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As climate change and human-induced disruptions threaten coral reefs worldwide, scientists are increasingly turning to the past to better understand these vital ecosystems, using coral cores from drowned reef terraces as time capsules to learn how reefs responded to environmental upheavals throughout history.

Close-up of a coral core aboard the Hawaiian Drowned Reefs Expedition
Close-up of a coral core collected aboard the Hawaiian Drowned Reefs Expedition in late 2023.

Akin to tree rings, coral cores exhibit growth banding that documents the ebb and flow of the surrounding environment. By scrutinizing these bands, scientists can reconstruct a month-to-month chronicle of events in a specific reef that may stretch back hundreds of thousands of years. This unique approach offers a window into the past, providing valuable insights into how coral reefs coped with disturbances such as flooding, storms, heat waves, and droughts. 

An international scientific research expedition, carried out on behalf of the International Ocean Discovery Program (IODP), aims to recover a record of past climate, sea-level change, and reef conditions off the coast of Hawai’i. USGS Research Oceanographer Nancy Prouty is part of a team of scientists from Australia, Austria, China, Denmark, France, Germany, India, Japan, the Netherlands, Great Britain, and the USA that is participating in the two-month research expedition aboard the multipurpose vessel MMA Valour, which left the port of Honolulu at the end of August 2023.   

Shallow water corals are considered “drowned” when sea-level rise submerges reefs to depths where they can no longer conduct photosynthesis. The exact depth where reefs “drown” depends on the species that live there, but generally no shallow water corals can survive below 40 meters (131 feet) of water. The drowned reef terraces studied on the Hawaiian Drowned Reefs Expedition are at depths between 130 to 1,200 meters (427 to 3,937 feet). 

This expedition focuses on the subtropical Pacific Ocean, a region of paramount importance due to its outsized influence on Earth's climate. The Pacific’s vast stores of heat give rise to numerous storm systems that can affect the entire planet. To date, scientists have lacked sufficient archives detailing the impacts of climate change to this region, and some are turning to ancient coral reefs for answers. 

One of the primary goals of USGS coral reef research is to understand how coral reefs contribute to coastal resilience, acting as natural barriers against extreme events like hurricanes and flooding. As climate change intensifies the frequency and severity of such events, the protective role of coral reefs becomes increasingly crucial. 

“Studying the past helps decision-makers triage the most pressing issues of today,” Prouty said. “We are living in a period where we desperately need to understand our planet’s response to climate change.”

Aboard the Hawaiian Drowned Reefs Expedition, Nancy Prouty examines a coral core
Aboard the Hawaiian Drowned Reefs Expedition, Nancy Prouty examines a coral core collected from a drowned reef terrace.

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