The boundary between the lower Pleistocene and the Middle Pleistocene, defined by the last geomagnetic reversal, dates to about 773,000 years ago. Yet currently there is no reference geological sequence (or stratotype) of this limit. Three sites are considered as candidates: Montalbano Jonico and Valle di Manche in Italy, and Chiba in Japan.
At Montalbano Jonico, in Basilicata, marine sediments were rapidly deposited, which supports a high time resolution of the stratigraphic analyses. Unfortunately, traditional paleomagnetic measurements are not an option due to a sediment remagnetization that occurred after the event of interest.
A team of researchers was able to bypass this difficulty by analyzing beryllium-10, produced by interaction between the atmosphere and cosmic rays. They were able to identify a twofold increase of the 10Be in the sedimentary section, due to a decrease in geomagnetic field intensity during the reversal of the Earth's magnetic field.
The volcanic ashes found on the site also allowed the researchers to accurately determine the absolute age of the signal by measuring the 40Ar/39Ar ratio signature, instead of indirectly, based on an "age model".
This study was carried out in collaboration with the European Centre for Research and Education in Environmental Geosciences (CEREGE) in Aix-en-Provence and Aldo Moro University in Bari, Italy.