Environmental changes in Korea/Tsushima Strait region for the last 40 years recorded in Iki faviid coral

Abstract

The Korea/Tsushima Strait is the major passage of the Tsushima and Korea warm currents and is the best place to monitor the effect of global climate change on Korean and Japanese marine environments. Preliminary field survey revealed abundant distribution of living and fossil hard corals in the region, nevertheless of its locationclose to the northern limit of coral distribution. It implies that high resolution (seasonal to annual) reconstruction of past marine environment is possible in the region using coral skeletons. For the reconstruction of past climate in the Korea/Tsushima Strait region, we collected 90-cm-long core of living faviid coral and 5-m-long drill core of coral mound in Iki Island, Japan. The living faviid coral core showed about 150 couplets of lighter and darker bands in soft x-ray images, indicating growth period of ~150 years. The fossil corals from mound showed C-14 age of several hundreds to 4,000 years b.p. Both oxygen and carbon isotopic ratios of the top 28cm section of the living coral showed clear seasonal variation consisting of 41 cycles, indicative of coral growth period from 1966 to 2007. Systematic variation of oxygen isotopic value was caused by seasonal fluctuation of sea surface temperature (SST) and sea surface salinity (SSS). However, the amplitude of seasonal variation in oxygen isotopic ratios was relatively small compared with observed SST data. It suggests the secession of skeletal growth during winter due to low SST below 18C, lower limit of coral growth. Therefore, the maximum oxygen isotope value of each year likelyreflects the late autumn and spring ocean condition rather than winter. These autumn-spring oxygen isotope values are relatively uniform from mid-1970’s to the present, but about 0.5 permil higher from 1966 to mid-1970s. It can be attributed to changes in precipitation pattern in Tsushima Current region or Chang Riverdrainage area due to northern hemisphere climate regime shift occurred in 1976-1977.