Geology Reference
In-Depth Information
the presence of the others. But these cosy familial embraces amongst the chemical be-
ings are both a blessing and a curse as far as living things are concerned: a blessing
because calcium is essential for the inner life of the cell, and because it can be used
to make tough protective armoured plates; but a curse because too much calcite spells
death through encrustation, or through poisoning of the cell's inner workings. Just about
every single creature that lives in the sea deals with these dangers by expending a lot of
energy in pumping excess calcium out of its cells, and by producing complex slimes re-
plete with specially crafted sugar molecules that stop the encrustation in its tracks. Some
marine organisms simply exude these chemicals onto their skins and into the surround-
ing sea water, but others, like the coccolithophores, crabs and sea urchins, have mastered
the art of controlling the precipitation within intracellular spaces in which the special
sugary slimes are used to direct the manufacture of a vast array of intricately sculptted
calcium carbonate artefacts such as carapaces, wheels, spicules, rods and stirrups. Even
land animals secrete these anti-calcification slimes—we lay them down on our teeth to
keep them clear of encrusting calcium phosphate. So replete is the ocean with dissolved
calcium bicarbonate from the weathering of rocks that every single surface would be-
come rapidly encrusted with calcite if the sugary anti-calcifying slimes were somehow
removed.
The control of calcification by living beings has become ever more subtle and soph-
isticated as Gaia's ability for self-regulation has evolved and strengthened over geolo-
gical time. The most basic kind of calcification happens without the intervention of life
when, in the absence of any chemical inhibitors, calcium precipitates out of the wa-
ter to form simple marine crusts or the stalactites and stalagmites in limestone caves.
But in the heyday of Gaia's infancy, some 3,000 million years ago, huge bacterial com-
munitiesies (the stromatolites) laid down vast crusty platforms of limestone wherever
they needed to grow nearer to the life-giving light at the ocean surface. Then, from 600
million years ago until about 80 million years ago, the main calcifying beings were the
corals, molluscs and crustaceans. Since then the main site of life-enhanced calcifica-
tion has shifted from the margins of the continents to the outer edges of the continental
shelves, as an infinitude of tiny floating beings such as the coccolithophores have laid
their chalky dead to rest on the seabed far below them. During this whole evolutionary
trajectory from crusts to coccolithophores, Gaia has become increasingly skilled at cal-
cification, so that today far greater quantities of chalk are precipitated out of the ocean
by microscopic algae than ever before, a development that has allowed Gaia to keep her-
self cool in the face of a dangerously bright sun. Once again, Gaia reveals herself as an
evolving
animate entity.
So far we have only considered the role of ocean life in Gaia's great chalk journey,
but life on land also makes a vitally important contribution to cooling the earth by
