Freshwater pearl mussels as stream water stable isotope recorders (MUSES)

径流的同位素记录对于提高我们对水文、生态、生物地球化学和大气过程的理解和建模至关重要，但它们的全部潜力的利用受到时间序列短和截断的阻碍。在这里，我们提出了一个创新的贡献来解决河流同位素记录限制的问题：使用淡水双壳类作为长期的河流水三角洲18o记录仪。我们通过分析软体动物壳delta18O数据（从10项研究中编译）进行了试点工作，跨越了全球16条河流的18个采样点的纬度序列。我们发现降水、水流和淡水软体动物的同位素特征之间存在很强的联系——水流和软体动物都表现出降水信号的强烈衰减。我们对淡水贻贝的探索工作开辟了一条道路，我们现在打算沿着这条道路进行几十年一次的河流三角洲重建。首先，我们将验证这样一个猜想，即水流温度——从蚌壳三角洲18o重建水流三角洲18o的先决条件——可以从(a)蚌壳生长速率、(b)蚌壳微观结构性质和/或(c)蚌壳的元素化学性质推断出来。其次，我们将对IRMS和SIMS进行基准测试，测试它们在软体贝壳delta18O测量精度方面的完全兼容性和互补性。虽然IRMS是测量软体动物壳中delta18O的标准工具，但SIMS可以以高测量精度将delta18O数据扩展到后来的个体发生壳部分。第三，我们将依靠测量的多年降水delta18O数据和重建的来自卢森堡、瑞典和德国三个研究集水区的水流delta18O信号来验证我们的假设，即基岩地质控制着集水区对气候变化的水文响应的长期敏感性。为了检验我们的假设，我们将依赖于年轻水的比例（Fyw）随着流量的增加而增加的机制关系，这表明快速流动路径对高流量的比例贡献增加。在我们研究的具有不同地理特征和气候的流域中，我们假设Fyw的流量敏感性（即Fyw与流量之间关系的线性斜率）对降水和随后的溪流三角洲18o信号的潜在年代际变化不敏感。

Isotope records of streamflow are pivotal for improving our understanding and modelling of hydrological, ecological, biogeochemical and atmospheric processes – yet the use of their full potential is hindered by short and truncated time series. Here we propose an innovative contribution to the problem of stream isotope record limitations: the use of freshwater bivalves as long-term stream water delta18O recorders. We have done pilot work by analysing mollusc shell delta18O data (compiled from 10 studies), spanning a latitudinal sequence of 18 sampling sites on 16 streams around the globe. We found strong links between isotope signatures in precipitation, stream water and freshwater molluscs – both stream water and mollusc showing a strong damping of the precipitation signal. Our exploratory work on freshwater mussels has blazed a trail that we now intend to follow for multi-decadal stream water delta18O reconstructions. First, we will test the conjecture that stream water temperature – as a prerequisite for reconstructing stream water delta18O from shell delta18O – can be inferred from (a) shell growth rate, (b) shell microstructural properties and/or (c) element chemical properties of the mussel shell. Second, we will benchmark IRMS and SIMS and test their full compatibility and complementarity in terms of mollusc shell delta18O measurement accuracy. While IRMS is a standard tool for measuring delta18O in mollusc shells, SIMS may extend – with high measurement accuracy – delta18O data to later ontogenetic shell portions. Third, we will rely on measured multi-decadal precipitation delta18O data and the reconstructed delta18O signals in stream water from three study catchments in Luxembourg, Sweden and Germany for testing our hypothesis that bedrock geology controls the long-term sensitivity of a catchment’s hydrological response to climate change. For testing our hypothesis, we will rely on the mechanistic relationship between the fraction of young water (Fyw) increasing with discharge – indicating an increase in the proportional contribution of rapid flow paths towards higher flows. In our study catchments with distinct physiographic characteristics and climates, we hypothesise the discharge sensitivity of Fyw (i.e., the linear slope of the relationship between Fyw and discharge) to be unresponsive to a potential inter-decadal variability in precipitation and subsequent stream water delta18O signals.