Abstract To better constrain the Sr isotope budget in marginal domains without any fluvial inputs, we analyzed the chemical composition and 87Sr/86Sr ratio of waters and shells from four locations: two coastal lagoons, one hemipelagic platform and one open marine shelf. Our results highlight homogeneous 87Sr/86Sr ratios typical of oligotrophic oceanic waters (OOW) (i.e., 0.709172 ± 0.000023) in the Pacific Tatakoto atoll and along a Mediterranean shore to offshore transect (~25 km off Banyuls-sur-Mer, BSM). This attests that oceanic inputs from oligotrophic areas remain the main Sr source in open shelf areas compared with submarine groundwater discharges (SGD) or particulate dissolution influences. In BSM, only foreshore data are more radiogenic, possibly due to rainwater mixing, local groundwater springs or more efficient particle dissolution in the intertidal zone. In restricted areas, we report variable 87Sr/86Sr ratios between the Salses-Leucate (France) and Oualidia (Morocco) lagoons. The first one has homogeneous 87Sr/86Sr ratio typical of OOW except close to SGD. In Oualidia, 87Sr/86Sr ratios decrease by 1.2 × 10−3 from OOW values close to the Atlantic inlet to progressively less radiogenic ones upstream within the interior of the lagoon. These differences depend on several factors including the leaky, restricted or choked morphology of lagoons modulating the oceanic Sr inputs, but also SGD fluxes whose 87Sr/86Sr ratios and Sr concentrations are highly variable according to the nature of rocks leached in karstic aquifer. In Oualidia, the low 87Sr/86Sr ratios correspond to high Sr concentrations (up to 150 μmol·l−1) issued from the dissolution of Mesozoic evaporites, leading to SGD fluxes accounting for 60% of the local Sr budget. Through data compilation, we show that similar 87Sr/86Sr gradients and processes prevail at the whole Mediterranean scale. Finally, we postulate that high coastal water retention times can also account for anomalous coastal 87Sr/86Sr ratios and that the combination of water mass restriction, SGD, bioadsorption and early diagenetic processes could decrease seawater Sr concentrations in some marginal areas.

中文翻译：

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大陆边缘海水 87Sr/86Sr 比率：开放和受限大陆架区域的模式和过程

摘要 为了在没有任何河流输入的情况下更好地限制边缘区域的 Sr 同位素收支，我们分析了四个位置的水和贝壳的化学成分和 87Sr/86Sr 比率：两个沿海泻湖、一个半远洋平台和一个开放的海洋大陆架。我们的结果突出了太平洋 Tatakoto 环礁和沿地中海沿岸到近海横断面（Banyuls-sur-Mer，BSM 约 25 公里）的典型贫营养海洋水域 (OOW) 的均匀 87Sr/86Sr 比率（即 0.709172 ± 0.000023）。这证明与海底地下水排放 (SGD) 或颗粒溶解影响相比，来自贫营养区的海洋输入仍然是开放大陆架区域的主要 Sr 来源。在 BSM 中，只有前滨数据更具辐射性，可能是由于雨水混合，当地地下水泉或更有效的颗粒溶解在潮间带。在限制区域，我们报告了 Salses-Leucate（法国）和 Oualidia（摩洛哥）泻湖之间的可变 87Sr/86Sr 比率。第一个具有 OOW 典型的均匀 87Sr/86Sr 比率，但接近 SGD。在 Oualidia，87Sr/86Sr 比值从靠近大西洋入口的 OOW 值降低 1.2 × 10−3 到在泻湖内部上游逐渐减少放射成因的值。这些差异取决于几个因素，包括调节海洋 Sr 输入的泻湖的泄漏、受限或阻塞形态，以及 SGD 通量，其 87Sr/86Sr 比率和 Sr 浓度根据岩溶含水层中浸出的岩石的性质而高度可变。在瓦利迪亚，低 87Sr/86Sr 比率对应于中生代蒸发岩溶解产生的高 Sr 浓度（高达 150 μmol·l-1），导致 SGD 通量占当地 Sr 预算的 60%。通过数据汇编，我们表明在整个地中海尺度上普遍存在类似的 87Sr/86Sr 梯度和过程。最后，我们假设沿海水滞留时间长也可以解释异常的沿海 87Sr/86Sr 比率，并且水量限制、SGD、生物吸附和早期成岩过程的结合可能会降低一些边缘区域的海水 Sr 浓度。