Chemical composition of ferromanganese micronodules and bulk sediments from ODP Hole 199-1216A

Distribution, size, mineral, and chemical compositions of ferromanganese micronodules (FMMNs) and chemical composition of host sediments were examined in a series of red clay samples with ages from Eocene to the present at Ocean Drilling Program Leg 199, Site 1216, south of the Molokai Fracture Zone in the Central Pacific Basin.
The number of FMMNs changed drastically throughout the 40-m-long red clay intervals. FMMNs are abundant in the upper 9 m of core, decrease between 9 and 25 meters below seafloor (mbsf) with depth, and are very rare from 30 to 40 mbsf. Chemical composition of FMMNs showed high Mn/Fe ratios and Ni and Cu contents and a distinct positive Ce anomaly because of the existence of buserite. This suggests that FMMNs in the red clay from 25 mbsf to the top of the cored interval were deposited continuously in an oxic diagenetic bottom environment. The red clay below 30 mbsf with higher Mn contents contains few FMMNs but abundant tiny Mn particles within brown silicates coated by Fe (oxy-hydro)oxides. This indicates that the mode of manganese deposition changed between 25 and 30 mbsf.

针对中太平洋盆地莫洛凯断裂带以南的大洋钻探计划（Ocean Drilling Program）199航次1216站位的一系列始新世至今年代的红黏土样品，本研究对铁锰微结核（ferromanganese micronodules, FMMNs）的分布、粒径、矿物与化学组成，以及宿主沉积物的化学组成开展了系统分析。
在总长40米的红黏土岩心段中，铁锰微结核的丰度随深度发生显著变化：岩心上段9米范围内结核极为富集，在海底以下9至25米（meters below seafloor, mbsf）区间随深度增加而逐渐减少，而30至40 mbsf段则极为稀少。铁锰微结核的化学组成表现出较高的Mn/Fe比值与镍、铜含量，且因水钠锰矿（buserite）的存在而呈现出显著的正铈异常。这表明，25 mbsf至岩心顶部的红黏土中的铁锰微结核，是在氧化成岩底水环境中持续沉积形成的。30 mbsf以下的红黏土锰含量更高，但其中铁锰微结核极少，仅在被铁（羟基）氧化物包覆的棕色硅酸盐中存在大量细小锰颗粒。这说明在25至30 mbsf区间内，锰的沉积模式发生了转变。