Sulfur speciation in growth layers of shell cross section of the long-lived bivalve Margaritifera laevis using synchrotron spectromicroscopy analysis

To reliably utilize the information on growth and environment stored within the hard carbonate shells of long-lived bivalves, it is necessary to accurately count the annual growth lines from their shell cross-sections. In this study, we investigated the relationship between the distribution of minor elements and visual lines in the shell cross-sections of the long-lived mussel Margaritifera laevis collected from the Chitose River in Hokkaido, Japan. The distribution of minor elements, such as sulfur (S), phosphorus (P), strontium (Sr), magnesium (Mg), and sodium (Na), in a cross-section of the umbo was determined by X-ray fluorescence analysis using synchrotron soft X-ray radiation. The chemical properties of S and P were analyzed in-situ using soft X-ray photoabsorption spectroscopy, and the chemical form dependence of S distribution was determined by speciation mapping. In addition, the annual and disturbance lines were visualized by staining shell cross-sections. Our results showed that the distribution of reduced S (i.e., SC and SH) corresponded to the annual lines, whereas the distribution of oxidized S (i.e., SO42−) corresponded to the disturbance lines. Overall, this study revealed that speciation analysis of S in shell cross-sections can be used to determine the origin of visual lines in chronological studies.