Investigation of Deep-Sea Volcanic Ash Layers from Equatorial Pacific Cores

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Trace and minor element analysis of 128 volcanic ash samples from 56 cores taken in the eastern equatorial and southeastern Pacific Ocean has allowed identification and correlation of individual volcanic ash layers over wide areas of the ocean floor. Three principal areas of ash deposition have been delineated, with probable source areas in Guatemala, El Salvador, Nicaragua, and Colombia or Ecuador. Analyses of continental pumice samples indicate vents near the Tecpan-Chimaltenango basin in the Guatemalan Highlands and the Lake Ilopango region of El Salvador. Many, perhaps all, of the ash layers do not date beyond 300,000 yrs. The two most extensive (400,000 and 300,000 km2) and voluminous (43 and 19 km3) ash falls have been tentatively dated at 54,000 and 220,000 yrs, respectively. Together these ash layers comprise the so-called “Worzel Ash.” The most extensive of the two layers occurs in all three ash areas and may also correlate with an ash horizon in the Gulf of Mexico dated at approximately 60,000 yrs. The correlation of some ash layers with a sequence of subbottom reflectors suggests that the entire sequence of reflectors seen in the upper 100 m of sediment off Central America can be attributed to discrete layers of volcanic ash.