Tree Story by Valerie Trouet

Author:Valerie Trouet
Language: eng
Format: azw3, epub
Publisher: Johns Hopkins University Press
Published: 2020-04-14T18:30:00+00:00

Figure 15 The 774–75 solar flare was recorded as a radiocarbon spike in the 775 ring of trees across the world. In ice cores, it was recorded as a spike in beryllium-10 levels. However, this spike in ice-core records was recorded in the 768 layer, revealing a 7-year dating error in the pre-1250 part of the ice-core record.

The sun constantly produces radiation that creates cosmogenic isotopes in the earth’s atmosphere. However, solar flares strong enough to result in C14 peaks such as seen in the 775 and 994 tree rings are rare, and the 774 superflare is estimated to have been the strongest over the past 11,000 years. The 774 event was recorded in the eighth-century section of the Anglo-Saxon Chronicle* as follows: “Annus Domini 774. . . . This year also appeared in the heavens a red crucifix, after sunset; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons.” Even though this particular chronicler seems to have been charmed by red crucifix auroras and wonderful serpents, the great infrequency of such massive solar storms is a good thing; they potentially deplete Earth’s ozone layer, disrupt its geomagnetic field, and seriously mess with our technology and telecommunication systems.

In addition to radiocarbon, beryllium-10 (Be10) is formed in the atmosphere when it is bombarded with the sun’s rays. Unlike C14, which is not typically captured in ice cores,† atmospheric Be10 is deposited in the snow and ice layers of Greenland and Antarctica. In the same way that sulfate peaks in ice cores can be used as a proxy for volcanic activity, Be10 peaks can be used as a proxy for the sun’s activity. Solar superflares, such as the ones in 774–75 and 994, create Be10 spikes in ice cores, which can be used to directly link the date of an ice layer to the date of a related C14 peak in the tree-ring record.

When Michael Sigl and his colleagues measured Be10 concentrations in ice cores from both Greenland and Antarctica, they found a Be10 peak in 768 and another peak in 987. Both peaks occurred exactly seven years prior to the tree-ring C14 peaks of 775 and 994, suggesting a seven-year offset in this early section of the ice-core record (see fig. 15). This seven-year offset explains the seven-year discrepancy between early volcanic eruptions in the ice-core record and cool years in the tree-ring record. Sure enough, when Michael and his team shifted the pre-1257 section of the ice-core chronology seven years toward the present, all of a sudden the ice-core and tree-ring data lined up perfectly. Fifteen of the sixteen coldest years in the tree-ring record for the period 500 BCE to 1000 CE now followed large volcanic sulfate peaks in the ice-core record by 1 to 2 years. This is how Joe McConnell describes the breakthrough in a Los Angeles Times interview: “Before this work, the tree rings and the ice core records diverged. In the new dating, they line right up.

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