Development of a protocol to obtain the composition of terrigenous detritus in marine sediments -a pilot study from International Ocean Discovery Program Expedition 361
Development of a protocol to obtain the composition of terrigenous detritus in marine sediments -a pilot study from International Ocean Discovery Program Expedition 361
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DOI：
10.1016/j.chemgeo.2019.119449
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发表时间：
2020-03
期刊：
影响因子：
3.9
通讯作者：
M. Simon;D. P. Babin;S. Goldstein;M. Cai;Tanzhuo Liu;Xibin Han;Anne A. Haws;M. Johns;C. Lear;S. Hemming
中科院分区：
文献类型：
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作者：
M. Simon;D. P. Babin;S. Goldstein;M. Cai;Tanzhuo Liu;Xibin Han;Anne A. Haws;M. Johns;C. Lear;S. Hemming
海洋沉积物中陆源粘土的地球化学和同位素组成可以提供有关沉积物来源和路径的重要信息。国际海洋发现计划考察队在南部非洲东部边缘钻探了361个地点，这些地点可能记录了非洲大陆的降雨情况以及阿古拉海流的扩散情况。我们使用标准方法去除碳酸盐和锰铁氧化物，并用斯托克斯沉降法分离粘土部分。与以前大多数旨在从海洋沉积物中提取碎屑信号的研究相比，我们额外应用了使用CSCL的阳离子交换洗涤作为样品制备的最后一步。额外步骤背后的动机并不经常使用，是为了去除粘土表面上的离子，这些离子是由于自生微量金属在海洋中的吸附或在浸出过程中获得的。如果不使用阳离子交换，这两种方法都会改变碎屑组分的组成。此外，使用CSCL将提供样品的阳离子交换能力(CEC)的额外测量。然而，到目前为止，还没有研究评估这种针对海洋沉积物的定向方案的潜力和局限性。在这里，我们探索了用Cs+去除和取代粘土表面吸附的阳离子的效果，并对一组8对粘土样品进行了化学成分和放射性同位素的测量。两组样品都经历了相同的完全浸出程序，只是其中一批样品经过了最后的CSCL洗涤步骤。在本研究中，由于IODP站点U1478的沉积物有机质含量相对较低，因此有机质没有淋溶。然而，总的来说，我们建议在浸出过程中包括这一步骤。正如预期的那样，海水中高浓度元素的很大一部分被Cs+(2SD 2.8%)取代。包括75%的钠和大约25%的钙，10%的镁和8%的钾。用于来源研究的微量金属，如锶和钕，在交换洗涤后，样品中的浓度也较低。交换洗涤影响了样品的放射性同位素组成。在所有洗涤的样品中，钕同位素比率的放射性略有降低。锶和铅的同位素在不同的样品中或多或少地偏离了放射性值。CsCl2处理组分的放射性同位素之间给出了更一致的相关性，我们认为这种处理提供了一种更好的来源测量方法。尽管我们观察到了同位素比率的变化，但数据的总体趋势和总体来源解释保持不变。然而，化学成分却有很大的不同。我们的结论是，包括阳离子交换洗涤的淋洗方案(例如CsCL)对于揭示陆地指纹是有用的。通过进一步的校准工作，CEC可以作为陆地化学风化的代用品。
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.