2:15 PM - 2:30 PM
[HTT16-15] Sedimentary environments of floodplains in East Asia during the Anthropocene: Exploring the effects of natural and social conditions using Pb-210 dating methods
Keywords:sedimentary environment, floodplain, Pb-210 dating method, Anthropocene
Floodplains are the first landscape to be significantly altered by humans in the Anthropocene. Particularly in the monsoon region of Asia, which has a warm and humid climate and has developed in tandem with flooding, the floodplains of major rivers are thought to have only begun to be exploited at a high level in the last century or so since modern times. From this perspective, floodplains can be seen as a symbolic social-ecological system in the environmental changes of the Anthropocene. This research focuses on the social-ecological systems of Anthropocene floodplains that have begun to coexist with rivers, and aims to clarify the impact of changes in natural and social conditions over the past century or so on the depositional environment, based on surveys of pond-like water bodies that remain in the major river systems of East Asia.
The study areas were the southern shore of Lake Taifu (Huzhou City, Zhejiang Province), which is located in the delta region of the Yangtze River system, the waterways within the Sanyang Wetland, which has been recently restored in the floodplain of the Oujiang River system (Wenzhou City, Zhejiang Province), and the pond-like water area connected to the Tsuya River, which flows through the floodplain of the Kiso River system (Kaizu City, Gifu Prefecture). At representative points in each study area, 50 cm long (5.4 cm diameter) bottom mud cores were collected using a column sampler. The collected column samples were cut into uniform thicknesses, and their wet and dry bulk densities were measured. The magnetic susceptibility of the dried samples was measured by applying a weak external magnetic field to the soil samples, and the samples were subjected to a loss-on-ignition test (for measuring ash free dry mass content) and heavy metal contain analysis, as well as lead-210 dating. After the polystyrene vials were filled to a certain height with the particles in a state of dense packing (sealed until the gas 222Rn emitted by the particles and the 214Pb and 210Pb produced by its decay reached radiological equilibrium), the specific radioactivity [Bq/g] of total 210Pb, 214Pb (with half-life in about 22 years) and 137Cs were quantified using an a g-ray spectrometer. Since the sediment also contains supported 210Pb derived from 226Ra in rocks, the specific radioactivity of the excess 210Pb derived from the atmosphere was calculated by subtracting the specific radioactivity of 214Pb, which has the same specific radioactivity as supported 210Pb in a state of radiochemical equilibrium, from the total 210Pb. The decay process of this excess 210Pb radioactivity was analyzed using the Constant Rate of Supply (CRS) model to estimate the depth distribution of the sedimentation age, and the increases in 137Cs radioactivity due to human activities such as nuclear testing and nuclear power plant accidents were verified with the literature records.
According to the analysis results for each water body, it was possible to clarify the sedimentation process over the past 20 to 80 years, reflecting the different formation ages and alteration histories of each water body. The estimated sedimentation rates (0.28-2.14 g cm-2 yr-1) did not differ significantly between regions, but were larger than previously reported values for lakes and inland bays (about 0.1 g cm-2 yr-1), and varied according to connection status with the main river channel, flood events and etc. The analytical values for each layer in the sediment cores showed large differences depending on the location, with dry bulk density, magnetic susceptibility, ash free dry mass and various metal contents showing different results depending on the depth, reflecting the characteristics of the floodplain landscape at each study site. For example, the bottom material (sediment) of the artificially maintained water area had a high bulk density and low magnetic susceptibility, loss on ignition, and metal content, which reflected the influence of the bottom material that was artificially introduced to secure spring water, suggesting the influence of garden-like maintenance and management. In addition, the layer estimated to have been deposited in the 1970s had a slightly higher Pb content than the layers before and after it, which coincides with the period when gasoline containing various organic lead compounds was used, and the results reflected past environmental pollution, and it was clear that the floodplain bottom sediments contain the effects of human activities that have changed dramatically in the Anthropocene.
The study areas were the southern shore of Lake Taifu (Huzhou City, Zhejiang Province), which is located in the delta region of the Yangtze River system, the waterways within the Sanyang Wetland, which has been recently restored in the floodplain of the Oujiang River system (Wenzhou City, Zhejiang Province), and the pond-like water area connected to the Tsuya River, which flows through the floodplain of the Kiso River system (Kaizu City, Gifu Prefecture). At representative points in each study area, 50 cm long (5.4 cm diameter) bottom mud cores were collected using a column sampler. The collected column samples were cut into uniform thicknesses, and their wet and dry bulk densities were measured. The magnetic susceptibility of the dried samples was measured by applying a weak external magnetic field to the soil samples, and the samples were subjected to a loss-on-ignition test (for measuring ash free dry mass content) and heavy metal contain analysis, as well as lead-210 dating. After the polystyrene vials were filled to a certain height with the particles in a state of dense packing (sealed until the gas 222Rn emitted by the particles and the 214Pb and 210Pb produced by its decay reached radiological equilibrium), the specific radioactivity [Bq/g] of total 210Pb, 214Pb (with half-life in about 22 years) and 137Cs were quantified using an a g-ray spectrometer. Since the sediment also contains supported 210Pb derived from 226Ra in rocks, the specific radioactivity of the excess 210Pb derived from the atmosphere was calculated by subtracting the specific radioactivity of 214Pb, which has the same specific radioactivity as supported 210Pb in a state of radiochemical equilibrium, from the total 210Pb. The decay process of this excess 210Pb radioactivity was analyzed using the Constant Rate of Supply (CRS) model to estimate the depth distribution of the sedimentation age, and the increases in 137Cs radioactivity due to human activities such as nuclear testing and nuclear power plant accidents were verified with the literature records.
According to the analysis results for each water body, it was possible to clarify the sedimentation process over the past 20 to 80 years, reflecting the different formation ages and alteration histories of each water body. The estimated sedimentation rates (0.28-2.14 g cm-2 yr-1) did not differ significantly between regions, but were larger than previously reported values for lakes and inland bays (about 0.1 g cm-2 yr-1), and varied according to connection status with the main river channel, flood events and etc. The analytical values for each layer in the sediment cores showed large differences depending on the location, with dry bulk density, magnetic susceptibility, ash free dry mass and various metal contents showing different results depending on the depth, reflecting the characteristics of the floodplain landscape at each study site. For example, the bottom material (sediment) of the artificially maintained water area had a high bulk density and low magnetic susceptibility, loss on ignition, and metal content, which reflected the influence of the bottom material that was artificially introduced to secure spring water, suggesting the influence of garden-like maintenance and management. In addition, the layer estimated to have been deposited in the 1970s had a slightly higher Pb content than the layers before and after it, which coincides with the period when gasoline containing various organic lead compounds was used, and the results reflected past environmental pollution, and it was clear that the floodplain bottom sediments contain the effects of human activities that have changed dramatically in the Anthropocene.