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ScienceWise - Autumn 2012

Rain, rain...

Article Illustration
Drilling sample cores from fossilised reef coral - photo Stewart Fallon

How scientists are building up a picture of Australa’s wet/dry climate cycles

It’s rained an awful lot over Eastern Australia in the past couple of years. The dams are full – too full in some cases – and the hills are as green as England. Yet not so long ago we were in the midst of an extreme drought that will no doubt return one day. No one could deny that Australia has a very variable climate.

Much of this variability comes from our proximity to the Indian Ocean. When the water is warm, evaporation from this massive body of water drives moisture south to Australia and when it’s cold, we experience droughts.

Naturally scientists interested in predicting future climate scenarios take a special interest in the Indian Ocean and how its temperature has varied in the past. However human records of ocean temperature only stretch back a few decades so how do you go about studying the history of an ocean’s temperature in the distant past?

Dr Nerilie Abram of the ANU Research School of Earth Sciences is one of the scientists with the answer. She’s about to head off to Sumatra to drill sample cores from corals.

“Because coral excretes a skeleton of calcium carbonate it creates a record of the ocean in which it was living” She says, “And a few quirks of biochemistry impregnate that record with information about not only the temperature of the ocean but also the rainfall.”

One of these quirks is strontium substitution. Strontium and calcium both occupy group two of the periodic table so in spite of a significant mass difference they both have two electrons in their outer shell giving them very similar chemical properties. Organisms generating a skeleton from calcium carbonate occasionally substitute strontium for calcium depending on both the relative availability of the two and importantly to the scientists, the ambient temperature. The warmer the water, the less strontium will be substituted for calcium.

By measuring this ratio of strontium to calcium in various parts of the fossilized coral skeleton, scientists like Dr Abram can build up a picture of how the temperature of the water has changed over the years.

But that’s not the whole story. Different isotopes of oxygen are also incorporated into marine skeletons again in varying proportions with both temperature and the availability of each isotope. Rain water tends to contain far less of the heavier isotopes of oxygen than sea water does. Because coral lives near the surface this means that when it rains a lot, there’s less heavy oxygen around and so less is included in its continually forming skeleton. If you already know the temperature from the proportion of strontium, it’s possible to correct for the temperature effect in the oxygen data and extract a rainfall record.

“A combination of data on oxygen isotopes and strontium enables us to build an incredibly detailed picture of temperature and rainfall” Dr Abram says, “Using corals growing on todays reefs, we can generate records that go back one or two centuries. Much beyond that and corals that have remained in the ocean tend to be either buried by new reef growth.”

Luckily for science, Sumatra’s intense volcanism and plate tectonic activity frequently shift coral out of the oceans where it can remain preserved for far longer. As part of her studies, Dr Abram will be examining blocks of coral that were washed deep inland by the tsunami that followed the massive explosive eruption of Krakatoa in 1883 that literally shook the world.

“The Krakatoa samples should enable us to look back a century or so more than we’ve been able to do so far,” Dr Abram explains, “But that still isn’t very long in climate science.”

The final pieces of the puzzle come from examination of stalagmites in the region’s caves. Forming over tens of thousands of years as rainwater seeps through limestone cave roofs, these spiky relics also record the isotopic concentrations of the local water.

“Stalagmites don’t give us anywhere near the detail that we can extract from coral, but they give us a very long term background to the climate over which we can superimpose the highly detailed information we extract from coral.”
“Australia has long been prone to cycles of drought and flood. So what were hoping to do here is to create an accurate scientific record of the regions climate history to see if the trends are shifting and how that might be influenced by factors such as increased atmospheric CO2.”

The recent widespread floods and the preceding decade of drought had huge impacts on our economy and on our people, so it makes sense that as a nation, we do all we can to better understand the climate we live in.

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