Post-bomb coral Δ14C record from Iki Island, Japan: possible evidence of oceanographic conditions on the northern East China Sea shelf
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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.
KeywordsKuroshio Current Shelf Area Coral Skeleton Tsushima Current Nansei Island
The authors thank Dr. Kaoru Sugihara for identifying the species of the coral sample (Dipsastraea speciosa) used in this study. The authors also thank the reviewers (J.R. Toggweiler and two anonymous) and the editors for their comments and suggestions on the manuscript.
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Conflict of interest
The authors declare that there is no conflict of interest with third parties.
- Chen CTA (1996) The Kuroshio intermediate water is the major source of nutrients on the East China Sea continental shelf. Oceanol Acta 19:523–527Google Scholar
- Konishi K, Tanaka T, Sakanoue M (1982) Secular variation of radiocarbon concentration in seawater: sclerochronological approach. Proc 4th Int Coral Reef Symp 1:181–185Google Scholar
- Levin I, Kromer B, Schoch-Fischer H, Bruns M, Münnich M, Berdau D, Vogel JC, Münnich KO (1994) Δ14CO2 records from sites in central Europe. In: Boden TA, Kaiser DP, Sepanski RJ, Stoss FW (eds) Trends ’93: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, ORNL/CDIAC-65:203–222Google Scholar
- Manning MR, Melhuish WH (1994) Atmospheric Δ14C record from Wellington. In: Boden TA, Kaiser DP, Sepanski RJ, Stoss FW (eds) Trends ’93: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, ORNL/CDIAC-65:193–202Google Scholar
- Mitsuguchi T (2000) Reconstruction of palaeoceanic environment from multi-element analysis of annually-banded coral skeletons. PhD thesis, Nagoya UniversityGoogle Scholar
- Mitsuguchi T, Matsumoto E, Abe O, Uchida T, Isdale PJ (1997) Magnesium/calcium ratio of coral skeletons as a palaeothermometer. Proc 8th Int Coral Reef Symp 2:1701–1706Google Scholar
- Yamano H, Sugihara K, Watanabe T, Shimamura M, Hyeong K (2012) Coral reefs at 34°N, Japan: exploring the end of environmental gradients. Geology 40:835–838. doi: 10.1130/G33293.1
- Zhao BR, Ren GF, Cao DM, Yang YL (2001) Characteristics of the ecological environment in upwelling area adjacent to the Changjiang River estuary. Oceanol Limnol Sin 32:327–333Google Scholar