The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 72

even in that universal solvent, water. Take iron for instance, in today's
oceans (in yesterday's oceans things here were quite different, as we
explore in Chapter 6). Wherever there is oxygen—which is nearly
everywhere in ocean and river waters—the iron converts to its triva-
lent form, Fe 3+ . This is almost insoluble, and simply converts into fine
particles of hydrated iron oxide, or rust, which falls to the floor of
the river or (if the iron has made it that far) the sea. There are many
elements that are very poorly soluble in water—cobalt, nickel, tin,
mercury, and gold for instance—and so their contents are very low,
and their contents in seawater may be measured in those milligrams,
or fractions of a milligram, per tonne.
Interestingly, the amounts of most of these trace elements in sea-
water is even lower than their simple solubilities, as measured in a
laboratory, would indicate. Hence, they are not simply being removed
from the water by precipitation. There are other processes at work.
Some can be purely inorganic—these elements can be adsorbed, or
'stick to', the surfaces of clay or rust particles where the water is
muddy, for instance, and be carried with them to the sea floor.
In others biology is at work; planktonic organisms can absorb these
elements (many are micro-nutrients) out of the seawater. They can
then be passed from prey organism to predator organism up the food
chain, until something somewhere along the chain dies and falls to
the sea floor, again taking those elements with it. Or the elements may
leave the water column yet earlier, in sinking faecal pellets or as dis-
carded gelatinous feeding webs that travel as 'marine snow' (also
known as the 'faecal express') to the sea bottom (see Chapter 6).
Life is thus a great regulator of the chemistry of the oceans—and
not only as regards trace elements. Living tissue is built of carbon,
nitrogen, and phosphorus, and if it has bones or teeth or scales then
yet more phosphorus, together with calcium, is needed to build that
skeleton. All of this derives, directly or indirectly, from seawater.