Slowly descending hundreds of meters into the Red Sea, Sam Purkis looked out the bubble-like window of a Triton submersible into the darkness. He was hunting for long-dead corals scattered on sea mounds.
“It’s like being on a spacecraft.”
“It’s like being on a spacecraft,” said Purkis, a marine geoscientist at the University of Miami. He and Ph.D. student Morgan Chakraborty have been collaborating with OceanX to collect fossil deep-sea corals throughout the northern Red Sea since 2020.
Some of these fossils revealed that corals had grown during a catastrophic drop in sea level during the Last Glacial Maximum, a time when scientists thought life couldn’t survive there. That’s according to a new study published in the Proceedings of the National Academy of Sciences of the United States of America.
During the Last Glacial Maximum (around 19,000–26,500 years ago) the water level of the Red Sea dropped to almost 120 meters below what it is today. The event isolated the gulf from the Indian Ocean and contributed to a salinity crisis in the Red Sea—water evaporated and, without inflow from the ocean, salt concentrated in the basin.
Because single-celled organisms known as planktonic foraminifera (forams) were absent from seafloor sediments deposited during this period, scientists thought that high salinity must have completely extinguished life in the Red Sea.
But forams tend to live in shallower water and move around, so they offer only a limited picture of the ocean ecosystem, said Beverly Goodman, a marine geoarchaeologist at the University of Haifa in Israel who has done research in the Red Sea but was not involved in the new study.
To find out whether other organisms survived the salinity crisis, Purkis, Chakraborty, and their colleagues turned to fossil corals. Unlike motile forams, “corals are sessile. So they were definitely alive in the particular position we were sampling,” Purkis said.
A Coral Time Machine
Over three expeditions in the submersible, the researchers collected 27 fossil coral samples belonging to two species, Rhizosmilia valida and Leptoseris striatus, which still exist today, Chakraborty said.
The corals started life in deep-sea mounds before they died and slowly cemented into rock. The now-fossilized corals sit within beds that are exposed in mounds on the seafloor. The submersible pilots tried to use a mechanical arm to sample fossils directly from the beds, but the corals were attached so strongly that “even the hammer of Thor wasn’t going to get [them] out of it,” Purkis said. Instead, the team gathered fossilized coral that fell onto the seafloor below.
In the laboratory, the researchers compared uranium-thorium ages from the corals, which reveal when they were living, with data from radiogenic strontium isotopes, which document sea level fluctuations, and oxygen isotopes, which indicate water salinity.
Leptoseris corals were 900–7,800 years old, younger than the salinity crisis. But to the researchers’ surprise, the Rhizosmilia corals were 800–17,800 years old. The older corals were alive during the Last Glacial Period. “That was the eureka moment of the study,” Purkis said.
Consistent with previous research, the strontium isotope ratio of the oldest coral indicated that the water level was below 110 meters from current levels when the corals were alive. Salinity was about 10% higher than it is today.
“It’s incredible to find these corals growing when conditions were so much different than they were now.”
“It’s incredible to find these corals growing when conditions were so much different than they are now,” Chakraborty said.
The results could mean that Red Sea corals evolved to be a little more tolerant than other corals, Goodman explained. She cited recent studies demonstrating the resilience of Red Sea corals.
But Purkis was hesitant to endorse this conclusion. “Typically, an organism as it exists today holds up to how it existed in the past,” he said. Such a pronounced change in biological traits would require rapid evolution, which he argued is unlikely within the relatively short time frame.
Their analyses could, instead, suggest that even when the sea level was at its lowest, changes in water temperature and salinity weren’t as drastic as previously reported and may well have been within a range that the deepwater coral could tolerate, Purkis said.
The findings put the current climate reality in context, Purkis said. The fossils show that these delicate corals were able to survive major changes on the planet, he said. “Yet today, it’s a complete massacre. I think it really shows how dramatic the human impact on present-day climate is.”
Both Purkis and Goodman agreed that more studies are needed to understand the ebbs and flows of the species that survived various conditions in the Red Sea. “The Red Sea is unique in just so many ways but so poorly understood,” Purkis said.
—Kristel Tjandra (@ktjandra.bsky.social), Science Writer
Citation: Tjandra, K. (2025), Red Sea corals survived the Late Glacial crisis, Eos, 106,https://doi.org/10.1029/2025EO250097. Published on 13 March 2025.
Text © 2025. The authors. CC BY-NC-ND 3.0
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