Shells of the bivalve Arctica islandica serve as high-resolution archive of past environmental conditions. Stable oxygen isotopes (δ18O) values from wellpreserved A. islandica shells are frequently used as a proxy for water temperature (and salinity). Hence, this species may improve distinctly our understanding of seasonal temperature dynamics in the past. We present the first stable isotope (δ18O & δ13C) analysis on a fossil semi-recrystallized A. islandica shell from the Tjörnes Beds of Iceland (Pliocene). Confocal Raman microscopy is used to identify areas of pristine aragonite and recrystallized calcite shell, which were then sampled by highresolution micro-milling. We compare paleo-water temperatures inferred from stable oxygen isotope ratios of both recrystallized and non-recrystallized portions of the shell to highlight and discuss the impacts of taphonomic alterations on a micro-scale and its implications for paleo-environmental reconstructions. Our findings emphasize the need for careful interpretation of carbonate-based water temperature reconstructions, because small-scale diagenesis can significantly modify the original stable oxygen isotope signature and substantially distort the paleoclimatic or paleoenvironmental signals inferred thereof.