Investigating variability in marine mollusk carbonates using infrared spectroscopy and radiocarbon analysis

Dusseault, Marisa (2021) Investigating variability in marine mollusk carbonates using infrared spectroscopy and radiocarbon analysis. Masters thesis, Memorial University of Newfoundland.

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Abstract

Marine bivalve mollusks are complex hierarchical calcium carbonate (CaCO₃) materials that are frequently used in archaeological studies and in chronologies that are used to study marine environment and climate change. Radiocarbon (¹⁴C) analysis of these materials is central to most of these studies. I study the variability of ¹⁴C signatures within marine bivalve mollusk shells from British Columbia, Canada by considering both the raw, uncalibrated fraction of modern carbon measurements and the calibrated age ranges. I use infrared (IR) spectroscopy as a method to screen for diagenesis - contamination or alteration of the original shell CaCO₃ - prior to radiocarbon analysis, and as a method to investigate the natural variability in the composition and structure of the shell CaCO₃. Using IR spectroscopy, I show that in three archaeological samples of the butter clam (Saxidomus gigantea) from Sechelt, British Columbia, only one shell contained contaminating calcite. However, analysis of the relative IR peak intensities showed a consistent difference between the inner nacreous aragonite layer and the outer crossed lamellar aragonite layer for all three shells. This consistent difference is likely correlated with that natural variability in aragonite microstructures. Three other archaeological shell samples from Deep Bay and Comox, British Columbia, were analyzed for ¹⁴C at three different locations within each of the shells. One sample displayed a variation in fraction of modern carbon within the shell while the others did not. I highlight that ¹⁴C measurements are meaningless without an understanding of date calibration and the choices that must be made when calibrating marine and mixed marine-terrestrial samples. My results are a starting point for developing best practices for ¹⁴C dating marine shell samples and for more focused studies on the links between crystallinity, diagenesis, and ¹⁴C.

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