Comparative oxygen and carbon isotopic records of Miocene and recent lacustrine unionid bivalves from Poland

The δ13C and δ18O isotope data from both fossil (Miocene) and modern freshwater bivalve shells of family Unionidae from Poland (species Margaritifera flabellatiformis and Unio tumidus, respectively) show a similar, truncated sinusoidal pattern.. The isotopic profiles of the whole shell are visibly marked by three growth stages, linked with a progressive loss of environmental record because of declining intra-annual biocarbonate accretion rate. The juvenile and gerontic phases exhibit generally more positive and stable (plateau) isotopic pattern than the mid-age stage. An increasing δ13C trend is typical for the final life stage, likely influenced by nutrient overloading, reversing the tendency towards δ13C depletion throughout the individual’s life induced by metabolic processes. Due to the progressive loss of environmental signals through ontogeny, these initial and final isotopic profile segments probably correspond to, respectively, an instant signature of the first season growth, and a multiyear value set of summer maxima during geriatric stage. Vague seasonal cyclic record is the striking feature of the mid-age δ18O and δ13C profile slices. In case of low-amplitude δ18O curve, this is probably promoted by a sensitivity of the lake ecosystem to many dynamic intra-annual factors affecting water budget balance. This consistent signature mode seems to be typical for lake-dwelling unionid shells at least since Miocene from different climatic zones, as confirmed by coeval lacustrine low-latitude mussels from Amazonia. Thus, this isotope record is relevant to obtain information on the habitat and life cycle of the fossil freshwater bivalves, as well as could help understand modern environmental change