Reconstructing stream water oxygen isotopes using freshwater pearl mussels (Margaritifera margaritifera): Toward an exploration of sub-seasonal watershed dynamics over the past 200 years

Stream water oxygen isotopes are a powerful natural tracer of watershed functions (e.g., water collection, storage, mixing, and release). However, instrumental δ¹⁸O_water records have limited spatial coverage and rarely span over more than a decade. Long-lived (> 200 years) freshwater pearl mussels (Margaritifera margaritifera) are a promising natural archive for complementing these δ¹⁸O chronicles because their shells faithfully capture the isotope composition of the water in which the mussel lived. The reconstruction of reliable shell-based δ¹⁸O_water chronologies relies on several steps: (i) identifying which layer of the shell biomineralizes in thermodynamic equilibrium with the ambient water, (ii) determining the seasonal timing and rate of shell growth to assign each isotope sample to a precise calendar date, and (iii) employing a temperature record or temperature reconstruction to resolve the thermodynamic relationship between δ¹⁸O_shell and δ¹⁸O_water. First, we assessed the oxygen isotope fractionation between the ambient water and both sublayers of the outer shell layer, i.e., the outer prismatic and inner nacreous portion. We found that reconstructions based on δ¹⁸O_shell of the prismatic sublayer provided excellent δ¹⁸O_water data, whereas the nacreous portions showed evidence of growth rate-induced (kinetic) effects. Second, we carried out tank experiments to quantify temperature controls on biomineralization rates, constrain the seasonal timing and rate of shell formation, and construct a monthly resolved seasonal growth model. Third, we assessed the temperature sensitivity of shell ultrastructural properties and observed a poor relationship between water temperature and the nacre table thickness, strongly masked by pH fluctuations in stream water. Based on these findings, we have reconstructed sub-seasonal δ¹⁸O_water records of streams in Luxembourg, Germany and Sweden extending back to the early 19^th century. These unprecedented reconstructions of stream water δ¹⁸O chronicles will open new vistas on multidecadal to centennial dynamics in the continental water cycle (e.g., for the assessment of watershed sensitivity to global change).