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.
Highlights
The hypercalcified demosponges (“sclerosponge” hereinafter), members of the earliest branching metazoan taxon (Porifera), have been dominant reef-building marine organisms throughout the Phanerozoic (e.g., Vacelet 1985; Reitner and Engeser 1987; Wood 1990; Reitner 1992)
We investigated 36 specimens of living sclerosponges collected in Kume Island, southwestern part of Japan in the North Pacific to clarify (1) within-skeletal variations in the δ18O, δ13C, and elements/Ca ratios (magnesium/ Ca (Mg/Ca), strontium/ calcium (Sr/Ca), barium/Ca (Ba/Ca), lead/Ca (Pb/Ca), and uranium/Ca (U/Ca)) and (2) their intraspecific variations
Skeletal aspects of our sclerosponge samples closely resemble those found in earlier studies (e.g., Wörheide 1998; Jackson et al 2010; Gilis et al 2013), clearly showing a typical macrostructure of reticulate skeletal organization with branches composed of aragonitic spherulitic structural elements (Ayling 1982; Reitner 1992; Allison et al 2012)
Summary
The hypercalcified demosponges (“sclerosponge” hereinafter), members of the earliest branching metazoan taxon (Porifera), have been dominant reef-building marine organisms throughout the Phanerozoic (e.g., Vacelet 1985; Reitner and Engeser 1987; Wood 1990; Reitner 1992). The stable oxygen (δ18O) and carbon (δ13C) isotope records of sclerosponges are a useful proxy for reconstructing seawater temperature and salinity (e.g., Wörheide 1998; Böhm et al 2000; Moore et al 2000; Rosenheim et al 2005a) and dissolved inorganic carbon δ13C of seawater (e.g., Böhm et al 1996; Swart et al 2010) for the past. Results of previous studies have shown that, like corals, the δ18O and δ13C values of sclerosponges have respectively reflected interannual climate variability (Swart et al 2002; Grottoli 2006; Grottoli et al 2010; Wu and Grottoli 2010) and increasing fossil fuel CO2 in seawater (Böhm et al 2002; Swart et al 2010). Some studies showed that sclerosponge strontium/ calcium (Sr/Ca) can offer a potential for paleotemperature determination of seawater (e.g., Rosenheim et al 2004; Waite et al 2018)
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