The Science of Discworld Revised Edition

from which it could be radiated away into space and lost. When the hot rocks turned the water back into vapour, the rocks got a lot cooler very suddenly. In a geologically short space of time, the rocks had cooled below the boiling point of water, and now the falling rain no longer got turned back into vapour – at least, not much of it did.
    It may well have rained for a million years. So it’s not surprising that Rincewind noticed that it was a bit wet.
    Thanks to gravity, water goes downhill, so all that rain accumulated in the lowest depressions in the Earth’s irregular surface. Because the atmosphere had a lot of carbon dioxide in it, those early oceans contained a lot of dissolved carbon dioxide, making the water slightly acidic. There may have been hydrochloric and sulphuric acids too. The acid ate away at the surface rocks, causing minerals to dissolve in the oceans; the sea began to get salty.
    At first the amount of oxygen in the atmosphere increased slowly, because the effect of incoming sunlight isn’t particularly dramatic. But now life got in on the act, bubbling off oxygen as a by-product of photosynthesis. The oxygen combined with any remaining hydrogen in the atmosphere, whether on its own or combined inside methane, to produce
more
water. This also fell as rain, and increased the amount of ocean, leading to more bacteria, more oxygen – and so it continued until the available hydrogen pretty much ran out.
    Originally it used to be thought that the oceans just kept dissolving the rocks of the continents, accumulating more and more minerals, getting saltier and saltier until the amount of salt reached its current value of about 3.5%. The evidence for this is the percentage of salt in the blood of fishes and mammals, which is about 1%. In effect, it was believed that fish and mammal blood were ‘fossilized’ ocean. Today we are still often told that we have ancient seas in our blood. This is probably wrong, but the argument is far from settled. It is true that our blood is salty, and so is the sea, but there are plenty of ways for biology to adjust salt content. That 1% may just be whatever level of salt makes best sense for the creature whose blood it is. Salt – more properly, the ions of sodium and chlorine into which it decomposes – have many biological uses: our nervous systems, for instance, wouldn’t work without them. So while it is entirely believable that evolution took advantage of the
existence
of salt in the sea, it need not be stuck with the same proportion. On the other hand, there is good reason to think that cells first evolved as tiny free-floating organisms in the oceans, and those early cells weren’t sophisticated enough to fight against a difference in salt concentration between their insides and their outsides, so they may well have settled on the same concentration because that was all they could initially manage – and having done so, they were rather stuck with it.
    Can we decide by taking a more careful look at the oceans? Oceans have ways to lose salt as well as gaining it. Seas can dry out; the Dead Sea in Israel is a famous example. There are salt mines all over the place, relics of ancient dried-up seas. And just as living creatures – bacteria – took out carbon dioxide, turning it into oxygen and sugar, so they can take out other dissolved minerals too. Calcium, carbon and oxygen go into shells, for instance, which fall to the ocean floor when their owner dies. The clincher is … time. The oceans are thought to have reached their current composition, and in particular their current degree of saltiness, about 2 to 1.5 billion years ago. The evidence is the chemical composition of sedimentary rocks – rocks formed from deposits of shells and other hard parts of organisms – which seems not to have changed much in the interim. (Though in 1998 Paul Knauth presented evidence that the early ocean may have been
more
salty than it is now, with somewhere between 1.5 to 2 times as much salt. His calculations indicate that salt could not have been deposited on the continents until about 2.5 billion years ago.) Simple calculations based on how much material dissolves in rivers and how fast rivers flow show that the entire salt content of the oceans can be supplied from dissolved continental rocks in twelve million years – the twinkling of a geological eye. If salt had just built up steadily, the oceans would now be far more salt than water. So the oceans are