Chemical weathering trends in fine-grained ephemeral stream sediments of the McMurdo Dry Valleys, Antarctica
Chemical weathering trends in fine-grained ephemeral stream sediments of the McMurdo Dry Valleys, Antarctica
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DOI：
10.1016/j.geomorph.2016.12.016
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发表时间：
2017-03
期刊：
影响因子：
3.9
通讯作者：
K. Marra;M. Madden;G. Soreghan;B. Hall
中科院分区：
文献类型：
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作者：
K. Marra;M. Madden;G. Soreghan;B. Hall
我们通过整合粒度、BET表面积和全岩地球化学，研究了从麦克默多干谷（MDV;赖特和泰勒谷）冰川融水流中收集的沉积物细粒（< 63 μm;粉砂和粘土）部分的化学风化趋势。虽然这两个山谷目前都有冷基冰川，但短暂的冰川溪流下的沉积物是在不同的冰川条件下沉积的。在赖特谷（克拉克冰川流），布朗沃思和三部曲漂移沉积通过冷基冰川作用，而罗斯海漂移下三角洲流在泰勒谷可能反映了湿基冰的贡献。与泰勒谷沉积物相比，赖特谷溪流沉积物通常颗粒较粗，且具有较高的淤泥含量。这些沉积物主要由辉石、石英和长石组成，辉石和石英的百分比在下游系统地增加。层状硅酸盐的百分比范围从4%到18%，并随下游距离而减少。相比之下，泰勒谷沉积物（三角洲流）颗粒较细，辉石和石英的百分比较低，层状硅酸盐的百分比明显较高（30-43%）。与克拉克冰川溪流沉积物相比，三角洲溪流横断面中所有矿物质相的浓度在丰度上与下游输送保持相对一致。标准化学风化指数，如化学蚀变指数（CIA），表明化学风化发生在南极河流沉积物的粉砂和粘土部分，特别是在三角洲河流沉积物中，BET表面积测量值> 40 m2/g。然而，利用MFW（镁铁质-长英质-风化）和A-CN-K（Al 2 O3-CaO + Na 2 O-K2 O）图更有效地辨别这些水系中化学风化的程度和性质。在三角洲流中表现出最高表面积的沉积物内观察到的钙和钠耗尽表明，化学风化可能导致辉石和辉石的点蚀和/或不一致溶解，以及无定形和/或纳米相风化产物的发展。相比之下，克拉克冰川流沉积物没有类似的细粒沉积物部分的沥滤趋势，并表现出最小的风化整体。这可能表明，细粒物质被困在克拉克冰川表面的顶部，由于采样时间的原因，尚未被运送到潜流带的风化环境中。或者，在该河流断面中可能发生细粒沉积物的完全溶解，因此未保存在细粒沉积物部分中。总的来说，两个流系统之间观察到的化学风化的幅度最终与底层漂移（冷和湿基漂移沉积）的性质，风成细粉的分散模式，和变量流排放率。因此，纳入当地细粒沉积物来自底层冰川漂流沉积物和分布通过不同的风制度在超干旱气候到活跃的河流通道可能会促进不一致的矿物溶解和风化产物的发展，并最终影响融水流溶质的组成和浓度。
We investigated chemical weathering trends within the fine-grained (< 63 μm; silt and clay) fraction of sediments collected from meltwater streams emanating from glaciers in the McMurdo Dry Valleys (MDV; Wright and Taylor Valleys) by integrating grain size, BET surface area, and whole-rock geochemistry. While both valleys currently host cold-based glaciers, the sediment underlying the ephemeral glacial streams was deposited under differing glacial conditions. In Wright Valley (Clark Glacier stream), Brownworth and Trilogy drifts were deposited via cold-based glaciation, whereas the Ross Sea drift that underlies Delta Stream in Taylor Valley likely reflects contributions from wet-based ice. Wright Valley stream sediments are typically coarser grained and have a higher silt content as compared to Taylor Valley sediments. These sediments consist primarily of pyroxenes, quartz, and feldspars, with the percentages of pyroxenes and quartz systematically increasing downstream. The percentage of phyllosilicates ranges from 4 to 18% and decreases with downstream distance. In contrast, Taylor Valley sediments (Delta Stream) are finer-grained and exhibit lower percentages of both pyroxene and quartz and a significantly higher percentage of phyllosilicates (30–43%). Concentrations of all mineral phases remain relatively consistent in abundance with downstream transport in the Delta Stream transect as compared to Clark Glacier stream sediments. Standard chemical weathering indices, such as the Chemical Index of Alteration (CIA), indicate that chemical weathering is occurring within the silt and clay fractions of Antarctic stream sediments and is particularly pronounced in Delta Stream sediments that have BET surface area measurements > 40 m2/g. Utilization of MFW (mafic-felsic-weathered) and A-CN-K (Al2O3-CaO + Na2O-K2O) plots, however, are more effective in discerning the extent and nature of chemical weathering in these stream systems. Ca and Na depletion observed within the sediments exhibiting the highest surface area in Delta Stream suggest that chemical weathering may result in pitting and/or incongruent dissolution of pyroxenes and feldspars, as well as the development of amorphous and/or nanophase weathering products. In contrast, Clark Glacier stream sediments do not have similar leaching trends in the fine-grained sediment fraction and exhibit minimal weathering overall. This may suggest that fine-grained material is being trapped on top of the Clark Glacier surface and has not yet been transported into the weathering environment of the hyporheic zone due to timing of sampling. Alternatively, complete dissolution of very fine-grained sediment could be occurring in this stream transect, and is therefore not preserved in the fine sediment fraction. Overall, the magnitude of chemical weathering observed between the two stream systems is ultimately related to the nature of the underlying drift (cold and wet-based drift deposition), dispersal patterns of eolian fines, and variable stream discharge rates. Thus, incorporation of local fine-grained sediment derived from the underlying glacial drift deposits and distributed via the varying wind regimes within the hyper-arid climate into active stream channels may facilitate incongruent mineral dissolution and development of weathering products, and ultimately influence the composition and concentration of meltwater stream solutes.