Tephrochronology in its broadest definition is a means of dating or correlating sedimentary signals of (paleo)environmental change. Rapid deposition of tephra on a wide variety of environmental surfaces, including lake beds, mires, soils, ice sheets, and deep and shallow ocean floors, ensures that tephra layers frequently define isochronous surfaces. In the terrestrial Japanese record, widespread tephras have been identified, inter alia, within archaeological horizons, soil horizons, interglacial raised beach deposits, lake sediments, and offshore (see Machida, 1999). Correlation of tephras is frequently heavily reliant upon geochemical analysis of component glass shards that can link tephra, uniquely, to a particular eruption of an identifiable volcano. In this manner tephras may possess a geochemical fingerprint. These geochemical data usually provide the highest degree of certainty that is possible in establishing correlation, particularly where continuous stratigraphic mapping is not possible. Firm correlation should only be made, however, where additional independent evidence such as absolute chronology, biostratigraphy, stratigraphical associations, grain size, and mineralogy is shown to be compatible (see Westgate and Gorton, 1981) and where investigations are of sufficient detail and coverage to be confident that discrete geochemical signatures are not shared by multiple eruptives produced hundreds or thousands of years or more apart. This geochemical repetition can be termed geochemical equifinality. Complete confidence in geochemical uniqueness is seldom wholly assured; however, it is highly desirable that its potential is evaluated prior to assigning ages to, or correlating between, analyzed tephras.
This pilot investigation of the tephra record from the Japan Trench focuses on sediments sampled from the Pliocene-Pleistocene sequence (Tables T1, T2) to evaluate the effects of alteration, broad geochemical changes, and the potential influence of differing modes of preservational style on the tephra record. At this initial stage, the criteria outlined above and required for rigorous tephrostratigraphical correlation have not been adhered to but await more detailed investigation. Therefore, this research cannot and should not be used to erect a chronological framework for the Pliocene-Pleistocene. It may be possible to suggest potential correlation to tephra isochrons defined in earlier studies, but these correlations must be treated with caution.