Research On The Growth And Elemental Geochemical Characteristics Of Large-scale Polymetallic Nodules From The Northern Continental Margin Of The South China Sea

The large-scale polymetallic nodules collected from the northern continental margin of the South China Sea (SCS) were selected to be researched in this study. From the analyses on the microstructure, mineral composition, geochronology, elemental geochemical characteristics etc and comparison with the nodules in the Pacific Ocean and the Atlantic Ocean, the connections between the growth rate of the polymetallic nodules and the paleoceanographic environment events in the northern continental margin of SCS were studied and recovered. Some conclusions were obtained as follows:1. The corresponding relationship of the polymetallic nodules'structures to the oceanic bottom current and the nodules'growth rate in the northern continental margin of SCS was accordant. When the bottom current was strong, the layer was loose and rich in terrigenous sediments, and the structure types mainly were arborescent and taxitic etc., while the layer grew quickly. When the bottom current was weak, the layer became dense and poor in terrigenous sediments, and the structure types mainly were laminated and columnar etc. Some parts had rhythm structure. The layer grew slowly at this period.2. The results of 10Be dating indicated that the mean growth rate of the nodules was 7.41mm/Ma, more higher than those of the nodules in the other sea. The fluctuation of the growth rate was very big in different periods and the growth rate appeared to be of the characteristics of steps and leaps obviously. These features showed that the nodule's growth process in SCS was obviously affected by the marginal sea environments.3. Compared with the nodules in the Pacific and the Indian Ocean, the nodules in the northern continental margin of SCS were rich in Fe but poor in Mn. The manganese phases mainly contained vernadite, and the origin of the nodules should be of hydrogenic. The contents of Fe, Co and Si were higher, but that of Cu and Ni were lower. Mn was strong negative correlation with Fe in the nodules. The content of the REE was very high and with the features of the enrichment of LREE but the depletion of HREE, Ce positive anomaly, the content of Ce in REE was above 70% etc, which showed that REE came from sea water. The strong positive correlation between Fe and REE indicated that REE mainly existed in ferric phases. The contents of Co, Mn and the ratio of Mn/Fe gradually increased and the contents of Si and Al decreased in the nodules of northern SCS with the far distance to the mainland, which revealed that the polymetallic nodules in the northern SCS was controlled by the marginal sea environment and the terrigenous sediments transported into SCS apparently diluted the chemical formation of the nodules.4. The growth of polymetallic nodules in the northern SCS responded to the paleoceanographic environment changing. Late Pliocene that the nodule started to grow (3.29Ma) was a fiercely turbulent period in which ice sheet of Arctic expanded, global climate cooled and oceanic current enhanced. The ore-forming period of nodules in the northern SCS and the Pacific was closely related. Both of them had the same regional ore-forming background. Meanwhile, late Pliocene was also a period of lower sedimentation rate in the evolution history of SCS, and at this period the abundant ore-forming material was in favor of the formation of the nodules in northern continental margin of SCS. The period of great change of the nodule's growth rate was also the period of the great change of the paleoceanic environment in SCS.