Geoscience Reference
In-Depth Information
deep-water manned and robot submersibles have visited many of the most interesting
places. But there is still a wealth of exploration ahead.
Where did the water come from?
It seems likely that the Earth's most primitive atmosphere was largely stripped away by the
strength of the newborn Sun's solar wind. It is almost certain that the heat generated by the
heavy bombardments that completed the formation of the Earth and the huge impact that
created the Moon must have melted the surface rocks and driven off most of the original
water. So where did our vast oceans come from? There are clues in the oldest rocks, from
4 billion years ago, that liquid water was around when they formed, and there is evidence
from not long after that of aquatic bacteria. The oldest fossilized imprints of rain drops are
in sediments about 3 billion years old in India. Some of the Earth's surface water may have
escaped from the planet's interior in volcanic gases, but most of it probably fell from space.
Even today about 30,000 tons of water falls to Earth each year in a fine rain of cometary
particles from deep space. In the early history of the solar system that flux must have been
significantly higher, and many of the late impacts are likely to have been from whole or
fragmentary comets, the composition of which has been likened to dirty snowballs, con-
taining abundant water ice.
Salty seas
Today, about 2.9% by weight of sea water is made up of dissolved salts, mostly common
salt, sodium chloride, but also sulphates and bicarbonates and chlorides of magnesium, po-
tassium, and calcium, plus trace elements. The salinity varies depending on the evaporation
rate and the influx of fresh water. So, for example, in the Baltic the salinity is low but in the
landlocked Dead Sea, it is about six times the average of 35 grams of solids per kilogram
of sea water. But the relative proportions of each of the main components of salinity remain
constant worldwide.
The oceans were not always that salty. Most of the salt is believed to have come from
rocks on land. Some of it was simply dissolved by rain and rivers and some was released
by chemical weathering, in which carbon dioxide dissolves in rain to make weak carbonic
acid. This slowly converts silicate minerals in rock into clay minerals. These tend to retain
potassium but release sodium, which is why sodium chloride is the biggest component of
sea salt. For the last few hundred million years, ocean salinity has been approximately con-
stant, with the input of salts from weathering balanced by their deposition in evaporite de-
posits and other sediments.
