ScienceWise - Nov/Dec 2009

Core Data on Climate

How the Great Barrier Reef Records Climate History

Australia and New Zealand have recently signed up to the world’s largest ocean research program - The Integrated Ocean Drilling Program (IODP). IODP is a multi-national scientific collaboration aimed at extracting data from core samples taken at numerous points throughout the world’s oceans. Data from these cores can be used to answer a wide range of questions about everything from the processes that shaped the Earth’s surface to marine biology and the effects of climate change.


Dr Mike Gagan of the Research School of Earth Sciences is one of the scientists involved in IODP. He’s currently preparing for an upcoming project to obtain core data from Australia’s Great Barrier Reef.


“The Great Barrier Reef is a very special place, not just because of its cultural and biological significance to Australia, but also because of the scientific information on the Earth’s past climate we can glean from its corals.” Dr Gagan explains.


What makes the reef so special from a scientific perspective is that it lies in tropical waters very remote from the ice caps and has had a heavy coral cover for many thousands of years. Its tropical location means that the crust below the reef has not been depressed by millions of tons of ice in the geologically recent past. So unlike Europe, the crust is not still rising after the last ice age and consequently, scientists can obtain reliable data on the historical depth of sea water there.


This makes the Great Barrier Reef an excellent measure of what’s known as the eustatic sea level – the actual volume of water in the Earth’s oceans. The reef has ancient coral at depths of over 100m. Since corals only grow near the ocean surface, this indicates that the sea level was very significantly lower in the past. It’s sobering to think that melting glacial ice caused the sea to rise 100m in the last 20,000 years and that there’s a whole lot more ice still out there!


Core samples taken from the reef can be very accurately dated because the corals absorb natural uranium from the seawater and over time this decays to thorium. The uranium to thorium ratio gives an indication of the age of the sample. When these data are combined with the position of the sample in the drill core, it’s possible to build up a picture of the change in sea level over time.


The resulting sea level data provide a good measure of the volume of ice on the planet and hence the global temperature.  But what’s really exciting about this work is the potential to determine sea temperature and even rainfall for Eastern Australia going back 20,000 years. This is all made possible by the biochemistry of the tiny polyps that make up living coral. As they feed, grow and secrete their mineral skeletons, they take up oxygen isotopes and trace elements such as strontium.


The amount of strontium absorbed by the polyps is strongly affected by the water temperature, so by measuring strontium concentrations in the core samples, scientists can determine the local sea temperature at that time. Amazingly, when this is done with fine enough resolution, a curve emerges showing the water temperature cycle from summer to winter each year, yielding an extraordinarily accurate and detailed measure of the surface temperature of the sea.


“The great thing about coral is that it only grows close to the surface where there is ample light. This means we know the data we derive from these cores is genuinely representative of the temperature of the ocean surface at that particular point,” Dr Gagan says.


Extraction of rainfall information from the same samples is possible because of the behaviour of different oxygen isotopes. The vast majority of atmospheric oxygen is oxygen16 but there is a small amount of oxygen17 and oxygen18 too. Water molecules incorporating oxygen18 are heavier and therefore very slightly less likely to evaporate than those containing oxygen16. This means that rainwater has a lower proportion of oxygen18 than seawater. When rain falls on the sea the incoming oxygen16 water dilutes the concentration of oxygen18 water molecules naturally present. Organisms like coral incorporate the different oxygen isotopes into their skeleton depending on their abundance so measuring the isotope ratio gives scientists a measure of local rainfall.


“If the operation goes as we plan, with this data set we should be able to build up a superbly detailed picture of the climate of eastern Australia over the last 20,000 years revealing everything including El Niño cycles. What we’re really keen to see is how well past changes in climate correspond with climate simulations derived from current models. If they tally well it really adds weight to future climate predictions and we hope this will provide an additional impetus to address environmental issues like emissions.”


Knowing how the reef has responded to changes in the past will also greatly help scientists to understand how it’s likely to react to the current threats it faces. And that knowledge may be vital in helping to ensure the survival of one of Australia’s greatest natural assets.




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