The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 76

present (and past) inputs of water from different rivers, or the chang-
ing patterns of ocean currents. To use this feature in practice, one
needs an element that can provide a distinct 'fingerprint', because
simply measuring the abundance of an element in seawater or in sedi-
ment does not provide precise enough information. Such an element
is neodymium, which has different isotopes, the proportions of which
change from place to place. The careful measurement of neodymium
isotopes in ancient seawater sediments has told stories of how the
outflow of water from the Nile into the Mediterranean has changed
over the climate vicissitudes of the Ice Ages, 49 or how the Gulf Stream
has changed its behaviour over the past few million years. 50
So seawater is a complex cocktail of chemical elements, some of
which can enter and exit the system almost on the scale of a human
lifetime, while others travel in cycles about as long as those of the
drifting continents. Given the enormous capacity of the ocean to
absorb ions of sodium, potassium, and chloride, it could potentially
become a brine denser than the Dead Sea, given that rivers have been
washing these salts into the sea for more than 4 billion years.
So why are the oceans not entirely saturated with salt? One answer
is to be found among a string of accidents. Wieliczka was one, a very
long time ago. The most dramatic example, though, is centred in
Messina, Sicily, by the beautiful blue Mediterranean Sea—or perhaps
that should be by the presently beautiful blue Mediterranean Sea.
Death of the Mediterranean
Messina has history. It has been invaded successively by the Cartha-
ginians, the Syracuse army, the Mamertines, the Romans, the Goths,
armies of the Byzantine and Arabian empires, the Normans, the forces
of Richard the Lionheart, the Spanish army (among them Miguel
de Cervantes, author of Don Quixote , who recovered in the local hos-
pital from wounds sustained in battle), and by Garibaldi's army. It