Post-bomb coral Δ¹⁴C record from Iki Island, Japan: possible evidence of oceanographic conditions on the northern East China Sea shelf

In this study, a sea-surface water Δ¹⁴C record of AD 1966–2000 (i.e., after the atmospheric nuclear-bomb testing period of the mid-1950s to early 1960s) was reconstructed from a coral sample collected from Iki Island, western Japan. The island is located in the Tsushima Strait where the Tsushima Current flows from the East China Sea (ECS) continental shelf into the Sea of Japan, indicating a strong influence of the ECS shelf water on the island. It is widely accepted that the Tsushima Current originates in the area between the ECS shelf break and the Nansei Islands further offshore as a branch of the Kuroshio Current, although another possible origin is the Taiwan–Tsushima Current System. The Δ¹⁴C record from Iki Island shows the following evidence of a response to the atmospheric nuclear testing: (1) an increase from ~55‰ in 1966 to ~133‰ in 1970, (2) a plateau ranging between ~123 and ~142‰ during the 1970s to the late 1980s, and (3) a gradual decrease from ~115‰ in 1990 to ~83‰ in 2000. Comparison of this record with coral Δ¹⁴C records from the Nansei Islands (Okinawa Island, Ishigaki Island and Kikai Island), located ~160–280 km off the ECS shelf break and little influenced by the shelf water, suggests that the surface-water Δ¹⁴C around Iki Island was ~30–45‰ lower than that of the Nansei Islands from the mid-1960s to late 1970s, and that the Δ¹⁴C difference between Iki Island and the Nansei Islands decreased from the end of the 1970s to ~0–15‰ in the mid-1980s to 2000. The lower Δ¹⁴C around Iki Island can be explained as follows: (1) in contrast to the Nansei Islands area, the ECS shelf area is a vertically mixed, highly concentrated carbon reservoir significantly connected to subsurface and deeper waters outside the shelf area, strongly suggesting that the surface-water Δ¹⁴C of the shelf area (perhaps excepting very shallow innermost shelf areas) was significantly less sensitive to the atmospheric nuclear-bomb ¹⁴C spike than that of the Nansei Islands area; (2) the shelf-surface water is conveyed from the northern ECS to Iki Island by the Tsushima Current. Thus, it can be suggested that the post-bomb coral Δ¹⁴C record from Iki Island reflects oceanographic conditions of the northern ECS shelf.