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Abstract
The geochemistry of the calcium carbonates of marine organisms is an excellent proxy for reconstruction of the paleoceanographic history. However, previous studies of hypercalcified demosponges (sclerosponges) are considerably fewer than those of corals, foraminifers, and bivalves. Here, we investigated stable oxygen (δ18O) and carbon (δ13C) isotopes and minor and trace element (Mg, Sr, Ba, Pb, and U) to Ca ratios of 36 living sclerosponges (Astrosclera willeyana) collected from Kume Island in the Ryukyu Islands, southwestern Japan, to evaluate the utility of geochemistry as a paleoenvironmental proxy. The δ18O, δ13C, and Sr/Ca deviations of the coevally precipitated skeleton were extremely small and almost constant at all skeletal portions, strongly suggesting that within-skeletal variations in the chemical components are negligibly small for non-symbiotic sclerosponges. Mean δ18O, δ13C, and Sr/Ca values (N = 36), falling within a quite narrow range, showed no significant evidence for intraspecific (inter-specimen) variations in the sclerosponges. The sclerosponges δ18O and δ13C were consistent with those of the aragonites precipitated in isotopic equilibrium with seawater at the growth site. The sclerosponge Sr/Ca was close to that of inorganically precipitated aragonite, and the estimated partition coefficient of 1.1 (almost unity) is identical to previously reported values for different species (Ceratoporella nicholsoni). Consequently, these results suggest that A. willeyana sclerosponge, having little vital effects on the geochemistry, is a robust indicator of paleocean environments (seawater δ18O, temperature, and dissolved inorganic carbon δ13C). Further, our evaluation study documents that sclerosponges living in deeper ocean environments can support the reconstruction of spatial and vertical paleoceanographic changes in conjunction with coral proxy records. The sclerosponge U/Ca showed little within-skeletal and intraspecific variations, but the heterogeneity and individual difference of the Mg/Ca, Ba/Ca, and Pb/Ca were relatively large, the reasons of which still remain unresolved.
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Details
1 Tohoku University, Institute of Geology and Paleontology, Graduate School of Science, Sendai, Japan (GRID:grid.69566.3a) (ISNI:0000 0001 2248 6943)
2 University of the Ryukyus, Department of Earth Science, Faculty of Science, Okinawa, Japan (GRID:grid.267625.2) (ISNI:0000 0001 0685 5104)
3 University of the Ryukyus, Tropical Biosphere Research Center, Iriomote Station, Okinawa, Japan (GRID:grid.267625.2) (ISNI:0000 0001 0685 5104)
4 University of the Ryukyus, Department of Chemistry, Biology, and Marine Science, Faculty of Science, Okinawa, Japan (GRID:grid.267625.2) (ISNI:0000 0001 0685 5104)
5 Nagoya University, Division of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya, Japan (GRID:grid.27476.30) (ISNI:0000 0001 0943 978X)
6 Universiti Sains Malaysia, Centre for Marine and Coastal Studies (CEMACS), Penang, Malaysia (GRID:grid.11875.3a) (ISNI:0000 0001 2294 3534)
7 Okinawa Prefectural University of Arts, Okinawa, Japan (GRID:grid.444807.b) (ISNI:0000 0000 9768 6969)
8 Tohoku University, Department of Civil and Environmental Engineering, Graduate School of Engineering, Sendai, Japan (GRID:grid.69566.3a) (ISNI:0000 0001 2248 6943)