Magnetostratigraphy and Radiometric Chronology

This paper briefly reviews the history and present status of magnetostratigraphy, and re-examines the usefulness of magnetostratigraphy in absolute chronology. At present, two different kinds of time scale are used as standards for magnetostratigraphic age determination. The first is that time scale developed from a combination of paleomagnetic polarity data and radiometric age dates derived from volcanic rock sequences on land. The second is based on the interrelationships between the sequence of geomagnetic reversals and marine planktonic microfossils. Usually, these time scales are assumed to be equivalent.Based on the collection of radiometric dates made on magneto- and biostratigraphically-studied sedimentary sections, the relationships between the geomagnetic-radiometric and geomagnetic-microbiostratigraphic time scales were examined. A distinct discrepancy was noticed between the radiometric age and the estimated absolute age from the magnetic-microbiostratigraphic scale, as indicated in Fig. 5. This discrepancy is worthy of serious consideration by uses of the geomagnetic time scale in absolute chronology. Further examination of various methods of measurement is needed to increase the reliability of magnetostratigraphy, microbiostratigraphy and radiometric chronology.In Japanese marine sedimentary sections, there exist many pyroclastic intercalations that may provide suitable materials for such examinations.