Researchers have identified a new method of precisely dating key prehistoric events using traces left by violent solar storms on trees.

A solar flare seen from the Solar Dynamics Observatory in 2012.

A developing branch of astrochronology — the study of the chronology and periodicity of celestial bodies — the new method records the specific timing of solar storms caused by eruptions on the sun’s surface.  When these rare storms occur on the earth-facing side of the sun, they leave a mark on every tree growing at the time, anywhere in the world.  Michael Dee and Benjamin Pope of the University of Oxford hope to use these ‘hidden clocks’ to tie known archaeological chronologies to exact calendar dates.

The new method relies on two well-established and complimentary archaeological dating techniques: dendrochronology and radiocarbon dating.  Dendrochronology records relative dates using the annual growth rings of trees, and radiocarbon dating compares the ratios of two isotopes of the element carbon, C-14 and C-12, to date organic material.  C-14 is a radioactive isotope which decays at a known rate after an organism dies, while C-12 isotopes remain stable.  The ratio between the two can indicate the date of an ancient ship timber, a scrap of papyrus, or a human bone, for example.  The only problems are that radiocarbon dating is expensive and often imprecise, with an error range of 50-100 years.  On the year of a sun storm, however, the amount of C-14 radioisotopes is remarkably higher — up to 20 times the normal level — creating a unique blip on a tree ring.

The real utility of the new dating technique is in relating so-called ‘floating chronologies’ such as those of the ancient Egyptian kingdoms and Mayan civilisation.  The sequence and general reign length of Egyptian kings in the Old Kingdom are well-established, for example, but the discovery of a few sun storms during the course of the third millennium BC would help pin down exact calendar years and allow archaeologists to check that the years between kingdoms add up without gaps.  Two ancient solar storms have already been identified in AD 775 and AD 994, but the laboratory team are employing complicated mathematical techniques to search for more spikes.

Image: NASA Goddard Space Flight Center

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