Marine Phosphorus Geochemistry In Nantuo Ice Age In Southern China

Phosphorus is one of the most important limiting nutrient elements of the ocean primary productivity, meanwhile, the burial of organic matter and the status of the redox atmosphere-ocean system plays an important role in regulating. Due to the marine phosphorus mainly comes from the chemical weathering of rock, therefore, in the geological history period the geochemical circulation of the marine phosphorus not only reflects the change of continental weathering, but also restrict the ocean primary productivity and the evolution of atmospheric oxygen plays an important role. It shows the concentration of the dissolved marine phosphate during the Neoproterozoic "Snowball Earth" period is significantly higher than the other time in Proterozoic. After the "Snowball Earth", it happens a large-scale global P-forming event during the Doushantuo Age, and it is developed the large-scale sedimentation of phosphorite in ocean, which is often the large or super-large deposits. The atmospheric oxygen concentration increases significantly in Neoproterozoic?called NOE?, and it is one of the most important moments in the history of earth. According to the traditional "Snowball Earth" point of view shows that the earth surface is completely covered by the ice during the Nantuo Ice Age?including the land and the sea?,while hydrologic cycle and chemical weathering are limited. Therefore, the cause of the high marine phosphorus concentration is a scientific problem that is needed to solve.In this paper, select from shallow water platform facies, continental slope facies to basin facies as the research objects in Guizhou province of Southern China, and the four profiles respectively are Liujing in Zunyi,Wuhe in Jianhe,Lijiapo in Congjiang,Dahebian in Tianzhu. Analysis of sedimentology, petrography and element geochemistry for weathering, transportation and deposition process of phosphorus in Southern China during Nantuo Ice Age are researched. In order to infer in the case of higer postglacial atmospheric oxygen concentration and temperature, changes of dissolved marine phosphorus. Therefore, a simulation experiment is designed for research the marine phosphorus in Nantuo Ice Age how to transformation in the ferruginous and hypoxic ocean during oxidation process. The studying gets several conclusions as follows:The observation of the profiles, Nantuo formation is mainly composed of moraine-breccia. There are several layers of pulveryte. Both gravels with good roundness and bedding structures in the profiles indicate warm and cool climate oscillation during Nantuo Ice Age, and water is the medium of transportation.Major element, trace element and rare-earth element geochemical analyses are performed on fine-grained parts of diamictite in Nantuo Formation. It suggests that four researched regions are formed from the same source rocks. A large range variation of Chemical Index of Alteration?CIA? in fine-grained parts of diamictite in Nantuo Formation indicates climate oscillation and changes of chemical geochemistry during Nantuo Ice Age. Significantly negative correlation between CIA values and P2O5/Al2O3 imply that diamictite undergo chemical wearthing on different degree and phosphorus released from source rocks during chemical weathering, which is an important source of marine phosphorus in Nantuo Ice Age.Iron components in fine-grained parts of diamictite are used to judge the sedimentary environment. The analysis shows that Feh/TFe of the most diamictite are higher than 0.38, and the pyrite content in the nantuo formation is few, so the sea is ferruginous and reducing during Nantuo Ice Age.With Feh combination of rare-earth elements geochemical analyses show that Liujing profile is characteristic of MREE obvious enrichment. The PAAS-normalized REE patterns of fine-grained parts of diamictite researched regions are different from typical ocean, but they are similar to the ferruginous and reducing water. The PAAS standard curve of REE in Wuhe profile and Dahebian profile reveal a deficit of light rare earth elements and enrichment of heavy one. It reflects light rare earth elements prefer to release heavy rare earth elements in the shallow-water sediments?platform facies?, and light rare earth elements enrich in deep water?slope facies and basin facies?.The results confirm the ocean is ferruginous and reducing during Nantuo Ice Age again. The samples of Dahebian profile show Ce positive anomaly to reflect the sedimentary environment of basin facies is more reduction than the other facies. During Nantuo Ice Age most places of the ocean are ferruginous and reducing, and this is another major cause of marine phosphorus enrichment.In order to reveal the occurrence of the marine phosphorus in Nantuo Ice Age, author analyses Fe-P in the red diamictite from Nantuo formation. There is little apatite in the diamictite, and Fe-P accounts for the bulk of TP. So they indicate most of marine phosphorus occurred in dissolved states. After water carrying dissolved oxygen into the sea, Fe²⁺ oxidizes for Fe³⁺ in the water, then dissolved phosphorus adsorption or precipitation on iron hydroxide formed Fe-P.As time goes on, pH and Eh of artificial Nantuo Ice Age water increase gradually in the high oxygen concentration and high temperature environment, and the oxidation degree of the water is higher and higher. Then, Fe²⁺ oxidizes for Fe³⁺, leading to dissolved phosphorus adsorb on highly active iron or precipitate with iron hydroxide.Based on the above research results, combined with overall climate characteristics during Nantuo Ice Age and the important discovery of the Antarctic region in recent years that conclude as follows: 1. Those analyses indicate that exit climate oscillation and subglacial weathering during Nantuo Ice Age in Southern China. At this time, transporting phosphorus from land to ocean mainly takes advantage of subglacial weathering. 2. Because the ocean is mainly ferruginous and reducing during Nantuo Ice Age, it means concentration of atmospheric oxygen is low. The sedimentary environment ensures the amount of phosphorus by subglacial weathering can move and concentrate in the ocean.