Records of palaeo-seawater condition from oxygen-isotope profiles of early Pleistocene fossil molluscs from the Seoguipo Formation (Korea)

Abstract

High-resolution delta 18O profiles of early Pleistocene fossil molluscs (Mizuhopecten tokyoensis hokurikuensis) from the shallow-marine sedimentary Seoguipo Formation (Korea) were found to exhibit distinct annual cycles identified by their unique seasonality (delta 18O amplitude). A direct comparison of fossil delta 18O profiles with that of living shells (Amusium japonicum japonicumi) indicated that the palaeoseawater conditions differed from present-day seawater. Specifically, the positive delta 18O shift in the isotope profile of the fossil specimens relative to that of the living mollusc shell reflected that palaeotemperature was lower than that today. However, a comparison of the coldest palaeotemperatures (determined from the heaviest delta 18O values of fossil shells), with the present-day winter temperatures indicates that temperature variation alone cannot account for the entire positive delta 18O offset. These findings indicate that variation in the seawater delta 18O(w) values plays a dominant role in the biogenic carbonate precipitation of fossils. Thus, the fossil shells obtained from stratigraphic units suggest different palaeoenvironmental conditions, including lower temperatures and 18O-enriched glacial seawater, when compared with the present-day conditions. The Seoguipo Formation records at least five cycles of relative sea-level fluctuations, with changes in fossil delta 18O amplitudes separated by sequence boundaries likely to reflect variations of unique palaeoseawater condition, although the oxygen-isotope profile of fossil molluscs appears to provide a snap-shot of the palaeoclimatic signature. square Early Pleistocene, mollusc fossils, oxygen isotope, palaeoenvironment, seawater temperature.