Effect of environmental conditions and skeletal ultrastructure on the Li isotopic composition of scleractinian corals

The lithium isotope compositions (7Li/6Li) and Li/Ca ratios of shallow-water and deep-sea corals (Porites lutea, Cladocora caespitosa, Lophelia pertusa and Desmophyllum cristagalli) were measured using a Caméca ims 1270 ion microprobe. The two C. caespitosa samples were grown under controlled conditions at CO2 partial pressures (pCO2) of 416 ± 29 µatm and 729 ± 30 µatm, respectively. In situ analyses show that all samples are isotopically homogeneous (within analytical precision, ± 1.1‰, 1σ) and display significantly lower δ7Li values relative to seawater, indicating a significant isotope fractionation during aragonite formation. In contrast to all other elements analysed so far, there is no relationship between the Li isotopic compositions and the skeletal ultrastructure. However, Li/Ca does show variation correlated with ultrastructure, albeit with significant differences between species. This implies that the biomineralization mechanisms, which are supposed to be different for the different skeletal components, do not influence the Li isotopic composition in corals. In particular, the model of Rayleigh fractionation in a semi-enclosed calcifying fluid is incompatible with the homogeneity of the Li isotope compositions at the micrometer scale. We also show that changes in pCO2 (and pH) do not significantly affect the Li isotope signature. Nevertheless, a small but significant and systematic difference in Li isotopic composition is observed between deep-sea azooxanthellate and shallow-water zooxanthellate corals. The lack of dependence on pH and pCO2 and on skeletal ultrastructure indicates that the Li isotopic signature of corals could be used as a proxy for reconstructing the paleo-δ7Li of seawater and, potentially, for deconvolving past continental weathering rates.