Comparison of oxygen isotope profile of a fossil bivale with the modern hydrographic condition: case study of the Seoguipo Formation (Korea)

Abstract

The early to middle Pleistocene Seoguipo Formation, cropping out along the southern coast of Cheju Island (Korea), consists of shallow marine deposits containing numerous macro-and micro-fossils with prolific trace fossils. Oxygen isotopic compositions of a well-preserved marine bivalve (Mizuhopecten tokyoensis hokrikuensis) from the formation were measured through a high-resolution sampling along the maximum shell growth to track the paleoceanographic conditions during the lifespan of the bivalve. The oxygen isotope profile shows two distinct cyclic patterns with seasonal contrast. The minimum (i.e., summer) and maximum (i.e., winter) δ18O values denote about 2.9 to 3.4‰ (PDB) of the seasonal variation, which can be converted to approximately 13 to 15°C in seasonal temperature variation. Using the paleotemperature equation for calcite, the bimonthly measurements of temperature and salinity at water depths of 10 m and 30 m surveyed by the National Fisheries Research and Development Agency (Korea) between 1991 and 1997 enable us to construct the theoretical oxygen isotope profile that represents the present-day environmental condition. The constant δ18O values of seawater are assumed in the equation, although the salinity varies seasonally to some extent. Direct comparison between the fossil mollusk profile (ancient condition) and the predicted isotope profile (present-day condition) explains that the summer and winter paleotemperatures during fossil shell growth were quite lower than the present. It is also possible that the bivalve might grow in the18O-enriched condition different from the present-day seawater.