The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 82

strong as modern seawater. Perhaps this was from the underground
dissolution of buried salt deposits? None, though, are known nearby.
Or by heating and evaporation related to the giant impact? That,
physically, could not work, and the brines extended well beyond the
crater. The only reasonable interpretation was that the buried brines
represent Early Cretaceous seawater of the North Atlantic—then
much narrower than today and sufficiently hemmed in by the
Americas, Europe, Asia, and Africa to develop its own, concentrated
version of seawater. 56
There must have been other kinds of changes too. It is not only the
quantity of salt in the oceans that matters. The quality of that salt is
important too. In fact, it controls life and death.
Limestone Patterns
Well-mixed as the seas are today, ocean chemistry is not quite uniform.
It varies across the wide surface of the oceans—from the coastlines,
where the water is full of trace elements of all kinds that help life grow
in abundance, to the ecological deserts in the centres of the oceans.
Ocean chemistry also varies from top to bottom. The surface
waters of the ocean are relatively alkaline because carbon dioxide
(that becomes carbonic acid when dissolved in water) is used up as
myriad tiny planktonic algae photosynthesize and grow. In those
sunlit waters it is easy for some of those planktonic organisms to
extract the calcium and carbonate ions 57 with which the water is satu-
rated, and make their intricate, jewel-like calcium carbonate skele-
tons. As these organisms die, their skeletons fall to the sea floor in
their billions to pile up as carbonate oozes that are limestone strata of
the future. As they fall, they take with them the iron, nitrogen, and
phosphorus in their dead tissues. The sunlit surface waters are thus
depleted in these elements, which puts a serious brake on further
biological growth.