Dating Thach Lac: cryptic CaCO3 diagenesis in archaeological food shells and implications for C-14

Petchey, Fiona, Piper, Phillip J., Dabell, Kathleen, Brock, Fiona, Turner, Helen, and Dzung, Lam Thi My (2022) Dating Thach Lac: cryptic CaCO3 diagenesis in archaeological food shells and implications for C-14. Radiocarbon, 64 (5). pp. 1093-1107.

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In many locations around the world, shell radiocarbon dates underpin archaeological research. The dating of shell brings the chronological relationship between the sample and target event (e.g., hunting and food preparation) into congruence, while shells are valuable geochemical proxies for understanding past climate dynamics and environments. However, this information can be lost as the shell, composites of biopolymers and carbonate minerals (mostly calcite and or aragonite), undergo diagenetic alteration. While studies into Pleistocene-age carbonates are common in the radiocarbon literature, there has been little research into the impact of alteration on Holocene-age shells used to interpret recent societal developments. The limits of our understanding of these diagenetic changes became evident when dating Placuna placenta (naturally calcitic) and Tegillarca granosa (naturally aragonitic) shells from the site of Thach Lac in Vietnam. These shells returned ages significantly younger than associated charcoal and terrestrial bone at the site, but standard tests for secondary recrystallization (XRD and staining techniques) did not indicate any alteration. Further investigation revealed that cryptic recrystallization (i.e., of the same crystal structure) had occurred in both the calcite and aragonite shells. This finding suggests recrystallization may have an undetected impact on some shell radiocarbon dates.

Item ID: 76082
Item Type: Article (Research - C1)
ISSN: 1945-5755
Keywords: aragonite, calcite, cryptic recrystallization, shell, Vietnam
Copyright Information: © The Author(s), 2022. Published by Cambridge University Press for the Arizona Board of Regents on behalf of the University of Arizona. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (, which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
Funders: Australian Research Council (ARC)
Projects and Grants: ARC DP140100384
Date Deposited: 21 Sep 2022 08:39
FoR Codes: 43 HISTORY, HERITAGE AND ARCHAEOLOGY > 4301 Archaeology > 430101 Archaeological science @ 100%
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