The Existing States Of Iron And Characteristics Of Magnetic Susceptibility In Sediments From Methane Hydrate Potential Area Of Northern South China Sea And Their Indication For Gas Hydrate

Natural gas hydrate is a new type energy with great potential,and it is receiving increasing attention from the scientific community and governments.However,it requires stable temperature and pressure conditions to preserve,geological events like sea level decline,oceanic crust structure movement,seabed slip may cause the hydrate to decompose and release methane.The anaerobic oxidation of upward methane with the electron acceptors like sulfate in seawater changes the phases of carbon,sulfur and iron in marine sediments.The changing of iron-based minerals,as the main carrier of sediment magnetics,will eventually lead to a "imprint" on magnetism.Therefore,the study of the existing states of iron and magnetic characteristics of the surface sediment and their evolution in the natural gas hydrate-bearing area will not only help us understand the influence of AOM(Anaerobic oxidation of methane)reaction in magnetism and phases of iron.And the study can also explore the coupling relationship between the magnetism and the existing states of iron in the early diagenesis process.This study can provide a new basis for the identification of the instability decomposition of deep gas hydrate reservoirs,and provide more basic data.In this paper,we aim to explore the existing states of iron and the evolution of magnetic characteristics and their relationship in shallow sediments in the hydrate-bearing area.We chose the sedimentary column samples SH1 and A27 in the Shenhu area of the hydrate potential area in the northern continental sloop in South China Sea as the research objects,to proceed a series of sedimentary,environmental magnetic,and geochemical researches.The main understandings are as followings:(1)The sediments of SH1 and A27 sites in the Shenhu sea area in the northern South China Sea are mainly made up with suspended semi-deep sea sediments.The silty component can provide the adsorption and preservation of sediment organic matter.The dating results of AMS14C show that the average deposition rate in the study area is at a high level(29.48 cm·ka-1),combined with background data such as structure,temperature and pressure and gas source conditions,indicating that the study area has the basic condition for gas hydrate formation.(2)The double peaks of the particle size distribution of the two stations indicate multiple sources of sediment,but are mainly from the Pearl River and Taiwan.From bottom to top,the characteristics of finer grain size and slower deposition rate reveal that the sea level of the study area has been raised from 17 ka cal.BP,which made the distance from the Pearl River estuary prolonged,so that the sediment was thinner and the deposition rate is slower.(3)Environmental magnetic studies at SH1 and A27 cores show that the magnetic properties of the two cores are affected by the early diagenesis and source changes,resulting in changes in the magnetic mineral and characteristics in each depth segment.The magnetism of the two cores are dominated by magnetite,but the content of magnetic minerals in the upper sediments is mainly iron(hydrogen)oxides such as goethite and hematite.Along with the process of pyrite mineralization,pyrite has gradually become the main magnetic mineral in the deeper layer.The deep layer of the A27 core is obviously influenced by the source changing,so that the iron oxide/hydroxide and the pyrite together become the main magnetic mineral form of the deep sediment.(4)The magnetic decline of the SH1 core at 10?130cm and the A27 core at 10?90cm is mainly controlled by the microbial-mediated dissimilation iron reduction(DIR).The magnetic reduction of the SH1 core 140-380cm and the A27 core 100?260cm layer are mainly caused by oxidation of organic matter-sulfate reduction(OM-SR).The magnetic properties of the two stations below 230 cm and 260 cm are affected by the combination of OM-SR and source changing.The A27 core is closer to the shoreline than SH1,so its magnetism is dominated by source changing.(5)The upper boundary of sulfate-methane transition zone(SMTZ)has been identified by the environmental magnetism and geochemical method in the SH 1 core is around 380 cnm.The sediment of A27 core below 430 cm was influenced by methane anaerobic oxidation of methane-sulfate reduction(AOM-SR),indicating a shallow methane-sulfate transition zone(SMTZ)at the A27 site.(6)The characteristics that magnetic susceptibility and Femang decrease from rapid to gradual of the surface can be used to identify the dissimilation iron reduction(DIR)process.OM-SR can be identified by the decrease of ?,SIRM,?ARM,Femag,Feox and the increase of Fepy content.And the boundaries of two processes above can be recognized.The significant decrease of ?,SIRM,?ARm,the contents of Feox and Femag,with the increasing of Fepy content,can be used to figure out the upper sulfate-methane transition zone(SMTZ),which also indicate the decomposition of natural gas hydrate.