Corals can tell us about climate change in the geological past

Understanding the effects that climate change is having on coral reefs is important for the future of marine ecosystems

 Studies have shown that 93 percent of the almost 3000 individual reefs that make up the Great Barrier Reef have been affected by bleaching.

Palaeoclimatology is the study of past climates. Scientists determine past environmental conditions via the use of climate proxies, the preserved physical characteristics of the past. 

Modern analogues are an essential part of palaeoclimatic studies because they provide the basis for understanding the geochemical signatures of fossils. Corals are extremely sensitive to variations in their environment so make excellent climate indicators.  

Corals have the potential to record several centuries of highly detailed environmental information in the form of chemical proxies such as trace elements and stable isotopes. They are fantastic sources for isotopes as their calcified skeletons are formed at or near to isotopic equilibrium with the surrounding seawater.

Research methods

Changes in chemical proxies are often correlated with thin growth bands, which are presumed to relate to annual growth cycles, just like tree rings. Larger corals commonly lay down density bands that are used as annual markers, allowing time-series records to be produced.

Scanning Electron Microscopy and backscattered electron imaging studies of the Caribbean species Montastraea faveolata, reveals small microbands that can be used to describe small variations in climate that provides researchers with a very accurate reading of past climates. 

 Studies have shown that 93 percent of the almost 3000 individual reefs that make up the Great Barrier Reef have been affected by bleaching.

Corals can also be used to help determine day length, Earth’s rotation, and diurnal cycles. By examining the growth rings of corals, we can determine that a year during the Devonian period was around 400 days long.

A limitation of using corals is that they are made almost entirely of calcium carbonate, so they dissolve with an increase in oceanic water acidification. Some parts of corals have also been altered by a process of diagenesis, and can yield false results. In some cases, diagenesis can erase environmental information.

The intermediate shell layers are the most likely to yield well-persevered sample materials and thus provide the most reliable results. This is true for both corals and macrofossils, myarea of research at CSM. 

No climate proxy is perfect so, to get the most reliable results possible, it is best to use multiple lines of evidence. This is especially true if important variables (e.g. sea-surface temperatures, eustatic changes, and ice-level fluctuations) are to be identified. Ideally, it is best to use a mixture of inorganic and organic data combined with model results.

Climate change

A quarter of marine ecosystems are dependent on coral reefs - these ecosystems directly support the livelihoods of millions of people worldwide, mostly from developing nations. 

Current anthropogenic climate change is having a devastating effect on coral colonies. Recent rises in ocean temperatures have already impacted reefs, and mass bleaching and disease outbreaks have already become more frequent. Bleaching occurs when the coral polyps expel the algae that that inside them, draining the corals of colour - hence the term “bleaching”.

These algae form an endosymbiotic relation with the coral: they are protected by the coral, and in turn they provide up to 90 percent of its energy. Without these source of energy the corals begin to starve and, given enough time, die. 

The increase in carbon dioxide in the oceans as decreased their pH, a process called ocean acidification. It is thought that between 30 to 40 percent of CO2 from human activity ends up in bodies of water and that some of it reacts with the water to form carbonic acid. This carbonic acid has already started to reduce calcification rates of coral-reefs.

Unless human-induced climate change is controlled, it will almost certainly destroy most of the coral reefs in the world.  Studies have shown that 93 percent of the almost 3000 individual reefs that make up the Great Barrier Reef have been affected by bleaching. Almost a quarter of the reef died during a mass bleaching event in 2016.

The study of ancient corals and their responses to past climatic changes helps place the modern biodiversity crisis, which has been dubbed the Sixth Mass Extinction, in to its historical context. 

The Author

Jack Wilkin is a graduate researcher in the Deep Time Global Change palaeoclimatology research group at the Camborne School of Mines, University of Exeter. His research focuses on the isotopic geochemistry of fossil shells from the South German Middle Jurassic. 

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