Potential of
submarine-cave sediments and oxygen isotope composition of cavernicolous
micro-bivalve as a late Holocene
paleoenvironmental record
Abstract
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 2,000 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 d18O-derived
temperature (Td18O) of C.
iejimensis suggests that the species grows between April and July; (5) the Td18O
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
Key words: submarine-cave
sediments;