Evidence for a rapid reversal of the geomagnetic field 41,000 years ago

Dr. habil., Norbert R. Nowaczyk / GFZ

Magnetic studies performed on sediment cores from the Black Sea by the GFZ German Research Centre for Geosciences show that Earth experienced a rapid (over 440 year period) and complete reversal of the geomagnetic field 41,000 years ago during the last ice age. Geomagnetic reversals result in the interchanging of the positions of magnetic north and magnetic south. As this was a brief and unsustained pole reversal, it is known as a geomagnetic excursion and not a full reversal. The evidence for this brief pole reversal is further bolstered by data obtained from additional studies performed in the North Atlantic, the South Pacific and Hawaii; together this shows the polarity reversal was a global event. The sediment cores also provide further evidence for the last ice age and a supervolcano eruption.

Earth has had several pole reversals in the last 20 million years, with a complete pole reversal occurring about every 200,000 to 300,000 years. The last major pole reversal occurred approximately 780,000 years ago and is known as the Brunhes–Matuyama reversal. A reversal does not happen instantly, and typically takes between 1,000 and 10,000 years. The Brunhes–Matuyama reversal caused no drastic changes in plant or animal life as provided by evidence from the fossil record. By looking at oxygen isotope ratios in deep ocean sediment cores, no changes in glacial activity were found for this period either. 

The field geometry of reversed polarity for the geomagnetic excursion of 41,000 years ago lasted about 440 years and the field strength was only about 25% of today’s field. The actual polarity lasted only 250 years, which is remarkably short. During this 250-year period, the magnetic field was only at 5% of today’s field strength. This significantly lowered Earth’s protection against hard cosmic rays, which in turn led to an increased exposure to radiation. Evidence for this is shown by peaks of radioactive beryllium (10Be) in ice cores recovered from the Greenland ice sheet. 

10Be is formed in Earth’s atmosphere through the collision of cosmic rays with atoms. 10Be has a half-life of 1.36 million years before decaying to 10Boron. Periods of high solar activity decrease the flux of cosmic rays that hit Earth, so the production of 10Be is inversely proportional to solar activity and increased solar wind. 

This geomagnetic excursion has been known about for 45 years, after the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central. The magnetisation of these lava flows differed significantly from the direction of the geomagnetic field today. This feature has since been known as the 'Laschamp event'. Before the latest work by the GFZ, the Laschamp event was shown only by point readings of the geomagnetic field during the last ice age. This new research gives a more complete view.

The sediment cores from the Black Sea not only show the geomagnetic excursion of 41,000 years ago, they also indicate several abrupt climate changes during the last ice age. These climate changes were already known from the Greenland ice cores, but now there is a high synchronisation between the data records from the Black Sea and Greenland. The sediment cores also document the largest volcanic eruption in the Northern hemisphere of the last 100,000 years: the eruption 39,400 years ago of a super volcano near Naples, Italy, known as the Campanian Ignimbrite super-eruption. About 350 cubic kilometres of rock and lava were expelled from the volcano and spread over the eastern Mediterranean and as far as central Russia. 

Interestingly, one of the Neanderthal extinction hypotheses involves climate change. During the last ice age, Europe changed into a semi-arid desert and it is speculated that Neanderthals did not adapt their hunting techniques to this new environment. Volcanic eruptions, such as the super eruption near Naples 39,400 years ago, also may have contributed to a reduction in food supply. 

Excavations at Mezmaiskaya Cave in the Caucasus Mountains of southern Russia showed there was also a reduction in plant pollen at the time. Two distinct layers of volcanic ash were observed in the cave, which coincided with large-scale volcanic events that occurred around 40,000 years ago. The first volcanic event is the Campanian Ignimbrite super-eruption. The second volcanic layer coincides with the end of Neanderthal presence at Mezmaiskaya and coincides with a smaller eruption thought to have occurred around the same time in the Caucasus Mountains.

NASA has more information on pole reversals here.


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