Science Advances

The PDFset includes:

• Supplementary discussion on quantifying mineralogy using XRD
• Table S1. XRD peak areas for calcite 104 (A_calc) and aragonite 111 (A_arag) of calibration materials and bivalves from the CRNWR excavation site.
• Fig. S1. XRD scans of pure aragonite and calcite.
• Fig. S2. Relative peak area for aragonite and calcite relative to the molar fraction of each mineral.
• Fig. S3. Photographs of all shells analyzed in this study.
• Fig. S4. Cross plot of δ¹³C and δ¹⁸O values for all modern and shell midden bivalves discussed in the main text.
• Legend for data S1

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Other Supplementary Material for this manuscript includes the following:

• Data S1 (Microsoft Excel format). Isotopic analyses of equilibrated gases, Cabo Rojo bivalves, and ETH carbonate standards.