Tephrochronology at Sites 502 and 503

Four hydraulic piston cores at Site 502 and two at Site 503 were analyzed for dispersed tephra.Peaks in dispersed tephra abundance were combined with megascopic ash layers to produce a tephrochronology based on the magnetostratigraphy and biostratigraphy for the section at each site.The frequency of tephra deposition at Site 502 increased in the late Miocene and Quaternary and may be related to the subduction rates of the Cocos Plate under Central America or major changes in wind direction over Central America.The much-less-complete section at Site 503 yielded only a few dispersed tephra layers for the same time interval, because the site was far from sources of ash until the middle Pliocene-when the movement of the plate brought the site into the dispersed ash-fall zone surrounding Central America.