Assessment of the mechanism of elemental incorporation into bivalve shells (Arctica islandica) based on elemental distribution at the microstructural scale

To address the mechanism of elemental incorporation into shells of the bivalve Arctica islandica, we performed two-dimensional elemental mapping of Sr, Mg, and S at a ∼3 μm spatial resolution using electron microprobe. These elemental distributions were precisely compared to the shell microstructure. The Sr distribution was intimately linked to the S concentration and/or shell microstructure. Sr showed a clear annual pattern with higher values at the annual growth lines and lower values in annual growth increments. The Sr and S concentrations were higher near sub-annual growth lines than in the adjacent regions, which were dominated by acicular microstructure (middle shell layer) or homogeneous microstructure (outer shell layer). Furthermore, the Sr concentration was lower in the middle shell layer than in the outer shell layer, but there was no clear difference in the S concentration between the two layers. The observed elemental distribution in the shell can be reasonably explained by the hypothesis that the organic composition at the calcification site, i.e. in the extrapallial space, can facilitate the preferential elemental incorporation into calcium carbonate. The Sr/Ca ratio shows micrometer scale variation within the shell, suggesting that fine spatial resolution, comparable to the shell microstructure, is required for the analysis of elemental distribution to avoid time-averaging and to correctly evaluate the magnitude of the compositional variation in the shell for past environmental reconstruction.