太湖现代沉积物中铁质结核特征:对太湖形成机制的探讨

太湖现代沉积物中发育大量各种形态的菱铁矿结核和褐铁矿结核。前者的矿物组成以菱铁矿为主，后者以针铁矿及纤铁矿为主，结核中混有陆源碎屑、黏土和大量各种水生植物碎片、细胞、花粉、植物蛋白石、少量动物介壳碎片以及大量细菌，表明铁质结核的形成与生物有关。菱铁矿结核形成于太湖现代沉积泥层较下部的封闭还原环境中，不同形状的菱铁矿结核的14C测年证明其形成于不同年代。褐铁矿结核形成于沉积泥层上部的氧化环境中，细菌起了关键作用。二者均为沉积成岩作用的产物。
  太湖的形成机制一直存在广泛争议，其中“陨石撞击说”最受关注。王鹤年等（2009）把前述的菱铁矿结核、褐铁矿结核、甚至黄土层中的钙结核当成“冲击坑溅射物”，并将之作为太湖形成于陨石撞击的确凿证据。陨石撞击地球是一种近乎瞬间的冲击变质作用，其相应的特征产物有:柯石英、斯石英，矿物中的变形页理和撞击玻璃（撞击岩）。迄今为止，在太湖及周边地区还没有发现石英的高压矿物相。观察到的石英砂岩中的石英变形纹是较低应力作用的产物，在沉积岩中常见，与冲击变质作用无关。太湖湖底平坦，水深不超过3 m，沉积层仅厚2 m，与陨石撞击造成的陨石坑地形特征不符。从沉积学的角度来看太湖形成的“洪水淹没说”更有说服力，不仅符合沉积物特征和沉积物年龄，也有考古学遗迹和史料的佐证。

Characteristics of Ferruginous Concretions in Modern Sediments of the Taihu Lake: Implications for the origin of Lake Taihu

A lot of sideritic and limouitic concretions with different shapes have been found in modern sediments of  Lake Taihu. The former mainly consists of siderite, while the latter is predominantly composed of goethite and lepidocrocite. They contain terrigenous clast, clay, numerous fragments of aquatic plants, cell, spore and opal phytolith, as well as some shell clast and a lot of bacteria. Clearly, their origin is related to organism. The sideritic concretions were formed in a closed reducing environment, just beneath the lower mud layer of modern sediments of  Lake Taihu. The 14C dating results of sideritic concretions with different shapes indicate different formation ages. In contrast, the limonitic concretions were formed in anoxidation environment in the upper mud layer of modern sediments, and bacteria play a key role in their aggregation. Therefore, these ferruginous concretions are products of  diagenetic processes.
  The origin of  Lake Taihu  is still under debate. Meteorite impact hypothesis has received a lot of attention. Wang et al. (2009) regarded the above sideritic and limouitic concretions, even calcareous concretions in loess as impact ejectain the Taihu area, and claimed that they provide evidence of meteorite impact hypothesis. However, the meteorite impact is associated with instantaneous shock metamorphism, which will produce some characteristic minerals and microstructure including coesite, stishovite, planar deformation features as well as impact glass. So far the high-pressure polymorphs of quartz have not been found in the Taihu and adjacent area. The deformed laminar structure in quartz from sandstone is a common feature in sedimentary clasts, which cannot be considered diagnostics of shock metamorphism. The bottom of  Lake Taihu  is quite flat. The water depth of Lake Taihu  does not exceed 3 m and the sediments are only about 2-m-thick, which do not agree with the topography of an impact crater. In the view of sedimentology, flood submerge is a more favorable hypothesis to interpret the origin of  Lake Taihu, because it is consistent with characteristics and ages of sediments, as well as archaeological relics and historical records.