Fig. 2: NanoSIMS imaging of nacre growth processes visualized via Sr pulse-chase labelling.

a ⁸⁸Sr/⁴⁰Ca ratio map sized 50 × 50 μm showing four Sr labels (magenta) and unlabelled nacre (blue) parallel to the nacre growth front (specimen: M1S1L). The combined unlabelled and Sr-labelled nacre layers have average Sr concentrations of 2,100 μg/g and 12,500 μg/g, respectively (Supplementary Table 2). The labelled nacre layers exhibit a stepped growth pattern of intercalated nacre tablets formed in seawater with normal Sr concentration (blue) and during Sr-labelling (magenta). b Magnified region of interest (circled in a) shows sharp (i.e., within 100 nm) near-vertical transitions, marked by white arrowheads, between labelled and unlabelled portions of nacre lamellae. The sizes of neighbouring Sr-labelled and unlabelled lamellae portions ranging between 3.0 and 4.2 µm indicate that these portions represent changes in Sr/Ca ratios within individual tablets as the total length of a nacre tablet is 10–20 µm. c Time-resolved schematic representation of the two components of nacre growth processes (area corresponds to that shown in b): extensional nacre growth across layers normal to the organic interlamellar sheets (white arrows) and space-filling nacre growth of individual tablets parallel with the interlamellar sheet within individual nacre layers (grey arrows). Growth between t₀ (yellow dotted line) and t₁ (green dotted line) was achieved within 3 days and between t₁ and t₂ (purple dotted line) within the following 6 days (see Supplementary Movie 1 for a full animation of the growth sequence). This time-resolved illustration of nacre growth demonstrates that extensional growth, thickening the shell, is followed by space-filling growth of separate individual nacre tablets. For additional NanoSIMS maps see Supplementary Figs. 2 and 3. Scale bar is 10 μm (a) and 3 μm (b).