Calcareous Nannofossils

Calcareous nannofossils are fossil remains of golden-brown, single-celled algae that live only in the oceans. Because they are plants they need sunlight, so they float near the surface of the water. There are billions and billions of them living in the oceans today, and they are eaten by anything that is bigger than they are. They are one of the primary organisms at the base of the food chain.

These algae make tiny calcite platelets inside their cells, and these platelets (the calcareous nannofossils or nannos for short) move to the surface of the cell. No one is certain why these platelets are formed, but after a while they fall off the cell and slowly drift down to the bottom of the ocean. These platelets are replaced by new ones that constantly are forming within each cell. As these platelets land on the bottom of the ocean, they are slowly covered up with remains of other plants and animals and bits of mud and sand that have washed out with the rivers of the world. At this point they are part of a mud or marl or sandy clay. Eventually, there are many sediments on the ocean bottom, and their weight is enough so that the lowest sediments are squeezed enough to become rocks. If these rocks are almost entirely made of nannos, they are a chalk. If there aren't so many nannos, they can be part of a limestone or shale. These calcite platelets are preserved in the rocks and are the fossils that paleontologists study.

Calcareous nannofossils have been living in the world's oceans for at least 200 million years (from the Triassic Period), and they have evolved and changed constantly over time. For example, if a paleontologist looks at an ocean bottom sediment from 1 million years ago and compares it to a sediment that was deposited 60 million years ago, most of the nannos in the two samples will be completely different species. A calcareous nannofossil specialist can look at ocean bottom sediment from anywhere in the world that was deposited less than 200 million years ago (one which does contain calcareous nannofossils) and be able to tell you how old that sediment is. And he/she would be accurate within 1-4 million years or even less.