[M-IS19] Paleoclimatology and paleoceanography

[MIS19-18] Terrestrial paleoclimate since 80 ka recorded in stable and clumped isotopes of two stalagmites obtained from the Pacific side of the Japanese islands

*Shota Amekawa¹, Hirokazu Kato¹, Akihiro Kano¹, Kenji Kashiwagi², Taiki Mori³, Ryoko Senda³, Chuan–Chou Shen⁴ (1.Graduate School of Science, The University of Tokyo, 2.Graduate School of Science and Engineering for Research, University of Toyama, 3.Graduate School of Integrated Sciences for Global Society, Kyushu University, 4.Department of Geosciences, National Taiwan University)

Keywords:clumped isotope, speleothem paleoclimatelogy

Terrestrial paleoclimate during late Quaternary has been broadly reconstructed by oxygen isotope (δ¹⁸O) of stalagmites. Previous studies have pointed out that stalagmite δ¹⁸O is largely associated with change in δ¹⁸O of meteoric water due to amount effect and shift of vapor sources, but also can be influenced by temperature change. One of solutions for evaluating the temperature effect is carbonate clumped isotope thermometer. Clumped isotopes are isotopologues in which rare isotopes are doubly or multiply substituted. In case of CO₂ generated from reaction of carbonate and phosphoric acis, the measuring target is ¹³C¹⁸O¹⁶O (mass of 47). Previous studies have shown that measured abundance of clumped isotopes is slightly more than the stochastic abundance and its excess or “clumping” (Δ₄₇) comes from thermodynamic stability, which allows us to reconstruct temperature during carbonate precipitation only by Δ₄₇ of the generated CO₂. In this study, Δ₄₇ and stable isotopes are applied to two stalagmites from the Pacific side of the Japanese islands, which should not be affected by vapor from Indian Ocean and Japan Sea.
Two analyzed stalagmites (KA01 and 03) were obtained from Taiki town, Mie prefecture, Japan. 32 U-Th age points determined at National Taiwan University reveal that KA01 was dated from ca. 14 ka to ca. 1 ka and KA03 (Mori et al., 2018) was from ca. 80 ka to ca. 20 ka as well as the Holocene. δ¹⁸O changes of KA01 and KA03 show similar trends with the cave records in south Chinese (e.g. Hulu cave), but the amplitude of the δ¹⁸O changes was reduced in half.
Results of our Δ₄₇ values require reconsideration of the previous interpretation for the Chinese stalagmite δ¹⁸O, dominant control by δ¹⁸O of meteoric water. A reconstructed temperature at the top of KA01 is consistent with modern temperature at the cave. The whole trend of reconstructed temperature combined with δ¹⁸O of modern rainfall indicates that δ¹⁸O changes of KA01 are mainly controlled by temperature changes instead of rainfall δ¹⁸O. Δ₄₇ of KA03 yields higher temperature during cool periods of Heinrich events (HE), which apparently contradicts with previous studies. Before and after HEs, the carbon isotopic value increases by 0.5–1 ‰, indicating degassing of ¹³C-depleted CO₂ from dripwater and carbonate precipitation in the conduit of the host rock because of reduced water supply. Drying during HEs could have made the surface of the stalagmite dry and resulted in apparent contradictory temperature. This study verifies that δ¹⁸O change should be used together with Δ₄₇ for robust interpretation of the stalagmite records.