Analysis of Manganese deposits from the South China Sea

In the northern South China Sea, the accumulation of enormous quantities of terrigenous sediment during Cenozoic rendered well-developed polymetallic nodules very rare. In this study, we analyzed a polymetallic nodule from the northwestern continental margin of the South China Sea using microscopic mineralogical observation, electron probes, X-ray diffraction (XRD), ICP-MS, and Be isotope dating. We found the nodule's shell layers rich in different types of microstructures, including columnar, laminar, stack-like, petal-like, and porphyritic structures. The major mineral components of the nodule are MnO2. Unlike nodules from the eastern Pacific basin, this nodule has high contents in Fe, Si, Al, and REEs but low contents in Mn, Cu, Co, and Ni. The Mn/Fe ratio is also low and the average REEs content is 1370.4 ppm. There is a strong positive anomaly of Ce; and the Be (beryllium) isotope dating shows the initial time of growth of the nodule to be about 3.29 Ma. The inner compact layer formed from 3.29 Ma to about 1.83 Ma. The laminar and stack-like structures and the low contents of the terrigenous elements such as Fe, Si, REE, and Al indicate the paleoceanographical environment with weak undersea currents and favorable oxidizing conditions. From 1.83 Ma to 0.73 Ma, the growth rate of the nodule increased by about 3%; the microstructures formed during this period are stack-like and columnar. The contents of Si and Al are increased by nearly 10%, indicating an increase of terrigenous sediment input in the northern South China Sea. The content of Ce is decreased by about 16%, indicating a significant weakening of the oxidizing conditions at the seabed. From 0.73 Ma to 0.69 Ma, the growth rate of the nodule rapidly rose up to 8.27 times that of the nodule's average growth rate, and the contents of Fe, Al, and REEs in the layer also increased, forming a loose layer characterized by oolitic, granular, porphyritic, and petal-like structures, indicating the paleoceanographical environment with a high sedimemtation rate and abundant supply of terrigenous sediment in the northern South China Sea. From 0.69 Ma to 0.22 Ma, the growth rate of the nodule suddenly slowed and the outer compact layer formed. Contents of Fe, Si, REE, Al, Mn, Cu, Co, and Ni in this layer were significantly lower than in other layers. The main structures of the layer are laminar and fissure filling structures. These reflect the paleoceanographical environment with stable undersea currents, poor oxidizing conditions, and other conditions not conducive to nodule growth. The growth process of nodule S04-1DG-1 was found to respond sensitively to the changes of the paleoceanographical environment of the northern South China Sea during the late Cenozoic.南海北部新生代时期因巨量陆源沉积物堆积，导致发育良好的多金属结核十分罕见。本研究利用显微矿物学观测、电子探针、X射线衍射（X-ray diffraction, XRD）、电感耦合等离子体质谱法（inductively coupled plasma mass spectrometry, ICP-MS）以及铍同位素定年（beryllium isotope dating）技术，对南海西北陆缘的一枚多金属结核开展分析。研究发现，该结核的壳层富含多种微观结构，包括柱状、层状、叠堆状、花瓣状及斑状结构。该结核的主要矿物成分为二氧化锰（MnO₂）。与东太平洋盆地的结核不同，本结核中铁、硅、铝及稀土元素（rare earth elements, REEs）含量较高，而锰、铜、钴、镍含量较低，且锰铁比偏低，稀土元素平均含量达1370.4 ppm，同时存在显著的铈正异常。铍同位素定年结果显示，该结核的初始生长时间约为3.29 Ma。其内致密层形成于3.29 Ma至约1.83 Ma期间，该层的层状、叠堆状结构以及较低的陆源元素（铁、硅、稀土元素、铝）含量，指示当时南海北部海底流较弱、氧化条件良好的古海洋环境。1.83 Ma至0.73 Ma期间，结核的生长速率提升约3%，此阶段形成的微观结构以叠堆状和柱状为主；硅、铝含量升高近10%，表明南海北部陆源沉积物输入量增加；铈含量降低约16%，指示海底氧化条件显著减弱。0.73 Ma至0.69 Ma期间，结核生长速率迅速升至平均生长速率的8.27倍，该层中铁、铝及稀土元素含量亦有所升高，形成以鲕状、粒状、斑状及花瓣状结构为特征的疏松层，指示南海北部当时处于高沉积速率、陆源沉积物供给充足的古海洋环境。0.69 Ma至0.22 Ma期间，结核生长速率骤降，形成外致密层。该层中铁、硅、稀土元素、铝、锰、铜、钴及镍的含量均显著低于其他层位，主要结构为层状及裂隙充填结构，反映当时海底流稳定、氧化条件较差且其他条件不利于结核生长的古海洋环境。研究表明，编号为S04-1DG-1的结核生长过程对晚新生代南海北部古海洋环境变化具有敏感响应。