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A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (31 m water depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. 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 show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the δ 18 O-derived temperature (Tδ18O )o fC. iejimensis suggests that the species grows between April and July; (5) the Tδ18O of C. iejimensis from cored sediments implies that there were two warmer intervals, at AD 340±40 and AD 1000±40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene. © 2006 Published by Elsevier B.V.

生きている化石の宝庫として海底洞窟は1960年代より世界的に知られていたが,そこの堆積物に関する研究は行われていなかった.サンゴ礁における古環境解析のための新たなる地質記録としての有効性を検討するため,我々は沖縄県伊江島の海底洞窟(通称“大洞窟”)から堆積物のコア試料(長さ43cm)を採取した.堆積物はシルトからなり,葉理や生痕などの堆積構造は見られない.堆積物粒子は石灰質岩片を主とし,コッコリス,底生有孔虫や浮遊性有孔虫などを含む.貝化石の産状と14C年代測定の結果,コア試料は1,500年間以上の記録を有していることが分かった.また,平均堆積速度は2～4cm/100年と算出された.以上のことから,海底洞窟堆積物はサンゴ礁の後期完新世の環境・気候変動を解明できる記録媒体として有効であるといえる.