Composition and formation age of amorphous silica coating glacially polished surfaces
Composition and formation age of amorphous silica coating glacially polished surfaces
复制标题
DOI：
10.1130/g45737.1
复制
发表时间：
2019-04-01
期刊：
影响因子：
5.8
通讯作者：
Hallet, Bernard
中科院分区：
文献类型：
--
作者：
Blackburn, Terrence;Siman-Tov, Shalev;Hallet, Bernard
最近拍摄的光滑、冰川磨损的硅质基岩的显微照片显示，基岩上附着有一层无定形涂层，其结构将其形成与冰川磨损联系在一起。目前尚不清楚的是，这种涂层是否是通过基岩的物理粉碎形成的，从而产生具有基岩成分的无定形材料，或者是通过硅酸盐矿物的化学溶解，然后沉淀出富含二氧化硅且相对于基岩而言贫阳离子的无定形层。在这里，我们报告了美国加利福尼亚州优胜美地国家公园的非晶涂层的成分和形成年龄。相对于基岩，涂层中的碱性阳离子较少（50%-90%），而富含二氧化硅（10%-50%）以及微量铁和铀（4至100倍），反映了冰下水的溶解和沉淀。非晶层的(234)U/(238)U活度比高于长期平衡200%-600%，反映了a反冲过程富集的地表U源，与冰下水域中观察到的U-234富集一致。 (230)Th/(238)U活度比低于长期平衡30%-100%，记录了30-10ka冰下水域中的Th-U分馏，与末次盛冰期(LGM)期间涂层的形成一致。这些无定形涂层是冰下沉淀物，记录了末次盛冰期期间冰川下方硅酸盐的化学风化。总的来说，这些观测结果将硅酸盐溶解和无定形二氧化硅的产生与冰川床的物理过程联系起来，这一结果可能对冰川-间冰期时间尺度上的全球硅和二氧化碳预算产生重大影响。
Recent micrographs of smooth, glacially abraded silicic bedrock reveal an amorphous coating layer adhering to the bedrock, with structures that tie its formation to glacial abrasion. What remains unclear is whether this coating is formed by the physical comminution of bedrock, resulting in amorphous material with a bedrock composition, or by chemical dissolution of silicate minerals followed by precipitation of an amorphous layer enriched in silica and depleted in cations relative to the bedrock. Here, we report the composition and formation age of the amorphous coatings in Yosemite National Park, California, USA. The coatings are depleted in base cations (50 %-90 %) and enriched in silica (10 %-50 %) as well as trace Fe and U (4- to 100-fold) relative to the bedrock, reflecting dissolution by and precipitation from subglacial waters. The(234)U/ (238) U activity ratio of the amorphous layer is 200 %-600 % above secular equilibrium, reflecting a surficial U source enriched by a-recoil processes and consistent with the U-234 enrichment observed in subglacial waters. The(230)Th/ (238) U activity ratio is 30 %-100 % below secular equilibrium and records Th-U fractionation in subglacial waters at 30-10 ka, consistent with coating formation during the Last Glacial Maximum (LGM). These amorphous coatings are subglacial precipitates that record the chemical weathering of silicates beneath glaciers during the LGM. Collectively, these observations link silicate dissolution and amorphous silica production to physical processes at the glacier bed, a result that may have significant implications for the global Si and CO2 budgets on glacial-interglacial time scales.