An Ode to Foraminifera

Foraminifera ( H. Spero, 2015)

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.

Foraminifera (E. Haeckel, 1899)

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).

Small Ice Sheets with more16O in the Ocean (MinuteEarth, 2016)
Large Ice Sheets with more18O in the Ocean (MinuteEarth, 2016)

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.

Marine Oxygen Isotope Curve (Lisiecki & Raymo, 2005)

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.

MinuteEarth. (2016) How these Seashells know the Weather in Greenland. Retrieved from:

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:

Wetmore, K. (N.A.). Foram Facts – An Introduction to Foraminifera. UC Museum of Paleontology. Retrieved from:

9 thoughts on “An Ode to Foraminifera

  1. The y-axis shows the ratio of 18O in per mil (‰, parts per thousand). The more enriched the ocean is in 18O, the higher the number. As the ratio of 18O increases, temperatures decrease as more 16O is being trapped in the ice sheets. An increase of 0.22‰ in the ratio is equivalent to approximately 1°C cooling.

    To visuallize colling as a drop in the graph, the y-axis is reversed which might be confusing at first sight.


  2. Yes, the x-axis represents time in Million years (Ma).
    So on the left is the present and on the right is 5 million years ago.


  3. I was wondering how you can be certain about the age of a specific Foraminifera? Because I understand how you are able to determine the amount of ice in the past, due to the different ratios of O16 and O18, but it is still unclear to me how a ratio can be matched with a specific year. Can the age of a Foraminifera also be determined from its shell composition?


  4. Yes, the age of the Foraminifera can partially be determined by the composition of the shell.
    This is done through radiocarbon-dating where you measure the amount of the carbon isotope 14C in the shell.
    Basically, living beings take up 14C during their lifetime mainting a constant amount in their body.
    As soon as they die the 14C starts decreasing as it radioactively decays with a half life of 5730 years.
    Therefore, the less 14C is measured, the older the material is.
    However, this method is limited to dating events only up to ~50.000 years.
    For older climate events other more complicated methods are used.


  5. Dear Christoph, I enjoyed your blog post! It’s very well articulated and explained in simple terms – understandable for someone with no prior knowledge of Foraminifera.

    You mention that if we were to find equatorial Foraminifera below the ocean floor off the coast of the Netherlands it would mean that in the past the Dutch sea must have been as warm as the water by the equator today. I was wondering whether this evaluation would be based on our knowledge of Foraminifera existing now? If so, couldn’t there be uncertainty with this since there must’ve been some level of evolution? More specifically, the Foraminifera which exist today are most likely not the exact same ones that existed in the past. Or is this notion false?

    Another quick question/though I had was do you think that ocean warming through climate change could have an effect on Foraminifera? I assume that Foraminifera adapted to a cooler ocean environment might negatively be affected by warming of its ocean environment. This could then potentially lead to migration of Foraminifera to other ocean waters – which from your post I understand does not currently occur.


  6. Hey Chris, I really liked reading your blogpost! Your writing style made it really entertaining and informative as you managed to explain the significance of foraminifera in ways people without prior knowledge could understand! Foraminifera are a very interesting topic to understand how we make ‘observations’ about past climate that inform our climate models and future predictions!

    I’d like to add to Nina’s question: I think that with ocean warming, the assemblages (collection) of foraminifera species in specific latitudes would change as their preferred temperatures are too low. This actually has happened a lot in the past. For example, for my capstone I look at different species of foraminifera which occur in different abundances over time. In my case, this indicates the occurrence of a Heinrich event, an event of abrupt climate change where a lot of freshwater was added to the ocean. This is reflected in the species of foraminifera I find in my sediment samples because there are species that prefer fresh and cold water over highly saline water.
    I hope this answers your second question Nina!

    As to the first one, I also wondered about that, but I don’t know! Maybe you know, Chris?


  7. Thanks for both your comments Nina and Sophie!
    As Sophie already explained, the assemblage of different foraminifera species in local settings will change due to ocean warming. Since there are a wide variety of Foraminifera species the overall population is not negatively affected. But indeed many species will probably migrate northwards as a result of increasing ocean temperatures.

    Concerning the evolutionary changes of foraminifera this is a very imporatant and intersting question to ask.
    It is mostly assumed that the species have remained relatively similar over time. However, this is not an assumption based on empty grounds but again backed up by scientific studies. Due to the constant deposition of Foraminifera on the ocean floor we have a very good catalogue of how the species changed or didn’t change throughout time. Furthermore, marine biologists study the evolution of foraminifera. Therefore, the interdisciplinary communication between paleoclimatologists and marine biologists is very important.
    I hope this answered your question 🙂

    Liked by 1 person

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