Occurrence Characteristics And Indicative Significance Of Iron And Manganese In Gas Hydrate Potential Areas In The Northern South China Sea

As an ideal alternative energy source in the future,the formation and occurrence of natural gas hydrate requires specific geological conditions.When the external geological conditions change,the hydrate may destabilize and decompose.The methane released by the hydrate is consumed in a large amount through the anaerobic oxidation of methane(AOM)during the upward escape process.At the same time,it will change the occurrence form of iron and manganese in the sediment.Therefore,by exploring the occurrence and evolution of iron and manganese in the sediments of hydrate occurrence areas,it is not only helpful to understand the effect of methane leakage on iron and manganese in the early diagenesis process,but also provides information for tracing the instability and decomposition of hydrates.Scientific basis.This paper aims to explore the occurrence characteristics and indications of iron and manganese in the gas hydrate potential area in the northern South China Sea.The column samples of stations SH1 and A21 in the gas hydrate potential area in the northern South China Sea are selected as the research objects,and the research area is established through preliminary experiments.The extraction method of active manganese in sediments,combined with the experimental methods of sedimentology,mineralogy and geochemistry,divided the diagenetic environment,explored the evolution of iron and manganese and their response to methane leakage,and obtained the following understanding.(1)The cores at stations SH1 and A21 have no obvious structural modification,sedimentary discontinuity,and biological disturbance.The grain size is mainly silt sand.The bottom-up average grain size becomes smaller,and the overall change is not large,indicating that its hydrodynamics The environment is relatively stable,which is characteristic of the transgressive sequence.The high sulfate flux(30.53 and 24.07 mmol/m2/yr),shallower SMI depth(9.63 and 15.13mbsf),high deposition rate(18.05～107cm/ka)and high The TOC content(0.47～1.23wt%)all indicate that the study area in this paper has good natural gas hydrate occurrence conditions.The doublepeak/multi-peak particle size frequency distribution curve shows that the sediments in the study area have multiple provenances.Combined with the whole rock analysis of the sediments and the analysis of the clay mineral composition,it can be seen that Fe and Mn mainly come from the Pearl River and Taiwan’s Zengwenxi,Gaopingxi,etc.In the terrestrial debris imported by rivers,Ca mainly comes from marine volcanic calcareous organisms.(2)According to the characteristics of sulfate content in the pore water,the diagenetic environment of stations SH1 and A21 is divided:the 385cm below station SH1 is divided into SMTZ;the 0～145cm layer of station A21 is the organic mineralization zone(OM),Where the 45～145cm horizon is the organic matter mineralization zone(OM-SR)coupled by sulfate reduction.The 285～485cm layer is the AOM-SR zone.(3)Iron and manganese(hydrogen)oxides are reduced with OM near the redox interface at stations SH1 and A21,forming siderite and rhodochrosite.In the OM-SR layer,the main iron sinks are pyrite and siderite.The pyrite has a small particle size,mostly in the shape of a single berry.This layer is limited by the sulfate reduction rate,and excess Fe2+ combines with HCO3-to form siderite.In the SMTZ layer,the main sinks of iron and manganese are pyrite,siderite and rhodochrosite respectively.The mineralogical characteristics of pyrite in this layer show that the content becomes larger and the particle size becomes larger with the increase of depth.Most of them are massive,filled with foraminifera,filled with carbonate crusts,and with ring structure.The rod-shaped and other forms are present,and the output forms are mostly raspberry spherical aggregates and a small amount of scattered single crystals.The pyrite in the SMTZ layer has a larger diameter and a larger diameter standard deviation than the spherulite clusters in the OM-SR layer.The mechanism of the enrichment of Mnkarb in the SMTZ layer is presumed to be that the product HCO3-of AOM-SR increases,the alkalinity in the environment increases,and Mn precipitates from the pore water and recrystallizes to form Mncarb.(4)The occurrence of iron and manganese in the study area of this paper is affected by methane leakage.In the SMTZ layer,the methane released by the decomposition of natural gas hydrate is consumed by the AOM-SR effect during the upward escape process.The AOM-SR effect produces a large amount of HS-and HCO3-.The iron and manganese elements are affected by the diagenetic environment to form Fepy and Mncarb of enrichment.The enrichment of Fepy and Mncarb has a certain indication for the underlying hydrate.