Big things in small shells

Contrary to popular belief, euphemisms, idioms and trite sayings often have little relevance to real life. Foraminifera on the other hand, lend credibility to the notion that big things do indeed come in small packages.

Foraminifera are small single-celled marine organisms; more specifically known as Protists. They exist at various depths within the oceans but are most commonly found in shallow waters above 50 m. Many feed on small marine plants and other detritus, but some are carnivorous and feed on other smaller species. They have lived in the oceans for millions of years and are found throughout every ocean on Earth. This fact makes them remarkably useful in studying ancient oceans.

Many species construct their shells out of calcium carbonate (CaCO₃), while others construct it using fragments of other shells, or even grains of sand. The CaCO₃ which makes up the shell contains a record of a large number of environmental variables which can help us understand the past ocean. Carbon and Oxygen both have 2 stable isotopes (¹²C, ¹³C, ¹⁶O and ¹⁸O respectively). The record of past changes in these isotopes locked in these Foraminifera shells can be used to infer changes in water mass distribution, changes in ice volume on land and nutrient distribution in the oceans. 

Aside from basic stable isotopes, Foraminifera also contain record radioactive isotopes. One of these isotopes called radiocarbon (¹⁴C), is particularly important to paleoceanographic studies. When Nitrogen 14 (¹⁴N) in the upper atmosphere is bombarded by incoming solar radiation, it gains a proton and becomes ¹⁴C. This radiocarbon diffuses into the oceans and is taken up in small amounts into the shells of the Forams. By measuring how much of this radiocarbon is left in an ancient shell we can know its age and the age of the sediment which surrounds it. This helps us tie down the ages of important ocean events like landslides and ice-sheet break-up.

Several additional proxies exist including Uranium decay series elements. These are used to detect the source of water masses by using their chemical fingerprint to trace the source of their terrestrial suspension material. Even the numbers of Forams can help us. Certain species thrive in cold water while others prefer the warm waters of the sub-tropics. Using the % of these different species in a sediment sample, we can compare this past assemblage with modern ones and infer sea surface temperature changes that help us understand de-glacial events

To sum up, these guys may only be the remains of tiny dead critters, but they provide us with a immensely powerful tool in our quest to understand the oceans and their role in the changing climate.

Josh Allin