1. Granitic anchor stones revealing contribution of Muslim traders to the second Genko in 1281
Genko (Mongol campaign against Japan in 1274 and 1281) is one of the most famous historical events in Japan. However, the question why Genko occurred still remains unclear despite extensive research works on the basis of historical documents. The second campaign was organized with c. 100,000 southern troops sailed by 3,500 ships from Chingyan, China and c. 35,000 Mongolian and Korean troops sailed by 900 ships from Hapu, Korea. Therefore, the important factor on this campaign is shipbuilding and navigation capacity. Identification of the shipbuilding may provide a new vista to solve the purpose of Genko. Because rocks used as anchor stones were possibly excavated from quarries near to the shipbuilding site, knowledge of the provenance of anchor stones enables us to place the building site.
Fig.1 Wooden anchor excavated form the submerged site off Kozaki port of Takashima-cho, KitaMatsuura-gun, Nagasaki Prefecture (offered from Takashima-town Archeological Center).
Wooden anchors were found and recovered from the submerged site off Kozaki port on Takashima Island in the Imari bay where Mongolian fleet encountered a fierce typhoon and wrecked during the second invasion. The anchors consist of red oak shank and flukes and bamboo rope with a pair of prismatic stones as the anchor stock. The largest one is 6 meters in length. This size of anchor might be mounted on a large ship of 40 m-long class. The calibrated ages of 1268-1284 cal. AD for bamboo rope and 1257-1279 cal. AD for wooden shank confirm that the wooden anchors are remains of wrecked Mongolian ships.
The anchor stones from different anchors are macroscopically and microscopically very similar, and consist of coarse-grained alkali-feldspar granite. The CHIME (U-Th-total Pb CHemical Isochron Method) zircon ages are 110±1, 108±3 and 108±2 Ma. Alkali-feldspar granites of c. 110 Ma age develop in the Quanzhou area of Fujian, southeastern China.
Fig.2 Plot of PbO vs. ThO2* for zircons in TS-08 granitic anchor stone. ThO2* represents sum of the measured ThO2 and ThO2 equivalent of the measured UO2.
Fig.3 Distribution of Jurassic and Cretaceous granitic rocks and pyroclastic rocks in southeastern China and southern Korean peninsula.
The building site of the wrecked ships including the large one of 40 m-long class was Quanzhou. Quanzhou was one of the largest trading ports in the world from the end of Nansong era to Yuan era and the base port of Muslim traders under the command of Pu Shou-Geng.
The present provenance study of anchor stones has brought considerable insight into that Muslim traders contributed significantly to the second Genko by providing ships and seamen. The second Genko in 1281 seems to be a combined action of the Yuan dynasty and the Muslim trader for the purpose of expanding the Silk Road via inland and overseas routes on the basis of Japanese gold and silver.
- Suzuki, K., Karakida, Y. and Kamada, Y., "Provenance of granitic anchor
stones recovered from the Takashima submerged site: an approach using the
CHIME method for dating of zircons," Proc. Japan Acad. 76 (2000) 139-144.
- Suzuki, K., Oda, H., Ogawa, M., Niu, E., Ikeda, A. and Nakamura, T., "AMS
14C dating of wooden anchors and planks excavated from submerged wrecks located
at Takashima in Imari Bay, Nagasaki Prefecture," Proc. Japan Acad.
77 (2001) 131-134.
2. A rock avalanche triggered by Tensho (1586) earthquake forming the legendary lake "Umi" in southeastern Gifu Prefecture
In the mountainous Kamiyahagi area of Ena city, southeastern Gifu prefecture, there is an area called "Umi" which means "sea" or "large lake". The area is a small river terrace along the Kamura River that flows through V-shaped valleys between mountain ranges with elevation of 1000 to 1270 m. There is no information about the origin of the name "Umi" in neither historical document nor folklore. Several families inhabited the river terrace during the Meiji and Taisho eras. In those days, the area had already been called "Umi." An old man said, "My grandfather and parents said that they wondered why it was called "Umi."
After the Tokai Gou (torrential rainfall in Tokai region) in 2000, stratified silt (clastics smaller than sand and larger than clay) layers typical of a lacustrine setting emerged from eroded riversides. This becomes the first solid evidence for the existence of a dammed lake in the Umi area. Because the stratified silt deposited directly on fallen leaves or squashed moss on regolith and coarse detritus typical of stream is absent, rock avalanche that dammed the river was not associated with a heavy rainfall.
The 14C ages of trees and moss excavated from the bottom of the silt layer are 1517 -1634 cal. AD. Therefore, the rock avalanche at "Umi" was possibly triggered by Tensho Earthquake of January 18, 1586 (13th year of the Tensho era).
The Tensho Earthquake, the largest inland seismic event with an estimated magnitude of 8.0-8.1 in history of central Japan, deeply affected a wide area including Tokai, Hokuriku and Kinki districts. There remains scant documentary evidence on the earthquake in the southeastern part of Gifu Prefecture and Nagano Prefecture. However, descriptions of damage during historic earthquakes are usually limited to major centers of population, culture and trading. Because large landslides occur normally in the area with the seismic intensities greater than 6, the discovery of a huge rock avalanche at Umi indicates that the disaster area of the Tensho Earthquake could extend further southeast than was previously thought. The revised disaster area is consistent with an epicenter in Ise bay, and supports the view that the Yoro and Isewan (Ise Bay) faults located to the west of Nagoya were responsible for the violent earthquake. From the enlarged disaster area, the magnitude of the Tensho Earthquake can be estimated to be over 8.1.
Fig.4 Location of Umi and estimated extent of the paleo-lake Umi (blue) on the Kamura River in the Kamiyahagi area. Base map id from the Yokomichi 1:25,000 Geographical Quadrangle Map published by the Geographical Survey Institute of Japan.
Fig.5 Distribution of seismic intensity of the Tensho Earthquake on January 18, 1586 (simplified Iida, 1987). Dots shows areas where seismic intensities were recorded in histrical documents. The red line defines the enlarged disaster area.
- Suzuki, K., Ikeda, A., Adachi. K., Dunkley, D., Niu, E., Kato, T., Mori,
S. and Fujii, N. "Finding of the legendary lake Umi in the Kamiyahagi
area of Gifu Prefecture; a lake-damming rock avalanche triggered by the
Tensho Earthquake in 1586," Proc. Japan Acad. 78 (2002) 111-116.
3. Development of age mapping method of monazite and zircon grains
For provenance study of stones used for prehistorical and historical artifacts, an important key will be provided by chronological data in addition to petrographical and geochemical characteristics of individual rocks. Most rocks contain U- and/or Th-bearing monazite and zircon grains in trace amounts. We have measured monazite and zircon ages by the CHIME method on the basis of precise determination of Th, U and Pb concentrations at micrometer scale by an electron-probe-micro- analyzer. Normally monazite and zircon occur in 0.03-0.2 mm size grain, and grains in particular rocks show evidence of multiple-growth. Because the geometry and distribution of age domains within a single grain differs between rocks that show almost the same monazite and/or zircon ages, visualization of age domains provides a significant advantage in provenance study.
For age mapping, it is necessary to scan the whole area step by step and to measure Th, U and Pb X-rays at individual pixels. We use JEOL JCXA-733 manufactured in 1984. It has 4 wavelength dispersive spectrometers with a 140 mm radius of the Rawland circle, and individual spectrometers are equipped with a PET (002 pentaerythritol) diffraction crystal and a sealed Xe X-ray detector. On the basis of the intrinsic response (0.37-0.32 cps/nA⋅wt% for PbMα at beam energy of 15 kV of the detector, a dwell tine of 0.5 to 1 s is sufficient for old minerals that contain sufficient amounts of Pb. However, the dwell time ranges from 50 to 100 s for monazite in Jurassic to Cretaceous rocks widespread in the Japanese Islands and the eastern margin of Asian continent. Alternatively, we use 3 spectrometers for the Pb measurement with a beam current of around 600 nA, and the rest spectrometer for acquiring a background map. Thus, the dwell time for Pb determination can be reduced from 50-100 to 2-3 s. The Th and U X-rays are measured with a dwell time of 2-3 seconds prior to the Pb measurement. We further shorten the measurement time by skipping the area outside the desired mineral. The age mapping technique can highlight the geometry and distribution of age domains as well as compositional domains within a single mineral grain.
Fig.6 Age mapping method of monazite and zircon grains
- Asami, M., Suzuki, K. and Grew, E.S., "Monazite and zircon dating
by the chemical Th-U-total Pb isochron method (CHIME) from Alasheyev Bight
to the Sør Rondane Mountains, East Antarctica: A reconnaissance study of
the Mozambique suture in eastern Queen Maud Land," Jour. Geol. 113
- Kato, T., "New accurate Bence-Albee a-factors for oxides and silicate
calculated from the PAP correction procedure," Geostandard and Geoanalytical
Research 29 (2005) 83-94.
- Suzuki, K. "CHIME (Chemical Th-U-total Pb isochron method) dating on the basis of electron microprobe analysis" Jour. Geol. Soc. Japan, 111 (2005) 509-526.
- Suzuki, K., Dunkley, D., Adachi, M., and Chwae, U., "Discovery of
a c.370 Ma granitic gneiss clast from the Hwanggangri pebble-bearing phyllite
in the Okcheon metamorphic belt, Korea," Gondwana Research 9 (2006) 85-94.