Abstract High-precision potassium (K) isotope ratios have recently been proposed as a new tool for tracing continental weathering and reconstructing Earth’s past climates. The premise is that the K isotopic composition of seawater is sensitive to terrestrial weathering changes. Modern seawater (δ41KNIST SRM3141a = +0.12±0.07‰) is significantly enriched in heavy K isotopes compared to the Bulk Silicate Earth (BSE) and the Upper Continental Crust (UCC). However, the controls causing such a large isotopic fractionation between these two major reservoirs are not well understood. Dissolved K in river water is one of the major inputs of K to seawater. To constrain the poorly defined K isotopic composition of riverine input to the global ocean and to understand the controlling factors of the K isotope composition of seawater, we analyzed the K isotopic composition of 32 river samples from 24 major rivers globally. These rivers drain all continents except Antarctica and collectively account for 40% of the annual global river discharge and 39% of the total K flux into the ocean. We observed a large range in K isotopic composition across all the rivers analyzed, ranging from δ41K = −0.59±0.04‰ to −0.08±0.04‰, but found no significant K isotopic variations among samples collected from the same river under differing flow conditions. We attribute the dissolved K isotopic composition of global rivers to the fraction of K retained in clay minerals during chemical weathering. Isotopically light K is retained with the clay fraction during weathering leading to heavy isotopic enrichment in the dissolved K load relative to the BSE and UCC. The flux-weighted and regionally-adjusted mean composition of all rivers studied here (−0.38±0.04‰) serves as a global estimate of the riverine δ41K value. The seawater K isotopic composition (i.e., +0.12±0.07‰) cannot be explained solely by the riverine input. Other mechanisms (hydrothermal input, reverse weathering, biological fractionation) are needed to explain seawater K isotopic composition.

中文翻译：

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世界主要河流的溶解钾同位素组成

摘要 高精度钾 (K) 同位素比值最近被提议作为追踪大陆风化和重建地球过去气候的新工具。前提是海水的K同位素组成对陆地风化变化敏感。与大块硅酸盐地球 (BSE) 和上大陆地壳 (UCC) 相比，现代海水 (δ41KNIST SRM3141a = +0.12±0.07‰) 显着富含重钾同位素。然而，导致这两个主要储层之间如此大的同位素分馏的控制因素尚不清楚。河水中溶解钾是海水中钾的主要输入之一。为了限制输入到全球海洋的河流中定义不明确的 K 同位素组成，并了解海水 K 同位素组成的控制因素，我们分析了来自全球 24 条主要河流的 32 条河流样本的 K 同位素组成。这些河流排放除南极洲以外的所有大陆，它们共同占全球河流年流量的 40% 和流入海洋的钾总通量的 39%。我们在所有分析的河流中观察到 K 同位素组成的很大范围，范围从 δ41K = -0.59±0.04‰ 到 -0.08±0.04‰，但发现在不同流量条件下从同一条河流收集的样本之间没有显着的 K 同位素变化。我们将全球河流溶解的 K 同位素组成归因于化学风化过程中保留在粘土矿物中的 K 分数。在风化过程中，同位素轻 K 保留在粘土部分中，导致溶解 K 负荷中相对于 BSE 和 UCC 的重同位素富集。此处研究的所有河流的通量加权和区域调整平均成分 (-0.38±0.04‰) 作为河流 δ41K 值的全球估计值。海水K同位素组成（即+0.12±0.07‰）不能仅由河流输入来解释。需要其他机制（热液输入、逆风化、生物分馏）来解释海水 K 同位素组成。