Have you ever wondered how humanity has reached a level of knowledge where we can say that 3 million years ago the temperature on our planet was 1.5°C colder? How can we make such precise statements about times when no human ancestor has existed yet? While it might seem impossible, science has found a peculiar way to answer questions about our past climate. And the fundamental building block for this are tiny organisms living in the oceans. More specifically we are talking about Foraminifera.
These are single-celled animals that float around in the oceans, waiting to run into something to eat. But while their everyday life might seem quite mundane, these creatures have a lot more to them than you might first think. So, let me introduce you to the fascinating world of Foraminifera.
What are Foraminifera?
As already said, in simple terms they are single-celled organisms floating through the oceans. To protect themselves against other marine life and the environment, they built beautiful complex shells around them. As there are thousands of different foraminifera there are also many different variations of these shells, each following a different pattern. From inside these shells they have tiny arms (called reticulopodia) reaching out to catch food or ‘walk’ on the ocean floor.
They are all highly adapted to different levels in the water columns and each species stays in their specific environment for its whole life.
Furthermore, they are very picky when it comes to the temperatures they live in. Some types of Foraminifera are only found in the surface oceans of Antarctica while another might be ‘walking’ around on the ocean floor around the equator.
With this basic understanding you might already see how these small guys are of great value when it comes to wanting to know something about the climate on Earth. But if not let me elaborate for you.
What do they tell us about past climates?
On most simple terms we can see what the climate was in the past in different places around the world by looking at the remaining shells of dead Foraminifera in the ocean sediments. As an example, imagine finding a equatorial Foraminifera below the ocean floor off the coast of the Netherlands. What you can tell from that is that at some point in the past the Dutch sea must have been as warm as the water at the equator today. Thanks to the wide variety of Foraminifera species and their distinct shells this is an easily accessible source of information about our past climate.
But this is not where the science stops. In fact, the really mind boggler comes when you start looking at what the shells are made from. This approach was developed by the climate scientist C. Emiliani in 1955 and is a bit more complicated. But hold on tight and you will have an amazing ride exploring one of the most important tools of climate scientists.
Basically, the shells of these small creatures are fingerprints of the environments during which they were formed. A million-year-old Foraminifera tells us what the composition of the oceans were 1 million years ago. One component in the shells that is particularly interesting is Oxygen. Oxygen is the main component of the shells, as their chemical formula is CaCO3. Furthermore, Oxygen is present all over the ocean as it is part of water molecules (H2O). What many people however don’t know, is that there are three types of Oxygen. These are called isotopes and are different in weight. For the sake of the argument just think of the lightest oxygen isotope (16O) and the heaviest (18O). When water from the ocean evaporates into the air to form clouds, obviously the light Oxygen isotopes are more likely to evaporate. Eventually these clouds will precipitate over the poles, adding to the polar ice caps. Therefore, the polar ice caps are mostly made from light Oxygen (16O). Hence, if the polar ice caps are significantly larger than normal, most of the 16O will be captured in the ice and therefore the oceans will be enriched in the heavy oxygen isotope (18O).
And exactly this difference in heavy and light oxygen isotopes in the oceans is recorded in the shells of Foraminifera. So, if you find a 20.000 year old Foraminifera with a high amount of 18O in its shell you know that the ice sheets must have been much larger, meaning that there was an ice age. This data has been collected by many scientists over the past and has led us to a detailed understanding of our past climate. When you plot this data of oxygen isotope ratios on a graph you get a beautiful curve visualizing the climate that our planet has went through in the past million years.
So next time someone asks you something about our climate and how it has changed, give a quick shout-out to these amazing creatures for helping us to get where we are today.
Emiliani, C. (1955). Pleistocene Temperatures. The Journal of Geology, 63 (6), 538-578.
Haeckel, E. (1899). Kunstformen der Natur. Leipzig, DL: Verlag des Bibliographischen Instituts.
Lisiecki, L.E., & Raymo M.E. (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic d180 records. Paleoceanography, 20(1), 1-17. https://doi.org/10.1029/2004PA001071
MinuteEarth. (2016) How these Seashells know the Weather in Greenland. Retrieved from: https://www.youtube.com/watch?v=oaOfeSJZ3lY
Ravelo, A.C., & Marcel, C.H. (2007). The Use of Oxygen and Carbon Isotopes of Foraminifera in Paleoceanography. In Marcel, C.H., & De Vernal, A. (Ed.), Develoments in Marine Geology (pp.735-764). Amsterdam, NL: Elsevier
Spero, H. (2015) What are Foraminifera? Retrieved from: https://factsfromthefield.wordpress.com/2015/10/28/what-are-foraminifera/
Wetmore, K. (N.A.). Foram Facts – An Introduction to Foraminifera. UC Museum of Paleontology. Retrieved from: https://ucmp.berkeley.edu/fosrec/Wetmore.html