17:15 〜 18:45
[MIS12-P08] ハマグリの貝殻を用いた高解像度古環境解析:種子島の完新世の水温変動と生活環境への影響
The shells of bivalves are primarily composed of calcium carbonate, and their oxygen isotope ratio (δ18O) serves as a proxy for water temperature, widely used for reconstructing ancient environments. Within the shell, the fine growth lines are engraved, potentially preserving a daily record of the environmental conditions and growth trends experienced during the growth process. Obtaining high-resolution environmental records from the shell's growth history and correlating chemical data with dates through growth line analysis is feasible. This enables the determination of the date of a bivalve's death, namely when it was collected by humans, contributing as a tool for reconstructing traces of human activities in the field of archaeology.
In this study, the utility of the Hamaguri genus (Meretrix), commonly unearthed in archaeological sites in Japan, was examined as indicators for paleoenvironmental studies and archaeology. Samples from archaeological sites in Kagoshima Prefecture on Tanegashima Island, including Hamaguri (M. lusoria) and Chosen Hamaguri (M. lamarckii), as well as Chosen Hamaguri collected in May 2022 from Nagahama Coast on Tanegashima, were subjects of geochemical analysis and growth line analysis. After mineral phase confirmation using X-ray diffraction (XRD), samples were cut along the maximum growth line. Powder samples were obtained from the outer layer of the shell at 0.2mm intervals using dental drills and analyzed with stable isotope ratio mass spectrometry. Additionally, staining and etching were performed on the shell's cross-section, and growth line width was measured using image analysis software. The fossil samples' ages were determined using radiocarbon dating.
The dating results indicated ages of 2378±23 yr BP, 2810±23 yr BP for Hamaguri, and 5462±21 yr BP for Chosen Hamaguri. The δ18Oshell exhibited periodic variations, particularly in Chosen Hamaguri, with individuals from 5.5 ka showing values of -3.77 to -0.04‰ and modern individuals displaying -3.45 to -0.41‰. Comparing the δ18Oshell, growth line width, and precipitation for modern Chosen Hamaguri revealed a decrease in δ18Oshell with increasing precipitation, accompanied by a reduction in growth line width. Both species' δ18Oshell recorded detailed variations during the summer, potentially reflecting increased precipitation due to typhoons. The water temperature calibration was derived from modern Chosen Hamaguri's δ18Oshell and salinity records. The δ18Owater of the surrounding sea varied by 0.65‰ annually, accounting for 21% of the entire range of δ18Oshell and influencing reconstructed water temperatures. The reconstructed water temperature exhibited an approximately 2°C discrepancy compared to observed water temperatures. However, in terms of relative temperature differences, it suggested that individuals from 5.5 ka lived in an environment 2°C colder during winter than the present.
In this study, the utility of the Hamaguri genus (Meretrix), commonly unearthed in archaeological sites in Japan, was examined as indicators for paleoenvironmental studies and archaeology. Samples from archaeological sites in Kagoshima Prefecture on Tanegashima Island, including Hamaguri (M. lusoria) and Chosen Hamaguri (M. lamarckii), as well as Chosen Hamaguri collected in May 2022 from Nagahama Coast on Tanegashima, were subjects of geochemical analysis and growth line analysis. After mineral phase confirmation using X-ray diffraction (XRD), samples were cut along the maximum growth line. Powder samples were obtained from the outer layer of the shell at 0.2mm intervals using dental drills and analyzed with stable isotope ratio mass spectrometry. Additionally, staining and etching were performed on the shell's cross-section, and growth line width was measured using image analysis software. The fossil samples' ages were determined using radiocarbon dating.
The dating results indicated ages of 2378±23 yr BP, 2810±23 yr BP for Hamaguri, and 5462±21 yr BP for Chosen Hamaguri. The δ18Oshell exhibited periodic variations, particularly in Chosen Hamaguri, with individuals from 5.5 ka showing values of -3.77 to -0.04‰ and modern individuals displaying -3.45 to -0.41‰. Comparing the δ18Oshell, growth line width, and precipitation for modern Chosen Hamaguri revealed a decrease in δ18Oshell with increasing precipitation, accompanied by a reduction in growth line width. Both species' δ18Oshell recorded detailed variations during the summer, potentially reflecting increased precipitation due to typhoons. The water temperature calibration was derived from modern Chosen Hamaguri's δ18Oshell and salinity records. The δ18Owater of the surrounding sea varied by 0.65‰ annually, accounting for 21% of the entire range of δ18Oshell and influencing reconstructed water temperatures. The reconstructed water temperature exhibited an approximately 2°C discrepancy compared to observed water temperatures. However, in terms of relative temperature differences, it suggested that individuals from 5.5 ka lived in an environment 2°C colder during winter than the present.