Abstract The geochemical and isotopic composition of terrigenous clays from marine sediments can provide important information on the sources and pathways of sediments. International Ocean Discovery Program Expedition 361 drilled sites along the eastern margin of southern Africa that potentially provide archives of rainfall on the continent as well as dispersal in the Agulhas Current. We used standard methods to remove carbonate and ferromanganese oxides and Stokes settling to isolate the clay fractions. In comparison to most previous studies that aimed to extract the detrital signal from marine sediments, we additionally applied a cation exchange wash using CsCl as a final step in the sample preparation. The motivation behind the extra step, not frequently applied, is to remove ions that are gained on the clay surface due to adsorption of authigenic trace metals in the ocean or during the leaching procedure. Either would alter the composition of the detrital fraction if no cation exchange was applied. Moreover, using CsCl will provide an additional measure of the cation exchange capacity (CEC) of the samples. However, no study so far has evaluated the potential and the limitations of such a targeted protocol for marine sediments. Here, we explore the effects of removing and replacing adsorbed cations on the clay surfaces with Cs+, conducting measurements of the chemical compositions, and radiogenic isotopes on a set of eight clay sample pairs. Both sets of samples underwent the same full leaching procedure except that one batch was treated with a final CsCl wash step. In this study, organic matter was not leached because sediments at IODP Site U1478 have relatively low organic content. However, in general, we recommend including that step in the leaching procedure. As expected, significant portions of elements with high concentrations in seawater were replaced by Cs+ (2SD 2.8%.) from the wash, including 75% of the sodium and approximately 25% of the calcium, 10% of the magnesium, and 8% of the potassium. Trace metals such as Sr and Nd, whose isotopes are used for provenance studies, are also found to be in lower concentrations in the samples after the exchange wash. The exchange wash affected the radiogenic isotope compositions of the samples. Neodymium isotope ratios are slightly less radiogenic in all the washed samples. Strontium and Pb isotopes showed significant deviations to either more or less radiogenic values in different samples. The radiogenic isotopes from the CsCl-treated fractions gave more consistent correlations with each other, and we suggest this treatment offers a superior measure of provenance. Although we observed changes in the isotope ratios, the general trend in the data and hence the overall provenance interpretations remained the same. However, the chemical compositions are significantly different. We conclude that a leaching protocol including a cation exchange wash (e.g. CsCl) is useful for revealing the terrestrial fingerprint. CEC could, with further calibration efforts, be useful as a terrestrial chemical weathering proxy.

中文翻译：

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制定获取海洋沉积物中陆源碎屑成分的协议——国际海洋发现计划远征 361 的一项试点研究

摘要 海洋沉积物中陆源粘土的地球化学和同位素组成可以为沉积物来源和途径提供重要信息。国际海洋发现计划远征 361 沿非洲南部东部边缘钻探了地点，这些地点可能提供该大陆降雨量以及厄加勒斯海流扩散的档案。我们使用标准方法去除碳酸盐和锰铁氧化物，并使用斯托克斯沉降来分离粘土部分。与以前旨在从海洋沉积物中提取碎屑信号的大多数研究相比，我们还使用 CsCl 作为样品制备的最后一步应用了阳离子交换洗涤。额外步骤背后的动机，不经常应用，是去除由于在海洋中或在浸出过程中吸附自生微量金属而在粘土表面获得的离子。如果不应用阳离子交换，两者都会改变碎屑部分的组成。此外，使用 CsCl 将提供样品的阳离子交换容量 (CEC) 的额外测量。然而，到目前为止，还没有任何研究评估过这种针对海洋沉积物的有针对性的协议的潜力和局限性。在这里，我们探索了用 Cs+ 去除和替换粘土表面上吸附的阳离子的影响，对一组八个粘土样品对的化学成分和放射性同位素进行测量。两组样品都经历了相同的完全浸出程序，只是一批样品经过最后的 CsCl 洗涤步骤处理。在这项研究中，由于 IODP Site U1478 的沉积物有机物含量相对较低，因此有机物没有浸出。但是，一般而言，我们建议在浸出程序中包括该步骤。正如预期的那样，海水中的大部分高浓度元素被洗涤中的 Cs+（2SD 2.8%）取代，包括 75% 的钠和大约 25% 的钙、10% 的镁和 8% 的钾。还发现在交换洗涤后的样品中，痕量金属（例如 Sr 和 Nd）的同位素用于来源研究，其浓度较低。交换洗涤影响样品的放射性同位素组成。在所有洗涤过的样品中，钕同位素比值略低。锶和铅同位素在不同样品中显示出或多或少的显着偏差。CsCl 处理的馏分中的放射性同位素相互之间具有更一致的相关性，我们建议这种处理提供了一种优越的来源测量方法。尽管我们观察到同位素比率的变化，但数据的总体趋势以及因此对整体来源的解释保持不变。但是，化学成分有很大的不同。我们得出结论，包括阳离子交换洗涤（例如 CsCl）的浸出方案对于揭示陆地指纹很有用。通过进一步的校准工作，CEC 可以用作陆地化学风化代理。来自 CsCl 处理部分的放射性同位素相互之间具有更一致的相关性，我们建议这种处理提供了一种优越的来源测量方法。尽管我们观察到同位素比率的变化，但数据的总体趋势以及因此对整体来源的解释保持不变。但是，化学成分有很大的不同。我们得出结论，包括阳离子交换洗涤（例如 CsCl）的浸出方案对于揭示陆地指纹很有用。通过进一步的校准工作，CEC 可以用作陆地化学风化代理。来自 CsCl 处理部分的放射性同位素相互之间具有更一致的相关性，我们建议这种处理提供了一种优越的来源测量方法。尽管我们观察到同位素比率的变化，但数据的总体趋势以及因此对整体来源的解释保持不变。但是，化学成分有很大的不同。我们得出结论，包括阳离子交换洗涤（例如 CsCl）的浸出方案对于揭示陆地指纹很有用。通过进一步的校准工作，CEC 可以用作陆地化学风化代理。但是，化学成分有很大的不同。我们得出结论，包括阳离子交换洗涤（例如 CsCl）的浸出方案对于揭示陆地指纹很有用。通过进一步的校准工作，CEC 可以用作陆地化学风化代理。但是，化学成分有很大的不同。我们得出结论，包括阳离子交换洗涤（例如 CsCl）的浸出方案对于揭示陆地指纹很有用。通过进一步的校准工作，CEC 可以用作陆地化学风化代理。