Geochemistry at DSDP Legs 56-57 Holes

The bulk chemical composition of 257 argillaceous sediments from cores around the Japan Trench which were drilled during Legs 56 and 57 was determined and examined. These samples are generally enriched in SiO2 and impoverished in MnO, compared with typical pelagic sediments. SiO2 is negatively correlated with other oxides. Statistical treatment of data shows that pure silica varies inversely with TiO2 and Al2O3. The silica was derived mainly from planktonic siliceous organisms such as diatoms and radiolarians. Siliceous tests of these organisms disappear during progressive diagenesis, but the resolved silica redeposited in the sediments and the total silica content of the sediments remain constant. Higher SiO2 in some samples may be due to increased volcanic glass content. On the west slope of the trench, much ferric iron was deposited but not oxidized. In contrast to iron, manganese originally present in the sediments was apparently reduced and not precipitated in this hemipelagic region. Abundant land-derived organic materials maintain a weakly reducing environment within the sediment, which permits the precipitation of ferric iron but not of manganese. The mutual relationship of conservative elements such as SiO2, TiO2, and Al2O3 indicates that sediments of the west slope of the trench were derived mostly from the land area to the west and that the original materials were chemically homogenized and diluted by biogenic silica. In contrast, the chemistry of core from the trench outer slope (Site 436) shows a vertical trend: SiO2 progressively decreases downhole, whereas MnO and the Fe2O3/FeO ratio increase significantly. The deeper samples are very similar to the pelagic sediments from the Pacific Plate, whereas upper samples resemble the trench inner slope sediments. The vertical trends at this site are plausibly explained by the progressive movement of the ocean plate toward the Japan Trench. When the oceanic plate was in the Central Pacific Ocean, much MnO and Fe2O3 were deposited under an oxidizing environment with slow sedimentation, owing to the paucity of organic materials. As the ocean plate moved westward, nearer to the Japanese Islands, hemipelagic sediment derived from these islands were deposited over the pelagic sediments. […]

本研究对日本海沟周边第56、57钻探航次钻取的257份泥质沉积物岩心样品的全岩化学成分进行了测定与分析。与典型远洋沉积物相比，这些样品普遍富集二氧化硅（SiO₂）而贫氧化锰（MnO）。二氧化硅与其他氧化物呈负相关关系。对数据的统计处理表明，纯硅质组分与二氧化钛（TiO₂）、三氧化二铝（Al₂O₃）呈负相关变化。该沉积物中的硅质主要来源于硅藻、放射虫等浮游硅质生物。这些生物的硅质介壳在持续成岩作用过程中会发生溶解，但溶解出的硅质会在沉积物中重新沉积，沉积物的总硅含量保持稳定。部分样品中较高的二氧化硅含量可能源于火山玻璃含量的增加。
在海沟西坡，大量铁质发生沉积但并未被氧化。与铁不同，该半远洋沉积区中原生沉积的锰元素经还原作用后并未发生沉淀。丰富的陆源有机质维持了沉积物内部弱还原环境，该环境仅利于三价铁的沉淀，而无法使锰元素发生沉淀。
二氧化硅、二氧化钛、三氧化二铝等保守元素的相互关联特征表明，海沟西坡的沉积物主要来自西侧陆域，其原始物质已发生化学均质化，并被生物成因硅质稀释。与之形成对比的是，海沟外坡436站位的岩心化学特征呈现垂向变化规律：随井深增加，二氧化硅含量逐渐降低，而氧化锰含量及三氧化二铁/氧化亚铁（Fe₂O₃/FeO）比值则显著升高。较深层的样品与太平洋板块的远洋沉积物极为相似，而浅层样品则与海沟内坡沉积物特征相近。
该站位的垂向变化规律可通过大洋板块向日本海沟的持续运动得到合理解释。当大洋板块位于中太平洋区域时，由于有机质匮乏，沉积物在氧化环境下以缓慢沉积速率形成，其中富集大量氧化锰与三氧化二铁。随着大洋板块向西移动并逐渐靠近日本列岛，源自日本列岛的半远洋沉积物逐渐覆盖于原远洋沉积物之上。[…]