ABSTRACT

This paper provides the first wide-ranging data for iodine concentrations of natural CaCO₃ samples (50 in total) from various geologic and marine-biogenic sources. The geologic samples are (i) calcite and aragonite crystals grown in underground or cave environments and (ii) calcite-aragonite coexisting travertines deposited from mountain spring water. The marine-biogenic samples are shells/skeletons/spicules of bivalves, gastropods, anthozoans, hydrozoans, bryozoans and foraminifera (36 extant and 3 extinct species), each consisting of either calcite, aragonite, or both, and including two geochemical reference materials GSJ CRM JCt-1 (giant clam) and JCp-1 (scleractinian coral). The iodine concentration data show distinct differences depending on the sample source and taxonomic group, probably indicating effects of (i) environmental conditions (e.g., redox conditions) and (ii) inter-taxonomic differences in the iodine-incorporation mechanism. Some samples with lowest iodine concentrations suggest that iodine (probably iodate) is incorporated more preferentially into calcite than into aragonite, as previously demonstrated in a laboratory experiment and a crystal-structure simulation. Measurements of Ca, Mg, Sr and Na were also made for all the samples, which revealed that some calcite samples (e.g., bryozoan skeletons) were composed of magnesian calcite with Mg and Ca concentrations of ~12,000–42,000 ppm and ~370,000–340,000 ppm, respectively (i.e., the higher the Mg concentration, the lower the Ca concentration). For more accurate expression of the iodine concentration in various natural CaCO₃ samples, including magnesian calcite, we propose the I/(Ca+Mg+Sr+Na) ratio instead of the I/Ca and I/(Ca+Mg) ratios; in future studies, it may be desirable to investigate, for some specific marine CaCO₃ materials, whether the I/(Ca+Mg+Sr+Na) ratio can practically be a more accurate paleo-redox proxy in comparison with the I/Ca and I/(Ca+Mg) ratios. Our dataset and detailed discussion will make a significant contribution to iodine-based geochemistry and marine biology.