09:15 〜 09:30
[MIS18-12] Reconstruction of Paleoenvironment of East Asia by Holocene Fossil Coral Skeletons: Uncovering of Prehistoric Archaeological Facts in Yilan, Taiwan
キーワード：古環境、化石サンゴ、晩全新世、考古学、台湾、喜界島
The development of human civilization would be closely linked to environmental changes influenced by climate, geographical conditions, and resources. Anthropological and archeological findings have shown that the environment was an important factor in population migration, urban abandonment, and state collapse. However, the direct link of these events to the paleoenvironmental conditions is poorly documented.
Due to its strategic geographic position, Taiwan is an essential island for human migration and cultural exchange of the Austronesian ethnocultural groups in East and Southeast Asia. Yilan in northeast Taiwan has a long, rich, and diverse history dating back to the early Neolithic Period. A 600-year gap (1400 to 2000 years BP) in Yilan’s cultural development could be attributed to social unrest brought by food shortage and harsh climate conditions. The hydroclimate of Yilan is strongly affected by the East Asian winter monsoon. Conventional paleoenvironmental reconstructions from pollen and lake sediments from Yilan have revealed inter-centennial resolution data through the late Holocene. However, the high-resolution records on the East Asian monsoon are required to compare with the distribution and pattern of human settlements in Yilan. This study aims to reconstruct the ocean temperature and salinity on a seasonal scale using coral geochemical records and reconstruct the East Asian monsoon development during the late Holocene.
Fossil corals are widely used in paleoclimate studies since the chemical compositions in coral skeletons reflect temperature and salinity changes in seawater. Strontium/calcium (Sr/Ca) ratio is a sea surface temperature (SST) proxy. Variations of oxygen isotopes in coral skeletons (δ18Ocoral) reflect the SST variations and hydrological cycles. Paleoclimate proxies, including fossil corals lake sediments around East Asia, were also used to demonstrate the density of the East Asia Monsoon.
This study analyzed fossil Porites core collected in Kikai Island in the northern part of the Ryukyu Islands, Japan, located in the East Asian monsoon area. Current meteorological data show a strong correlation in daily, and monthly temperatures between Kikai Island and Yilan, and the correlation is higher in winter than in summer. The coral specimen was dated by the U-Th method to 1701 ± ten years BP and composed of aragonite based on SEM and XRD. X-ray photos were taken to determine the main growth direction of the coral skeletons. The coral skeletal powder was collected at 0.2 mm intervals along the maximum coral growth axis. Stable carbon and oxygen isotope ratios (δ13Ccoral, δ18Ocoral) were measured every four samples (1,5,9…) using an isotope ratio mass spectrometer (IRMS). The distance from the top of the coral to each data point was converted into time, and 32 oscillations (32 years of data) were observed.
δ18Ocoral and δ13Ccoral values of Porites coral range from -2.4‰ to -4.7‰ and 0.9‰ to -2.1‰ (V-PDB) with average δ18Ocoral and δ13Ccoral value of -3.6±0.5‰ and -0.5±0.5‰ (1σ, N = 477). The results were compared with isotope data from another period: modern and fossil coral in 3235, 4960, and 5712 years BP. According to the development, the average value of δ18Ocoral in 1701 years BP was lower than in 3235, 4960, and 5712 years BP, and much lower than the average value from the modern coral. In addition, low values of δ18Ocoral were observed in the middle of the 32 years, indicating the strengthening of the East Asia winter monsoon. In future studies , we also collect more detailed paleoclimate and archaeological data in Taiwan, and further comparison of historical data from the mainland for the same period is also considered. The interrelationships between climate and human activities like historical incidents will be revealed in the East Asian region.
Due to its strategic geographic position, Taiwan is an essential island for human migration and cultural exchange of the Austronesian ethnocultural groups in East and Southeast Asia. Yilan in northeast Taiwan has a long, rich, and diverse history dating back to the early Neolithic Period. A 600-year gap (1400 to 2000 years BP) in Yilan’s cultural development could be attributed to social unrest brought by food shortage and harsh climate conditions. The hydroclimate of Yilan is strongly affected by the East Asian winter monsoon. Conventional paleoenvironmental reconstructions from pollen and lake sediments from Yilan have revealed inter-centennial resolution data through the late Holocene. However, the high-resolution records on the East Asian monsoon are required to compare with the distribution and pattern of human settlements in Yilan. This study aims to reconstruct the ocean temperature and salinity on a seasonal scale using coral geochemical records and reconstruct the East Asian monsoon development during the late Holocene.
Fossil corals are widely used in paleoclimate studies since the chemical compositions in coral skeletons reflect temperature and salinity changes in seawater. Strontium/calcium (Sr/Ca) ratio is a sea surface temperature (SST) proxy. Variations of oxygen isotopes in coral skeletons (δ18Ocoral) reflect the SST variations and hydrological cycles. Paleoclimate proxies, including fossil corals lake sediments around East Asia, were also used to demonstrate the density of the East Asia Monsoon.
This study analyzed fossil Porites core collected in Kikai Island in the northern part of the Ryukyu Islands, Japan, located in the East Asian monsoon area. Current meteorological data show a strong correlation in daily, and monthly temperatures between Kikai Island and Yilan, and the correlation is higher in winter than in summer. The coral specimen was dated by the U-Th method to 1701 ± ten years BP and composed of aragonite based on SEM and XRD. X-ray photos were taken to determine the main growth direction of the coral skeletons. The coral skeletal powder was collected at 0.2 mm intervals along the maximum coral growth axis. Stable carbon and oxygen isotope ratios (δ13Ccoral, δ18Ocoral) were measured every four samples (1,5,9…) using an isotope ratio mass spectrometer (IRMS). The distance from the top of the coral to each data point was converted into time, and 32 oscillations (32 years of data) were observed.
δ18Ocoral and δ13Ccoral values of Porites coral range from -2.4‰ to -4.7‰ and 0.9‰ to -2.1‰ (V-PDB) with average δ18Ocoral and δ13Ccoral value of -3.6±0.5‰ and -0.5±0.5‰ (1σ, N = 477). The results were compared with isotope data from another period: modern and fossil coral in 3235, 4960, and 5712 years BP. According to the development, the average value of δ18Ocoral in 1701 years BP was lower than in 3235, 4960, and 5712 years BP, and much lower than the average value from the modern coral. In addition, low values of δ18Ocoral were observed in the middle of the 32 years, indicating the strengthening of the East Asia winter monsoon. In future studies , we also collect more detailed paleoclimate and archaeological data in Taiwan, and further comparison of historical data from the mainland for the same period is also considered. The interrelationships between climate and human activities like historical incidents will be revealed in the East Asian region.