5:15 PM - 7:15 PM
[SVC34-P09] Eruptive Age of the Kirishima Miike Tephra and its Chronological Significance in Southern Kyushu, Japan
Keywords:Kirishima Volcano, Miike, 14C dating, tephrochronology
It is extremely important to study fallout tephra deposits in order to know the geological history of the area, because tephra is dispersed and deposited over a wide area in a short time. This contributes to the evaluation and understanding of long-term volcanic activity, and also plays a major role in paleoenvironmental and archaeological chronology. However, in order to fully demonstrate their effectiveness, it is necessary to accurately determine stratigraphic relationships and tephrochronological positions of each tephra by describing their lithologies in detail and dating the appropriate samples.
Miike tephra is one of the largest plinian deposit of Kirishima Volcano, which erupted from a maar about 1 km in diameter in the southeastern part of the volcano. The tephra has been dated by 14C dating from the soil immediately below and above the tephra and from peat layers, and is estimated to have erupted at 4.6 cal ka BP. On the other hand, the ages of charcoals from the interior of the tephra has not been reported. Paleosols are not likely to be closed systems because of bioturbation and other factors, so the 14C ages obtained may not necessarily reflect the actual age of the eruption, even if the soil sample is directly beneath the tephra. Here, we found the charcoal in the Miike tephra and dated them to 4078±28 BP by 14C dating. This age is consistent with previously reported ages of paleosols above and below the Miike tephra, so the eruption age of the Miike tephra is estimated to be 4.6 cal ka BP. The outcrop where the charcoal was collected is located about 0.8 km south of the Miike crater rim, and many red and high-vesiculated pumice, indicating high-temperature oxidation, were observed. In addition, there is no strong evidence of organic substances contamination due to disturbance of the layers after the tephra deposits has settled. Based on these characteristics, it is considered that this charcoal was carbonized by the deposition of this tephra under high temperature. Therefore, the reported age is considered to be quite accurate as the eruption age of the Miike tephra.
While the age obtained in this study are expected to be useful for tephrochronology in southern Kyushu, there are cases where tephra cannot be distinguished based on only age and lithology. For example, the distribution of this tephra overlaps with the P5 tephra from Sakurajima volcano near Fukuyama-cho, Kirishima, Kagoshima. Several Jomon-period sites have been discovered in this region, and identification of tephra is also important in dating the sites. As a result of a pliminary analysis, the phenocryst composition of the tephra may be used as an indicator of tephra discrimination, because the compositions of most orthopyroxenes in the P5 tephra and the Miike tephra are distinctly different, with Mg#63-66 and Mg#45-55 in the P5 and Miike tephra, respectively. Therefore, in order to perform accurate tephrochronology in the future, it is necessary to improve the data set of geochemical characteristics such as chemical composition and refractive indices, as well as lithological description and dating data.
Miike tephra is one of the largest plinian deposit of Kirishima Volcano, which erupted from a maar about 1 km in diameter in the southeastern part of the volcano. The tephra has been dated by 14C dating from the soil immediately below and above the tephra and from peat layers, and is estimated to have erupted at 4.6 cal ka BP. On the other hand, the ages of charcoals from the interior of the tephra has not been reported. Paleosols are not likely to be closed systems because of bioturbation and other factors, so the 14C ages obtained may not necessarily reflect the actual age of the eruption, even if the soil sample is directly beneath the tephra. Here, we found the charcoal in the Miike tephra and dated them to 4078±28 BP by 14C dating. This age is consistent with previously reported ages of paleosols above and below the Miike tephra, so the eruption age of the Miike tephra is estimated to be 4.6 cal ka BP. The outcrop where the charcoal was collected is located about 0.8 km south of the Miike crater rim, and many red and high-vesiculated pumice, indicating high-temperature oxidation, were observed. In addition, there is no strong evidence of organic substances contamination due to disturbance of the layers after the tephra deposits has settled. Based on these characteristics, it is considered that this charcoal was carbonized by the deposition of this tephra under high temperature. Therefore, the reported age is considered to be quite accurate as the eruption age of the Miike tephra.
While the age obtained in this study are expected to be useful for tephrochronology in southern Kyushu, there are cases where tephra cannot be distinguished based on only age and lithology. For example, the distribution of this tephra overlaps with the P5 tephra from Sakurajima volcano near Fukuyama-cho, Kirishima, Kagoshima. Several Jomon-period sites have been discovered in this region, and identification of tephra is also important in dating the sites. As a result of a pliminary analysis, the phenocryst composition of the tephra may be used as an indicator of tephra discrimination, because the compositions of most orthopyroxenes in the P5 tephra and the Miike tephra are distinctly different, with Mg#63-66 and Mg#45-55 in the P5 and Miike tephra, respectively. Therefore, in order to perform accurate tephrochronology in the future, it is necessary to improve the data set of geochemical characteristics such as chemical composition and refractive indices, as well as lithological description and dating data.