Global Records of the Iceland Basin Geomagnetic Excursion

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The Earth's magnetic field varies on a number of different timescales. On short timescales of years to centuries the magnetic poles drift around the geographic poles, in a process known as secular variation. Hence, the magnetic pole is rarely at the geographic pole, and this why magnetic compasses are adjusted for this variation before they can be used for navigation. If the location of the magnetic poles is averaged over longer timescales of about 10,000 years the mean positions coincide with those of the geographic poles. On very long time scales of millions of years the Earth's magnetic field is known to reverse its polarity, and compasses would point to the south rather than to the north. These reversals do not occur regularly and the duration of the different polarities varies enormously. We are currently in a period of 'normal polarity' (i.e. compasses point to the north) and the last reversal of the field was about 780,000 years ago. Geomagnetic excursions occur on timescales between secular variation and field reversals. During a geomagnetic excursion the magnetic poles wander far away (>45 dgerees) from the geographic poles, but return to their previous location. This distinguishes them from full reversals where the magnetic pole wanders far away from the geographic pole, before settling at the opposite geographic pole. There have been suggestions that there have been up to 12 such excursions in the last 780,000 years (i.e. since the last reversal), and because they are global events, they can be used to correlate different sedimentary sequences worldwide. The excursions are also marked by a decrease in the overall intensity of the Earth's magnetic field. This field shields us from incoming solar radiation, and we have recently shown that excursions are marked by an increase in the rate of production of radionuclides in the atmosphere, due to the increased incoming solar radiation that results from the reduction in field strength. Such radionuclides are preserved in sediments and in ice cores, and potentially offer a way of correlating ice-core records of climate change with patterns of climate change recognised in marine sediments. An understanding of the causes of geomagnetic excursions, how long they last, and how useful they can be for correlating different climate events has been hampered by a lack of good records of these events. They are typically recognised in the magnetic signal from sediments, where the Earth's magnetic field aligns magnetic minerals during their incorporation into the sediment, or when magnetic minerals grow during formation of the rocks. Because they are thought to be of short duration we need sediments that accumulate at a rapid rate. We have recently shown that one of these excursions known as the Iceland Basin Event, which took place about 186,000 years ago, lasted for about 7000 years, based on analyses of two sediment cores from the Atlantic. Other authors have proposed shorter and longer durations for the same event. Our proposal aims to study the magnetic signal for five other cores, scattered through the Atlantic and Pacific oceans, and spread over a range of latitudes. This will enable us to assess the global nature of the signal. We will also be carrying our geochemical analyses that will enable us to calculate the duration of the event. Recent work has shown that the magnetic instabilities associated with reversals appear to only last 2,000 years at the equator, and about 10,000 years at the geographic poles. We will be able to test if the same holds true for geomagnetic excursions. Finally, a knowledge of the duration of these events will inform climate scientists as to the resolution that excursions can offer their correlations of climate records. Short durations will offer very precise correlation, but are harder to identify, whereas long durations should be easier to find and identify but offer correlation at a much lower precision.