Palaeoclimatic and chronostratigraphic interpretations from strontium, carbon and oxygen isotopic ratios in molluscan fossils of Quaternary Seoguipo and Shinyangri Formations, Cheju Island, Korea

Palaeotemperatures and chronostratigraphies of the Quaternary Seoguipo and Shinyangri Formations, Cheju Island, South Korea were investigated in terms of the strontium, oxygen and carbon isotopic compositions of carbonate mollusc shells. The 87Sr/86Sr ratios of mollusc shells from the Seoguipo Formation range from 0.70911 to 0.70914, which yield carbonate ages of 0.5–1.2 Ma (Early to middle Pleistocene) using the linear equation between ages and 87Sr/86Sr ratios of Quaternary seawater. The 87Sr/86Sr ratio (0.70917) of aragonitic shells from the Shinyangri Formation provide late Pleistocene ages (0.04 Ma). Based on AMS 14C dates of mollusc fossil shells from the Shinyangri Formation, the depositional age of the formation has been extended to 1570 yr BP. Palaeotemperatures calculated on the basis of the oxygen isotopic composition of the calcitic mollusc shells from the Seoguipo Formation give a range of values from 12.6 to 19.1°C, which is lower than the present-day shallow seawater temperature (14.9–25.5°C) in the vicinity of the Seoguipo coast. Aragonitic mollusc shells from the Shinyangri Formation provide oceanic temperatures ranging from 22.5 to 29.8°C, which is higher than present-day seawater temperatures (14.1–23.2°C). The calculated palaeotemperatures of the Seoguipo and Shinyangri Formations are inferred to reflect cold and warm currents flowing over Cheju Island during the Pleistocene to Holocene.