greenland, in 1860, of starfish clinging to a rope brought up from depths of 2,300 metres. It was only when, in the late 1860s, newly developed dredging nets recovered abundant life from the abyss that Forbes’s theory was reluctantly abandoned.

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The size of the abyss is stupendous. The deep sea, with its abyssal plains lying between 3,000 and 5,000 metres below the ocean surface, is the largest habitat on Earth, giving each of the world’s 500-odd deep-sea researchers around two million cubic kilometres to explore. The region is also incredibly varied, from seamounts whose summits are covered in corals thousands of years old, to the volcanic vents known as black smokers that are surrounded by dancing yeti crabs, prawns with eyes in their backs, two-metre-long vent worms, and snails with iron shells. Between such hotspots, vast mud plains stretch endlessly, interrupted by great canyons plunging down to up to 11 kilometres below the surface.

Just how life survives in the freezing, pressurised and eternally dark abyss is a source of wonder. With the exception of the mid-ocean vent faunas (where life survives on minerals carried up from deep in the Earth), all subsistence in the deep sea arrives from the sunlit zone above, mostly in the form of “marine snow”. The snow consists of fluffy particles of mucus, faeces and dead bodies, the smallest of which are fragments of single-celled plankton, and the largest the bodies of whales. The density of the marine snowfall changes with the seasons and currents, and armies of sea cucumbers, starfish and other creatures march across the abyssal plain in search of fresh deposits. Much of the snowfall is intercepted on the way down, just 2 per cent making it to the deep sea floor.

Among the mid-water harvesters is the vampire squid. Blood-red of skin, with domed red eyes and a bone-white beak, it feeds by unfurling a filament eight times the length of its body. Marine snow adheres to this strange appendage, which the squid reels in and wipes over its hook-covered arms, prior to consuming.

When the body of a whale drops into the abyss, the first guests at the gargantuan feast are some of the largest predators of the deep – Greenland sharks – along with other flesh-eating fish. When only the skeleton remains, the bone-eaters settle in. These curious creatures are three-centimetre-long worms, with red feathery tentacles at one end and vivid green branching “roots” at the other. They have no mouths, guts or anus, their green “roots” producing an acid that dissolves the bone, which they then absorb. All are female, the males being minute creatures that take up residence inside the female’s body. A single female will host a harem consisting of hundreds of these tiny males, each living off its egg yolk and dutifully producing sperm for its alien-like mistress.

In the deepest parts of the ocean, some life at least finds its limits. Fish, for example, cannot exist below nine kilometres depth because the pressure is so great that the metabolic activities of vertebrates cannot be completed. But other kinds of life flourish, even at the very bottom of the ocean (known as the Challenger Deep). Here the pressure is so severe that calcium carbonate skeletons and exoskeletons dissolve away. The aluminium amphipod flourishes because it has evolved the ability to enshroud itself in a gel made from aluminium recovered from deep-sea sediments, which protects its shell.

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In the first half of her book, Scales does an excellent job of animating the almost unbelievable panoply of life in the deep. As an explorer herself, she has seen things first-hand that few others will ever witness. But it is the second part of her book, devoted to the human impacts on the abyss, which brought gasps to my throat. Improving technologies have allowed fishing in ever deeper waters, and sometimes fishers discover valuable resources in the abyss. The slimehead, Scales tells us, had its name changed to “orange roughy” “so people would eat it”. These fish swarm in abundance on seamounts – indeed, in such great numbers that they once filled nets so quickly the catch could not be processed, and the dead fish were thrown away.

It takes these plate-sized fish between 20 and 40 years to reach sexual maturity, and they can live for 250 years. While it seems crime enough to eat such venerable creatures, it’s the damage done to the deep in order to catch them that’s the greater sin. Seamounts are covered with forests of deep-sea corals that can grow many metres tall. These forests are home to countless creatures – from tiny seahorses to sea anemones found nowhere else. And they grow slowly. When I was director of the South Australian Museum in 2000, a fisherman chasing orange roughy brought in the base of a large coral that had come up in his net. We radiocarbon-dated it, and it turned out to be more than 4,000 years old. You can’t bottom trawl a seamount without first knocking down its coral forest: the great, glorious groves are flattened so that, at the cost of a few dollars, the flesh of a fish that is most likely older than you can be placed before you on the table.

[see also: The butterfly effect]

Bottom trawling involves dragging huge lumps of steel and net across the sea floor. It has destroyed some of the most precious oases of life in the deep, though thankfully it’s now banned in some regions such as in the North Sea. And remarkably, it seems that once the trawls cease, life can return. It may take decades, and perhaps centuries, for devastated seamounts to recover. Some may never do so fully. But with no days or seasons, time passes differently in the deep sea, and there is no doubt that the effort to conserve such wonders is worthwhile.

The deep oceans are the ultimate repository for much atmospheric carbon. Once marine snow descends below a kilometre in depth, the carbon in it cannot easily rise back to the surface. This means that, for centuries if not millennia, the carbon is locked out of the atmosphere.

Because the deep ocean is so large, it can store colossal volumes of carbon. For example, if we were able to take 50 per cent of the CO2 in the atmosphere and place it in the deep ocean (below 3,000 metres), it would increase the CO2 concentration in the deep by just 2 per cent. Lots of carbon reaches the deep in the form of seaweed that has drifted down submarine canyons, and in fallen whale carcasses. Some scientists think that by growing seaweed in strategic locations, and by encouraging the health of the ocean overall, we can increase the rate at which carbon is sequestered in the deep.

But a terrible threat to the abyss is now emerging, which could fatally damage the entire planet. With the increasing need for rare metals, and huge advances in technology, it seems the world is on the brink of a deep-sea mining boom. Black smokers are chimney-like deposits of minerals that form along mid-ocean ridges. They are also home to unique ecosystems that rely not on energy from the sun, but on minerals brought up from the Earth’s mantle. Is the world willing to let the only known home of the giant tube worm, the scaly-foot snail (which makes its shell from iron), and the hairy-chested Hoff crab (which is named after David Hasselhoff and which dances ceaselessly in the deep, waving its narrow pincers to its own internal rhythm), for a haul of gold, silver and other precious metals?

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As damaging as deep-sea vent mining would be, another kind of mining is proposed that strikes at the heart of the deep itself. Large areas of the abyssal plain are covered with fist-sized nodules of manganese. These nodules take hundreds of thousands of years to form and are home to many unique creatures, from sea anemones to sponges, that use them as holdfasts. They are also rich in the metals used to make the batteries of electric vehicles. The International Seabed Authority (ISA) is tasked with overseeing seabed mining in the high seas. Once its “mining code” is finalised (which was supposed to be negotiated in 2020, but has been delayed), exploration permits can be converted into mining permits. By 2019, China held more exploration permits than any other country, covering an area roughly the size of the UK.

The jewel in the crown of such holdings is the Clarion Clipperton Zone (CCZ). Stretching across a gently undulating section of the abyssal plain between Mexico and Kiribati, the CCZ’s metallic nodules can be as dense as cobbles on a street. Astonishingly, the ISA – the supposed regulator of ocean mining – has the right to mine. And according to Scales, its secretary-general Michael Lodge sees seabed mining as desirable and indeed a foregone conclusion. For the creatures of the CCZ, the nodules are their forest. Remove them and the creatures of the zone, which include fist-sized single cells that are among the largest on the planet (known as xenophyophores) will vanish. As horrific as this prospect is, it is eclipsed by the potential damage mining could do to the ocean’s carbon sequestration capacity.

Clouds of toxic sediment will be stirred up, and they will not easily disperse. Migrating turtles, sharks and other creatures will be affected. Most worryingly, the disturbed sediments may allow enormous volumes of sequestered carbon to re-enter the atmosphere. And if that happens, we may be tipped into a climatic disaster.

It is hard to imagine a more timely or important book than The Brilliant Abyss. Carefully conceived and luminously written, it is certain to be a bestseller, which gives me hope that its urgent message might help save the world.

Tim Flannery is an Australian zoologist and environmentalist

The Brilliant Abyss: True Tales of Exploring the Deep Sea, Discovering Hidden Life and Selling the Seabed
Helen Scales
Bloomsbury, 352pp, £16.99