Shell and gravel layers caused by storm-induced rip currents during the Medieval Warm Period and Little Ice Age in South Korea

Five shell-gravel layers (2.2-2.9 m in elevation, 70 cm in thickness) formed by storm-induced rip currents along the southern coast of the Korean Peninsula over the past 1300 years are described. The three radiocarbon dates (S1-S3) and the two optically stimulated luminescence (S4 and S5) dates correspond well with the succession of the five shell-gravel layers. The age of S1, CE 720 ± 60, corresponds to the early Medieval Warm Period (MWP). The dates of the three overlying layers (S2-S4), CE 880 ± 110, 950 ± 70, and 995 ± 120, respectively, coincide with the middle MWP. The fifth date of the uppermost shell-gravel layer (S5), CE 1535 ± 40, coincides with the Little Ice Age (LIA) of Western Europe. The characteristics of shell habitats based on 16 shell species from each layer suggest that the first four layers formed in a slightly erosional to dominantly depositional environment during the MWP, whereas the youngest layer formed in a dominantly erosional environment during the LIA. Analyses of grain size distribution, magnetic susceptibility, and geochemistry of the sediments, and gravel morphology indicate that storm activity at this site during the MWP and LIA occurred within active storm periods in the western North Atlantic (WNA). Comparison of the five storm events and related climatic factors suggests that, although the typhoon track type at the site is similar to recurving tracks typical for El Niño conditions in North China and Japan, tracks during the MWP and LIA were likely shifted not only by changes in El Niño conditions but by other climatic factors such as sea surface temperature, the North Pacific High, and the westerly jet stream. However, storm energy under El Niño years during the middle MWP was likely stronger than that produced in normal years during the early MWP.