The stories that corals tell us

By Carla Elliff

Our planet has gone through countless changes over the course of its geological history. We’ve had oceans where there are now deserts, some of our current mountain tops used to be at sea level, our climate was completely different! All these changes leave behind clues, pieces of evidence that allow scientists to know what on Earth happened – and help us understand where we might be headed.

The study of past environmental conditions includes names like paleontology, paleoceanography and paleoclimatology. Paleo- comes from a Greek word meaning old or ancient, which is why it is used for all sorts of different fields.

And guess who has lent a helping hand to scientists in telling the story of our planet?

Corals are excellent storytellers if we just learn to listen. (Wakatobi, Indonesia. Credit: Warren Baverstock / Coral Reef Image Bank).

Back in the 1970s, researchers found that, much like the growth rings in a tree, corals have alternating patterns in their skeletons that follow seasonal changes. This was the beginning of a field of study known as sclerochronology (record of coral skeleton growth).

These alternating patterns in coral skeletons reflect either poorer or better growth conditions. Tropical corals, as you may have guessed, prefer warmer conditions. Water temperature below about 18 °C is too cold for most reef-building tropical corals. This means that during warm months we can expect a coral to be doing well, with abundant energy, happily growing bigger and bigger. In turn, during cold months, this energy has to be redistributed to help keep the coral’s physiological functions in order – so there’s not much energy left for growing. These alternating periods of more and less growth create the pattern etched into the coral’s skeleton: thinner layers of carbonate (the material that makes up the skeleton) during cold months and thicker layers during warm months.

Tree growth-rings to the left and coral skeleton “growth-rings” to the right.

Since we know that each year there is a winter season and  summer season, by looking at these carbonate layers we can tell how many seasons a coral colony has experienced, which represents how old this organism is. But that is only the first piece of the puzzle on the history of a coral reef.

We can also tell for example if a winter was especially harsh based on marks it will have left on the coral, helping us understand climate patterns from thousands of years ago. Yes, thousands of years! These records are kept in coral skeletons even after they have died and become fossils. The use of an indicator like this is called a proxy.

To make analyses of this sort more precise, scientists soon began to also apply geochemical methods to sclerochronology. This means that coral skeletons started to be assessed regarding how much of specific chemical isotopes, such as oxygen and carbon, were present. These elements are now commonly used proxies for a number of environmental conditions like temperature, salinity, cloud cover, river discharge, upwelling, and ocean circulation.

What was our ocean like hundreds or even thousands of years ago? (Credit: Smithsonian Institution)

These conditions set the scene for our coral heroes’ adventures. And, spoiler alert, there are plenty of obstacles and plots twists along the way…

For example, every few years our climate shows natural variations that have important consequences to our oceans. This includes the El Niño-Southern Oscillation (ENSO) System, which is an important climate phenomenon that can be traced based on geochemical and sclerochronological studies. One of the most disastrous consequences of this climate phenomenon to modern coral reefs is the triggering of coral bleaching through higher than normal seawater temperatures, which can lead to mass mortality. This stressful period will be recorded in the coral skeleton as a moment of very poor growth, since most of the organism’s energy is going into staying alive!

Current climate changes are also leaving their mark and we are yet to see the end of this tale.

One of the largest El Niño events on record (1997-1998) and the risk of coral bleaching around the world (Credit: NOAA)

It is clear that corals have lots to tell us. However, we must keep in mind that as any storytellers, they are speaking from their own perspective. Different coral species from different reefs are likely to react to environmental conditions in different ways. Some corals can withstand much higher temperatures than others without the same negative effects on their growth, for example. If we only hear the story of this single narrator, we will be missing out on the big picture. This means we must strive to become better listeners ourselves.

To learn more about the stories corals tell us, check out:

Eakin & Grottoli. 2006. Coral Reef Records of Past Climatic Change.

Coral Cores: Ocean Timelines.

Paleoclimatology Data:

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