Initiation and internal structure of temperate coral reef community over the past 7.3 kyr in Kushimoto, mainland of Japan

The initiation timing and formation process of coral community in areas where coral reefs have not developed beyond 30°N are still unclear. We conducted underwater drilling of carbonate rocks to the bedrock in Kushimoto area (33.45°N, 135.75°E), Wakayama Prefecture, located on the Pacific side of mainland of Japan, and first discovered that the initiation of coral community formation in this area dates back to 7.3 ka (7487–7210 cal. yr BP, within 2σ range). This onset could be because of the warming resulting from the Kuroshio warm current approaching the Pacific side of mainland of Japan around 10–7 ka. Interestingly, we found that this period aligns closely with the onset of coral reef formation in subtropical regions of the northwestern Pacific. While Acroporidae is the primary builders of coral reefs in tropical to subtropical regions, the coral communities in Kushimoto area were formed predominantly by domal Merulinidae (Cyphastrea, Favites, and Dipsastraea), representing a single facies formed in low wave energy conditions at depths of approximately 10–20 m. The facies formed by domal corals persisted from 7.3 ka to 3.7 ka, whereas the current dominant coral community is composed of Acropora. However, it remains unclear when this Acropora community began and whether there was a hiatus between 3.7 ka (3849–3525 cal. yr BP, within 2σ range) and the present. The study area has lower SSTs compared to tropical and subtropical regions, which reduces the precipitation of calcium carbonate, hindering the upward growth of corals. Additionally, the relatively smooth rocky substrate of the study area suggests that even if coral communities were to form mounds and reach near the sea surface, they would likely be dislodged by high waves due to the open-ocean environment. As a result, although corals have been inhabiting the area since 7.3 ka, the coral reef remains underdeveloped.