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THE SCIENCE CORNER

by Nigel Bunce and Jim Hunt College of Physical Science University of Guelph Thur. Apr. 19, 1984

TREE RINGS AND VOLCANOS

Since the eruption of Mt. St. Helens and El Chichon in Mexico in 1982, there has been a renewed interest among scientists on a possible connection between large volcanic eruptions and significant effects on the climate. Some volcanos inject sufficient material into the stratosphere that the amount of radiation received at the surface of the Earth from the Sun is materially altered. Large volcanic explosions deposit fine silicate ash and sulphur aerosols in the stratosphere, and these may remain there for some time.

Models of this type of event suggest that there would be an expansion of the region of circulating arctic air over western North America in January and a July weather pattern that would resemble those of a normal mid-May. One result of such weather would be to produce frosts in the height of the growing season in western North America.

Frost damage to mature trees is rare but can occur during the growing season with two successive nights at -5 C and days not above freezing. The freezing of water outside the immature growing cells crushes them and leaves a permanent record of frost in the tree ring for that year. In fact, the type of damage observed in the ring is different for the early part of the growing season and the late part. In a given tree ring, the date of the frost can sometimes be determined to within a week or two.

In several locations of the western United States lives the oldest known living thing on Earth: this is the Bristlecone pine (Pinus longaeva). In one location at Campito Mountain in the White Mountains of California, living trees and deadwood pieces provide an accurate year-by-year tree ring sequence back to 3435 BC, a continuous record for five and one-half thousand years!

V. C. LaMarche and Katherine Hirsckboeck have recently reported, in the magazine Nature, on a study of the frost damage in the rings of the Bristlecone pine. In the recent tree-ring records, they find a remarkable correlation between frost damage rings and the known date of large eruptions. For example, in the past 100 years, there have been four climactically important events: Krakatoa (1883), Pelee, Soufriere (1902), Katami (1912), and Agung (1963). In each case, a ring of frost damage was found and always in the same year if the eruption was early in the year or, otherwise, the next year. The frost ring never preceded the volcanic event, which seems to prove that the frost rings are the result of the eruption.

When the tree ring record is examined back in time, there are some interesting results. Seventeen events are found in the rings between 2035 BC and 1884 AD. Some of these are known from other paleontological or historical evidence. For example, Mt. St. Helens erupted in 1500 AD and 2035 BC and Mt. Etna in 42 BC.

Of particular interest is the explosion of Santorini or Thera in the Agean Sea. This great explosion has been suggested as the cause of the decline of the Cretan civilization about 1450 BC. However, radio carbon dating of material found on Thera has given 1688 BC within an uncertainty of 50 years. Now the tree ring frost tells us that Santorini exploded in 1626 BC or, at most, one or two years earlier; i.e., about 275 years before the sudden decline of Crete, which thus still remains a mystery.