Constraining the long-term lowering rates of shore platforms on volcanic islands in the East Sea of the Korean Peninsula, using cosmogenic 36Cl

The occurrence of global warming and concomitant rises in sea level means it is important to assess the vertical lowering rates and evolution of shore platforms. Most previous studies, however, were based on empirical measurements and limited to the decadal scale, implying there is great necessity to learn about long-term rates of change on rocky coasts. In particular, because it is relatively easy to monitor soft rocks such as chalk, there is little information about hard rocky coasts that consist of volcanic, igneous, and metamorphic rocks. In this study, we used cosmogenic 36Cl exposure dating to overcome the spatial and temporal limitations of previous studies that have restricted progress in rocky coast research. Results show that the exposure ages of samples collected from the shore platforms along volcanic islands in the middle of the East Sea (Sea of Japan) range from the mid-Holocene (ca. 4 ka) to the last century (ca. 0.1 ka). The large range in ages along the outer platform edges may be related to stochastic, differential, and mechanical wave erosion. We also calculated the rates of vertical lowering from the 36Cl concentrations of the platform surfaces, yielding a maximum rate, excluding outliers, of 0.68 mm/yr. The rates of vertical lowering inferred from the cosmogenic 36Cl abundances are comparable with previous empirical values, and will improve our understanding of the long-term evolution of rocky coasts.