海洋地质与环境重点实验室知识库

    作为沉积物最容易被观察记录的物理性质，沉积物颜色对沉积环境的重建具有特殊指示意义。沉积物颜色主要受控于含铁矿物、有机质和碳酸盐等组分的含量高低，而沉积物中铁的价态和有机质含量都极大程度上受到海底氧化还原环境的影响，因此，地质记录中海洋沉积物颜色变化在一定程度上可能反映了沉积环境尤其是海底氧化还原环境的演化。

        2017年在南海北部，国际大洋发现计划（IODP）368航次的科学家们在钻取的岩芯中首次发现了大洋沉积物中厚达数百米的红-绿颜色的韵律交替现象。其中本次研究所用U1502站位岩芯中的红褐色沉积层与绿灰色沉积层以平行接触的方式呈韵律交替，各层厚度从5厘米到50厘米不等。根据微体古生物组合定年可知，本次研究所用岩芯段的年代跨度大致为中-晚中新世（15-7 Ma）。为查明U1502站位红褐色-绿灰色韵律沉积层的物质来源、形成环境和成因机制，我们对U1502站位晚中新世红-绿韵律层沉积物样品进行了详细的矿物学和地球化学分析，包括粒度、粘土矿物、常微量元素、总有机碳含量、Sr-Nd-Fe同位素组成和碳酸盐组分的碳氧同位素组成分析。

    对比U1502站位红褐色-绿灰色层粒度及物质组成发现，整体以粘土质粉砂为主，粒度变化较小，但红褐色层相对绿灰色层粒度更细，碎屑组分总含量更低。沉积物的粘土矿物组成、Sr-Nd同位素组成、稀土元素配分模式和La-Th-Sc三角图、Fe同位素组成等指标指示研究站位沉积物陆源输入总体以河流碎屑物质为主，不同物源区对研究站位沉积物相对贡献以古珠江为主、吕宋河流次之。其中，绿灰色层相对红褐色层有稍多的古珠江物质输入和稍少的吕宋物质输入。沉积物的Cu、Zn、ƩREE含量、碳酸盐碳氧同位素等地球化学指标对比均表明U1502站沉积物红绿颜色转变与热液和冷泉等还原性流体无关。U/Th比值、Mo和U富集因子等氧化还原指标，结合生源Ba、TOC等古生产力指标，指示研究站位中红褐色层对应偏氧化的底层水氧化还原环境，而绿灰色层对应偏还原的底层水氧化还原环境。

    综合构造、气候和古海洋因素分析认为，U1502站位红绿韵律层颜色变化是南海晚中新世构造演化和冰期-间冰期旋回中水体通风、底层水氧化还原环境改变及早期成岩作用共同影响的结果。随着中中新世时欧亚板块与吕宋岛弧发生旋转碰撞，原先开放的南海海盆逐渐变得相对封闭，其深部通风和氧化还原条件开始呈现典型的冰期-间冰期旋回模式。沉积物颜色反射率a*的频谱分析以及南海北部的物源分析表明，红褐色层和绿灰色层可能分别对应于间冰期时南海深层水通风良好的氧化环境和冰期时南海层化加剧，通风不良的还原环境，并最终以赤铁矿含量的多少反映在沉积物颜色变化中。

    As the most easily observed and recorded physical property of sediments, sediment color has special indicating significance for the reconstruction of sedimentary environment. The color of sediment is mainly controlled by the content of iron bearing minerals, organic matter and carbonate. As the valence state of iron and content of organic matter in sediments are largely affected by the submarine redox environment. the change of sediment color of marine sediments in geological records may reflect the evolution of the sedimentary environment, especially the seabed redox environment to a certain extent.

    During the International Ocean Discovery Program (IODP) 368 Expedition in the northern South China Sea (SCS) in 2017, it was the first time that the ocean red-green rhythmic sediments were widely found. Among them, the rhythmic sedimentary layers of reddish brown-greenish gray alternating repeatedly appeared in the sedimentary layers in parallel contact at IODP Site U1502. The thickness of single reddish brown sedimentary layer and greenish gray sedimentary layer varies from 5cm to 50cm. According to the micropaleontology dating, the age span of the studied core is roughly middle-late Miocene (15-7 Ma). In order to understand the sediment provenance, redox environment change and formation mechanism of reddish brown-greenish gray rhythmic sedimentary layer, we carried out detailed mineralogical and geochemical analysis on the sediment samples of Site U1502, including changes of particle size, clay minerals, major and trace elements, Sr-Nd-Fe isotopes composition, total organic carbon content and C-O isotopic composition of carbonate components.

    Comparing the particle size and material composition of the reddish brown-greenish gray layer at Site U1502, the overall is mainly clayey silt and the particle size change is small, the particle size of reddish brown layer is finer than that of greenish gray layer, the total content of clastic components is lower. The clay mineral composition, Sr-Nd isotopic composition, rare earth element distribution model, La-Th-Sc triangle and Fe isotopic composition of the sediments indicate that the terrestrial input of the sediments at the research station is mainly River clastic materials. The ancient Pearl River is the main contributor to the terrigenous input of sediments at Site U1502, and the Luzon river is the secondary contributor. The reddish brown layer has slightly more Luzon material input than the greenish gray layer, and the greenish gray layer has slightly more ancient Pearl River material input than the reddish brown layer. Multiple geochemical indicators at Site U1502, such as Cu, Zn, ƩREE content, carbonate C-O isotope and other geochemical indicators, indicate that the formation of reddish-greenish rhythmic sedimentary layer at U1502 station has nothing to do with the influence of later hydrothermal and cold seep fluid. Redox indicators such as U/Th ratio, U_EF and Mo_EF, combined with paleoproductivity indicators such as BA_EF and TOC, indicate that the reddish brown layer is formed in the relatively oxidized water, while the greenish gray layer is formed in the relatively reduced water.

    Its color change is the result of the combined influence of the South China Sea in the late Miocene affected by the glacial interglacial cycle, the periodic changes of deep seawater ventilation and redox environment, and the early diagenesis in the sediments. With the rotational collision between the Eurasian plate and the Luzon island arc in the Middle Miocene, the originally open South China Sea basin gradually evolved into a relatively closed state, resulting in the periodic changes of deep seawater ventilation and redox environment under the control of glacial interglacial cycle. The spectral analysis of sediment color reflectance a* in depth and provenance analysis in the northern South China Sea show that the reddish-brown layer and the greenish-gray layer may respectively correspond to the oxidation environment with good ventilation of deep seawater during interglacial period and the reduction environment with intensified stratification and poor ventilation of deep seawater during glacial period, and finally reflected in the change of sediment color with the content of hematite.