Appearance
1.11 — The Ocean Floor and the World Ocean
In 1977 a submersible called Alvin descended to 2,500 metres near the Galápagos Islands, where a research team expected to find warm water seeping out of cracks in the sea floor. What they found instead were chimneys of mineral gushing hot water, surrounded by dense communities of animals nobody had ever seen: tube worms over two metres long with blood-red plumes, blind white crabs, clams the size of dinner plates.
There is no sunlight at 2,500 metres, and there had been no reason to expect anything to be living there at all beyond a thin scattering of scavengers living off scraps drifting down from above. The geologists on board had brought no biologist and no preservative; they ended up storing specimens in the vodka from the ship's stores.
What they had found was an ecosystem that runs on chemistry rather than light. Bacteria at the base of that food web oxidise hydrogen sulphide coming out of the vent and use the energy to build sugars — a process called chemosynthesis, the alternative to photosynthesis. The tube worms have no mouth and no gut; they farm those bacteria inside their own bodies.
That discovery changed the estimate of where life can exist, on this planet and elsewhere, and it happened because people finally started looking at the sea floor. This page is about what is down there.
The real shape of the sea bed
Going out from any coast, you pass through the same sequence.
The continental shelf. A gently sloping submerged edge of the continent, typically extending tens to a few hundred kilometres out and down to about 130 metres. This is not really ocean — it is flooded continent, made of continental crust, and during ice ages when sea level was 120 metres lower it was dry land people walked on. Chapter 3.6 uses that. Shelves are where sunlight still reaches the bottom, where rivers deliver nutrients, and consequently where most of the world's fisheries and almost all offshore oil are.
The continental slope. The real edge, dropping from the shelf break down to the deep floor at a much steeper angle. Slopes are cut by submarine canyons, some as large as the Grand Canyon, carved by turbidity currents — dense avalanches of sediment-laden water that race downslope and can travel hundreds of kilometres across the abyssal plain. These are known to be violent: a 1929 earthquake off Newfoundland triggered one that snapped transatlantic telegraph cables in sequence down the slope, and the timing of the breaks let engineers calculate the current's speed at over 60 kilometres per hour.
The abyssal plain. Flat, dark, cold, 4,000 to 6,000 metres down, covered in fine sediment — clay blown or washed off the continents, plus the microscopic shells of plankton raining down continuously. These are the flattest surfaces on Earth, because sediment has been settling on them undisturbed for tens of millions of years, burying the original volcanic topography.
Seamounts are underwater volcanoes that never reached the surface — there are tens of thousands of them. Because they force deep, nutrient-rich water upward, they are oases of marine life, and consequently they are heavily fished.
The mid-ocean ridge runs 65,000 kilometres through every ocean basin, rising 2 to 3 kilometres above the abyssal plain, and it is where all of this crust is made.
The trenches are where it is destroyed. The deepest, the Challenger Deep in the Mariana Trench, is about 10,935 metres below sea level. Drop Everest into it and there would be over two kilometres of water above the summit.
Why the sea is salty when rivers are not
Rivers are fresh, and rivers are what fills the ocean. So where does the salt come from?
It comes from the rivers. River water is not pure — it carries dissolved ions weathered out of rock (Chapter 1.4), just at concentrations too low to taste. The ocean has no outlet. Water leaves it by evaporation, and evaporation takes the water and leaves the salt behind. Repeat for four billion years and the dissolved load concentrates.
But it does not simply climb forever, and this is where the story gets interesting. Ocean salinity has been roughly stable for a very long time, which means salt is being removed as fast as it arrives. The removal routes are real and measurable: salts precipitate out where shallow seas dry up, leaving beds of rock salt and gypsum; ions are adsorbed onto clay particles that settle; marine organisms take calcium and silicon out to build shells; and — the route that surprised everyone — seawater circulates down through the hot new crust at mid-ocean ridges and comes back out chemically altered, having exchanged magnesium and sulphate for calcium and other elements. The entire volume of the ocean passes through that hydrothermal circuit roughly every few million years.
The composition is startlingly constant. Salinity varies from place to place — the Baltic is nearly fresh, the Red Sea is very salty — but the proportions of the major ions are the same everywhere in the open ocean, to a fraction of a percent. That constancy is what allows a single measurement of one property, such as electrical conductivity, to give total salinity, and it is a consequence of the ocean being mixed thoroughly on a timescale of about a thousand years, which is much faster than the salt is added or removed.
Average salinity is about 35 grams of dissolved salt per kilogram of seawater, mostly sodium chloride, with magnesium, sulphate, calcium and potassium making up most of the rest.
The ocean as the planet's heat and carbon store
Two numbers explain most of the ocean's role in climate, and both come from ordinary physics.
Water has an exceptionally high heat capacity. It takes about four times as much energy to raise the temperature of a kilogram of water by one degree as to do the same to a kilogram of air. Combine that with the fact that the ocean is enormously more massive than the atmosphere, and you get the result that matters: the top three metres of the ocean hold about as much heat as the entire atmosphere. The whole ocean holds roughly a thousand times more.
Consequences follow immediately. The ocean is why coastal climates are mild and continental interiors have brutal seasonal extremes. It is why the Earth's climate has enormous thermal inertia and responds slowly. And it is why over 90 percent of the extra heat trapped by increased greenhouse gases since the mid-twentieth century has gone into the ocean rather than into the air — a fact Part 14 needs, and one that is measured directly by a fleet of nearly 4,000 free-drifting Argo floats that cycle between the surface and 2,000 metres, reporting temperature and salinity by satellite.
Carbon dioxide dissolves in water, and once dissolved it mostly converts to bicarbonate and carbonate ions, which lets far more of it in than simple dissolution would allow. The ocean holds roughly fifty times as much carbon as the atmosphere. It has absorbed something like a quarter to a third of all carbon dioxide humans have emitted.
That absorption has a price, and it is straightforward chemistry. Dissolved carbon dioxide forms carbonic acid, which releases hydrogen ions, which lowers pH. Ocean surface pH has fallen from about 8.2 to about 8.1 since pre-industrial times. A drop of 0.1 on a logarithmic scale is about a 30 percent increase in hydrogen ion concentration, and the effect that matters biologically is that those extra hydrogen ions mop up carbonate ions, which is the raw material corals, shellfish, and many plankton need to build their skeletons. This is the same mechanism named in the end-Permian and end-Cretaceous sections of Chapter 1.10, running now at a rate that is slower than those events but faster than anything else in the recent record.
Life in the water column, and why the sea is not uniform
Light runs out fast. The photic zone, where there is enough light for photosynthesis, is at most 200 metres deep and often far less in murky coastal water. Below that is a twilight zone, and below about 1,000 metres, total darkness apart from the light animals make themselves. Bioluminescence is not exotic in the deep sea; it is the norm — a large majority of deep-sea animals produce light, for hunting, mating, camouflage and startling predators.
Almost all ocean food starts with phytoplankton — microscopic photosynthesising organisms drifting in the sunlit surface. They produce something on the order of half of all the oxygen generated by photosynthesis on Earth. Roughly every second breath you take was produced in the sea.
But most of the ocean is a desert, and the reason is nutrients, not light. Phytoplankton need nitrogen, phosphorus and iron. Those sink with dead material into the deep, and the surface is left stripped. So the productive regions are precisely the places where deep water is brought back up:
- Coastal upwelling zones, where wind blowing along a coast pushes surface water offshore and deep water rises to replace it. Peru, Namibia, California and Somalia have these, and they support enormous fisheries in tiny areas.
- Polar seas, where cold surface water sinks and vigorous mixing keeps nutrients circulating.
- Continental shelves, fed directly by rivers.
Chapter 2.6 takes up the currents that drive this, and why the collapse of upwelling off Peru during an El Niño year is felt in food prices around the world.
The vents, and possibly the origin of life
Back to the chimneys. There are two kinds and the difference matters.
Black smokers sit directly on ridge axes. Seawater sinks into cracks in the young crust, is heated to 300–400 °C, dissolves metals out of the rock, and jets back out. On meeting near-freezing seawater the dissolved metal sulphides precipitate instantly as a black cloud, and build chimneys of sulphide minerals. These are ore bodies forming in real time, and many of the copper, zinc and gold deposits mined on land today are ancient black smoker systems that were later scraped onto a continent.
Alkaline vents — the field called Lost City, found in 2000 on the Atlantic sea floor away from the ridge axis — are different and, for the question of life's origin, more interesting. They are cooler, around 40 to 90 °C, and they are driven not by magma but by a chemical reaction: seawater reacting with mantle rock, a process called serpentinisation, which produces hydrogen and alkaline fluids and can run for tens of thousands of years without any volcanic heat.
Why that combination is attractive as a cradle for life: the vent produces hydrogen-rich alkaline fluid, the surrounding ocean of the early Earth was acidic and rich in carbon dioxide, and the chimney walls are microscopically porous mineral. So you have two chemically different fluids separated by a thin mineral barrier — which is a natural version of the arrangement every living cell uses to make energy, where a difference in hydrogen ion concentration across a membrane drives the synthesis of ATP. Volume V develops this fully; the geological point here is that the setting is real, it is common, and it existed on the early Earth.
What we still do not know
Around a quarter of the sea floor has been mapped at high resolution. The rest is known only from satellite altimetry, which infers the shape of the bottom from tiny bumps in the sea surface caused by the gravity of underwater mountains — a clever trick with a resolution of kilometres, not metres. We have better maps of the surface of Mars than of most of our own sea floor. The Seabed 2030 project aims to close that gap.
Deep-sea mining is now the live argument. Abyssal plains are scattered with nodules of manganese, nickel, cobalt and rare earths, grown over millions of years, and they sit in exactly the metals that battery manufacturing wants. The seabed beyond national waters is legally "the common heritage of mankind" under the Law of the Sea, administered by the International Seabed Authority, and the ecological case against is that abyssal communities grow extraordinarily slowly and there is no evidence they recover on any human timescale. Test tracks scraped in the Pacific in the 1970s are still visible and still nearly lifeless.
Where this shows up in your life
Fishing. Around 3 billion people get a significant share of their protein from the sea, and the great majority of the catch comes from continental shelves and upwelling zones — a small fraction of the ocean's area. This is why fishing disputes are always about shelves: the North Sea, the Grand Banks, the South China Sea, the Bay of Bengal. Chapter 12.10 shows how a shelf full of fish and gas turns into a geopolitical crisis.
Your internet. Over 95 percent of international data traffic runs through submarine fibre-optic cables lying on the sea floor, not through satellites. Their routes are chosen around the topography in this chapter, avoiding trenches, turbidity current paths and fishing grounds, and the commonest cause of a cable break is a ship's anchor or a trawl net on a continental shelf.
Your weather. The ocean supplies the water vapour for essentially all rainfall, and the heat that powers cyclones. Part 2 is the direct continuation.
And your salt. Sea salt is evaporated ocean, and the salt tax that Gandhi's 1930 march was aimed at was a tax on people making it themselves from the sea a few steps from their homes. Chapter 6.21 tells that story, and it starts here.
What the next page covers
Geology decided where the metals, the fuel and the fertile soil ended up, and those endowments were distributed with no regard whatever for where people would later draw borders. Chapter 1.12 closes this Part by connecting the rocks to the money — why the Persian Gulf holds so much oil, why southern Africa holds so much of the world's platinum and diamonds, why India has coal and iron but almost no oil, what the "resource curse" is and whether it is real, and how a country's geological luck becomes its politics.