Abstract Carbonates in loess-paleosol sequences are considered as a valuable paleoenvironmental proxy for climate change during the Quaternary period. In this study we propose a new dataset of δ44/40Ca, δ88/86Sr and 87Sr/86Sr to understand the formation of rhizoliths (roots encrusted with CaCO3) within these kinds of sequences from Nussloch region (Germany). Our results indicate that the leached fractions of rhizoliths (sampled at 3.25 m, 6.65 m, and 8.45 m depths and leached with HCl) have lower δ44/40Ca and δ88/86Sr, and higher 87Sr/86Sr isotopic values than the leached fractions of rhizosphere and root-free loess of the same stratigraphic levels. This suggests different mechanisms of carbonate formation and different sources of Sr and Ca. A three steps model of formation of rhizoliths from Nussloch has been proposed. First of all, the roots grow in an existent stratigraphic level and the biological activity associated with roots allows for the dissolution of carbonate and non-carbonate minerals. A part of the available Ca and Sr is taken up by roots, which absorb preferentially the light isotopes (e.g., 40Ca and 86Sr). A higher rate of water absorption by roots than nutrients, in association with CO2 released by biological respiration, allows to reach supersaturation with respect to CaCO3. The precipitation of rhizolith carbonate selects preferentially the light isotopes (e.g., 40Ca and 86Sr) leading to lower δ44/40Ca and δ88/86Sr isotope values than the ones recorded on the closest rhizosphere and loess. The higher 87Sr/86Sr signatures of rhizolith carbonates, which is not modified by root uptake or by precipitation processes, is the result of a mixture between 87Sr/86Sr signatures of carbonate and non-carbonate minerals digested around the roots. Nevertheless, grey zones remain, in this main hypothesis, about the very high contribution of residue materials to the isotopic signatures of rhizoliths. In this light, other mechanisms of downward transfer of Ca and Sr from upper stratigraphic level, that could imprint the leached fractions of rhizoliths, have been discussed.

中文翻译：

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多同位素方法（δ44/40Ca、δ88/86Sr 和 87Sr/86Sr）深入了解 Nussloch（德国）陆地沉积物中的根石形成机制

摘要 黄土-古土壤层序中的碳酸盐岩被认为是第四纪气候变化的重要古环境代表。在这项研究中，我们提出了一个新的 δ44/40Ca、δ88/86Sr 和 87Sr/86Sr 数据集，以了解来自 Nussloch 地区（德国）的这些类型序列中根石（根部包裹着 CaCO3）的形成。我们的结果表明，根石的浸出部分（在 3.25 m、6.65 m 和 8.45 m 深度采样并用 HCl 浸出）具有较低的 δ44/40Ca 和 δ88/86Sr，以及高于根际浸出部分的 87Sr/86Sr 同位素值。和相同地层的无根黄土。这表明碳酸盐形成的不同机制以及 Sr 和 Ca 的不同来源。已经提出了从 Nussloch 形成根石的三步模型。首先，根系在现有的地层水平上生长，与根系相关的生物活动允许碳酸盐和非碳酸盐矿物溶解。部分可用的 Ca 和 Sr 被根吸收，优先吸收轻同位素（例如 40Ca 和 86Sr）。根系吸水率高于养分，再加上生物呼吸释放的 CO2，可使 CaCO3 达到过饱和。根际碳酸盐的沉淀优先选择轻同位素（例如 40Ca 和 86Sr），导致 δ44/40Ca 和 δ88/86Sr 同位素值低于在最近的根际和黄土上记录的同位素值。根石碳酸盐具有较高的 87Sr/86Sr 特征，不会因根吸收或沉淀过程而改变，是碳酸盐和非碳酸盐矿物在根部周围消化的 87Sr/86Sr 特征混合的结果。尽管如此，在这个主要假设中，关于残留物质对根石同位素特征的非常高的贡献，灰色地带仍然存在。有鉴于此，已经讨论了从上地层向下转移 Ca 和 Sr 的其他机制，这些机制可能会影响根石的浸出部分。