Relationships Between Composition of Organic Matter, Depositional Environments, and Sea-Level Changes in Backarc Basins, Central Japan
Relationships Between Composition of Organic Matter, Depositional Environments, and Sea-Level Changes in Backarc Basins, Central Japan
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日本中部弧后盆地有机质组成、沉积环境与海平面变化的关系
DOI：
10.1306/021304740620
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发表时间：
2004
影响因子：
2
通讯作者：
K. Hoyanagi
中科院分区：
文献类型：
--
作者：
A. Omura;K. Hoyanagi
沉积有机质是古环境的潜在指示物。本文研究了日本中部新泻和秋田弧后盆地中新世至更新世沉积物中干酪根（不溶性有机质）组成、沉积环境和海平面变化之间的关系。我们的主要分析工具是三元图，其顶点由木质煤质有机质组成，草本有机质含有花粉和孢子，无定形有机质（AOM）含有褐藻（有机壁海洋微化石）。干酪根的组成在三元图上划分为不同的区域：河流到河口、原生三角洲、陆架、斜坡和盆地底海底扇，以及远端盆地底沉积物。河流和河口沉积物中含花粉和孢子的木质煤质和草本有机质比例较高，AOM +藻酸盐比例较低，因为花粉和孢子主要沉积在河口。由于三角洲分流体提供了丰富的粗粒陆源有机质，故前三角洲沉积物中木煤有机质比例较高。陆架沉积物的干酪根组成与斜坡和盆地底海底扇沉积物的干酪根组成相似，如图所示为三元图。两种类型的干酪根聚集均具有较高比例的木质煤质有机质和含褐藻质的AOM，以及较少比例的含花粉和孢子的草本有机质。这种关系表明，浊度流提供了陆源沉积物。远端盆底沉积物中AOM含量较高。三元图中的每种模式都反映了水动力行为、与陆地的距离和陆源有机质供应的差异。以WFA（弱荧光无定形有机物）、NFA（非荧光无定形有机物）+ FA（荧光无定形有机物）和褐藻酸盐为顶点的亚三元图进一步提示了AOM的起源。推断陆架沉积物的NFA和远端盆地底沉积物的WFA分别由陆生高等植物和海相有机质组成。富NFA干酪根的13C值（-24.6 ~ -27.3‰）提示陆生植物，富WFA干酪根的13C值（-20.0 ~ -23.6‰）提示海洋浮游生物。这些推论与从次三元图得出的推论一致。浊积岩中干酪根成分的变化反映了相对海平面的变化，从三元图上的成分变化可以看出。在今后的干酪根、沉积环境和海平面关系的研究中，建议使用这里所使用的三元图。
ABSTRACT Sedimentary organic matter is a potential indicator of paleoenvironments. In this study we examine the relationship among composition of kerogen (insoluble organic matter), sedimentary environments, and sea-level changes in Miocene to Pleistocene sediments of the Niigata and Akita backarc basins, central Japan. Our primary analytical tool is a ternary diagram with apexes consisting of woody-coaly organic matter, herbaceous with pollen and spores, and amorphous organic matter (AOM) with alginite (organic-walled marine microfossils). The composition of kerogen plots into distinct regions on the ternary diagram: fluvial to estuarine, prodeltaic, shelf, slope and basin-floor submarine fans, and distal basin-floor sediments. The fluvial and estuarine sediments have high proportions of woody-coaly and herbaceous organic matter with pollen and spores, and a lesser proportion of AOM + alginite, because pollen and spores were mainly deposited in estuaries. Because abundant coarse-grained, terrigenous organic matter was supplied by delta distributaries, the prodeltaic sediments have high proportions of woody-coaly organic matter. The composition of kerogen in the shelf sediments is similar to the kerogen in slope and basin-floor submarine-fan sediments, as plotted on the ternary diagram. Both kinds of kerogen accumulations have high proportions of woody-coaly organic matter and AOM with alginite and lesser proportions of herbaceous organic matter with pollen and spores. This relationship suggests that turbidity currents supplied the terrigenous sediments. The sediments on the distal basin-floor contain high proportions of AOM. Each pattern in the ternary diagram reflects a difference in hydrodynamic behavior, distance from land, and the supply of terrigenous organic matter. The sub-ternary diagram, which has apexes of WFA (weakly fluorescent amorphous organic matter), NFA (nonfluorescent amorphous organic matter) + FA (fluorescent amorphous organic matter), and alginite, further suggests the origin of AOM. The NFA in shelf sediments and WFA in distal basin-floor sediments are inferred to consist of terrestrial higher plant and marine organic matter, respectively. A 13C value of kerogen rich in NFA (-24.6 and -27.3‰) suggests land plants, whereas kerogen rich in WFA (-20.0 to -23.6‰) suggests marine plankton. These inferences agree with those derived from the sub-ternary diagram. Furthermore, compositional changes of the kerogen in turbidites reflect relative sea-level changes, as seen by shifts in compositions on the ternary diagram. The use of ternary diagrams like those used here is recommended for future studies of kerogen, depositional environment, and sea-level relationship.