Cómo los microfósiles ayudan en los estudios sobre el cambio climático

Did you know that even the largest habitat on Earth is filled with microscopic organisms? In the ocean, bacteria, protists, and other animals of no more than 1 mm have lived together and balanced the ocean’s very complex chemistry for the last 541 million years. Such balance ends up having a great influence on the ocean-atmosphere relationship that in part controls the global climate.

Several microfossil groups remain within marine rocks. Radiolarian, diatoms, and the widely known foraminifera are some of the most representative. If you have never heard of foraminifera (or “forams”), you are not the only one. Even though micropaleontology has several applications in both earth and environmental sciences, there are not many experts, and we haven’t done such a great job communicating the importance of our field to society.

Foraminifera are unicellular protists—eukaryotic organisms that are neither animal, fungi or plants [1]—that first appeared on the fossil record during the Cambrian as benthic forms (forms that live on the ocean floor; if they live near the ocean surface, we call them planktic), and they quickly diversified and colonized all marine environments. Foraminifera are the most abundant microfossil group in marine rocks, and they produce a hard shell of different compositions (most commonly calcium carbonate), which allows them to become fossils after being buried at the bottom of the sea. To this day 60,000 to 80,000 species of foraminifera have been identified, and of these, 99 % are benthic foraminifera [2].

Benthic foraminifera have a special quality—species distribution is conditioned by environmental parameters of the habitat they occupy. As a result, we find certain species at specific depths and/or environments, and this environmental response has been an advantage for scientists that use taxonomical, quantitative, and geochemical analysis of foraminifera, to unravel how global climate has changed throughout time. 

As mentioned before, thousands of species exist, of these, some are limited to coastal settings, others to deeper environments, and others are restricted to low-oxygen environments. For example, the widely known Cibicidoides wuellerstorfi is a species most commonly found in paleodepths—the water depths at which the rock that contains the microfossils was formed—of more than 200 m. Species that have certain environmental restrictions can be considered eco-marker species [3], that is the case of the above-mentioned C. wuellerstorfi.

The correct taxonomical identification of these eco-marker species on the fossil record allows us to determine how sea level has changed, identify low-oxygenation intervals, and interpret changes in food supply, salinity, and even water temperature. In addition, foraminifera produces their shells in equilibrium with ocean chemistry, capturing the chemical conditions of where they lived; like taking a picture of what the temperature was and how much food and oxygen were available in that specific time. Through stable isotope analysis of these shells, earth scientists and biologists can reveal this picture and identify how these parameters have changed in the past, and when they have been critical.

Benthic foraminifera in paleoenvironmental and paleoclimatic studies, are a very recent branch of applied micropaleontology. They have proven to be an invaluable tool in assessing global climatic crises, reliably telling the story of how marine paleoenvironmental conditions abruptly changed after the Chicxulub impact 66 million years ago [4], or how marine microorganisms responded to deep climatic changes like the temperature peaks that occurred 55.8 million years ago, or more recently to the Last Glacial Maximum 19,000 years ago [5].

Being able to decipher how the complex ocean-atmosphere relationship shapes the global climate has led to a better understanding of the profound climatic changes we are going through today, and the part foraminifera play in this complex puzzle seems to be of great importance so far.

References

1. Vidyasagar, A (2016) What Are Protists? in Live Science Blog https://www.livescience.com/54242-protists.html#:~:text=Protists%20are%20a%20diverse%20collection,specialized%20cellular%20machinery%20called%20organelles
2. Lipps, J H. Learning From the Fossil Record https://ucmp.berkeley.edu/fosrec/Lipps1.html 
3. Joen G.V. Widmark, Robert P. Speijer (1997). Benthic foraminiferal ecomarker species of the terminal Cretaceous (late Maastrichtian) deep-sea Tethys. Marine Micropaleontology 31 (3–4): 135-155.
4. Alegret, L., Kaminski, M. A., Molina, E. (2004). Paleoenvironmental Recovery After the Cretaceous/ Paleogene Boundary Crisis: Evidence From the Marine Bidart Section (SW France).
5. Psheneva, O.Y., Gorbarenko, S.A. (2017) The responses of benthic foraminifera to paleoceanographic changes during the last glacial maximum, deglaciation, and the Holocene in the northwestern Pacific. Russ J Mar Biol 43, 65–75