What 40 million year old tropical reef corals tell us about climate change

View inside the drawers of the collection: 40 million year old tropical reef corals from present-day France. Credit: Stefan Kruger

The exceptionally well-preserved reef corals of the Geological and Paleontological Collection of the University of Leipzig hold a great secret: they allow us to travel far into the past and reconstruct the climatic conditions at our latitudes. Researchers from the University of Leipzig, the universities of Bremen and Greifswald and UniLaSalle in Beauvais have achieved this. Thanks to chemical analyses, they were able to model the seasonal temperature fluctuations of this period and show for the first time that corals were already living in symbiosis with algae 40 million years ago. Their results, which can also be used to improve current climate predictions, have been published in the journal Scientists progress.

In the Middle Eocene, some 40 million years ago, a climate prevailed in our latitudes: it was hot and humid, as evidenced by the fossils of Lake Geiseltal near Halle, for example. It was so hot, in fact, that Coral reefs extended far to the north – up to about the 45th degree of latitude, roughly at the level of present-day southern France. Some of these tropical reef corals are now fossils in the geological and paleontological collection of the University of Leipzig. They come from the Parisian basin, a large marine bay which extended to present-day France.

Among these fossils, Professor Thomas Brachert and his team found very particular corals: they were not fossilized like many others, but escaped this process. “It makes for a wonderful environmental archive. A coral skeleton grows every year much like a tree. But what’s special is that the skeleton is something of an archive of hundreds to a thousand years of climate history. “, Professor Thomas said. Brachert from the Institute of Geophysics and Geology of the University of Leipzig.

What 40 million year old tropical reef corals tell us about climate change

Professor Thomas Brachert discovered the special corals of tropical reefs in the Geological and Paleontological Collection of the University of Leipzig. Credit: Stefan Kruger

Skeleton reveals small seasonal temperature fluctuations

The geologist and his team took samples of the limestone skeleton of the coral and analyzed the material using geochemical methods. Based on the chemical properties, the scientists were able to deduce the temperature of the water in which the corals lived. The ratio of oxygen isotopes in the samples showed that seasonal temperature differences were very small for this latitude. They correspond to about half of the current value of 15 degrees Celsius difference between the seasons. “This means that our study confirms what was expected but could never be measured so well: the fact that there were very small seasonal differences in the warm periods of the planet,” explains Brachert.

New discovery: corals already lived in symbiosis at the time

The researchers also studied the eating habits of corals 40 million years ago. By analyzing carbon isotopes, they were able to show for the first time that even then corals lived in a symbiosis with unicellular “algae“, supposedly zooxanthellae. These carry out photosynthesis, giving the sugar they produce to the coral. The coral in turn digests the sugar and returns important nutrients to the algae for photosynthesis. But if the sea water gets too hot, the corals expel the algae and starve to death. They were therefore already susceptible to coral bleaching at the time and probably affected on several occasions.

What 40 million year old tropical reef corals tell us about climate change

Skeletal structure of tropical reef corals at 20x magnification. A growth interruption, clearly visible in the middle of the image, is due to a partial death of the coral colony and may have been caused by coral bleaching. Credit: Professor Thomas Brachert

Study the data to improve current climate calculations

The research team’s data not only allow conclusions to be drawn about the Middle Eocene climate, but also improve current climate models. “We can use our new findings on periods of extreme heat as a comparison for the future. Our current computer models are based on assumptions that are not necessarily all correct. Based on our data, we can assess how measurement these models provide useful results,” summarized Professor Thomas Brachert.

Climate-resistant microalgae could help restore coral reefs

More information:
Thomas C. Brachert et al, Slow-Growing Reef Corals as Climate Records: A Case Study of the Middle Eocene Climatic Optimum 40 Ma Ago, Scientists progress (2022). DOI: 10.1126/sciadv.abm3875

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Leipzig University

Quote: What 40 Million Year Old Tropical Reef Corals Tell Us About Climate Change (2022, May 23) Retrieved May 23, 2022 from https://phys.org/news/2022-05-million-year -old-tropical-reef-corals-climate.html

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