Q&A: What ancient coral reefs can tell us about our changing climate

The once-rich ancient ecosystems hold records of past global temperatures, sea levels, and ocean chemistry

Thanks to nearly £1m of new funding from NERC, Imperial researchers will study ancient coral reefs to tap into lost records of our changing climate.

Led by Dr Cedric John at the Department of Earth Science and Engineering, scientists will take a seismic vessel to the Pacific Ocean to study ‘drowned’ coral reefs – reefs formed millions of years ago that stopped growing and sunk into the ocean. The team includes researchers from Imperial College London and the Universities of Oxford, Cambridge, Manchester, Southampton, Naples, Sao Paulo, and Houston.  

We'll use advanced technology that's improved significantly since these reefs were last imaged. Dr Cedric John Department of Earth Science and Engineering

These reefs carry records of past global warming and cooling as well as changes in sea level and ocean chemistry. Understanding how the climate changed in the past can help us understand the current climate crisis – but there are gaps in our knowledge.

Studies of the effects of sea level change on ancient coral reefs, for example, extend to about 30 million years in the past. Now, thanks to funding from Natural Environment Research Council (NERC), part of UKRI, researchers hope to tap into these ancient records from up to 120 million years ago. 

Caroline Brogan spoke to Dr John about the new project. 

Why do we want to study drowned coral reefs? 

Formed by strong structures that can withstand waves and time, ancient reefs can be used as time capsules to understand the climate of the past.  

Our youngest target is 40 million years old and grew during the Eocene period, when whales first appeared and primates were limited to small, tree-dwelling creatures.  Dr Cedric John Department of Earth Science and Engineering

Though they drowned years ago, reefs have stuck around to experience our planet’s warming and cooling over millions of years. These changes have left clues on the reefs, which we intend to uncover with our new funding. 

The creatures that built the reefs can themselves also tell us about the ecological conditions at the time they were formed. By studying reefs, we can try to understand how ocean temperature, atmospheric carbon dioxide levels, and sea level changed in over millions of years. 

Coral reefs are also rich ecosystems. Although they grow in regions of the oceans that are akin to watery deserts, where there is little food supply, the corals use sunlight to grow and require very little nutrients. In turn, they create around them a very rich ecosystem of organisms that feed off the corals and each other, like fish, sharks, and seashells.  

This rich ecosystem is important for the overall health of the ocean, and for millions of people living from traditional fishing on tropical islands. Climate change is causing much stress to coral reefs, and we hope that understanding ancient reefs formed during past episodes of global warming will help us mitigate the worst consequences for coral reefs, and the planet, today. 

Where can they be found, and which ones will you be targeting? 

Today, coral reefs are typically found in the tropical to sub-tropical oceans, away from sources of nutrients and pollution, and grow on top of a hard shallow-water substrate such as a volcanic island. Good modern examples include Hawaii, the Bahamas and the Maldives.  

However, we will be targeting drowned reefs, which are much older: our oldest target grew about 120 million years ago during the Cretaceous period, when dinosaurs were roaming the Earth. Our youngest target is 40 million years old and grew during the Eocene period, when whales first appeared and primates were limited to small, tree-dwelling creatures. 

The six drowned reefs we’ll target are all in the Northern Pacific Ocean. The eldest two are in what is known as the ‘Mid-Pacific Mountains’, roughly west of Hawaii. The other four are North of Hawaii, in what is known as the ‘Emperor Chain’. All six reefs grew on now extinct volcanoes. 

What will the funding allow you to achieve? 

We’ll take a seismic vessel to Pacific and spend some 40 days getting detailed images of the internal structures of the drowned reefs. We'll use advanced technology that's improved significantly since these reefs were last imaged. The new seismic data will be of much higher resolution, and we will be able to see the details of the reef structure much better.  

The funding will also allow us to do work on the seismic data once we are back onshore. With this data we'll document how the internal architecture of ancient reefs compares to modern reefs. We will also trace the trajectory of the reef top to reconstruct changes in sea level during periods of warming. 

How can studying ocean changes help current climate science? 

We will offer a window on what reefs did when Earth was in a global warming phase with few or no ice caps. Two key aspects of study are: 

Chemical differences  

We know that current global warming results in ocean acidification, and this in turn leads to difficulties for organisms such as corals to build their carbonate skeleton. Our work will address what strategies ancient reef-builders used to construct vast reefs in an ocean that was globally warmer and chemically different to the modern ocean. 

Sea level rise 

One of the biggest threats to coastal communities is global sea level rise due to climate warming, but precisely how much sea level will rise is debated among scientists. Our current records of sea level rise are spotty, and our work will provide much needed examples, from much older records than we until now have been able to access.

What will you do once you process the seismic data? 

The funding from NERC is only the beginning of a long-term, collaborative research effort with international partners. Once we obtain the seismic images, we will decide which part of the ancient reefs we want to recover and take for further study in the lab. For this, we need a drill ship capable of drilling into a kilometre or more of water. 

Only by combining the seismic images with measurements of the chemical, physical, and magnetic properties of the rocks will we be able to fully answer our research questions.  

This further work on cores will be conducted as part of the UK IODP – a major international research program focused on recovering sediment and rocks from the ocean.

Images: Shutterstock