A study published on Scientific Reports and carried out by an international team lead by the University of Florence chemists Rita Traversi, Roberto Udisti, Mirko Severi and Silvia Becagli, has proved the reliability of nitrate's stratigraphy found in polar ice cores as an indicator to reconstruct the cosmic ray flux in past eras.
The research was part of EPICA - European Project for Ice Coring in Antarctica. ("The Laschamp geomagnetic excursion featured in nitrate record from EPICA-Dome C ice core", doi:10.1038/srep20235)"The indicators so far used to study the cosmic ray flux on Earth in past eras, cosmogenic radionuclides <sup>10</sup>Be (Beryllium 10) and <sup>14</sup>C (Carbon 14), have some limitations - explains Traversi, researcher in Analytical Chemistry at the Department of Chemistry "Ugo Schiff" - and the scientific community has been at work for some time to identify new markers".
The Florentine researchers - in collaboration with their colleagues Sami Solanski of Max Planck Institute for Solar System Research in Goettingen, Germany and Ilya Usoskin of the Department of Physics, University of Oulu, Finland - have verified the sensitivity of nitrate stratigraphy to cosmic ray flux proving their usefulness as indicators of a particular event, known as "Laschamp", a strong anomaly in the concentration profile of <sup>10</sup>Be occurred roughly 41,000 years ago and due to an important geomagnetic field excursion. This event has produced a weakening of the Earth screen towards cosmic rays that have therefore penetrated inside the atmosphere, this is demonstrated by the presence in the ice core of cosmogenic isotopes such as <sup>10</sup>Be.
"Laschamp was the most intense geomagnetic event of the last 50,000 years - continues Traversi - It caused a quasi-reversal of polarity of the geomagnetic field and has been observed since the 1980s in both hemispheres in numerous climatic archives, such as volcanic rocks, marine sediments cores and South and North polar ice cores. Nitrates found in the EPICA ice core showed a sensitivity to cosmic ray flux comparable to that of <sup>10</sup>Be during Laschamp and can be therefore used as markers to reconstruct past variations of solar activity".
The ice core, extracted at the Italian-French research base Concordia (in the Dome C site on the Antarctic plateau) with its 3,233 metres in length covers the widest time range ever obtained from such a natural archive, almost a million years, including the latest 9 glacial cycles. It has become in fact, a key source of paleoclimatic information on the duration and timing of glacial and interglacial periods.