PALEOCEANOGRAPHIC INVESTIGATION OF THE MOLLUSK FOSSILS FROM THE SEOGUIPO FORMATION, KOREA

Stable oxygen and carbon isotopic compositions of a living pectenid (Amusium japonicum japonicum) collected from the shallow waters and the fossil petenids (Mizuhopecten tokyoensis hokurikuensis) taken from three units of the early Pleistocene Seoguipo Formation in the Jeju Island have been analyzed to reconstruct the paleoceanographic condition of the shallow marine environment during the deposition of the Seoguipo Formation. Based on the high-resolution d¹⁸O profiles, all pectenids show the distinct and seasonal variations (the lowest d¹⁸O values during summer and the highest d¹⁸O values during winter). In particular, one pectenid exhibits oxygen isotopic compositions out of isotopic equilibrium as an aging effect. More interesting is a feature that the pectenid, collected at stratigraphic higher position, provides larger seasonal variations. This may suggest that the summer paleotemperatures became higher and the winter paleotemperatures lower as the Seoguipo Formation was deposited progressively. The enriched d¹⁸O values of the fossil pectenids by ca. 2‰, compared to those of a living pectenid, strongly imply that the paleoceanographic condition seems to be quitely different from the present-day when the Seoguipo Formation was deposited. It is notable that the seasonality is more prominent during the glacial period. Carbon isotopic compositions of a living and fossil pectenids also show seasonal variations. The individual sharp depletion of d¹³C values during the summer may indicate kinetic fractionation due to the faster growth rate of the shell. Otherwise, it marks the enhanced decomposition of organic matters that occurs seasonally and reduces ¹³C/¹²C ratio of the DIC in seawater.