Major and rare earth element composition of ODP Site 191-1179 sediments, northwest Pacific

Depth profiles of major element and rare earth element (REE) abundances in sediment samples (mainly siliceous ooze and clay) recovered from Holes 1179B and 1179C at Site 1179, Ocean Drilling Program Leg 191 (41.4°N, 159.6°E) were determined. The oxidation states of Mn and Ce were determined by X-ray absorption near-edge structure. Some geochemical indicators were tested, including the MnO/TiO2 ratios, a bivariate diagram of La/Ce vs. Al2O3/(Al2O3+Fe2O3), and other discrimination diagrams. The oxidation state of Mn is reduced Mn(II) in the depth profile below 0.60 meters below seafloor (mbsf), which is consistent with relatively low and high abundances of Mn in the sediments and pore waters, respectively. It is possible that the diagenetic effect on the oxidation state and abundance of Mn makes it difficult for the MnO/TiO2 ratio to reflect the depositional environment. The normalized ratio of La and Ce does not change very much with depth, suggesting that the diagenetic effect does not affect the REE signature in the sediments. On the diagram of La/Ce vs. Al2O3/(Al2O3+Fe2O3), the sediments studied here plot at the boundary of the pelagic and continental margin fields. This suggests that continental material has contributed to the sediment to some degree, even though Site 1179 is in a pelagic region of the northwestern Pacific Ocean, >1600 km from Japan.

本研究测定了大洋钻探计划（Ocean Drilling Program）第191航次1179站位1179B、1179C钻孔获取的沉积物样品（以硅质软泥和黏土为主）中的主量元素与稀土元素（Rare Earth Element, REE）丰度深度剖面。本研究采用X射线吸收近边结构（X-ray absorption near-edge structure, XANES）技术测定了锰（Mn）与铈（Ce）的氧化态，并测试了多项地球化学指标，包括MnO/TiO₂比值、La/Ce与Al₂O₃/(Al₂O₃+Fe₂O₃)的二元判别图解以及其他判别图件。在海底以下0.60米（mbsf）以深的深度剖面中，锰的氧化态为还原态Mn(II)，该特征与沉积物中锰丰度偏低、孔隙水中锰丰度偏高的分布规律相一致。成岩作用对锰的氧化态与丰度产生的影响，可能使得MnO/TiO₂比值难以准确反映沉积环境。镧（La）与铈（Ce）的标准化比值未随深度发生显著变化，表明成岩作用并未对沉积物中的稀土元素特征产生干扰。在La/Ce与Al₂O₃/(Al₂O₃+Fe₂O₃)的二元判别图解中，本次研究的沉积物样品投点落在远洋沉积与大陆边缘沉积的分界区域。尽管1179站位位于距离日本本土超过1600千米的西北太平洋远洋区域，但该结果表明大陆物质在一定程度上对沉积物有贡献。