Lamina-scale analysis of sedimentary components in Cretaceous black shales by chemical compositional mapping: Implications for paleoenvironmental changes during the Oceanic Anoxic Events
Lamina-scale analysis of sedimentary components in Cretaceous black shales by chemical compositional mapping: Implications for paleoenvironmental changes during the Oceanic Anoxic Events
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DOI：
10.1016/j.gca.2004.06.039
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发表时间：
2005-03-15
影响因子：
5
通讯作者：
Taira, A
中科院分区：
文献类型：
--
作者：
Kuroda, J;Ohkouchi, N;Taira, A
利用10 μ m分辨率的电子探针微量分析仪，研究了白垩纪Cenomanian/Turonian边界“Livello Bonarelli （LB）”精细层状黑色页岩中矿物组分的相对丰度和分布规律。通过对沉积物成分图的图像处理，重建了生物成因硅（Si-bio）、磷酸钙、碳酸钙、黄铁矿等沉积成分。总有机碳（C-org）含量也以1.5 mm的采样间隔测定。这些沉积组分的深度剖面覆盖了约60%的总104cm LB层序，表明它们的沉积具有高度的变异性。特别是C-org的含量差异很大，从< 0.1%到26%不等。富含C-org的音程在LB中清晰可见，与“深色”音程相对应。富C-org和富Si-bio层段的反复出现是LB最显著的特征之一。这些特征可以部分解释为硅-bio对C-org的稀释作用，但主要是由于存在两种不同的古环境背景：一种以有机质沉积为主，另一种以硅-bio沉积为主。各层间厚度变化较大，大多在1.5 ~ 35 mm之间，代表10(3)~ 10(4)年量级的海洋变化。此外，我们还发现了100 μ m尺度的细粒黄铁矿、硅酸铝和碳酸钙层状，当这些层状叠加在富碳有机碳和富硅生物硅的交替层状上时，表明海洋环境的10(2)年尺度波动。版权所有(c) 2005 Elsevier Ltd
Relative abundances and distribution patterns of mineral components in finely laminated "Livello Bonarelli (LB)" black shale from the Cenomanian/Turonian boundary, Cretaceous, were investigated by 10 mu m-resolution compositional mapping using an electron probe microanalyzer. Sedimentary components such as biogenic silica (Si-bio), calcium phosphate, calcium carbonate, and pyrite were reconstructed by image processing of the compositional maps. Total organic carbon (C-org) content was also determined at a sampling interval of 1.5 mm. The depth profiles of these sedimentary components, covering similar to 60% of the total 104-cm LB sequence, indicate the highly variable nature of their deposition. Especially, the C-org contents vary widely, ranging from < 0.1% to 26%. Intervals enriched in C-org are clearly discernible in the LB, and correspond to "dark" colored intervals. Repeated occurrences of C-org-rich and Si-bio-rich intervals are one of the most prominent features in the LB. These features could be explained partly by dilution of C-org by Si-bio, but largely by the presence of two distinct paleoenvironmental settings: one involves predominant deposition of organic matter and the other is dominated by Si-bio deposition. The thickness of individual intervals varies widely, mostly ranging from 1.5 to 35 mm, representing oceanic changes on the order of 10(3)-10(4) years. Furthermore, we found 100-mu m-scale laminations of fine-grained pyrite, aluminosilicate, and calcium carbonate, which when superimposed on the alternation of C-org-rich and Si-bio-rich intervals, suggests 10(2)-year scale fluctuations in oceanic environments. Copyright (c) 2005 Elsevier Ltd.