Fossils and our Future Climate

Fig.1. 40 million year old fossil planktonic foraminifera from Tanzania 
(Credit: Paul Pearson/Cardiff University)

According to a study published this week in the journal Nature, tiny fossils from 53 million to 36 million years ago may help to predict the future of climate change. This research, led by scientists from the University of Southampton and partners around the United Kingdom, sheds light on the relationship between atmospheric carbon dioxide (CO2) and climate during an ancient period in Earth’s history known as the Eocene Epoch.

The beginning of the Eocene around 55 million years ago was marked by dramatic global warming, with ice-free poles and temperatures averaging more than 14℃ warmer than today. By the epoch’s end around 34 million years ago, the planet was much cooler and drier.

Lead author Dr. Eleni Anagnostou, a postdoctoral researcher at the University of Southampton, worked with her colleagues to determine that ancient atmospheric carbon dioxide drove major climatic shifts during the Eocene. Since we cannot go back in time to collect air samples, researchers must use proxies to measure changes in carbon dioxide levels. This is where the fossils come in.

Trapped in ocean sediment cores are planktonic foraminifera fossils, tiny marine creatures that lived near the ocean surface during the Eocene. Their delicate shells were made of calcium carbonate (CaCO3), which captured changes in the chemical makeup of the seawater around them. As atmospheric carbon dioxide mixed with surface ocean water, carbonic acid and hydrogen ion concentrations increased, thus increasing the water’s acidity. An overabundance of carbonic acid reduced available carbonate ions, affecting the growth of planktonic foraminifera shells.

This ocean acidification could be deadly to shelled sea life - and is certainly a problem today - but it can be used to understand ancient atmospheric carbon dioxide levels. The researchers studied isotopes of the element boron in foraminifera fossils, which act as good indicators of acidity and therefore carbon dioxide concentrations.

Anagnostou et al. built off previous work that connected elevated carbon dioxide levels to higher temperatures and found that the Early Eocene Epoch’s extreme warmth was due to very high concentrations of atmospheric carbon dioxide. They also determined that between the Early and Late Eocene, carbon dioxide levels halved. The researchers think this sharp decrease can explain a majority of the cooling that occurred and led to the establishment of today’s polar ice sheets.

Even though this cooling event happened millions of years ago, the sensitivity of the ancient climate system to changes in carbon dioxide is similar to today. Those tiny fossils in sediment cores from ancient times are essential to our understanding of the current climate and how it may change in the future.


References
Anagnostou, E., et al. (2016). Changing atmospheric CO2 concentration was the primary driver of early Cenozoic climate. Nature. doi: 10.1038/nature17423.

Eocene Epoch. (2009). University of California Museum of Paleontology. Retrieved from: http://www.ucmp.berkeley.edu/tertiary/eocene.php

Glossary

Eocene Epoch
A time in Earth's history that lasted from 55.8 to 33.9 million years ago, marked by dramatic warming during the Early Eocene and a much cooler, drier climate during the Late Eocene.
planktonic foraminifera
Single-celled organisms called foraminifera (forams) have been part of the fossil record for 540 million years and an estimated 4,000 species live today. Planktonic forams float in water.
ocean acidification
Ocean acidification is the process by which the uptake of carbon dioxide from the atmosphere lowers the ocean’s pH (increases acidity)