Foraminifera (or forams for short) are small, short-lived marine organisms. They are just one of million diverse species that call the ocean their home. In the paleoclimate realm, they are crucial tools in reconstructing past climatic parameters. How? Through the calcium carbonate (CaCO3) shells (or tests) that they secrete during their 2-4 week lifespan. The tests fall to the ocean floor after the forams die and are preserved in ocean sediments as microfossils for years to come. Analysis on these tests reveal to us a plethora of information from times past. While they secrete their tests, information about oceanic conditions is chemically locked in the calcium carbonate. Hence, they are a crucial tool in paleoceanography. Here is a (very) short history of foraminifera and man’s knowledge about these creatures.
In 1821, Alcide d’Orbigny, the famous 19th Century French naturalist considered to be the father of modern micropaleontology, was sifting through some marine sediments under a microscope when he came across strange creatures with intricately constructed calcareous shells. Most of the different types he saw had perforated pores. Owing to this he named them foraminifers, which in Latin means ‘hole-bearers’.
However, by no means were these noble creatures unseen by the human eye before the 19th century. Ancient historians such as Herodotus, Strabo and Pliny the Elder had noticed numerous ‘lentil-like’ forms in the calcareous pyramids of Giza and thought them to be pulse food dropped by the workmen. In the late middle ages, Leonardo da Vinci and Georgius Agricola deduced the organic origin of foram shells and Antonie van Leeuwenhoek wrote about foram shells ‘no bigger than a grain of sand’ as he peered through his microscope. Forams were formally (and correctly) described as single-celled protozoa in 1835 by one Félix Dujardin.
Today, forams are described as single-celled amoeboid protists. The most definitive classification thus far was conducted by Loeblich and Tappan (1988). They distinguished 12 sub-orders after the nature of the test and its formation methods, 74 super-families after the internal nature and the spatial distribution of the chambers, and after apertures, 296 families, 302 sub-families and 2446 genera. A total of 275,000 species extinct and extant have been estimated (woah!)
Forams are abundant all over the ocean and are tolerant to a wide range of salinities and temperatures. They either live on the sea bottom (benthic) or drift in the upper water column (planktic). Of the estimated 4,000 species living today, 40 are planktic. A few benthic species have been recorded from terrestrial environments including ground water. The size of the foraminiferal test typically ranges from 0.05mm to 0.5mm although some forams may be as large as several centimeters with a recorded maximum of 18cm in diameter. Remember these are single-celled creatures! Benthic forams have been around since Cambrian times (roundabout 550 million years ago) and planktic forams appeared around 220 million years ago.
A question that popped into my head was how do we know whether a particular species (based on its shell) is planktic or benthic? This is essentially a paleontology problem. I asked foraminiferal expert, Dr. Richard Poore of the USGS and this is what he had to say:
Many of the early workers studied modern sediment and water column samples – it was almost self-evident that modern sediments from the deep ocean contained mostly planktonic foraminifers (the challenger expedition provided a wealth of material) – provenance (shallow versus deep water environments), shell character (planktic foraminifers typically have spherical chambers with spinose texture and simple apertures) – benthic foraminifers tend to have more complicated chambers with a smooth surface and complicated apertures). The morphology of many calcareous species is similar to agglutinated species (that’s a good clue). Mistakes were made and it is not uncommon to find that some planktic foraminifers were originally described as benthic e.g., some of the flattened smooth surfaced forms of the Paleocene and Eocene were originally considered benthic forms.
The ubiquity of forams in the ocean makes them a very useful tool in reconstructing climatic parameters in different parts of the world. After their lifespan, the foram test drifts to the ocean floor. Marine sediment cores usually contain abundant foraminifera of different species. One can use an assemblage of forams as a proxy through abundance analysis via counting – applying the concept that particular species thrive in selective environments. Supposing tests of a species that prefers warmer waters are found in a sediment core from a presently colder area, it can be inferred that in the past, the area was warm enough to allow that species to thrive. With ensuing scientific progress, quantitative reconstructions of climatic parameters (salinity, temperature, ice volume etc.) through geochemical analysis (isotopic, trace metal) and transfer functions of species abundances using sophisticated algorithms were developed. Another plus point as a proxy, forams are dated relatively easily through radiocarbon dating, as their tests are carbonate based.
Foraminifera are the proxy of choice for my investigations into the paleoclimate and paleoceanography of the Gulf of Mexico over the last few thousand years. They make for marvelously interesting microfossils. Though it is tedious to sit by the microscope for hours picking particular species from marine mud, the information stored in them is invaluable. The famed geologist/micropaleontologist, Cesare Emiliani, a student of Harold Urey, was the first person to analyze forams in a mass spectrometer and worked out that they were proxies for temperature and ice volume over long time scales. In 1955, he published his seminal work Pleistocene Temperatures [pdf] in the Journal of Geology.
With that, I will leave you with a stereotypically poetic quote from the Frenchman d’Orbigny:
“No animal series offers more facilities and benefits to the geologist and zoologist: first, to determine the temperature of places and second, by their wonderful diversity. Through the elegance of their forms and the uniqueness of their organization, they play, despite their smallness, a role vast in nature.”