Beach ridge formation and Holocene relative sea-level changes in the southern Sendai coastal plain, northern Japan

Beach ridges are common on wave-dominated coasts in Japanese coastal plains. However, because sandy beach-ridge deposits generally lack materials suitable for radiocarbon dating, the timing of their formation remains unclear, except in a limited number of plains. This study investigates the sedimentary facies and infrared-stimulated luminescence (IRSL) chronology of the beach ridge sequences and their subsurface deposits in the southern Sendai coastal plain, near the epicenter of the 2011 Tohoku earthquake. Borehole core sediments (10 m in length) retrieved from the four beach ridges primarily consist of three facies, in ascending order: lower shoreface, upper shoreface, and foreshore/backshore facies. Fading-corrected IRSL ages from foreshore/backshore deposits suggest that the most landward beach ridge formed around 6 ka, while the other three deposited within the last 4 ka. Additionally, corrected IRSL ages from the near-surface deposits corresponding to foreshore/backshore deposits in numerous beach ridges provide a more detailed timeline of their formation. The succession from the uppermost lower shoreface to the foreshore/backshore likely occurred within a few hundred years for each beach ridge. The base elevation of the foreshore deposits, which likely corresponds to spring low tide levels, is approximately 1.2 m above present sea level in the most landward beach ridges, and more than 2.5 m higher than in the three seaward ridges. The elevation difference, along with evidence for a middle Holocene sea-level highstand, is primarily attributed to glacio-hydro isostasic adjustment of the Earth; however, tectonic movements associated with subsurface fault activity beneath the plain are also considered a possible contributing factor.