Foraminifera: The tiny and mighty

This post is reproduced from a 2014 cruise blog: http://slidesinthedeep.blogspot.co.uk/2014/07/big-things-in-small-shells.html

 

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 plankton; more specifically known as Protists. They live at various depths within the oceans but are most commonly found in shallow water depths (<50 m). Many species of Foraminifera feed on small marine plants and other detritus, while 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. Their ubiquity and persistence through time makes them remarkably useful in studying ancient oceans.

Many species construct their shells out of calcium carbonate (CaCO₃). Other species construct it using fragments of other shells, or even grains of sand. The CaCO₃ which makes up the shell of many species can contain a record of a large number of environmental variables that can help us understand the past ocean. Carbon and Oxygen both have 2 dominant 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 terrestrial ice volume, and nutrient distribution in the oceans.

Aside from basic stable isotopes, Foraminifera also contain useful 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 (CaCO₃) shells of the Foraminifera. 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 glacial changes on land.

 

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 suspended and dissolved material. Even the numbers of Foraminifera can help us. Certain species thrive in cold water, while others prefer the warm waters of the sub-tropics and equatorial regions. Using the percentage 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 the effect of glacial cycles and climate change on the worlds oceans. Calcium/Magnesium ratios in formainiferal shells can also give us an indicator of temperature, while Barium/Calcium can be used to indicate palaeosalinity. The list of these ‘proxies’ goes on, and existing ones are often refined or replaced with new ones.

To sum up, these grains of sand 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