Filling the palaeomagnetic gap
As our planet’s magnetic field is constantly changing, it is difficult to know how the field behaved at any given point in the past. However, there is one way to examine the more recent palaeomagnetic history; through our ancestors’ love of fire.
Palaeomagnetism has had an incredibly important role in the history of geology, but palaeomagnetic data can also be crucial as a dating tool for human settlement sites.
To establish an accurate palaeomagnetic chronology, researchers often rely on human pottery-making or metalwork furnaces for example.
It does however require activities that involve firing minerals to temperatures exceeding the Curie Temperature; the point above which objects become demagnetized. When the objects cool, the finished pot remagnetizes, locking in a record of the direction and strength of the magnetic field in the region at that specific point in time and space.
“We have very good palaeomagnetic data from across the world recording field strength and direction – especially in the Northern Hemisphere,” said Gillian Turner from Victoria University, Wellington, New Zealand.
“The southwest Pacific is the gap, and in order to complete global models, we’re rather desperate for good, high-resolved data from our part of the world,” she told BBC News.
Dr Turner was speaking at the recent American Geophysical Union (AGU) Fall meeting, the world’s largest annual gathering of Earth scientists.
Finding palaeomagnetic data
The Maori, who first settled New Zealand around 1200 CE, are one such example of a culture that left behind no pottery and this may suggest that there is no palaeomagnetic data to gather.
However the Maori are famous for hāngi, large steam pit ovens using heated rocks.
The Maori legends tell how the stones would glow white hot and by placing thermocouples into a test firing of a hāngi the team was able to show temperatures as high as 1,100 C were achieved.
Hāngi stones were carefully chosen and the Maori preferred andesite rocks found in Central North Island as they were less likely to shatter in the fire; fortunately for the team, they also contain a high concentration of magnetite.
Dr. Turner and her fellow researchers experimented with modern hāngi to confirm that the rocks did indeed exceed the Curie Temperature, which means that all rocks from ancient hāngi sites potentially hold a wealth of information on Earth’s magnetic fields. By radiocarbon dating charcoal recovered at the hāngi sites, Dr. Turner can identify exactly when the rocks were first fired and thus match magnetic data with timescales.
Understanding current climatic systems
Dr Turner and her team propose to gather data recorded by magnetic particles deposited in lake and marine sediments in New Zealand and archaeological artefacts such as fired pottery from the Lapita culture for example, will be part of the study too. The outcome will be a detailed history of the southwest Pacific’s magnetic field over the last 10,000 years.
This history will enable a high-resolution tool for dating recent events, such as early settlement of New Zealand and climate change, to be developed. And, by combining their measurements with similar observations from the Northern Hemisphere it is hoped to provide a globally consistent model of Earth’s dynamic magnetic field.
Global earth and climate systems have recently dominated national and international forums. They are beginning to impact on the way we live, and we need to understand how they work. New Zealand’s unique geological evolution offers important insight into these natural earth and climate systems, providing some of the most significant archives – both long and short-term.
The challenge facing geologists is to interpret these records in a time scale that means something to us – and to work out which intervals of the archive are most relevant to the specific problems we face.
Past Horizons. 2013. “Maori ovens to provide missing data on Earth’s magnetic field”. Past Horizons. Posted: December 22, 2012. Available online: http://www.pasthorizonspr.com/index.php/archives/12/2012/maori-ovens-to-provide-missing-data-on-earths-magnetic-field