A global assessment of freshwater mollusk shell oxygen isotope signatures and their relation to precipitation and stream water

Records of δ18O in stream flow are critical for understanding and modeling hydrological, ecological, biogeochemical and atmospheric processes. However, the number of such records are extremely limited globally and the length of such time series are usually less than a decade. This situation severely handicaps their use in model testing and evaluation. Here we present a global assessment of freshwater mollusk (bivalves & gastropods) isotope data from 25 river basins that have stream water isotope values, water temperature data and shell material isotope signatures. Our data span a latitude range of 37.50°S to 52.06°N. We show that δ18O signatures in freshwater mollusks are able to explain 95% of the variance of stream water δ18O. We use shell δ18O values and water temperature data to reconstruct stream water δ18O signatures. With freshwater mussel life expectancy ranging from a few years up to 200 years, this translation of mollusk metabolic properties into long term stream water isotope records is a promising approach for substantially extending global stream water isotope records in time and space.


Biology of mollusk shells
Mollusks have been used extensively as environmental sentinel organisms and archivesextending paleotemperature records on the basis of shell growth rates and  18 O signatures 11 . A limitation and source of uncertainty in the use of mollusk shell paleorecords of environmental signatures has been related to the irregular growth rates 16 . The shell material is produced by the mollusk's mantle as long as the environmental conditions remain favorable. In the case of excessively high or low water temperatures or water pollution, the mantle withdraws from the edges of the shell and the mollusk stops producing shell material.
As soon as shell growth resumes, a new growth band is formed and gradually covers the previous one. Darker and thinner bands are assumed to be produced during winter months 17 . Based on their growth bands, it is possible to estimate the age of a mollusk 17 . The average lifespan of freshwater mussels is largely species dependent -freshwater pearl mussels (Margaritifera margaritifera) living up to 200 years 16,17 .

On  18 O signatures in freshwater mollusks and their use in environmental research
While records of isotopic signatures in precipitation are available at global scale and over many decades, historical data on stream water isotope signatures is still patchy. Similar situations of limited data on past environmental conditions are typically overcome by using tree rings or sediment layers as natural archives -leading to the reconstruction of temperature chronicles or trends in soil and air chemical composition 36,37 .
For overcoming the very limited availability of long stream water  18 O series, we rely here on organisms living in freshwater bodies and potentially assimilating for decades the seasonal fluctuations in  18 O of stream water. Mollusks precipitate their calcium carbonate shell in equilibrium with the surrounding water 14,18,38 . The oxygen isotopic composition of calcium carbonate in a mollusk's shell thus depends on water temperature (controlling the fractionation process) and water isotopic composition. As a consequence, mollusks record isotope signatures in stream water 39,40 .
While the results from our global assessment are encouraging, more work needs to be done on the extraction of  18 O values from mollusk shells and within-shell trends in isotopic amplitudes and averages and how this affects extracted isotope profiles from successive years 18 . Secondary Ion Mass Spectrometry (SIMS) may be a promising technique for future global testing of mollusks as a hydrological isotope recorder as shown by Linzmeier et al. 32 and Pfister et al. 34 . Figure S1. World map of studies on isotope signatures in freshwater mollusks. The map was created in ArcGIS version 10

Shell analyses Isotope data source and extraction
Waghorne et al. 41 Crushed shell, Mass spec. analyses Average and standard deviation values directly taken from text (no raw data provided in article). Versteegh et al. 7 Shell growth bands, Micromill, Mass spec. analyses Meuse River: Stream water (article Figure 4) and shell isotope data (article Figure 11-B) extracted via PlotDigitizer. Rhine River: Stream water (article Figure 4) and shell isotope data (article Figure 12-B) extracted via PlotDigitizer. Demény et al. 42 Shell growth bands, Dental drill, Mass spec. analyses Stream water taken from Table 3 and shell isotope data (article Figure 1)  Shell growth bands, Micromill, Mass spec. analyses Stream water isotope data and temperatures extracted via PlotDigitizer (article Figure 3A). Shell isotope data taken from Table 3.
Yan et al. 45 Shell growth bands, Scalpel blade, Mass spec. analyses HuaXi River: Stream water temperature and isotope data (article Figure 3) and shell isotope data (article Figure 4) Table 2 and shell isotope data taken from article Table 4. Gomti River: Stream water isotope data taken from article article Table 2 and shell isotope data taken from Table 3. Ganga River: Stream water isotope data taken from article Table  2 and shell isotope data (article Figure 9) extracted via PlotDigitizer. Kelemen et al. 9 Shell growth bands, Micromill, Mass spec. analyses Niger River: Stream water (article Figure 2) and shell isotope data (Appendix; Shell N37B) extracted via PlotDigitizer. Oubangui River: Stream water (article Figure 2) and shell isotope data (article Figure 6) extracted via PlotDigitizer. Vonhof et al. 46 Shell growth bands, Micromill, Mass spec. analyses Omo River: Stream water taken from article Table 2 and shell isotope data (article Figure 4)  Crushed shells, Mass spec. analyses North-western Mendoza: stream water isotope data taken from Panarello & Dapeña (1996) and shell isotope data taken from article Table 4. Central Eastern Mendoza: stream water and shell isotope data from article Table 4. Southern Mendoza: stream water and shell isotope data from article Table 4. Bonadonna et al. 49 Crushed shells, Mass spec. analyses Stream water taken from Table 2 and shell isotope data taken  from Table 1.   Table S3: Shell material analyses techniques and stream water and shell isotope data sources for the 15 selected studies.