Chemical weathering signatures at Mt. Achernar, Central Transantarctic Mountains II: Surface exposed sediments
Chemical weathering signatures at Mt. Achernar, Central Transantarctic Mountains II: Surface exposed sediments
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横贯南极山脉中部阿赫纳尔山的化学风化特征 II：地表暴露沉积物
DOI：
10.1016/j.gca.2022.06.024
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发表时间：
2022
影响因子：
5
通讯作者：
Kaplan, Michael R
中科院分区：
文献类型：
--
作者：
Graly, Joseph A;Licht, Kathy J;Bader, Nicole A;Kassab, Christine M;Bish, David L;Kaplan, Michael R
Achernar Moraine山是一个高海拔、高纬度的蓝色冰原，在典型的条件下，不存在液态水。宇宙成因和盐分积累测年表明，冰川表面远离活动冰缘的年龄逐渐变老，表面暴露年龄高达1 Ma。我们分析了沿冰雹横断面的<；63微米组分中的化学和矿物学变化。数据包括整体化学成分、X射线衍射法的结晶矿物学，以及顺序提取的化学风化的无定形或低丰度产物的组成。对这些数据进行了多元回归分析，这些数据是暴露年龄的函数，也是冰雹卵石和鹅卵石大小的碎屑组成的函数。随暴露年龄的变化取决于盐类和碳酸盐矿物的发育以及碎屑物质的输入，碎屑物质主要来自沉积岩。粘土矿物和无定形胶结物以碎屑物质的形式分解，其比例远远高于它们在岩石碎屑中的丰度，而骨架硅酸盐(即长石和石英)的分解比例相对较小。碳酸盐矿物和一些盐都是由大气中的酸(如H_2CO_3)形成的，而大气酸又与其他矿物发生反应。质量平衡表明，这些大气酸的输入与自生蒙脱石、沸石和无定形物质的收益相平衡。这些矿物中的许多也形成于冰下环境，但在下面的岩石中很少出现，这表明在这种极端寒冷和干旱的环境中，冰下和地表环境都存在类似的化学风化制度。CO2向碳酸盐矿物的下沉速率随着冰雹的逐渐增厚而增加，从新出现的沉积物中的3 mg·m2·a−1增加到暴露500kA后的∼50 mg·m2·a−1。酸性气溶胶的风化作用与冰芯记录的大气通量成正比，不随冰雹厚度变化。碳酸盐矿物的形成速率比冰下环境低一个数量级以上，比在温暖的沙漠土壤中发现的低两个数量级。然而，大气中的二氧化碳被吸收成碳酸盐矿物是在陆地环境中发生的，在那里水只以蒸汽的形式存在。
Mt Achernar Moraine is a high altitude, high latitude blue ice moraine where typical conditions preclude the presence of liquid water. Cosmogenic and salt accumulation dating indicate that the moraine’s surface is progressively older away from the active ice margin, with surface exposure ages up to 1 Ma. We analyze the chemical and mineralogical transformations in the <63 µm fraction along transects across the moraine. Data include bulk chemical composition, crystalline mineralogy by X-ray diffraction (XRD), and the composition of amorphous or low abundance products of chemical weathering by sequential extraction. These data are analyzed by multiple regression as a function of exposure age and as a function of composition of the moraine’s cobble and pebble-sized clasts. Change with exposure age is defined by the development of salts and carbonate minerals along with the input of detrital material, principally from sedimentary rocks. Clay minerals and amorphous cements breakdown as detrital material in proportions far above their abundance in the rock clasts, whereas framework silicates (i.e. feldspars and quartz) break down in relatively small proportions. Both the carbonate minerals and some of the salts form from atmospheric acids (i.e. H2CO3) that in turn react with other minerals. Mass balance shows that the input of these atmospheric acids balances with gains in authigenic smectites, zeolites, and amorphous material. Many of these minerals also form in the subglacial environment, but are poorly represented in the underlying rock, suggesting a similar chemical weathering regime in both the subglacial and surface environments of this hyper cold and arid setting. The rate of CO2drawdown into carbonate minerals increases as the moraine progressively thickens, from 3 mg·m2·a−1in freshly emerging sediments to ∼50 mg·m2·a−1after 500 ka of exposure. Weathering from acidic aerosols is proportional to atmospheric flux documented in ice cores and does not vary with moraine thickness. The carbonate mineral formation rates are more than an order of magnitude below those of the subglacial environment and as much as two orders of magnitude below those found in warm desert soils. Nevertheless, the drawdown of atmospheric CO2into carbonate minerals occurs in a terrestrial setting where water exists only in vapor form.
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