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[MIS18-P02] Climatic changes in the Tono region during the past 400 years as revealed by cellulose oxygen isotope ratios in tree rings
Keywords:Okute shinmei cedar tree, Tree rings, Cellulose oxyzen isotope, The Kansei drought
Introduction
Paleoclimatology is the study of climates before modern climate observation, and various climatic reconstructions have been carried out using tree rings and ice sheets. In particular, tree rings are widely used because they are easy to obtain and store. In recent years, isotope ratios of cellulose in tree rings have been shown to record past climate changes.
Cellulose in tree rings is a chemically stable material, and is thought to maintain its composition at the time of ring formation. Oxygen molecules in cellulose are thought to be derived from precipitation, and climate reconstructions using their isotopic ratios have been developed. However, most of these reconstructions are based on annual precision.
In this study, the annual cellulose oxygen isotope ratio (δ18O) of Japanese cedar (Cryptomeria japonica) samples collected in Okute Town, Mizunami City, Gifu Prefecture, Japan, was divided into equal parts to improve the temporal resolution of the reconstruction. A negative correlation between the oxygen isotope ratio of cellulose annual rings and relative humidity in the atmosphere was utilized to study the long-term climate change based on the variation of oxygen isotope ratio for about 400 years in the corresponding region.
Materials and method
In this study, samples were collected from a cedar tree that felled in July 2020 at Okute Shinmei Shrine in Okute Town, Mizunami City, Gifu Prefecture. The cedar was about 670 years old, but the center of the trunk was heavily corroded, so the annual rings after 1609 were studied. A cellulose thin plate was made from the sample, and after dividing one annual ring into equal parts, the oxygen isotope ratio was measured using an on-line system of pyrolysis elemental analyzer and isotope ratio mass spectrometer installed in the Graduate School of Environmental Studies, Nagoya University.
Results and Discussion
The results of oxygen isotope ratio measurements are shown in Figures A through F. The oxygen isotope ratios measured in this study showed a sawtooth pattern, with high values at the beginning of one year, decreasing toward the end of the year, and rapidly increasing at the beginning of the next year. As for the long-term variation, no clear trend was observed, but Figure A shows that the intra-annual variation was relatively small from 1780 to 1800, and the period of low isotope ratio persisted. This period was marked by the famine of the Tenmei era (1780-1800), suggesting that wet weather persisted throughout the growth period of the sample trees. The correlations between the oxygen isotope ratios measured in this study and the relative humidity observed at the Gifu District Meteorological Observatory since 1883 were examined using data for 74 years from 1883 to 2020, when the annual layer could be divided into six segments. The negative correlations between the first and second segments and mid-April, the third and fourth segments and late May, and the fifth segment and early July were relatively strong. The growth period of the sample trees was estimated to be from mid-April to early July.
Since the Tono region prospered as a post station on the Nakasendo , several old diaries have been preserved. For example, the "Chronicle of Tsukuchi-mura, Ena-gun," included in the "History of Gifu Prefecture, Historical Records, Early Modern Period VI," mentions that there was drought in the summers of 1790, 1793, and 1794. Figure F shows that the oxygen isotope ratios were high in 1790, 1793, and 1794. Therefore, it is considered that the area was dry throughout the growth period.
Summary
In this study, we investigated climatic changes in the Tono region in the early modern period based on cellulose oxygen isotope ratios of annual tree rings over a period of about 400 years. As a result, it was confirmed that detailed climatic reconstruction was possible by dividing the annual layer into sections, which revealed oxygen isotope ratio variations with a higher resolution than that of annual units. In some years, the oxygen isotope ratios of the annual rings of the trees coincided with those of the old diary, indicating that the trees recorded the climate at the time of annual ring formation.
Paleoclimatology is the study of climates before modern climate observation, and various climatic reconstructions have been carried out using tree rings and ice sheets. In particular, tree rings are widely used because they are easy to obtain and store. In recent years, isotope ratios of cellulose in tree rings have been shown to record past climate changes.
Cellulose in tree rings is a chemically stable material, and is thought to maintain its composition at the time of ring formation. Oxygen molecules in cellulose are thought to be derived from precipitation, and climate reconstructions using their isotopic ratios have been developed. However, most of these reconstructions are based on annual precision.
In this study, the annual cellulose oxygen isotope ratio (δ18O) of Japanese cedar (Cryptomeria japonica) samples collected in Okute Town, Mizunami City, Gifu Prefecture, Japan, was divided into equal parts to improve the temporal resolution of the reconstruction. A negative correlation between the oxygen isotope ratio of cellulose annual rings and relative humidity in the atmosphere was utilized to study the long-term climate change based on the variation of oxygen isotope ratio for about 400 years in the corresponding region.
Materials and method
In this study, samples were collected from a cedar tree that felled in July 2020 at Okute Shinmei Shrine in Okute Town, Mizunami City, Gifu Prefecture. The cedar was about 670 years old, but the center of the trunk was heavily corroded, so the annual rings after 1609 were studied. A cellulose thin plate was made from the sample, and after dividing one annual ring into equal parts, the oxygen isotope ratio was measured using an on-line system of pyrolysis elemental analyzer and isotope ratio mass spectrometer installed in the Graduate School of Environmental Studies, Nagoya University.
Results and Discussion
The results of oxygen isotope ratio measurements are shown in Figures A through F. The oxygen isotope ratios measured in this study showed a sawtooth pattern, with high values at the beginning of one year, decreasing toward the end of the year, and rapidly increasing at the beginning of the next year. As for the long-term variation, no clear trend was observed, but Figure A shows that the intra-annual variation was relatively small from 1780 to 1800, and the period of low isotope ratio persisted. This period was marked by the famine of the Tenmei era (1780-1800), suggesting that wet weather persisted throughout the growth period of the sample trees. The correlations between the oxygen isotope ratios measured in this study and the relative humidity observed at the Gifu District Meteorological Observatory since 1883 were examined using data for 74 years from 1883 to 2020, when the annual layer could be divided into six segments. The negative correlations between the first and second segments and mid-April, the third and fourth segments and late May, and the fifth segment and early July were relatively strong. The growth period of the sample trees was estimated to be from mid-April to early July.
Since the Tono region prospered as a post station on the Nakasendo , several old diaries have been preserved. For example, the "Chronicle of Tsukuchi-mura, Ena-gun," included in the "History of Gifu Prefecture, Historical Records, Early Modern Period VI," mentions that there was drought in the summers of 1790, 1793, and 1794. Figure F shows that the oxygen isotope ratios were high in 1790, 1793, and 1794. Therefore, it is considered that the area was dry throughout the growth period.
Summary
In this study, we investigated climatic changes in the Tono region in the early modern period based on cellulose oxygen isotope ratios of annual tree rings over a period of about 400 years. As a result, it was confirmed that detailed climatic reconstruction was possible by dividing the annual layer into sections, which revealed oxygen isotope ratio variations with a higher resolution than that of annual units. In some years, the oxygen isotope ratios of the annual rings of the trees coincided with those of the old diary, indicating that the trees recorded the climate at the time of annual ring formation.