Abstract

Three distinct volcanic-ash layers were identified in peat deposits through visual, microscopic, and chemical means. These layers were related to known volcanic events through the 14C dating of 34 peat samples. The upper layer was probably Bridge River ash, deposited about 2350 years BP. The middle layer was possibly St. Helens Y tephra, deposited around 3400 years BP. The lower ash was related to Mazama tephra, deposited about 6600 years BP. Ash-enriched layers were formed in the accumulating peat as aerial deposition of tephra occurred, often for several hundred years before and after the presumed main eruption, as shown by 14C dates and peat accumulation rates. This implies periodic eruptions by the volcanoes, not all of which produced distinct ash layers in areas distant from the source. Redeposition from wind-eroded beds of Mazama ash during the dry postglacial climatic maximum is a possibility.

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