Coastal dynamics and sea-level change at 4 ka: A case study from the Wakayama Plain, Japan

Coastal depositional systems can be classified into transgressive and regressive systems, but their dynamics in relation to multi-segment sea level change remains largely unknown. In this study, we examined five sediment cores and 57 radiocarbon dates from the Wakayama Plain, western Japan. We have elucidated that the maximum flooding surface is dated at 7.6 cal kyr BP (8 ka), and the depositional system changed from a wave-dominated estuary to a wave-dominated delta at around 4 ka. This two-step change in the coastal depositional system can be explained by the decreasing in the rate of sea-level rise at 8 ka and the onset of sea-level fall at 4 ka, respectively. The coastal dynamics of the Wakayama Plain suggests that in areas where the mid-Holocene sea-level was higher than present sea levels, the sediment stacking pattern was controlled by the "rate" of sea-level rise, and the change in the geometry of the depositional system was controlled by the "height" of sea level.