Coastal Transport Processes of Sand Grains Assumed from Residual Optically Stimulated Luminescence Intensities of Alkali-Feldspar Grains around the Ohkouzu Diversion Channel Mouth in Niigata Prefecture, Central Japan

Abstract

Optically stimulated luminescence (OSL) intensities are useful to estimate the mineral burial duration since previous resetting (bleaching) by sufficient sunlight exposure, and hence they are applied in OSL dating. OSL dating assumes that minerals are bleached at the time of deposition, although complete bleaching before deposition is not always achieved. Opportunities for minerals to receive sunlight exposure generally increase in proportion to the sediment transport distance, and therefore the trend in the decrease in residual intensities and increase in percentage of bleaching minerals (bleaching percentage, BLP) suggest the sediment transport direction. Although residual OSL intensities are an obstacle to accurate depositional age estimation, they may be useful for estimating sediment transport processes because these suggest variations in the sunlight exposure of individual grains. The present study attempted to determine the coastal sand transport process around the Ohkouzu Diversion Channel (the diversion channel of the Shinano River) mouth using residual OSL intensities and BLP of alkali-feldspar grains.

It was found that a decrease in residual OSL intensities and an increase in BLPs with transport distances occurred around the river mouth as a whole. It is concluded that sandy sediments supplied from the channel are transported at least to Kakuta-hama beach, 17km northeast of the channel mouth. However, it was found that residual OSL intensities and BLPs had no clear changing trend at Nodumi beach on the northern side of the channel mouth. This did not support our assumption, although the transport was probably affected by recent coastal erosion around the channel mouth. Coastal erosion supplies buried unbleached grains, which have high residual OSL intensities, to the current beach. These grains might have disrupted the trend in the changing residual OSL intensities and BLPs with coastal transport along Nodumi beach.

This study showed that the residual OSL intensities and BLPs are useful for estimating recent coastal transport processes of sandy sediments when those processes are difficult to understand using previous techniques.