Earth’s invisible shield can help reveal our past and our future

I study the history of the Earth’s magnetic field. Understanding the past may help us to predict and mitigate the impact of future changes in the field’s strength and direction. My field of research is palaeomagnetism – I examine magnetic materials in sedimentary rocks to track the changes in the field over time and to develop more accurate techniques for determining the age of rocks and archaeological materials.

The Earth’s magnetic field shields our infrastructure and technology from harmful cosmic rays (energetic particles moving through space at close to the speed of light). Large bursts of particles from the sun, called ‘solar storms’, can deform our magnetic shield, disrupting global communications, navigation systems and power grids.

To predict field changes and mitigate the risk of technological breakdowns, we need a better understanding of the mechanisms of change and how fast it can happen.

The dynamics of the Earth’s liquid iron core govern the magnetic field. As molten rock in the planet’s core moves, the field’s strength and alignment change, and the north and south magnetic poles drift.

Our record of field changes includes around 20 years of satellite data, 200 years of data from magnetic observatories on the ground, and 400 years of navigational data. We can gather information about older field fluctuations – palaeomagnetic data – from rocks or other materials that have magnetic properties, including human artefacts.

Scientists have modelled the field’s behaviour using palaeomagnetic data from the Northern Hemisphere but we have inadequate data from the Southern Hemisphere. Predicting magnetic field behaviour without data is like forecasting rain without any meteorological records or view of the sky. As part of the University of Melbourne’s Rock and Palaeomagnetism research group, I’m working to fill in this critical blind spot, focusing on Australia, the South Pacific and Antarctica.

I uncover the history of the magnetic field by drilling sediment cores out of ocean or lake floors. Imagine a little magnetite dust particle eroded from rock and flushed into a lake by rainfall. After settling on the lake floor, the particle orients itself with the Earth’s magnetic field. Over time, the magnetite particle becomes buried by more sediment, which locks it in place like a compass needle frozen in time.

As a kid in Montreal, Canada, I liked being outdoors, asked lots of questions and dreamt of exploring the world. I did my undergraduate degree in geography and then a masters degree and PhD in oceanography. I became interested in palaeomagnetism and participated in drilling projects in the Arctic Ocean, Baffin Bay in Canada, and Patagonia.

The Earth’s magnetic field extends into space but is not consistently strong around the world. For example, in Australia, the field is intense, but around Brazil, the field is three times weaker: this is called the South Atlantic Anomaly. When the International Space Station or satellites pass through this region, they need to shut down their instruments or shield them to prevent electronic damage from cosmic radiation.

Global changes in the magnetic field are excellent chronology markers. For example, around 41,000 years ago we had the Laschamp Event. This was the most recent ‘geomagnetic excursion’: the Earth’s magnetic field became dramatically weaker, and the magnetic poles reversed, eventually migrating back.

By understanding this global event, we can identify its impact in the magnetic properties of a rock and say, “This is 41,000 years old.” Paleomagnetic techniques can help accurately date the things we find in sedimentary rock layers, like fossils or the remains of human activity.

I came to Australia to address the lack of paleomagnetic data from the Southern Hemisphere. One of my career goals is to solve this long-standing problem and reconstruct the history of Earth’s magnetic field in Australia. I analyse multiple archives that preserve this information – a wide range of rocks and magnetic materials – to build up a comprehensive record.

I recently started studying the magnetic properties of stalagmites. Australia’s lack of active tectonic and glacial processes means our lakes are usually shallow and dry out periodically, which affects the preservation of magnetic information in sediments.

Stalagmites are a good alternative archive because they can give us detailed information about magnetic field changes over time, at a resolution typically higher than that of lake sediments. There are plenty of stalagmites in karst systems around Australia. I focus on Victoria, South Australia and Western Australia.

I am developing a new method for dating rocks and artefacts at sites of human occupation. Artefacts like bricks, ceramics, fireplaces and fire-treated stone tools have magnetic properties. Having a more comprehensive palaeomagnetic record will enable better interpretation of these archaeological finds.

The aim of my Australian Research Council Discovery Early-Career Research Award project is to create an easy-to-use, online, paleomagnetic dating tool for south-east Australia. If an archaeologist finds a stone fireplace, for example, they can send oriented samples of the stone to a laboratory for magnetic analysis.

The archaeologist could then use online software to compare the lab results with the reference curve that I’m developing, and produce an age estimate for the samples. Tools like this exist in Europe and other places, but it would be a first for Australia.

I would tell anyone considering a career in science to follow your dream. It’s important that along the way you seek feedback and learn to work well in a team. Most of all, be creative: only innovative ideas can uncover the unknown.

- As told to Bagas Aditya and Jonah Nelson

This story was written by masters students as part of the Science Communication subject.

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