Volcanic ash deposits in the Gulf of Alaska and problems of correlation of deep-sea ash deposits

Abstract

The study of sediment cores from the Gulf of Alaska reveals the occurrence of three distinct ash layers or sediment intervals abundant in volcanic ash. The refractive index of the glass fragments of the upper layer is 1.485 ± 0.002. The ash is composed of 95% glass with an approximate silica content of 74%. The Katmai volcanic eruption of 1912 is considered to be the source of the ash which is more widespread than previously described. The middle ash is dark brown and of basaltic composition with a refractive index of 1.565. It occurs in some areas as a distinct layer and in others, mixed with non-volcanic sediment. The physical properties, stratigraphic relationships and composition suggest that the ash in this unit is derived from a common but unknown source. The lower layer of volcanic ash, approximately andesitic in composition, is recorded in two cores. The age of this layer, as determined by radiocarbon dating of carbonate sediments above and below the ash is inferred to be older than 25,000 years. The petrography of these deposits is discussed. Included in this study are brief discussions of deep-sea ash deposits in the north Atlantic Ocean, Mediterranean Sea, Gulf of Mexico and equatorial Pacific Ocean (Worzel ash). Some of the famous volcanic eruptions of historic record are briefly reviewed such as the Tambora in the East Indies, the Volcan Quizapú in Chile, the Hekla eruption of 1947 in Iceland, and the Glacier Peak eruption of 6,700 years ago in the northern Cascades, U.S.A. Some of the problems of correlation of deep-sea ash deposits are discussed. The possible world-wide occurrence of Worzel deep-sea ash ( Ewing et al., 1959) and the cometary origin of the ash is discounted.