Distinguishing sediment transport modes in the eastern continental margin of the Korean Peninsula through end-member analysis of surface sediment grain-size distributions.

Abstract

 This study presents a detailed analysis of grain-size distributions of closely spaced surface sediment samples from the eastern continental margin of the Korean Peninsula off Gangneung and Donghae. This region is characterized by a wave-dominated beach, narrow shelf less than 10 km wide, and slopes with varying gradients ranging from 0.7 to 6.3 degrees. Spatial variations in dominant sediment transport modes were identified using end-member analysis (EMA) of the grain-size distributions.  The EMA revealed five distinct end-member distributions (EMD) with mean grain sizes of 221.2 μm (EMD1), 89.2 μm (EMD2), 52.4 μm (EMD3), 22.0 μm (EMD4), and 4.5 μm (EMD5), respectively. EMD1, is significant only at two shallow sample sites near the coast, adjacent to the exposed rocky seafloor, indicating an origin from a relict sand during the post-glacial transgression. EMD2 predominates on the shelf and upper slope (40 to 150 m water depths) along the margin, particularly near a local river mouth, suggesting bedload transport of riverine sand by longshore drifts or episodic storm surges. EMD3, potentially representing coarser suspended load, dominates the upper middle slope (200 to 400 m water depths) where the slope gradient is relatively constant, and the isobaths generally run parallel to the shoreline about 15 km apart. EMD4, potentially representing finer suspended load, prevails in the deeper middle slope (400 to 800 m water depths) characterized by varying morphology: narrower and relatively steeper in the northern part, and wider and gentler in the southern part of the margin. The higher proportions of EMD4 extend far offshore in the wider and gentler southern part more than about 35 km, whereas they are limited to within 30 km from the shoreline at the base of the steeper and narrower slope in the northern part. The morphologically controlled EMD4 distributions suggest that a density current was responsible for the offshore fine-grained sediment transport in this margin, rather than diffusion or advection by ocean currents. Finally, the finest EMD5, predominates in the deepest part of the study area, showing no significant further offshore variations, and is interpreted to represent aeolian dust from the Asian inland.