WIGGLE-MATCH DATING OF THE LAST LARGEST ERUPTION OF TIANCHI VOLCANO

The Tianchi Volcano is located in the Jilin province, northeastern China and also on the border between China and North Korea, and lies within the active zone of the eastern margin of the Eurasia plate. Its characteristics and activities have been studied for many years (Wei et al., 2005). This volcano is a Cenozoic polygenetic central type, and experienced early basalt shield-building in the interval of 1.10-1.66 Ma, which was followed by a trachytic composite cone-building in the period 1.12-0.04 Ma. Most recently, the volcano produced comendite composite debris-formed sheet stages during the Holocene (Liu et al., 1998). Among these eruptions one large-magnitude, violent explosive Plinian eruption was found and suspected to have a similar magnitude as the 1815’s eruption of Tambora, Indonesia (Wei et al., 2005). A remarkable feature is that charcoal and carbonized wood are ubiquitous in the pumice fallout and in an associated ignimbrite from the area adjacent to Tianchi volcano. This paper shows a reliable high-resolution geochronology based on carefully-collected tree-ring samples from Yalujiang (in the south of Tianchi crater) and Dongfanghong (in the north of Tianchi crater) section that can allow the reconstruction of a clearer eruption history about this event. Previous radiocarbon dating was primarily based on single charcoal sample and gave larger ranges of calendar dates.

Two carbonized wood samples were collected from each site. 10-24 samples were cut from transects A and/or B in one tree, with 5-year or 10-year intervals, and numbered from edge to center. Tree-ring widths were measured and cross-dated. Almost 69 AMS ages were obtained from Yalujiang section. These dates allowed estimation of the time of the eruption with high precision by wiggle-match analysis to 935-942 AD. The 52 AMS ages for Dongfanghong section have been prepared, and data will be reported at the meeting. The primary 14 AMS dates of Dongfanghong site showed that the estimated age for volcanic eruption was 846-895 AD. There are significant discrepancies (69 years) between the two sites. Therefore, we believe this may record different eruptions, as opposed to the one previously-proposed event. This result was supported by the different growth pattern from the Dongfanghong site and the Yalujiang site.