Geoscience Reference
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carbonate skeletons dissolved sooner and the CCD rose by about
a kilometre.
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There were other, more subtle changes in the oceans that affected
the way in which calcium carbonate precipitated out of the seawater.
One involves changes in the relative proportions of calcium and mag-
nesium in the ocean waters, where the balance between the two has
swung to and fro over at least the past half-billion years—and prob-
ably since the oceans first formed. This ratio has proved surprisingly
difficult to measure in ancient rocks, not least because the chemical
patterns are prone to change as the strata lie buried deep underground
over millions of years. However, traces of these ancient chemical pat-
terns may be preserved in particular types of calcium carbonate (such
as that in the crystals of sea urchin skeletons), and also in what seem
to be true samples of ancient (albeit concentrated) seawater, in fluid
bubbles trapped within fossil crystals of rock salt that once crystal-
lized on a sea floor.
These patterns show oscillations between times of magnesium
dominance (such as now) and calcium dominance (such as 100 mil-
lion years ago in the Cretaceous Period, when dinosaurs roamed the
Earth). This magnesium-calcium oscillation seems to reflect different
plate tectonic modes of the Earth. When sea floor spreading is vigor-
ous, the abundant, newly erupted underwater basalts will absorb
magnesium from the seawater leaving it rich in calcium. When sea
floor spreading slows the magnesium absorption slows, leaving the
seawater richer in this element.
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This slow chemical oscillation seems
to exert a strong influence on global climate,
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and so on the course of
life's evolution, both in the oceans and on land.
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Life, to reciprocate,
has also changed the pattern of ocean chemistry, as we shall see in
Chapter 6.
There has been another sea change linked with life's evolution on
Earth that was just as profound: the slow spread of oxygen through
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