Late Holocene paleoenvironmental analysis from submarine cave deposits

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We examined a sediment layer and surface sediments in a submarine limestone cave (31 m depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan. In addition, oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data lead to the following conclusions : 1) although the biotic and non-biotic environments within the cave have persisted for the past 5,000 years, the environmental conditions of the innermost cave area gradually spread to the entrance of the cave ; 2) the alternation of water masses within the cave is caused by the tidal cycle ; 3) the δ^<18>O-derived temperature of C. iejimensis indicates spring temperature at 30 m deep outside the cave ; 4) the δ^<18>O-derived temperature of C. iejimensis from cored sediments implies that there were two warmer intervals, at about AD 280 and AD 1050 ; 5) a sediment layer containing pumice grains (BC 440 ± 40 to AD 640 ± 80) was identified and named as Daidokutsu pumice. These suggest that submarine-cave sediments and oxygen isotope records of cavernicolous shells are a useful tool to reconstruct century-scale paleoenvironmental and climatic variability for the subtropical Okinawa Islands during the Holocene.