Tracing the seasonal origin of boreal streamwater over two centuries: Insights from oxygen isotopes of freshwater pearl mussel shells from northern Sweden

1 Title of dataset Dataset: Tracing the seasonal origin of boreal streamwater over two centuries: Insights from oxygen isotopes of freshwater pearl mussel shells from northern Sweden Abstract Global warming profoundly affects snowmelt-driven runoff regimes in northern Sweden. In particular, shifts in timing and magnitude of spring floods, which often account for more than half of annual streamflow, have the potential to significantly affect summer drought vulnerability. Assessing these changes requires analyzing historical seasonal water budgets, particularly the contributions of winter vs summer precipitation to streamflow. To quantify these contributions, oxygen isotopes (δ18O) can serve as natural tracers when comparing the seasonal oxygen isotope signatures in precipitation with those in streamwater. The dataset presents shell-based estimates of the seasonal origin index (SOI), developed by Allen et al. (2019a, b), for four northern Swedish streams from 1836 to 2023. Keywords Seasonal Origin Index (SOI); Bivalve sclerochronology, Streamflow chronologies ; Streamwater oxygen isotopes; Margaritifera margaritifera; Climate change; Droughts; Catchment hydrology; Boreal regions Lead author for the dataset Guilhem Türk1 Christoph J. Gey 2 Title and position of lead author PhD student1 PhD student 2 Organization and address of lead author 1Catchment and eco-hydrology group/ Environmental sensing and modelling unit, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg 2Institute of Geosciences, University of Mainz, Joh.-Joachim-BecherWeg 21, 55128 Mainz, Germany Email address of lead authors [email protected] [email protected] Additional authors or contributors to the dataset Laurent Pfister2,3, Hjalmar Laudon4, Frankie Thielen5, Bernd R. Schöne1 Organization associated with the data 1 Institute of Geosciences, University of Mainz, Joh.-Joachim-BecherWeg 21, 55128 Mainz, Germany 2 Catchment and ecohydrology group/ Environmental sensing and modelling unit, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg 3 University of Luxembourg, Faculty of Science, Technology and Medicine, 2 Avenue de l’Université, L-4365 Esch-sur-Alzette, Luxembourg 4 Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Skogsmarksgränd 17, 90183 Umeå, Sweden. 5 natur&ëmwelt Fondation Hëllef fir d’Natur, 14 Haaptstrooss, L-9764 Marnach, Luxembourg Funding This study was made possible by a German Research Foundation (DFG) grant (SCHO793/25) to B. R. Schöne and a Luxembourg Fonds 2 National de la Recherche (FNR) grant (C20/SR/14757154) to L. Pfister within the framework of the joint D-LUX – CORE 2020 project MUSES. License CCO Geographic location – verbal description Northern Sweden: Västerbotten and Norbotten County Geographic coverage bounding coordinates Lat: 64.75 – 66.37 Lon: 19.00 – 22.08 (WGS 84) Time frame - Begin date 1836 Time frame - End date 2023 General study design In this study, 13 freshwater pearl mussel (Margaritifera margaritifera) shells from four catchments in northern Sweden are used to reconstruct δ18Ostreamwater from 1836 to 2023 CE. These reconstructions are combined with model based δ18Oprecipitation predictions to determine seasonal contributions of precipitation to streamflow and to investigate catchment-specific evolution of the seasonal origin of stream water over the past two centuries. Laboratory, field, or other analytical methods For the isotope analysis powder samples of the bivalve shells of 50 to 120 µg were dissolved with anhydrous phosphoric acid in He-flushed exetainers at 72°C. The released CO2 was measured using a ThermoFisher MAT 253 isotope ratio mass spectrometer in continuous flow mode, coupled to a GasBench II. Data were two-point calibrated against a Carrara Marble by IVA Analysensysteme GmbH (δ13C = +2.01 ‰; δ18O = 1.91 ‰) and NBS-18 (δ13C = - 5.014 ‰, δ18O = -23.20 ‰). To derive d18OQ from δ18Oshell, the empirical equation by Grossman and Ku (1986) was employed, as described in Schöne et al. (2020) δ18OQ =19.43−4.34 × 𝛿𝛿18𝑂𝑂𝑠𝑠ℎ𝑒𝑒𝑒𝑒𝑒𝑒−𝑇𝑇𝑠𝑠𝑠𝑠𝑠𝑠𝑒𝑒𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑒𝑒𝑠𝑠 −4.34 The δ18Oprecipitation was obtained with the model developed in Türk et al. (2025) or directly downloaded from https://isotope.bot.unibas.ch/PisoAI/ ; last accessed August 13, 2024. Isotope values are based on modeling by Nelson et al. 2021. The SOI was calculated using the in the dataset available parameter following the equation developed by Allen et al. (2019a, b): SOI =�d18OQ−d18OP d18OPsummer−d18OP,if d18OQ >d18OP d18OQ−d18OP d18OP−d18OPwinter,if d18OQ <d18OP Taxonomic species or groups Margaritifera margaritifera Quality control As a quality control for shell isotope analysis we used the IAEA 603 as reference material (δ18O = -2.37 ‰, δ13C = +2.46 ‰). Based on longterm analysis of NBS-19, the machine has an accuracy of 0.04 ‰ for δ13C values and an average internal precision error of 0.03 ‰ based on 8 injections per analysed sample. For δ18O values, the accuracy is better than 0.05 ‰ and the internal precision error is better than 0.08 %. 3 Additional information For information on how to derive oxygen isotopes of stream water from shells, see: Grossman, E.L., Ku, T.- L., 1986. Oxygen and carbon isotope fractionation in biogenic aragonite: Temperature effects. Chem. Geol. 59, 59–74. https://doi.org/10.1016/0168-9622(86)90057-6 Schöne, B.R., Meret, A.E., Baier, S.M., Fiebig, J., Esper, J., McDonnell, J., Pfister, L., 2020. Freshwater pearl mussels from northern Sweden serve as long-term, highresolution stream water isotope recorders. Hydrol. Earth Syst. Sci. 24, 673–696. https://doi.org/10.5194/hess-24-673-2020 For information on modeled precipitation oxygen isotope data, see: Türk, G., Gey, C.J., Schöne, B.R., Pfister, L., 2025. Effects of synoptic atmospheric variability on sub-daily precipitation & δ18O-air temperature functions in reconstructions of preinstrumental δ18O chronicles across Europe. EGUsphere 1–26. https://doi.org/10.5194/egusphere-2024-4169 Nelson, D.B., Basler, D., Kahmen, A., 2021. Machine learning predictions of precipitation isotope time series applied to Europe. Proc. Natl. Acad. Sci. U.S.A. 118, e2024107118. https://doi.org/10.1073/pnas.2024107118 For information on the Seasonal Origin Index (SOI), see: Allen, S.T., Kirchner, J.W., Braun, S., Siegwolf, R.T.W., Goldsmith, G.R., 2019a. Seasonal origins of soil water use by trees. Hydrol. Earth Syst. Sci. 23, 1199-1210. https://doi.org/10.5194/hess-23-1199-2019 Allen, S.T., von Freyberg, J., Weiler, M., Goldsmith, G.R., Kirchner, J.W., 2019b. The seasonal origin of stream water in Switzerland. Geophys. Res. Lett. 46, 10425-10434. https://doi.org/10.1029/2019GL084552 Dataset filename: 2025_06_soi_data_1836-2023.xlsx Sheet: 1_d18Op_sinfits Dataset description: Model-derived stable oxygen isotopes of precipitation Column name Description Units id Unique integer identifier model Model used to derive precipitation δ18O stream id Identifier of the stream dO_mean Mean precipitation δ18O (VSMOW2 ‰) dOs Summer precipitation δ18O obtained with the sine wave curve fittings (VSMOW2 ‰) 4 dOw Winter precipitation δ18O obtained with the sine wave curve fittings (VSMOW2 ‰) rsq Coefficient of determination (R²) of sine wave fit n Number of months included in the sine wave fitting period months date Reference date assigned to the fitted window YYYY-MM-DD start_date Start date of the data segment used for fitting YYYY-MM-DD end_date End date of the data segment used for fitting YYYY-MM-DD Dataset filename: 2025_06_soi_data_1836-2023.xlsx Sheet: 2_d18Oq Dataset description: Shell-derived stable oxygen isotopes of streamwater Column name Description Units id Unique integer identifier stream id Identifier of the stream where the bivalve was collected lat Latitude Decimal degrees lon Longitude Decimal degrees elevation Catchment average elevation m a.s.l. taxon Species shell id Identifier of the shell shell portion Shell sublayer from which isotope sample was taken, oOSL: outer portion of outer shell layer year Year for which the SOI was calculated yr CE d18OQ Streamwater δ18O derived from bivalve shell (VSMOW2 ‰) Dataset filename: 2025_06_soi_data_1836-2023.xlsx Sheet: 3_soi Dataset description: Seasonal origin index Column name Description Units id Unique integer identifier stream id Identifier of the stream where the bivalve was collected year Year for which the SOI was calculated yr CE soi Seasonal origin index based on δ18O Pmean , δ18OPsummer, δ18OPwinter, δ18OQ - here representing the SOI during the main growing season of the bivalves from June to August soi.sd Standard deviation of the seasonal origin index