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[ACG48-04] Sediment transport off the Kujukuri coast revealed by luminescence
Keywords:lower shoreface, shelf, coastal area, sediment tracer
The lower shoreface and shelf off the sandy beach exhibit too little topographic change for the water depths and in many of engineering projects are supposed to be inactive in terms of sediment transport. However, some recent bathymetric surveys with multi-beam ecosounder systems have suggested a considerable amount of sand isexchanged between these offshore areas and the beach. It is necessary to gain more detailed understanding of sediment transport in offshore areas. In this study, we attempt at utilizing luminescence signals of mineral particles, which are considered as a potential sedimenttracer.
Luminescence signals of mineral particles are depleted as they are exposed to sunlight during sediment transport. Therefore, spatial variations in the intensity of the signalsemitted from coastal sediments reflects the distance and path of transport and can be used as a source of information for discussing sediment transport. In addition, because the rate of signal depletion and the sensitivity to light vary for different types of luminescence signals, trends in multiple signals may provide further information.In this study, we measured multiple luminescence signals from feldspar sand grains (IR50, pIRIR150, pIRIR290) contained in beach/offshore surface samples and sedimentcores collected from the Kujukuri coast. We also examined if the ratio of the two of the signals (pIRIR150/pIRIR290) provide further insights.
The foreshore samples, collected along the coast, are characterized by all signals decreasing downdrift from the Byobugaura, the sediment source, to the central coast. The pIRIR150/pIRIR290 ratio of the foreshore samples also decreases from 0.2 at the Byobugaura to 0.05 at Shirasato, where the longshore drift is supposed to converge. On the other hand, along several shore-normal transects, the pIRIR150/pIRIR290 ratio is almost constant onshore from the foreshore to the dune. Therefore, the pIRIR150/pIRIR290 ratio is considered to reflect the mode of sediment transport.
The pIRIR150/pIRIR290 ratio was also evaluated for offshore sediment cores. In cores collected at water depths of 10 and 15 m (75 and 60 cm long, respectively), the pIRIR150/pIRIR290 ratio was approximately 0.05 for the entire core. The sediments in these cores were transported from the central beach to offshore. The luminescence ages are less than a few decades, suggesting that frequent sediment transport has occurred. In cores collected at water depths of 20 and 25 m (60 and 65 cm long, respectively), the pIRIR150/pIRIR290 ratio was approximately 0.05 for the interval shallower than 30 to 40 cm deep, also suggesting that the sediment was transported from the center beach. For the deeper interval, in contrast, the ratio increases and the luminescence agesare around 100 years old. Therefore, it is thought that the sediment deeper than 30 to 40 cm in cores at depths of 20 and 25 m has not undergone recent sediment movement. Our study shows that luminescence signals of offshore sediments provide information of offshore sedimenttransport that is otherwise difficult to obtain.
Luminescence signals of mineral particles are depleted as they are exposed to sunlight during sediment transport. Therefore, spatial variations in the intensity of the signalsemitted from coastal sediments reflects the distance and path of transport and can be used as a source of information for discussing sediment transport. In addition, because the rate of signal depletion and the sensitivity to light vary for different types of luminescence signals, trends in multiple signals may provide further information.In this study, we measured multiple luminescence signals from feldspar sand grains (IR50, pIRIR150, pIRIR290) contained in beach/offshore surface samples and sedimentcores collected from the Kujukuri coast. We also examined if the ratio of the two of the signals (pIRIR150/pIRIR290) provide further insights.
The foreshore samples, collected along the coast, are characterized by all signals decreasing downdrift from the Byobugaura, the sediment source, to the central coast. The pIRIR150/pIRIR290 ratio of the foreshore samples also decreases from 0.2 at the Byobugaura to 0.05 at Shirasato, where the longshore drift is supposed to converge. On the other hand, along several shore-normal transects, the pIRIR150/pIRIR290 ratio is almost constant onshore from the foreshore to the dune. Therefore, the pIRIR150/pIRIR290 ratio is considered to reflect the mode of sediment transport.
The pIRIR150/pIRIR290 ratio was also evaluated for offshore sediment cores. In cores collected at water depths of 10 and 15 m (75 and 60 cm long, respectively), the pIRIR150/pIRIR290 ratio was approximately 0.05 for the entire core. The sediments in these cores were transported from the central beach to offshore. The luminescence ages are less than a few decades, suggesting that frequent sediment transport has occurred. In cores collected at water depths of 20 and 25 m (60 and 65 cm long, respectively), the pIRIR150/pIRIR290 ratio was approximately 0.05 for the interval shallower than 30 to 40 cm deep, also suggesting that the sediment was transported from the center beach. For the deeper interval, in contrast, the ratio increases and the luminescence agesare around 100 years old. Therefore, it is thought that the sediment deeper than 30 to 40 cm in cores at depths of 20 and 25 m has not undergone recent sediment movement. Our study shows that luminescence signals of offshore sediments provide information of offshore sedimenttransport that is otherwise difficult to obtain.