Chemical weathering signatures from Mt. Achernar Moraine, Central Transantarctic Mountains I: Subglacial sediments compared with underlying rock
Chemical weathering signatures from Mt. Achernar Moraine, Central Transantarctic Mountains I: Subglacial sediments compared with underlying rock
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DOI：
10.1016/j.gca.2020.06.005
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发表时间：
2020-08
影响因子：
5
通讯作者：
J. Graly;K. Licht;N. Bader;D. Bish
中科院分区：
文献类型：
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作者：
J. Graly;K. Licht;N. Bader;D. Bish
为了确定南极洲冰下陆地表面的化学风化速率，我们将新出现的细沉积物的成分和矿物学与底层基岩（以冰川衍生的鹅卵石大小的碎屑为代表）的成分和矿物学进行了比较。样品是从阿赫纳冰碛山采集的，这是一个巨大的蓝色冰碛，冰下物质通过周围冰的升华自然出现。通过 X 射线衍射以及针对化学风化产物的连续提取，分析了岩石和沉积物的总元素组成、矿物丰度。与卵石碎屑相比，细粒沉积物部分的化学风化产物显着富集，而原生矿物则减少。蚀变途径主要包括蒙皂石、高岭石、碳酸盐矿物和无定形物质的发育。磁化率下降和可萃取铁增加证明了铁的广泛氧化。如果我们假设冰下系统的唯一输入是由基底熔体提供的水和冰困气体，则净化学变化可以通过相当于基岩质量约 0.7% 的有机物的氧化和随后的碳化风化来解释。下面的沉积岩含有足够丰富的有机质，因此这条路径是合理的。对于由基底融水提供的氧化有机物的氧气，水通量需要比沉积物通量大三个数量级。我们现场的基底融化和沉积物迁移的独立模型证实，研究地点可能存在水和沉积物通量之间的这种差异。从阿赫纳山冰碛遗址推断出的冰下碳化风化速率可能与高纬度地下环境中发现的速率相当。如果阿赫纳尔冰碛山是其他南极地区的典型，那么南极洲的冰下陆地表面确实在全球地球化学循环中发挥着作用。
In order to determine chemical weathering rates on the subglacial land surface of Antarctica, we compare the composition and mineralogy of freshly emerging fine sediments to that of the underlying bedrock, as represented by glacially derived cobble-sized clasts. Samples were collected from Mt. Achernar Moraine, a large blue ice moraine, where subglacial material naturally emerges through sublimation of the surrounding ice. Both rocks and sediments were analyzed for total elemental composition, mineral abundance by X-ray diffraction, and by sequential extractions targeting chemical weathering products. The fine sediment fraction is significantly enriched in chemical weathering products and depleted in primary minerals compared with the cobble clasts. The alteration pathways consist primarily of the development of smectite, kaolinite, carbonate minerals, and amorphous material. Extensive Fe oxidation is evidenced by a decline in magnetic susceptibility and by increases in extractable Fe. If we assume the only input into the subglacial system is the water and ice-trapped gas supplied by basal melt, the net chemical alteration is explained through oxidation of organic matter equal to ∼0.7% of the bedrock mass and subsequent carbonation weathering. The underlying sedimentary rock is sufficiently rich in organic matter for this pathway to be plausible. For the O2that is oxidizing organic matter to be supplied by basal meltwater, water fluxes would need to be three orders of magnitude larger than sediment fluxes. Independent models of basal melt and sediment transport at our field site confirm that such a difference between water and sediment flux is likely at the study site. The rate of subglacial carbonation weathering inferred from the Mt. Achernar Moraine site may be comparable to that found in high latitude subaerial environments. If Mt. Achernar Moraine is typical of other Antarctic sites, the subglacial land surface of Antarctica does play a role in global geochemical cycling.