HR: 08:30h
AN: PP31F-03    [Abstracts]
TI: 14C-Dating and Paleoclimate Reconstruction Using the Holocene Tufa Developed in Ehime Prefecture, Southwestern Japan.
AU: * Hori, M
EM: horizon@hiroshima-u.ac.jp
AF: Hiroshima University, Kagami-yama 1-3-1, Higashi-Hiroshima, 739-8526, Japan
AU: Kano, A
EM: kano@geol.sci.hiroshima-u.ac.jp
AF: Hiroshima University, Kagami-yama 1-3-1, Higashi-Hiroshima, 739-8526, Japan
AU: Okumura, K
EM: kojiok@hiroshima-u.ac.jp
AF: Hiroshima University, Kagami-yama 1-2-3, Higashi-Hiroshima, 739-8522, Japan
AB: Tufas are freshwater carbonates widely developed in limestone areas under temperate and tropical climates. The stable isotopic and the other chemical properties of the tufas changes with climatic conditions. Because of the relatively high depositional rate and development of annual lamination, the tufas can be ideal paleoclimatic archives that supply high-resolution records on the past temperature and precipitation change. We have analysed 1.45 m-thick paleo-tufa section along a stream in Seiyo City in Ehime Prefecture, SW Japan. We determined the ages of the deposits by carbonate ¹⁴C because the section lacks suitable material for ¹⁴C dating of organic carbon and for U-Th dating. We collected ten samples from the section and measured the ¹⁴C composition by benzene-liquid scintillation counting. To evaluate initial value of dead carbon proportion (dcp), we also analyzed the recent tufa. The paleo-tufa section appears a clear discontinuity that devides the section into the lower part (sample of ps-1 to 4, 80 cm thick) and the upper part (ps-5 to 10, 65 cm). Annual lamination of 1-3 mm thick is clearly developed in the upper part, but obscure in the lower part. The ¹⁴C value of the recent tufa is -37.7 ‰ (vs. NIST) and corresponding to the apparent age of 309 B.P. On the other hand, the 14C values of the paleo-tufas range from - 410.7 ‰ in the lowermost sample (ps-1) to -242.6 ‰ in the uppermost sample. Assuming that initial dcp of the recent tufa can be applied to the paleo-tufa samples, the 1.45 m-section was considered to have been deposited from 3940 to 1900 B.P. The assumed age of the sample below the discontinuity (ps-4) was 3300 B.P. This suggests that the depositional rate of the 80-cm-thick lower part was approximately 1 mm/year. Both of oxygen and carbon stable isotopic values increased from the oldest sample (-7.30 and -10.96 ‰, respectively) to the youngest sample (-6.99 and -6.85 ‰, respectively). This might be ascribed to the cooling trend in this period. Carbon stable isotopic value of the recent tufa is as low as -9.87 ‰ that may indicate a warmer condition than 1900 B.P. However, a slightly higher oxygen isotopic value (-6.68 ‰) of the recent tufa may have resulted from cooling if the water isotopic value have been stable in the last thousands years.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1115 Radioisotope geochronology
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting