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Correlation of boron isotopic composition with ultrastructure in the deep‐sea coral Lophelia pertusa: Implications for biomineralization and paleo‐pH

Using the CRPG‐CNRS Cameca 1270 ion microprobe facility, we have measured boron isotopic compositions (11B/10B) in different ultrastructural components of the deep‐sea aragonitic scleractinian coral Lophelia pertusa. We observe a systematic difference in B isotopic composition between the Early Mineralization Zone (EMZ) and adjacent fibrous skeleton. In EMZ the measured δ11B values are consistently low. Fibrous aragonite is characterized by systematically higher δ11B values but also displays B isotopic heterogeneity associated with specific growth bands in the calyx wall. The magnitude of the observed B isotopic variations cannot be explained by changes in environmental conditions and is likely caused by biological processes involved in the biomineralization of new skeleton, i.e., “vital” effects. The observed B isotopic variations are opposite to the predictions of geochemical models for vital effects. These models are based on the idea that stable isotopic fractionations (including C and O) in coral skeleton are driven by changes in pH of the fluid from which the skeleton is presumed to precipitate. Our data indicate that pH variations are not responsible for the observed stable isotopic fractionations. Geochemical models therefore do not provide an adequate framework within which to understand coral skeletal formation. Without a better understanding of these processes the use of B isotopic composition to reconstruct paleo‐pH variations in the oceans must be considered problematic, at least as far as Lophelia pertusa is concerned.