The analysis of the stable isotopic composition of bivalve shells provides the data needed to construct climate records at high temporal resolution. Yet, the reproducibility of the results and the effect of microstructural organization on the isotopic signature and measurements have not been extensively studied. Here, we examine the architectural changes within Arctica islandica shells, specifically if samples from microstructurally different shell layers differ in respect to stable oxygen isotope values. The oxygen isotope profiles of two microstructurally different shell layers, each sampled at different temporal resolution, were compared to each other. Our results show that aragonite, collected from the layer that is dominated by cross-acicular/lamellar structures, tends to be enriched in heavier oxygen isotopes compared to samples from portions of the outer shell layer dominated by homogeneous microstructure. In some cases, this difference exceeded 0.3‰, which can significantly affect the interpretation of a recorded environmental signal. Observed differences in stable oxygen isotope data may be associated with the physiology of the mollusk and the physical and chemical composition of studied shell layers
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