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.
Biostratigraphy - Calcareous nannofossils are the most useful age indicator for marine sediments from the Jurassic to the Recent because of their rapid rate of evolution and wide geographic distribution. The first appearance datums (FAD's) and last appearance datums (LAD's) for calcareous nannofossil species usually occur at the same horizon globally and often can date sediments to accuracies of one million years or less. Accurate dating of sedimentary deposits has many practical applications. For example, calcareous nannofossils have been used successfully to help map both the surface and subsurface geographic extent of lithologic units, particularly in regions with complex facies patterns like New Jersey and Alabama. They also are used in regional geologic studies to correlate time- equivalent but lithologically distinct deposits from state to state and region to region.
Paleoceanography, Paleobiogeography, and Paleoecology - Calcareous nannofossils can be used to help determine the temperature and current patterns of ancient oceans. For example, if a species lived only in warm tropical waters, then its presence can be used to trace the geographic extent of tropical waters through time. Thus, species B, which has been found in Campanian sediments only off the coast of present-day Florida, ranged from Florida to New Jersey during the Maastrichtian. From this we can conclude that in the region north of Florida, ocean temperatures were warmer during the Maastrichtian than during the Campanian. Similar paleobiogeographic fossil data are useful in determining ancient climates, species migration patterns, and continental relationships.
Forensic, Art, and History Studies - Calcareous nannofossils have been used to help police solve criminal cases. For example, clay scraped from the shoes of a murder suspect in England contained calcareous nannofossil species that were unique enough to lead the police to the scene of the crime. Calcareous nannofossils have been used to determine the origin of building stones for Medieval churches in Denmark and to check authenticity of paintings. In Norway, which has no native chalk, calcareous nannofossils were used to determine the origin of white chalk that was used to prepare the surfaces of Medieval wooden sculptures and panels before painting. The pattern and changes through time of the chalk trading routes probably can be used to interpret general trading patterns in northern Europe at the same time.