QUATERNARY TEPHROCHRONOLOGY: PROGRESS, PROBLEMS, AND PROSPECTS

Tephrochronology is “a geological chronology based on the measuring, interconnecting, and dating of volcanic ash layers” (Thorarinsson, 1944). Its roots can be found in palynological and soil science investigations in South America, Iceland, Japan, and New Zealand during the 1920s and 1930s. The founding of its own INQUA commission in 1960, under the leadership of Professor K. Kobayashi, was an important milestone in that it provided an international forum for tephrochronological studies, fostering its application to the Quaternary Sciences. A distinctive tephra bed demarcates a synchronous horizon in the sedimentary record and facilitates accurate correlation of strata over distances that can approach continental dimensions. Such beds can be used to establish reliable, regional stratigraphic sequences with a relative chronology, as with fossils, but also chronometrically calibrated sequences, because their age can be determined by a variety of methods. Complications arise due to weathering of tephra beds and the ease with which they can be reworked into younger sediments. Confident identification of tephra beds requires a comprehensive array of physical and chemical criteria. Earlier studies focused on stratigraphic and sedimentological context, color, thickness, grain-size, mineralogy, morphology and refractive indices of glass and minerals, and elemental composition of bulk tephra and separates by wet chemical and XRF methods. Bulk chemical analyses must be considered suspect, however, because of the likely presence of lithic, xenocrystic, and detrital grains. For this reason, characterization and age determination are best done by grain-specific methods with high spatial resolution. Modern tephrochronological studies, therefore, commonly involve use of an electron microprobe for major-element analyses, laser ablation ICP-MS equipment for trace-element analyses, and single-crystal laser fusion 40Ar/39Ar (feldspar) or fission-track (glass, zircon) systems for dating, augmented by paleomagnetic data. These tools permit the comprehensive characterization of single glass shards and minerals as small as 50 microns, extending the reach of the tephrochronologic method, and opening up exciting possibilities for accurate linkages between terrestrial, marine, and ice-core records.