2:30 PM - 2:45 PM
[MIS18-03] Reconstruction of Monthly rainfall variability during 1798 and 2015: Analysis of Hydrological signature and Coral record in Kikai Island
Keywords:coral, freshwater discharge, Precipitation, Ba/Ca, δ18Osw
In the synoptic-scale of hydrological cycle, precipitation is one of the most important climatic variables and has a significant impact on society and ecosystems. In kikai island, the site of this study, is affected by the Asian monsoon and its annual precipitation is 1892.7 mm, which is higher than the national average. However, precipitation immediately seeps into the ground, and people have suffered from a water shortage problem for a long time. In the 1820's to 1850's, there are many records of famines caused by drought, and the human and social impacts of water shortage are serious. Therefore, quantitative reconstruction of past precipitation records is scientific and social goal for understanding natural cycles.
We investigated salinity, oxygen isotope (δ18O), hydrogen isotope (δD), d-excess and their relations for understanding the process of river and precipitation influence in Shiomichi Bay. From the water isotope results, the precipitation-δ18Osw equation was derived from coral records and reconstructed of Monthly rainfall variability during 1798 and 2015.
Water isotope ratio results show a strong positive correlation between δ18Osw and salinity (r2=0.99), quantitatively clarified that δ18Osw in Shiomichi Bay is a binary mixing of seawater and freshwater. This line does not pass through the end-member values of the Kuroshio surface water and Changjiang River water, indicating that the Kuroshio surface water and Changjiang River water was not the source of water for Shiomichi Bay.
The δ18O-δD relationship for freshwater shows that the isotope ratios of precipitation and river water are plotted lower than Global Meteoric Water Line. This indicates that both freshwaters are not affected by evaporation. It is suggested that the process of precipitation recharging underground and discharging into rivers is so fast that evaporation does not occur.
Monthly variations of paleo-environmental data from coral skeletons (1979-2015) revealed that Ba/Ca and δ18Osw were associated with increased freshwater runoff during the rainy season (May-June) and typhoon season (September). In particular, δ18Osw is related to precipitation, with a correlation of r2=0.47 during April-September. A comparison of precipitation variability and δ18Osw variability during 1979-2015 shows that the variability period and amplitude intensity are generally consistent. We have calibrated this relationship using 35 data points to obtain:
δ18Osw=-0.0024(±36.5688)×P + 0.37 (±19.89) (P: Monthly precip. mm/mo.)
Using this equation to reconstruct precipitation for the period 1798-2015, it was revealed that many heavy rainfalls over 500 mm/mo. during rainy season in 1950-1955. On the other hand, the 1930s and 1960s resulted in less precipitation and is consistent with the local drought records. In addition, the 1820-30s coincided with a period of low precipitation and very cold weather, which supported the scientific aspect that one of the factors of famine occurrence was unseasonable weather.
We investigated salinity, oxygen isotope (δ18O), hydrogen isotope (δD), d-excess and their relations for understanding the process of river and precipitation influence in Shiomichi Bay. From the water isotope results, the precipitation-δ18Osw equation was derived from coral records and reconstructed of Monthly rainfall variability during 1798 and 2015.
Water isotope ratio results show a strong positive correlation between δ18Osw and salinity (r2=0.99), quantitatively clarified that δ18Osw in Shiomichi Bay is a binary mixing of seawater and freshwater. This line does not pass through the end-member values of the Kuroshio surface water and Changjiang River water, indicating that the Kuroshio surface water and Changjiang River water was not the source of water for Shiomichi Bay.
The δ18O-δD relationship for freshwater shows that the isotope ratios of precipitation and river water are plotted lower than Global Meteoric Water Line. This indicates that both freshwaters are not affected by evaporation. It is suggested that the process of precipitation recharging underground and discharging into rivers is so fast that evaporation does not occur.
Monthly variations of paleo-environmental data from coral skeletons (1979-2015) revealed that Ba/Ca and δ18Osw were associated with increased freshwater runoff during the rainy season (May-June) and typhoon season (September). In particular, δ18Osw is related to precipitation, with a correlation of r2=0.47 during April-September. A comparison of precipitation variability and δ18Osw variability during 1979-2015 shows that the variability period and amplitude intensity are generally consistent. We have calibrated this relationship using 35 data points to obtain:
δ18Osw=-0.0024(±36.5688)×P + 0.37 (±19.89) (P: Monthly precip. mm/mo.)
Using this equation to reconstruct precipitation for the period 1798-2015, it was revealed that many heavy rainfalls over 500 mm/mo. during rainy season in 1950-1955. On the other hand, the 1930s and 1960s resulted in less precipitation and is consistent with the local drought records. In addition, the 1820-30s coincided with a period of low precipitation and very cold weather, which supported the scientific aspect that one of the factors of famine occurrence was unseasonable weather.