Climatic and hydrologic variability in the East China Sea during the last 7000 years based on oxygen isotope records of the submarine cavernicolous micro-bivalve Carditella iejimensis

摘要

The micro-bivalve Carditella iejimensis inhabits the sediment surface within submarine caves at Ie Island, Okinawa, Japan. A comparison of the δ~(18)O values (δ~(18)O_(aragonite)) of empty and living shells indicates that the shell is formed over several seasons, and that the main cause of mortality is low water temperature. According to this hypothesis, it is likely that the shells with heaviest δ~(18)O_(aragonite) values (-0.4‰) among the recent dataset formed under or close to the lower limit of growth temperature for the species. Assuming an unchanging temperature tolerance of the species during the Holocene, samples with δ~(18)O_(aragonite) values heavier than -0.4‰ indicate unusually low temperatures and enrichment of δ~(18)O of sea water (δ~(18)O_(seawater)). The δ~(18)O_(aragonite) record of C. iejimensis from sediment cores recovered from Daidokutsu cave shows no clear long-term trend in sea surface temperature or δ~(18)O_(seawater) in the East China Sea during the past 7000 years, and indicates anomalously cool and dry events (enrichment in δ~(18)O_(seawater)) at around 6300 and 5550 cal. years BP. These events may have been related to changes in the activity of the East Asian monsoon, related in turn to weakening solar activity. In contrast, these anomalies appear to be obscured during the last 1000 years, including a weak Asian summer monsoon event during the Little Ice Age, thereby indicating that the mode of the East China Sea climatic and hydrologic response to decadal- to centennial-scale variability in the intensity of East Asian monsoon has varied over the past 7000 years.