Organic carbon (Corg) storage and the absorption of atmospheric carbon dioxide (CO2) are among the valuable functions in vegetated coastal habitats (e.g., seagrass meadows, saltmarshes, and mangroves). The conservation and restoration of these habitats enhance atmospheric CO2 sinks, which is considered as important measures for mitigating global climate change. Corg accumulation into sediment is a key process contributing to the function of atmospheric CO2 absorption. An increasing number of studies have examined the factors controlling the Corg stocks in vegetated coastal habitats. However, the regulating factors of Corg accumulation rate and its spatiotemporal variations are poorly understood in these habitats. In this study, we examined the sediment profiles of geological and biogeochemical features in two coastal lagoons in eastern Hokkaido, aiming to investigate the regulating factors of Corg accumulation rate in vegetated coastal habitats. For assessing the variation in the depositional environments, the historical change in the source of sedimentary Corg was estimated by using isotopic and elemental signatures. Our geological and biogeochemical approaches indicated that the Corg accumulation rate was regulated by the depositional environment (relative sea-level, sediment supply, and calmness) and the habitat relocation. The depositional environment and the habitat were affected by relative sea-level fluctuations driven by seismic land-level change and climate change. In particular, relative sea-level rise accelerated sediment accretion and increased Corg accumulation rate. Our findings help planning the conservation and restoration of these habitats for enhancing their carbon storage function.

Key Words: Blue carbon, Seagrass meadows, Saltmarshes, Carbon storage, Relative sea-level, Isotopic analyses