Foraminifera

“The case of the three species of protozoan which apparently select differently sized grains of sand, etc., is almost the most wonderful fact I ever heard of. One cannot believe that they have mental power enough to do so, and how any structure or kind of viscidity can lead to this result passes all understanding.” -Charles Darwin, in a letter written to W. B. Carpenter in 1872, speaking of agglutinated foraminifera that use other parts of organisms and sediment to build their shells

Foraminifera are the group of fossils that Adriane works on, and are also an important paleoclimate proxy.

Assorted planktic foraminifera species. Images are 60-100x.

What are foraminifera?

An agglutinated foraminifera from the Smithsonian Museum of Natural History collections. This particular specimen is the holotype for the species. Image courtesy of the Smithsonian Learning Lab.

Foraminifera (or forams for short) are single-celled marine plankton that live in the open ocean. They are unique in that they secrete a calcite shell (or test), which can have spines or holes, and comes in a variety of different shapes and sizes. Most forams are about the size of a grain of sand. Some larger species (greater than 1 mm) can be found living in tropical marine environments. Forams reproduce sexually, so that each individual has its own unique DNA! The foraminifera are partitioned into two main groups: planktic forams, those that live in the upper water column, and benthic forams, those that live on the seafloor or within ocean sediments.

Forams have inhabited the Earth’s oceans since at least the Cambrian (>500 million years ago), and the earliest forms might have been ‘naked’ (they lacked a shell). There are more than 4,000 species of extinct (no longer living or fossil) foraminifera, and only 40 extant (still living) species. Forams have an excellent fossil record, one that is more complete than any other fossil taxa known. This is because they occur everywhere in the world’s oceans, are very common, and their shells are easily preserved on the seafloor. Most foram species, especially the surface-dwelling planktic foraminifera, evolved very quickly through time.

A generalized temperature profile of the surface ocean, thermocline, and deep ocean. Planktic foraminifera live within the upper water column along the thermocline.

Since their evolution in the Cambrian, benthic and planktic foram species have evolved to live in a wide range of habitats. So much so that today, forams inhabit oceans from polar regions to the equator. Much like modern animals, only certain foram species are typically found in very cold habitats, while other species live in very warm, tropical waters. Some species inhabit a wide range of environments, but need more nutrients than other species to live successfully.

Planktic forams have also evolved to live in different layers of the ocean. Some species of planktic forams, such as Globogerinoides ruber, live in the uppermost surface waters (upper 50 meters) of the ocean. Other species live deeper in the water column near the thermocline, an oceanic boundary that is marked by a steep change in water temperature.

What are foraminifera important?

An incomplete specimen of a larger benthic foraminifera, Nummulites irregularis.

Because forams evolved so quickly in the geologic past, the first occurrences (when a species first evolved or appeared in the fossil record) and last occurrences (the last time a species is present in the fossil record) of species can be important markers to subdivide our geologic timescale. Use of fossil taxa to date ocean sediments, rocks, and correlate formations among areas is called biostratigraphy (refer back to the Geologic Time page for more information on biostratigraphy).

Although forams are super cool plankton because they are so unique and differ vastly among species, they are also important in reconstructing ancient ocean conditions. Here is a brief list of the ways in which paleoclimatologists use foraminifera as proxies:

A benthic foraminifera, Citharinella compara. Image courtesy of Smithsonian Learning Lab.

1. Forams are useful in biostratigraphic studies to tell how old ocean sediments or rock formations are. They are also used to correlate rock units and drilled ocean cores from one place to another.

2. The number or amount of certain types of species compared to the number or amount of other species in a sample (a sample of ocean sediment or from a rock formation represents a limited window of time) can tell us something about the structure of the water column. Certain species like to live in the thermocline, whereas others prefer to live in the warm mixed layer (upper 50 meters) of the ocean. If there are more thermocline-dwelling species in a sample compared to mixed layer species, this indicates that the thermocline moved up in the water column, there was increased nutrients in the water column, or deep ocean waters were being upwelled (brought from deeper parts of the ocean to the surface). On the other hand, if there are more mixed layer species in a sample than thermocline species, this may indicate the thermocline was located deeper in the water column and perhaps less nutrients in the water.

3. Because planktic foraminifera species have a zonal distribution (certain species occur in certain water temperatures which correlate with bands of latitude), their fossil shells can be used to reconstruct ancient climate belts.

4. The assemblages (what species are present, and in what percent, in a sample) of benthic foraminifera can be used to tell us about the bottom water or deep ocean conditions. These include: water depth, amount of nutrients, and amount of oxygen.

5. When forams build their shell, the shell captures the chemical signature of the water around them through isotopes of carbon (C) and oxygen (O).

Thus, because foraminifera are abundant in ocean sediments, have an excellent fossil record, and evolve quickly, they have been used extensively over at least the last century to study our oceans and evolution.