Here’s a gorgeous image of the southwest coast of Britain and northwest coast of France off which, towards the bottom left of the picture, you can clearly see the algal bloom, caused by the back scatter of light from billions of tiny white calcite tests (miniature shells) attached to countless individual algae known as coccolithophores. All the algae - a form of phytoplankton - are photosynthesising, extracting Carbon Dioxide from the atmosphere and making their food from it. This is how chalk was made during the Cretaceous period, only in a shallower sea, with more available Carbon Dioxide and a lot more coccolithophores.


Photo NASA/Earth Laboratory