Fossilized corals help shed light on our climate future

Field research along the Red Sea: Paleontologists investigate a number of fossil and living coral reefs, in order to forecast the evolution of these important ocean ecosystems in the face of global warming. Source: University of Vienna

One of the current climate catastrophe’s first major impacts on biodiversity is the change it wreaks on the global coral reef ecosystems. While being at the same time a tourist attraction, the primary importance of the reefs lies in the fact that they provide coastline protection and a source of nutrition. Coral bleaching, migration or, even worse, the extinction of these living organisms will have far-reaching consequences. In order to predict how coral populations will change with increasing global warming, researchers find it worthwhile to take a look at the geological past. About 125,000 years ago, during the last warm period before the current Holocene, temperatures rose to a level that corresponds to an increase of about two degrees above pre-industrial levels today. That rise in temperature during the so-called Eemian period was, however, nowhere near as rapid as today’s climate change driven by human activity.

Fossil reefs are messengers to us from that climate change of long ago. Together with colleagues, Martin Zuschin and Angelina Ivkić from the Department of Paleontology at the University of Vienna are working on deciphering the messages these reefs carry. In an FWF-funded project, they studied fossil reefs in the Red Sea and compared their findings on diversity at that time with current coral communities. Their research is set to provide a glimpse into a future where reef biodiversity has advanced considerably due to climate change. “One hypothesis about the evolution of reefs in the Red Sea holds that increasing temperatures cause coral populations to migrate to the slightly cooler northern part of this body of water,” Zuschin explains. “That is what prompted the search for fossil reef terraces in this northern area.”

Research in a restricted military area

Accordingly, the paleontologists visited a number of sites along the Red Sea in Egypt and Sudan for their field research. In the north, they investigated fossil reef terraces on land. In the south, present-day coral reefs were studied underwater. “125,000 years ago, the sea level was several meters above current levels. This is why the fossil coral reefs are found along the coastline today,” Zuschin notes. Not all of the challenges in the research expedition are of a scientific nature: “The fossil deposits are all located in restricted military areas. This made our work more difficult since the authorities kept interrupting our research to check our documents,” recalls Zuschin. “Our partners from Egypt were a great help with the logistics in this desert area.”

One thing the research team aimed at finding out was which methods were best suited for recording the corals – both fossil and living. Standard methods include the so-called line transect method, which involves extending a tape in a straight line along a reef to survey the structures lying under the tape very precisely. “We were able to show that the results from using line transects are definitely preferable for the fossil reef. Employing the alternative photo grid method, in which image segments are evaluated, we came up with greatly deviating results,” Ivkić notes. “On the other hand, we also found out how to make line transects more efficient. It is not necessary to take measurements along the entire length continuously. It’s sufficient to measure the structures under the line every 20 centimeters to derive meaningful results.”

A coral genus characteristic of the Red Sea, the fire coral or Millepora, helped the paleontologists gain an important insight. Source: University of Vienna

Fire corals as a crucial genus

The researchers thus studied the reefs by means of line transects at two elevation levels. They also took samples for dating in the laboratory. Zuschin emphasizes that his Viennese research group is one of only a few in the world that has been given permission to export fossils from Egypt. A coral genus characteristic of the Red Sea, the fire coral or Millepora, helped the paleontologists gain an important insight. “We were able to refute claims that Millepora has a very poor preservation potential in the fossil reef,” Zuschin explains.

“It is a challenge to identify fossil fire corals, however, and in addition they need a habitat with shallow water and strong water-current energy. When you explore these habitats – typically reef edges – with the necessary expertise for identifying these organisms, it turns out that they are present in similar amounts in fossil as well as modern-day reefs,” says Zuschin. These findings have a major impact on further research: “Millepora can be used as a key genus to better distinguish between different habitats in terms of their ocean-current energy and water depth, which was otherwise difficult in the fossil environment,” says Ivkić.

Prehistoric biodiversity

The researchers analyzed the fossil data and were able to actually identify a slight “diversity gradient” increasing from south to north in both habitats – i.e. both on reef edges and reef slopes. “We can conclude from this that diversity in the habitats increased slightly towards the north during the Eemian warming,” explains Zuschin. “While the study covers just three latitudes for the time being – meaning we have to wait for further results – the outcome is nevertheless a strong argument for the hypothesis of northward migration of species in the wake of climate change.”

In upcoming research, the team will further extend the prehistoric diversity gradient. In addition, they will investigate whether a similar gradient can also be found on the current reefs in Sudan that could provide information about a decrease in diversity there. This part of the research was delayed by the Covid pandemic and political unrest in Sudan. Angelina Ivkić is also thinking about appropriate science communication to make schoolchildren aware of the endangered coral reefs. “The idea is to design a game for young investigators from age 13 that will help them better understand threats to present coral reefs. The game will task them with ‘interrogating’ fossil reefs, reef inhabitants and scientists. The goal is to use the newly gained knowledge to save the last remaining reef in the Red Sea.”

Personal details

Martin Zuschin was appointed Professor of Paleontology at the Faculty of Earth Sciences, Geography and Astronomy at the University of Vienna in 2011. His research interests include paleodiversity, fauna gradients, and the ecology of reef habitats in the Earth’s modern era. Angelina Ivkić, who completed her Master’s Degree in Marine Sciences at the University of Utrecht in the Netherlands, is a PhD student in Zuschin’s research group. The project “Red Sea coral reefs: A Pleistocene-Recent comparison” was awarded EUR 262,000 in funding from the Austrian Science Fund FWF.


Zuschin Martin: Challenges of Conservation Paleobiology: From Baselines to novel communities to the necessity for granting rights to nature, in: Palaios 38 (6) 259-263, 2023

Ivkić A., Puff F., Kroh A., Mansour A., Osman M., Hassan M., Ahmed A., Zuschin M.: Three common sampling techniques in Pleistocene coral reefs of the Red Sea: a comparison, in: Geological Society, London, Special Publications, Vol. 529, 2023

Tomasovych A., Dominici St., Nawrot R., Zuschin, M.: Temporal scales, sampling designs, and age distributions in marine conservation palaeobiology, in: Geological Society, London, Special Publications, Vol. 529, 2023

Ivkić A., Kroh A., Mansour A., Osman M., Hassan M., Zuschin M.: Millepora in Pleistocene coral reefs of Egypt, in: Lethaia. Vol. 55, 2022

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