Coral seer

Corals hold records of past climate change, but they could also reveal much about major earthquakes, a palaeoclimatologist predicts.

There are events that, like the hours on a clock, measure the passage of a career. One’s first day on the job is followed by a review, promotions, changes of workplace and overseas stints – a steady ascent, until a gradual slowing eases into the golden handshake and retirement. Some of these markers can be found in most CVs, including that of Dr Mike Gagan from the Research School of Earth Sciences. But they pale beside the awesome events that have marked his career in palaeoclimatology: cyclones, droughts, earthquakes and coral deaths.

“It seems like my career has been punctuated by natural disasters,” Gagan says. “When I was a PhD student, we were working on the Great Barrier Reef. We had collected all these samples of the sea floor, when category three Cyclone Winifred came right over our study area. The chances of that happening are pretty remote.”

The corals in the path of the cyclone were broken and scattered by its tremendous force. Where some would see ruin, Gagan and his colleagues saw a rare opportunity to look at the effects of cyclones on coral reefs, comparing the data they had gathered before and after the event.

This double vision – looking forwards and backwards in time – has underpinned Gagan’s academic career. By studying records of past climate change, he is contributing to a better understanding of how the climate could behave in future. He is also concerned with records of more sudden events – major earthquakes, such as the one that caused the Boxing Day tsunamis in late 2004. For want of a functioning crystal ball, Gagan uses living and fossil corals to peer into the past.

Nature’s tape recorders

“Corals are terrific because they grow right near the ocean’s surface. The ones that we’re working on grow between two and 10 metres beneath the surface, so they’re monitoring the surface temperature and salinity. These two variables are the fundamental parameters that oceanographers monitor now to see what the climate is doing.

“We drill living corals, some of which can grow for 400 years. They commonly live for 200 years. They grow continuously, so we basically have a ‘tape recorder’ of the climate over the last 200 to 400 years.

“Corals have skeletons like we do, and live tissue on the outer surface. What we’re looking at is changes in the chemistry of the skeletons through time. Corals have nice annual density bands, so we can count back year after year. That’s important when you’re looking at climate, to have chronology.”

Another benefit to studying corals is that they are concentrated in the warm waters of the tropics, especially in an area to Australia’s north known as the Indo-Pacific Warm Pool. Climatologists believe that temperature variations in this part of the ocean are linked to weather patterns all around the world.

“The warmest water on earth is right in our backyard. It turns out that subtle changes in our ocean’s temperature are really important for climate. We have an important part of the ocean, the heat engine for atmospheric circulation, and a recording device growing right in it. It’s a fantastic opportunity for Australian science.”

In order to see how changes in sea surface temperature and salinity have affected samples of coral, Gagan and his colleagues drill cores of living and fossilised corals in the field. Back at the lab, these cores are sliced into thin slabs, X-rayed, and then examined in minute detail using specialised microsampling devices and mass spectrometers. Gagan says this equipment allows the researchers to date chemical signals in chronological bands in the coral skeleton to within weeks. He says it’s also important that the samples are very clean, as contamination can make it difficult to read the isotopes and trace elements under inspection.

“We measure oxygen isotope ratios in the coral skeletons, which are a function of temperature, but superimposed on that is a salinity effect. What happens is that when you evaporate water from the ocean’s surface, you take out the lighter isotope O16. If you measure oxygen isotope ratios in rainfall, they’re very different from the ocean’s surface. The heavier O18 is left behind in the ocean, while the lighter O16 tends to be evaporated to the atmosphere. When it comes down in rainfall, the coral records it because there is an isotopic contrast between the ambient sea surface and the rainwater coming in.

“We can use the strontium – an element similar to calcium – found in coral skeletons to subtract off the temperature signal in the oxygen. As the water’s temperature changes, the coral’s growth rate changes slightly. When you change the coral’s growth rate, the amount of strontium inside it changes. What we’re left with is the salinity measure, whether it’s from direct rainfall, river runoff – we’ve even detected groundwater.”

Plotting droughts

It’s a long way from the bejewelled tropical waters north of Australia to its parched plains in the centre and south. But the record of the El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole systems, which are crucially linked to periods of drought in Australia, is conveniently stored in corals. In an Australian Research Council (ARC) Discovery Project, Gagan and his team from Australia, North America and Indonesia will draw on coral samples to create a record of past climate changes, with a particular focus on ENSO and the recently discovered Indian Ocean Dipole.

“My goal is to document the full range of natural variability in the ENSO and Dipole climate systems, and how they respond to different background climate states. These perspectives from the past will act as a guide for understanding the nature of drought across Australia.

“We have coral records going back almost continuously 7,000 years. If you look at those, you can see distinct patterns in the magnitude and frequency of El Niño and Dipole events. We have records from Indonesia, the Philippines and northern Australia, and then we have records from the central equatorial Pacific, which is where the rain goes during an El Niño event.

“One of the primary controls of the strength of El Niño in that time scale is changes in insulation seasonality. Six thousand years ago the earth was in a situation where the tropics were getting less solar radiation at the time of year when El Niños develop. Computer models suggest that El Niños should be weaker at this time, but there has been no data to back it up. The corals are showing that it’s probably true. What we get is very weak El Niños about 6,000 years ago. About 2,000 years ago it’s the opposite, when there were very strong El Niños.

“The big surprise, however, is that the coral records show Indian Ocean Dipole events were strong when El Niño was weak. The Dipole events 6,000 years ago were stronger than any we’ve seen today and produced protracted droughts in western Indonesia. The corals suggest that any future strengthening of the Dipole could increase the severity of drought in Indonesia and Australia, even if the ENSO system doesn’t change at all.”

Gagan says because of the high time fidelity in coral records, researchers can see individual El Niño and Dipole events, leading to a greater understanding of the history and future of droughts in Australia. Corals also hold a record of another present danger: earthquakes.

Earthquake library

The latest ARC Discovery funding has enabled Gagan and his team to follow up on their serendipitous discovery that corals record earthquake activity from the past. When an earthquake causes the ocean floor to shift upwards or downwards, this information is stored in the structure of the coral.

“About seven years ago, we were working near Sumatra and we happened to analyse a coral that grew through the 1797 earthquake, which was a magnitude eight event. Historical records show that in this area the uplift was about 70cm. This is not very much, given that the uplift in 2004–2005 [the Boxing Day tsunami earthquake and its successor] was as great as three metres. Nevertheless, we saw this magnificent shift in the carbon isotope record for 1797. The quakes occur, the coral is suddenly in a lighter environment, and the carbon shifts.

“We measure in corals the relationship of carbon isotopes C13 to C12, which is driven by light intensity. Corals get their carbon from symbiotic algae living in the coral. The algae photosynthesise, and the carbon is then transferred to the coral. The rate of photosynthesis depends on the light intensity in the water. If the coral’s growing deep, the light is dim; if it’s growing shallow, there’s more light.”

The researchers have found that coral is a remarkably sensitive recorder of light. They hope to reconstruct a record of earthquake events over the last 7,000 years. As coral can focus down on very small time periods, it’s also hoped that they’ll be able to see the exact intervals between seismic events, perhaps demonstrating the frequency of compound earthquakes, like the 2004–2005 double event in Sumatra.

“The worry now is that another great earthquake will soon follow the 2004–2005 event. As the earthquake centres migrate to the south, towards Java, then the tsunamis start to affect Australia more. Last time, most of the energy was directed west and north from Sumatra. A southern earthquake could be a worry for Broome, Dampier, and Perth, for example.”

If it can be shown that living corals carry extensive records of earthquakes, Gagan plans to extend the study to fossil corals too, meaning the seismic library could be extended even further into the past. The corals could prove remarkably useful for climate and earthquake studies, thanks to their versatility and durability. But they are not indestructible.

Coral deaths

In 1997 massive wildfires in Indonesia sent columns of smoke and ash into the air. The thick smoke released thousands of tonnes of iron into the atmosphere. Around the same time, a red tide – a massive plankton bloom – was observed off Sumatra. The corals in the area perished.

“With the coral reef deaths, we didn’t even know that they had happened. Imagine an area that’s equal to about one third of the Great Barrier Reef, from MacKay up to Cairns, dead. That’s what happed in 1997.

“The first time we surveyed the Sumatran reefs was in 1999. Just to show you how remote it is, nobody had reported the death of the reef. If something like that happened in the Great Barrier Reef, the whole world would hear about it. I was absolutely gobsmacked to see something of that magnitude go unreported.”

There was a massive cooling that occurred off Sumatra in 1997, and an upwelling of cool, nutrient-rich water drove phytoplankton production. The locals said that when the plankton appeared, the corals died almost simultaneously. The plankton oxidised in the water column, effectively strangling the coral.

“Our coral records show that these plankton blooms have occurred in the past, but the coral didn’t suffer. An event like the massive coral death in 1997 probably hadn’t happened in the last 6,000 years. There had to be something else driving the plankton bloom. Iron fertilisation is important to the growth of plankton and was caused by the fallout from the fires.”

Aware that precious records of past climate and earthquake activities could be damaged and even lost through a disregard for environmental preservation, Gagan has also been involved in community education programs in Indonesia. By emphasising the important role that corals can play in understanding the past and future of this planet, he hopes that others will learn to see the precious advantage of nature’s tape recorders.

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ANU Reporter Spring 2006