堀 真子

We report here hourly variations of Mg/Ca, Sr/Ca, and Ba/Ca ratios in a Mediterranean mussel shell (Mytilus galloprovincialis) collected at the Otsuchi bay, on the Pacific coast of northeastern Japan. This bivalve was living in the intertidal zone, where such organisms are known to form a daily or bidaily growth line comprised of abundant organic matter. Mg/Ca ratios of the inner surface of the outer shell layer, corresponding to the most recent date, show cyclic changes at 25-90 mu m intervals, while no interpretable variations are observed in Sr/Ca and Ba/Ca ratios. High Mg/Ca ratios were probably established by (1) cessation of the external supply of Ca and organic layer forming when the shell is closed at low tide, and (2) the strong binding of Mg to the organic layer, but not of Sr and Ba. Immediately following the great tsunami induced by the 2011 Tohoku earthquake, Mg/Ca enrichment occurred, up to 10 times that of normal low tide, while apparent Ba/Ca enrichment was observed for only a few days following the event, therefore serving a proxy of the past tsunami. Following the tsunami, periodic peaks and troughs in Mg/Ca continued, perhaps due to a biological memory effect as an endogenous clock.

In the East Asian monsoon area, stalagmites generally record lower and higher oxygen isotope (delta O-18) levels during warm humid interglacial and cold dry glacial periods, respectively. Here, we report unusually low stalagmite delta O-18 from the last glacial period (ca. 32.2-22.3 ka) in Fukugaguchi Cave, Niigata Prefecture, Japan, where a major moisture source is the East Asian winter monsoon (EAWM) that carries vapor from the warm surface of the Japan Sea. The delta O-18 profile of this stalagmite may imply millennial-scale changes, and high delta O-18 intervals that are related to Dansgaard-Oeschger (D-O) interstadials. More importantly, the stalagmite exhibits low overall delta O-18 values; the mean delta O-18 (- 8.87 parts per thousand) is distinctly lower than the mid-Holocene mean of another stalagmite from the same cave (4.2-8.2 ka, - 7.64 parts per thousand). An interpretation assuming a more intense EAWM and greater vapor transportation during the last glacial period, compared with the mid-Holocene, contradicts the limited inflow of the Tsushima Warm Current into the Japan Sea because of lowered sea level. Additionally, our model calculation using delta O-18 data from meteoric water indicated that the amount effect of winter meteoric water was insignificant (1.2 parts per thousand/1000 mm). Low stalagmite delta O-18 for the last glacial period in Fukugaguchi Cave most likely resulted from O-18-depleted surface water, which developed in the isolated Japan Sea. The estimated amplitude of the delta O-18 decrease in surface water was similar to 3 parts per thousand at most, consistent with the abnormally low values for foraminifera (by similar to 2.5 parts per thousand) in sediment during the last glacial period, shown by samples collected from the Japan Sea. This is the first terrestrial evidence of O-18 depletion in Japan Sea surface water during the last glacial period.

Our understanding of stalagmite delta O-18 paleoclimate records would be significantly improved by determining the relative influence of the two factors, temperature and water delta O-18 Here we apply carbonate clumped isotope thermometry to a well-dated (18.1-4.5 ka) stalagmite, Hiro-1 from Maboroshi Cave in Hiroshima Prefecture, southwestern Japan. The Delta(47) values from 50 stalagmite layers, calibrated using Delta(47) -temperature relationships observed in modern stream tufa from Japan (Kato et al., 2019) yield estimates of paleo-temperature and water delta O-18 variability. Several layers affected by significant prior calcite precipitation under a dry climate display strong kinetic isotope influences: positive delta O-18 and negative Delta(47) deviations. Except for these layers, Hiro-Delta(47) records temperature changes that are broadly consistent with climatic stages after the last glacial maximum (LGM) to the mid-Holocene, although the significant Bolling-Allerod warming does not appear in Delta(47) values. The estimated temperature difference between the period after the LGM (18.0-16.0 ka) and mid-Holocene (7.7-4.9 ka) was 8.3 degrees C. Using Delta(47) temperature, stalagmite delta O-18 values, and change in seawater delta O-18(SW), the comprehensive isotopic depletion from vapor source (seawater) to meteoric water was reconstructed. We obtained larger 18(O)-depletion (-9.5 to -10.0 parts per thousand) in the pre-Holocene section than in the middle Holocene section (around -8.5 parts per thousand). The larger 18O(-)depletion of the pre-Holocene period was ascribed to a larger fractionation during vapor generation under a lower temperature, change in seasonality of precipitation, or a longer vapor trajectory due to the subaerially exposed Seto Inland Sea. Unlike Chinese stalagmites, delta O-18 record of Hiro-1 stalagmite appear largely unaffected by rainfall amount. (C) 2020 The Author(s). Published by Elsevier Ltd.