Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation
Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation
复制标题
格林河组碱性古湖中生物诱发的层状燧石形成
DOI：
10.1038/s41598-019-52862-7
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发表时间：
2019
影响因子：
4.6
通讯作者：
Ikeda Masayuki
中科院分区：
文献类型：
--
作者：
Kuma Ryusei;Hasegawa Hitoshi;Yamamoto Koshi;Yoshida Hidekazu;Whiteside Jessica H.;Katsuta Nagayoshi;Ikeda Masayuki
韵律层状硅质岩在远洋海洋和湖泊沉积中都有发现，但后者的形成机制仍高度不确定。我们对美国犹他州始新世绿色河组中硅质岩-白云岩交替层的研究揭示了硅质岩层中可能起源于藻孢囊的有机物球体（直径30-50 μm）的密集堆积，以及硅质岩层沉积的百年至千年尺度的周期性。有机质微球的降解程度与硅的分布呈正相关，说明藻类有机质的分解导致了湖相硅质的沉淀。由于碱性和溶解硅可能是高的古湖沃茨的绿色河组，我们推测，藻类有机物的分解降低了沉积物孔隙沃茨的pH值，并导致二氧化硅沉淀。我们提出了一个形成模式，在沉积物中的藻类有机质的初始丰度随生产力在湖面变化，这种藻类物质的分解控制沉积物中的二氧化硅沉淀的程度。有节奏的层状燧石白云岩的形成可能与百年至千年尺度的气候/环境因素有关，这些因素调节藻类的生产力，而这些因素可能与已知具有类似周期性的太阳活动周期有关。
Rhythmically bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain. Our study of alternating chert–dolomite beds in the Eocene Green River Formation, Utah, USA reveals dense accumulations of organic-matter spheres (30–50 μm diameter) of probable algal cyst origin in the chert layers, and centennial- to millennial-scale periodicities in chert layer deposition. A positive correlation between the degree of degradation of the organic spheres and Si distribution implies decomposition of algal organic matter lead to precipitation of lacustrine chert. As both alkalinity and dissolved silica were likely high in the palaeo-lake waters of the Green River Formation, we hypothesize that decomposition of algal organic matter lowered the pH of sediment pore waters and caused silica precipitation. We propose a formation model in which the initial abundance of algal organic matter in sediment varies with productivity at the lake surface, and the decomposition of this algal matter controls the extent of silica precipitation in sediment. The formation of rhythmically bedded chert–dolomite may be linked to centennial- to millennial-scale climatic/environmental factors that modulate algal productivity, which are possibly tied to solar activity cycles known to have similar periodicities.