The world’s chalk deposits are made of fossilized remains of the shells of tiny microscopic algae.
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The Coccolithophores, such as Emiliana huxleyi shown here, are microscopic algae that are members of the phytoplankton at the base of marine food chains. The surfaces of these organisms are covered in calcium carbonate plates called coccoliths that are secreted by the algae. When they die, their shells sink to the seafloor and form sediments— a process that has been going on for millions of years, and that is responsible for the vast deposits of chalk around the world.
In 1868 the scientist and educator, Thomas Henry Huxley, gave a public lecture to the working people of Norwich, England titled “On A Piece Of Chalk” during which he described that the chalk he was using to write on the blackboard was made up of fossilized skeletons of tiny organisms. He concluded that a shallow sea once covered Norwich, and this was evidence of change. While Huxley could not view the coccololithophores in as much detail as that pictured here, they continue to provide evidence of change—some species of coccolithophores have recently been shown to be sensitive to increasing seawater acidity.
The sample of Emiliania huxleyi was collected in the English Channel, and filtered from the seawater onto a polycarbonate microfilter. It was then imaged with a scanning electron microscope at the Natural History Museum in London, and the image was subsequently colorized using a computer. Each coccolithophore is approximately 4 micrometers in diameter—around half the size of a red blood cell.
Jeremy Young PhD., Dept. Earth Sciences, University College, London, UK