Formation of tafoni-like depressions and salt accretion distribution in the seawater splash zone

Although wave erosion mediated by salt weathering due to seawater supply and solar radiation has been suggested to be the main factor in the growth of tafoni-like depressions in the seawater splash zone, there are no quantitative measurements of salt deposition or deposition distribution. In this study, we analyzed the distribution of salt deposition on the rock surface by mapping the deposition and adhesion of salts and examined the relationship between the distribution of salt deposition and the growth of depressions. The study area is the Yayoi Bridge over Aoshima, Miyazaki Prefecture, Japan. The bridge was built in 1951 and the piers are made of blocks of the same sandstone as the bedrock of Aoshima. There are 144 sandstone blocks forming the wall, set in 11 layers. The layers are numbered from the bottom to the top, with the lowest being the 1st layer and the highest being the 11th layer. Continuous studies on the formation of tafoni-like depressions have been conducted here since 1970s. In this study, the spectral reflectances of rock surfaces were measured using a portable spectrometer. The results showed that the effect of salt on the Original Reflectance (OR), which was processed by Continuum Removal (CR), and Normalized Difference Spectral Index (NDSI), was remarkable. The salt distribution on the south face of the piers was estimated to be thicker in layers 4 to 6 and thinner in layers 8 to 11. The CR-treated values showed a positive correlation (0.55) with depression volume, while NDSI showed a negative correlation (−0.52), suggesting that salt accretion distribution, or salt weathering, is the main cause of weathering of the sandstone blocks. In addition, a comparison of depression depths at 20, 38, 50, and 73 years after completion showed a decreasing trend in the overall rate of increase, but a high rate of increase was still observed in certain layers. The main reason for this is that the potential for salt weathering is still high in the deeper part of the depression, which continues to expand. In the future, it is desirable to develop a measurement method that can acquire salt distribution more simply and efficiently using a camera or other equipment.