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For all treatments, exposure times ranged from 15 minutes to 6 hours. These samples comprised four catfish otoliths (Carlarius spp.) and seven bivalve shells (Senilia senilis, Linnaeus 1758). Alteration of mineralogy and geochemical proxies in modern Mercenaria campechiensis shells by simulated prehistoric cooking methods: Untreated (grey triangles and grey striped areas), boiled (blue triangles), roasted (green circles), and burned shells (orange diamonds).

Afterwards, we measured the bulk clumped-isotopic composition (Δ values of mid-Holocene shell midden constituents excavated in the northern part of the Mauritanian coast (Banc d’Arguin), radiocarbon dated to 5,020–5,320 cal. We also analysed additional samples drilled from the hinge region of each bivalve shell, as well as high-resolution ontogenetic oxygen-isotope records of three exemplary shells. (a) Conversion of primary aragonite into secondary calcite with the predicted aragonite-calcite conversion based on the Arrhenius model for biogenic aragonite of Staudigel & Swart values of all treatments were significantly different from the control shells, there was no significant difference between the different cooking treatments.

Depending upon the cooking temperature, variable changes in shell mineralogy, visual appearance and outer shell structure occurred within the different cooking treatments (Fig. Boiling and roasting of the shells caused bleaching and blackening of the outer organic layer, respectively. We observed no statistically significant alteration trend with respect to shell δ values.

While the cooked shells consistently showed a pristine outer shell structure, some roasted shells exhibited small cracks (Visual comparison of untreated left valves and the corresponding right valves exposed to the different treatments prior to hydrogen peroxide cleaning. All mid-Holocene otoliths are mainly composed of aragonite (96–100%) which is in agreement with the initial mineralogy of modern Carlarius spp.

Regardless of experimental uncertainties, the lower Δ values of the shells in the roasting and burning treatments are significantly different from the control as well as from each other.

(c) Oxygen isotopic composition of bulk fish otoliths, bulk bivalve shells and hinge samples from the bivalve shells.

The composition, internal structure or the spatial and temporal distribution of these shell middens represent a valuable source of information about human dispersal, site-specific occupation pattern, subsistence strategies, associated dietary preferences or fishing and foraging seasonality.

Moreover, mollusc shells or fish otoliths derived from shell middens can provide proxy records such as oxygen isotopes which enable the reconstruction of highest-resolution (sub-seasonal) palaeoenvironmental conditions of ancient coastal environments.

Thus, our experimental data strongly support the aforementioned hypothesis that clumped-isotope thermometry applied on aragonitic skeletal structures represents a suitable diagnostic tool for detecting and differentiating between certain prehistoric cooking techniques in shell midden deposits.

The measured alterations of δ values generally agree with previous studies in terms of magnitude, however, they differ in the minimum temperature required for a measurable alteration of oxygen isotopes in aragonitic skeletal structures by prehistoric cooking practices.

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