Dendrochronological study of past volcanic activity with fossil petrified wood.

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Tree rings of petrified woods in volcanic sediments were analyzed to resolve a volcanic activity of 20 Ma ago with a relative accurcy of 1 yr. Field observations indicate that these petrified woods are the fossils of the trees that were carried away from their growth sites with volcanic breccia ejected upon explosive eruption and included in it. Using a dendrochronological procedure we measured the relative years of the last rings of the petrified woods. The relative years of the last rings are concentrated in 17 yrs, where the ring-width patterns show an anomalous behavior similar to one observed in firs growing in the area devastated by Mt.St.Helens. We suggest that the last-formed tree ring of a petrified wood is a record of volcanic eruption and that the activity of volcano continued over a period of 17 yrs, in which eruption occurred at least five times.A fossil wood found in volcanic sediments of the 1783-eruption of Asama volcano is used to test the usefulness of tree-ring records for high resolution dating of an eruption event. The dendrochronological dating is first applied by using the Chamaecyparis master chronology. High correlation is found at the position of 1783 AD as tree death though some other years have also high correlation. In order to overcome the difficulty to give uniqueness of dating the 14C dating was then applied. We extracted 14C age variation within the fossil wood, and then searched the matching position between this 14C age variation and the calibration curve of Stuiver and Becker (1993). Uncertainty of the calendar year of death of the fossil wood was greatly reduced by this procedure compared with the conventional method. In combination of the results of dendrochronological dating and of 14C dating it is suggested that the death of the fossil wood occured in 1783 AD, a date consistent with document records. Our result indicates the usefulness of tree-ring records for high resolution dating of an eruption event.