The first record of the white, southern face of the British Isles at Dover, and along the coast, is by Julius Caesar when he found this nearest entry from France to be too well guarded. The cliffs have long served England as a first defence against invasion and they now enclose a network of tunnels and chambers that housed a hospital during World War I and troops and a command station for the Battle of Britain during World War II.  While a large part of the British Isles consists of the same chalk deposit, the chalk strata continues across the channel and can be seen from atop the cliff at Dover, at Cap Gris Nez on the French coast.

The chalk deposit is known to be the skeletal remains of algae, or phytoplankton, photosynthetic organisms found in the sea. Some  hundred million years ago, this part of the earth was submerged by water and the hard, outer remains called cocoliths, of a variety of seaweed, descended and gradually formed a 500-metre mound on the seabed, a mound that rose above sea level, as the British Isles, due to movements of the earth’s crust.

A group of scientists from Bigelow Laboratory for Ocean Sciences, Maine, the Bermuda Institute of Ocean Sciences, the University of Southampton, the Woods Hole Oceanographic Institution, California, and the Massachusetts Institute of Technology has studied a vast growth of cocolith-creating organisms, collectively called the Great Calcite Belt, in the Southern Ocean, the sea south of 38° South and till 60° South latitude.  They describe in the American Geophysical Union journal, Global Geophysical Cycles, why the Belt arises and the conditions that allow the algae to flourish, which suggests the conditions in which calcite of the White Cliffs mound may have come to be.

The cocolithophores, as the single-celled algae are called, create crystals of calcium carbonate that enclose the cells as an exoskeleton. The scale formation is by the reaction of calcium atoms with hydrogenated carbonate, an alkaline combination of carbon dioxide gas and water, a product of photosynthesis. Of the two carbon atoms consumed in the reaction, one gets fixed as calcite, and the other is released as carbon dioxide. While some of the CO2 is again used for photosynthesis, reducing the alkalinity and releasing CO2 has the effect of sending CO2 back to the atmosphere. The net effect, however, is to fix carbon as calcite. The calcite shells are partly shed during cell growth and fully when the cell dies, and they sink to the seabed, effectively sequestering carbon.

Read more at http://www.thestatesman.com/news/supplements/seaweed-and-the-great-chalk-mound/167697.html#fZD116XcTVxS8vem.99