Tephra study in the Lake Biwa

Details Tephra study in the Lake Biwa Tephra study in the Lake Biwa

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v23a0624f&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #V23A-0624

Physics

8400 Volcanology, 8404 Ash Deposits, 8450 Planetary Volcanism (5480), 9320 Asia, 0370 Volcanic Effects (8409)

Scientific paper

Lake Biwa is one of important reservoirs for the volcanic ash in Japan. During 1982-1983, the 1422 m core was drilled in southern basin of the lake (35øX13'06"N, 136øX00'49"E). The most upper part of core was a continuous 250 m thick lacustrine clay and dated as within 430 ka (Meyers et al.,1993; Takemura, 1990). Total of 18 tephra layers were distinguished from the upper 640 m of the core. However, the fore 14 tephra layers were from the most upper 250 m. Pure glass shards were handpicked out from each tephra layer under the microscope and analyzed by the electron microprobe in the IES for chemical compositions. The K2O vs. SiO2 plot of the glass shards was used to verify the source of each tephra layer. Tephra Layer-2 at depth 13.5m with lower SiO2 and higher K2O was supposed from Ulreung-Oki in southern Japan Sea. Furuta (1986) claimed that glass shards of the caldera eruptions in central Kyushu revealed the high alkali contents (K2O + Na2O = 8.8wt%). Therefore, the tephra Layer-4 (K2O =4.6wt%) at depth of 66.85 m can be corresponded to the Aso-4 eruption well, while the tephra Layer-10 (K2O =4.8wt%) and tephra Layer-11 (K2O =4.98wt%) at around 169.2-170.9 m depth can be identified as the product of the Aso-1 eruption. Comparison of chemical compositions of glass shards in tephra layers in Lake Biwa with those in marine cores around Japan (Machida, 1999), they clearly showed that tephra Layer-1 at depth of 12.9 m, tephra Layer-5 at depth of 74.5 m, and tephra Layer-9 at depth of 158.6 m can be correlated with the caldera eruptions of K-Ah, Ata and Ata-Th in southern Kyushu, respectively. However, some tephra layers with lower K2O content (<4 wt%) are hard to distinguish their sources. Advanced isotopic analyses are needed to distinguish their sources in the future.

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