07 August 2015 Volume :3 Issue :36

Geology Professor Co-Authors Study Published in Nature Communications

Geology Professor Co-Authors Study Published in Nature Communications
Professor Mike Watkeys.

Geology’s Professor Mike Watkeys of the School of Agricultural, Earth and Environmental Sciences (SAEES) at UKZN is a co-author of a notable study published in the prestigious Nature Communications open access last month.

In the study titled: “Antiquity of the South Atlantic Anomaly: Evidence for Top-Down Control on the Geodynamo”, the eight authors make use of geological and archaeological techniques to examine why the earth’s magnetic field is getting weaker over South Africa.

This weakening, explains Watkeys, follows a pattern one would expect before a reversal of the earth’s south and north magnetic poles, a phenomenon which occurs roughly every 100 000 years. The group’s data suggest that an area of earth’s core beneath southern Africa may be a trigger for past and future magnetic pole reversals, previously thought to have started at random locations. Interestingly, Watkeys added that the present weakening had resulted in satellites passing over this region being damaged by solar particles, so that they are now shut down for the duration of the passage.

There has been a lot of speculation about an imminent reversal of this kind, but there is a lack of rocks in South Africa that preserve a record of the earth’s magnetic field. This led Watkeys and pre-eminent Iron Age archaeologist in southern Africa, Professor Emeritus Tom Huffman of the University of the Witwatersrand, to explore using archaeological material instead.

Watkeys explained how the team analysed baked pottery and the mud floors of huts and grain bins that were baked as a result of burning.  During the baking process, the grains of magnetic minerals in the clay and mud were heated to the point where they lost their magnetism, and then cooled and regained their magnetism, recording the intensity and orientation of the earth’s magnetic field at that position at that time.

‘It is certainly the first time that a mechanism has been proposed for magnetic reversals being caused, not by processes within the liquid outer core, but by the outer core circulation being affected by a slab on the core-mantle interface,’ said Watkeys.

All the analyses were undertaken at Professor John Tarduno’s labs at the University of Rochester in the United States. Tarduno and Watkeys have been studying the Earth’s magnetic field for about 10 years to establish the strength of the field during the early history of the Earth about 3.5 billion years ago. About five years ago, they turned their attention to using similar techniques to study the weakening of the earth’s magnetic field over the South Atlantic and South Africa in the historical past.

Watkeys and the team took archaeological samples from sites in the northern areas of South Africa and southern Zimbabwe, most notably around Mapungubwe, which is just south of the triple junction between South Africa, Zimbabwe and Botswana.

Watkeys said there was room for further investigation, since their study revealed that two periods of weakening had taken place in the past 700 years, necessitating going further back in time to establish whether there were older pulses.  This is constrained by a lack of archaeological evidence, with Iron Age farmers having only arrived in South Africa around about AD250, with some data only available from fireplaces of hunter-gatherers.

The study has generated a buzz in the science community, with its release being covered by the New Scientist magazine and a German radio station.

Christine Cuénod

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