Stable isotope and chemical composition of pearls: Biomineralization in cultured pearl oysters in Ago Bay, Japan

The δ¹⁸O, δ¹³C and trace element composition of pearls collected from Ago Bay, Japan, were investigated in order to evaluate biomineralization in the cultured pearl oyster (Pinctada fucata martensii). The oxygen isotopic data suggest that the pearls were produced around 23-24°C, mainly in June to early July, which is consistent with their occurrence in the field. Therefore the pearls were produced under or close to isotopic equilibrium conditions, although they showed high calcification rates (higher than 0.2-1.0 g cm^-2yr^-1) under which, for example, coral skeletons (calcification rate ∼0.28 g cm^-2yr^-1) often show non-equilibrium isotope partitioning. The δ¹³C values were ∼-2.9‰ lower than those calculated for offshore waters under equilibrium conditions. This may be due to low-δ¹³C bottom waters resulting from the degradation of organic matter (OM) or to a contribution of low-δ¹³C food. In the latter case, a simple mass balance calculation gives a respiration component of 14%. Twelve trace elements of bulk pearl samples were classified into four groups on the basis of their enrichment/depletion patterns relative to seawater and inter-element relationships: group 1, Co, Cr, Pb; group 2, Ba, Cs, U; group 3, Cu, Sn, V, and group 4, Mn, Rb, Mo. Comparison with coral skeletons suggests that Ba and Mn (groups 2 and 4) were definitely much enriched in proteinaceous OM relative to aragonite crystals in pearls and that V (group 3) in pearls showed only slight enrichment in the organicrich layer. By contrast, the other elements showed small differences between both layers (enrichment factor of <3), suggesting that these elements occur largely in aragonite crystals.