Geology Reference
In-Depth Information
plants. In the ocean, however, we find plenty of sulphur, brought there by the rivers that
garner it from the weathering of rocks exposed to the air. There might at first seem to
be a contradiction here: how is it possible that sulphur is scarce in the soil if its initial
source is the weathering of sulphur-rich rocks? The answer is that most of the sulphur is
whisked away by rivers to the sea before life on land can get its roots into it.
Gaia faces the critical problem of transporting the sulphur from the ocean where it
is abundant to the land where it is scarce; for without this vital transfer, terrestrial life
would be impossible. James Lovelock predicted that organisms in the ocean had to be in-
volved in this process by producing a sulphur-carrying gas that would blow in on ocean
winds to fertilise the land. But what could the gas be? An obvious candidate is hydrogen
sulphide, the gas beloved by children in their early dabblings with chemistry because of
its pungent odour of rotten eggs. Hydrogen sulphide, however, can't do the trick. Firstly,
not enough of it is produced, and secondly it is quickly and passionately dismantled by
hydroxyl ions, the aerial children of oxygen, which find it irresistibly attractive.
Next time you go down to the sea, take a deep breath. The delicious tangy, uplifting
aroma that greets your senses is the gas dimethyl sulphide, produced by marine algae.
The tanginess comes from sulphur in the gas, which gives it a slightly acidic bouquet.
During a long sea voyage, Lovelock found plenty of DMS in the air above the ocean,
and calculations by his colleagues later confirmed that the sea produces more than
enough DMS to close the sulphur cycle.
But DMS is important not just as the carrier gas for sulphur; it also plays a vital
role in seeding planet-cooling clouds. Seen from space, the Earth looks like a beautiful
blue marble streaked with swirling pearly white mountains of water vapour. These are
Gaia's clouds, the silent captains of the sky. At any given moment, much of her surface
is covered by them, and some, generally the low ones such as marine stratus, cool the
Earth by reflecting the sun's light to space from their dense white upper surfaces, whilst
others, the high fliers such as cirrus, warm the Earth by delaying the exodus of heat ra-
diated from the surface.
Everyone knows that clouds appear when water vapour from the ocean and the land
condenses in the air above us, but it takes more than just water vapour to make a
cloud. The water molecules swarming about in water vapour would like nothing better
than to clump up very close together to make a cloud, but they can't do this by them-
selves—they must first condense on small particles in the air known as cloud condensa-
tion nuclei (CCN). Particles of dust blown in from the land do this job well, as can salt
spray sucked up by winds from the ocean surface, but both are nowhere near common
enough to account for the abundant swirling whiteness that cloaks our planet.
For a long time no one knew what the mysterious cloud-seeding particles were, or
where they might be coming from, but Lovelock had a strong intuition that organisms
