10:00 AM - 10:15 AM
[HQR05-05] Potential and Accuracy of OSL Dating for Holocene Fluvial Deposits
Keywords:OSL dating, Holocene
OSL dating has been widely applied to a variety of sediments due to the ubiquitous presence of feldspar and quartz, making it one of the most important methods for dating Holocene deposits. OSL dating is intended for sediments that were well exposed during deposition. Therefore, OSL dating of fluvial deposits requires several innovations. In particular, Japanese fluvial deposits have been considered unsuitable for OSL dating due to their short river lengths compared to continental rivers, which may not receive sufficient exposure. However, feldspar particles in certain types of fluvial deposits, such as riverbeds, point bars, and crevasse splays, are partially exposed in contemporary fluvial environments, suggesting the utility of OSL dating for these deposits (Shirai et al. 2008). The purpose of this study is to assess the effectiveness of IRSL dating in fluvial deposits in Japan. By analyzing the measured ages and trends within the data, we seek to determine which fluvial deposits are suitable for IRSL dating and evaluate its potential as a method for distinguishing between marine and fluvial deposits.
The study area is near the river mouth of Abukuma River, located in the southern Sendai Plain. The Abukuma River drains the Abukuma Granite Belt and the igneous rocks of the Ou Mountains. A strandplain extends around the river mouth. The formation age of the beach ridges were discussed by Matsumoto (1984) and Fujimoto and Matsumoto (2012). The area is also characterized by the occurrence of several abandoned river channels and associated natural levees.
Sediment samples were collected by mechanical boring at three sites (W5, W6, W7) and by hand auger excavation at one site (EB3). The samples were transported to the National Institute of Advanced Industrial Science and Technology (AIST) without exposure to light. The sediments were classified into Unit A (abandoned channel deposits), Unit B (natural levee deposits), Unit C (riverbed deposits), Unit D (upper shoreface deposits), and Unit E (lower shoreface deposits) based on color, grain size, and sedimentary structure. OSL dating samples were collected from one horizon in Unit A (W6-270), two horizons in Unit B (W7-280, W7-440), four horizons in Unit C (W5-320, W5-930, W6-335, W6-765), two horizons in Unit E (W5-960, W5-810), and two horizons in Unit E (W5-960 and W5-810). Feldspar grains of 180–250 μm diameter were extracted under subdued red light to avoid the bleaching of the luminescence signal. The age of each sample was calculated by measuring IR50, pIR150, and fading rate for each of the 8 disks using a RISO instrument, along with U, Th, Rb, and K concentrations, and water content for each sample. A plant fragment obtained from the lowest part of Unit A (EB3) was radiocarbon-dated at the Korea Institute of Geoscience and Mineral Resources (KIGAM).
The OSL age obtained for the IR50 signal from the lower section of Unit A was 2.44 ± 0.20 ka. Ages for deposits in Unit B ranged from approximately 0.8 to 4.6 ka, while those from the upper part of Unit C yielded ages between 1.6 and 1.9 ka. The lower part of Unit C showed ages between 4.3 and 5.7 ka, and the upper part of Unit E displayed ages ranging from 5.1 to 7.0 ka. The 14C age from the lowest part of Unit A was approximately 1.3 ka.
A comparison of the De values for fluvial deposits in Units A, B, and C with those of marine deposits in Unit E shows that the latter has a smaller variation in De. The IR50 and pIR150 ages of the marine deposits were nearly identical, whereas the pIR150 ages of the fluvial deposits were consistently older than the IR50 ages by 1.2 to 5 ka. This discrepancy is likely due to the insufficient exposure of the fluvial deposits to sunlight, which affects the pIR150 signal. These clear differences indicate that OSL measurements are effective in distinguishing between fluvial and marine deposits.
The IR50 age of the Unit C deposits is consistent with the 14C age obtained from the lowest part of Unit A and the IRSL age of marine deposits from the upper end of Unit E, suggesting that the IR50 age for the riverbed deposits is generally accurate. However, the IR50 age from the lower part of Unit A is about 1 ka older than the 14C age obtained from the same location and is older than the IR50 age from the underlying Unit C deposits. Additionally, the IR50 age for the Unit B deposits are older than the IR50 age of the underlying Unit C deposits. Both of these deposits may have formed in an environment with low light exposure, such as flood deposits. These results suggest that IR50 dating of feldspars is useful for riverbed deposits, but that IR50 dating is difficult to apply to abandoned channel-fill deposits and natural levee deposits.
The study area is near the river mouth of Abukuma River, located in the southern Sendai Plain. The Abukuma River drains the Abukuma Granite Belt and the igneous rocks of the Ou Mountains. A strandplain extends around the river mouth. The formation age of the beach ridges were discussed by Matsumoto (1984) and Fujimoto and Matsumoto (2012). The area is also characterized by the occurrence of several abandoned river channels and associated natural levees.
Sediment samples were collected by mechanical boring at three sites (W5, W6, W7) and by hand auger excavation at one site (EB3). The samples were transported to the National Institute of Advanced Industrial Science and Technology (AIST) without exposure to light. The sediments were classified into Unit A (abandoned channel deposits), Unit B (natural levee deposits), Unit C (riverbed deposits), Unit D (upper shoreface deposits), and Unit E (lower shoreface deposits) based on color, grain size, and sedimentary structure. OSL dating samples were collected from one horizon in Unit A (W6-270), two horizons in Unit B (W7-280, W7-440), four horizons in Unit C (W5-320, W5-930, W6-335, W6-765), two horizons in Unit E (W5-960, W5-810), and two horizons in Unit E (W5-960 and W5-810). Feldspar grains of 180–250 μm diameter were extracted under subdued red light to avoid the bleaching of the luminescence signal. The age of each sample was calculated by measuring IR50, pIR150, and fading rate for each of the 8 disks using a RISO instrument, along with U, Th, Rb, and K concentrations, and water content for each sample. A plant fragment obtained from the lowest part of Unit A (EB3) was radiocarbon-dated at the Korea Institute of Geoscience and Mineral Resources (KIGAM).
The OSL age obtained for the IR50 signal from the lower section of Unit A was 2.44 ± 0.20 ka. Ages for deposits in Unit B ranged from approximately 0.8 to 4.6 ka, while those from the upper part of Unit C yielded ages between 1.6 and 1.9 ka. The lower part of Unit C showed ages between 4.3 and 5.7 ka, and the upper part of Unit E displayed ages ranging from 5.1 to 7.0 ka. The 14C age from the lowest part of Unit A was approximately 1.3 ka.
A comparison of the De values for fluvial deposits in Units A, B, and C with those of marine deposits in Unit E shows that the latter has a smaller variation in De. The IR50 and pIR150 ages of the marine deposits were nearly identical, whereas the pIR150 ages of the fluvial deposits were consistently older than the IR50 ages by 1.2 to 5 ka. This discrepancy is likely due to the insufficient exposure of the fluvial deposits to sunlight, which affects the pIR150 signal. These clear differences indicate that OSL measurements are effective in distinguishing between fluvial and marine deposits.
The IR50 age of the Unit C deposits is consistent with the 14C age obtained from the lowest part of Unit A and the IRSL age of marine deposits from the upper end of Unit E, suggesting that the IR50 age for the riverbed deposits is generally accurate. However, the IR50 age from the lower part of Unit A is about 1 ka older than the 14C age obtained from the same location and is older than the IR50 age from the underlying Unit C deposits. Additionally, the IR50 age for the Unit B deposits are older than the IR50 age of the underlying Unit C deposits. Both of these deposits may have formed in an environment with low light exposure, such as flood deposits. These results suggest that IR50 dating of feldspars is useful for riverbed deposits, but that IR50 dating is difficult to apply to abandoned channel-fill deposits and natural levee deposits.