The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 83

However, to build future limestone strata those tiny skeletons
must not sink too deep, for if they do they will enter a new realm
and simply disappear. In the cold, dark depths of the oceans, about
4 kilometres below the sea's surface, the water is both cold and
old. Hundreds of years have elapsed since the water has been at the
surface—it is charged with carbon dioxide from decaying animal
and plant tissues, and part of this gas dissolves to form carbonic
acid. As they enter these deep, more acid waters the calcium car-
bonate shells simply dissolve and disappear, even before they hit
the ocean floor.
There is therefore a kind of snow line known as the carbonate com-
pensation depth (or CCD) in the oceans, above which pale oozes
made of microscopic calcium carbonate skeletons accumulate. Below
this only fine, insoluble sediment can very slowly build up, which is
made of material such as windblown desert dust, tiny particles of vol-
canic ash, and the tiny silica skeletons of such organisms as radiolaria
or diatoms. One of the fundamental boundaries in the oceans, the
CCD varies from place to place. It is generally shallower in the Pacific
than the Atlantic, because Pacific deep waters are more sluggish and
therefore older and more acid. And above productive areas of surface
water the carbonate snow line can be pushed downwards, because
the rain of skeletons from above can overpower the ability of the deep
water to dissolve them.
The CCD has also changed through geological time. There have
been, for instance, times in the past when the atmosphere was sud-
denly invaded by large amounts of carbon dioxide ('suddenly' here
means over tens of thousands of years, by comparison with what is
happening today: see Chapter 7). One such event took place 55 million
years ago, perhaps as a result of an intense burst of magmatism asso-
ciated with the opening of a part of the Atlantic Ocean. As well
as warming the world it acidified the ocean, so that the sinking