18:15 〜 19:30
[BBG21-P06] 琉球列島宮古島産の硬骨海綿の骨格記録
キーワード：硬骨海綿, 骨格, 酸素同位体比, 炭素同位体比, 古環境, 琉球列島
Sclerosponges, living in dark environments of tropical to subtropical shallow oceans, precipitate calcium carbonate skeleton with growth bands. They grow slowly at an approximate rate of <1 mm/year unlike corals (about 1 cm/year) but can be so long-lived for several decades to hundred years like corals (e.g., Benavides and Druffel, 1986). Skeletal oxygen isotopic ratios (δ18O) reflect variations in sea surface temperature and seawater δ18O with the latter being closely related to salinity reflecting the precipitation-evaporation balance at the sea surface and changes in water mass transport (e.g., Wu and Grottoli, 2009). In contrast to zooxanthellate corals, which commonly show positive correlations between skeletal δ18O and carbon isotopic ratios (δ13C), there do not exist vital effects in the secretion of sclerosponge skeleton (Druffel and Benavides, 1986). Previous studies showed significant decrease trends in the δ13C records toward the present, which is probably a result of 12CO2 added into the atmosphere/ocean from fossil fuel burning (e.g., Bohm et al. 1996). Therefore, sclerosponges are shown to provide annually resolved time series of proxy records of ocean environments since the Industrial Revolution. However, longer (>100 year) proxy records from sclerosponges were derived only from the Atlantic Ocean.
Here we present δ18O and δ13C records from high-Mg calcite skeleton of two sclerosponges (Acanthochaetetes wellsi) collected at a water depth of about 10 m from Miyako-jima, Ryukyu Islands in the North Pacific. The samples were slabbed to a thickness of 5 mm parallel to the skeletal growth and subsamples for stable isotope measurements were taken every 1 mm. External precision of replicate measurements of interlaboratory calcite material throughout the stable isotope analysis using a continuous flow isotope ratio mass spectrometer system (Delta V Advantage and Gasbench II: Thermofisher Scientific Inc.) of Ryukyu University was ±0.05 per mil for δ18O and δ13C. Soft X-ray images showed highly developed skeletal growth bands with >100 high/low density layers. The secular changes in δ13C of the two sclerosponges were quite similar to previously reported δ13C records from Atlantic and Pacific corals and sclerosponges. The long-term δ18O trends of the two samples are characterized by slight depletions throughout their living periods, indicative of an overall trend toward warmer ocean environment around Miyako-jima. Our sclerosponge-based estimates of sea surface temperature and salinity may document thermal and hydrologic variations in the Ryukyu Islands, furthering a good understanding of northwestern tropical-subtropical Pacific climate change for the last several centuries in conjunction with coral-based long proxy records.
Here we present δ18O and δ13C records from high-Mg calcite skeleton of two sclerosponges (Acanthochaetetes wellsi) collected at a water depth of about 10 m from Miyako-jima, Ryukyu Islands in the North Pacific. The samples were slabbed to a thickness of 5 mm parallel to the skeletal growth and subsamples for stable isotope measurements were taken every 1 mm. External precision of replicate measurements of interlaboratory calcite material throughout the stable isotope analysis using a continuous flow isotope ratio mass spectrometer system (Delta V Advantage and Gasbench II: Thermofisher Scientific Inc.) of Ryukyu University was ±0.05 per mil for δ18O and δ13C. Soft X-ray images showed highly developed skeletal growth bands with >100 high/low density layers. The secular changes in δ13C of the two sclerosponges were quite similar to previously reported δ13C records from Atlantic and Pacific corals and sclerosponges. The long-term δ18O trends of the two samples are characterized by slight depletions throughout their living periods, indicative of an overall trend toward warmer ocean environment around Miyako-jima. Our sclerosponge-based estimates of sea surface temperature and salinity may document thermal and hydrologic variations in the Ryukyu Islands, furthering a good understanding of northwestern tropical-subtropical Pacific climate change for the last several centuries in conjunction with coral-based long proxy records.