Stable isotope profiles of bivalve shells: seasonal temperature variations, latitudinal temperature gradients and biological carbon cycling along the east coast of Korea

The stable oxygen (δ18O) and carbon (δ13C) isotopes of the living bivalve shells collected from four localities (Keojin, Jumunjin, Pohang and Kampo) in the shallow waters along the east coast of Korea (Sea of Japan) were measured at close intervals along the maximum growth line of bivalve shells to obtain the isotope profiles. High-resolution δ18O profiles exhibit distinct and annual cycles from which the seasonal temperature variations of each locality can be compared. The amplitude of δ18O profiles as well as δ18Omax and δ18Omin values demonstrates that each bivalve clearly represents the seasonal temperature variations at the collection site and the latitudinal temperature gradients can be deduced. The δ13C profiles show less distinct cyclicity, in contrast to the δ18O profiles, which may reflect the stability of available carbon isotope in the water column throughout the collection sites. The small range of variation of the δ13C values is likely to be attributed to the shallow water depth above the thermocline where the seasonal change of carbon isotope may not be well recorded. The insignificant level of seasonal blooming of primary production is likely to result in the steady biological cycling of carbon isotope associated with consistent δ13C values. However, the northward decrease of mean δ13C values may imply either the gradual increase in light carbon (12C) due to the organic matter oxidation or the different level of primary production activity on the carbon isotope.