Characteristics Of Formed Authigenic Phosphorus Minerals In A Cold Seep Area Of The Qiongdongnan Region Of The South China Sea And Its Implications For Carbon Cycling In Methanogenic Environments

Deep-sea cold seeps,which serve as unique material cycling pathways connecting the seafloor and the ocean,are widely distributed along the continental margins worldwide.The biogeochemical reactions occurring at these cold seeps may have significant impacts on the element cycling of regional and even global oceans.Therefore,a thorough understanding of the element cycling and biogeochemical processes at cold seeps is of great importance for understanding deep-sea processes and material cycling.Phosphorus（P）is a necessary nutrient for the biological communities at deep-sea cold seeps,but limited knowledge exists regarding the sources,fate,and involvement of P in the biogeochemical cycling processes at seeps.Currently,there is a significant uncertainty regarding how multi-episode cold seepages affect P-Fe coupling and P cycling,and little is known about the characteristics of P-Fe coupling in environments rich in natural gas hydrates and hydrogen sulfides.This study focuses on sediment and pore-water samples collected at four cold seep sites in the Qiongdongnan Basin of the South China Sea,using a continuous chemical extraction method to investigate the iron and phosphorus components preserved in the sediments.By combining stable isotopes of carbon,sulfur,and other relevant geochemical parameters,the study explores the biogeochemical cycling processes of phosphorus in the deep-sea cold seep environments.Two sediment cores affected by methane seepage,ROV02-PC01 and QDN14B,as well as a background sediment cores,QDN31,were analyzed for sediment,pore-water geochemical parameters,and different phosphorus and iron components in the Qiongdongnan Basin of the South China Sea.It was found that below the sulfate-methane transition zone at the QDN14B site,in a non-sulfidic,methanogenic environment,the content of iron oxides(Fe _Ox)in the sediment decreased,but the content of iron-bound P(P_Fe)and authigenic P(P_Auth)increased,which may reflect the presence of authigenic vivianite.The P_Fe/Fe _Ox and P_Fe/Fe _Ox scatter plots in the sediment at this depth clearly indicate the influence of authigenic vivianite,suggesting the formation of vivianite in the sediment.The high P_Auth may be due to the reduction of iron oxides,leading to the release of a large amount of dissolved phosphate,resulting in the formation of authigenic apatite under the conditions of high alkalinity,high phosphate,and high calcium ion concentration in the cold seep environment.Below the sulfate-methane transition zone at the ROV02-PC01 site,the high concentration of hydroge n sulfide indicates a sulfidic environment,and the significant increase in pore-water phosphate concentration and decrease in Fe _Ox content indicate that some of the pore-water phosphate may be derived from the reduction of iron oxides by hydrogen sulfide or methane.Similarly,the decrease in organic phosphorus(P_Org)indicates that some of the pore-water phosphate may come from the catalytic degradation of organic matter.The high P_Fe content within the sulfate-methane transition zone indicates the formation of other forms of iron-phosphorus minerals.The ratios of P_Fe/Fe _Ox,P_Fe/Fe _Ox1（iron of amorphous Fe-oxides）,and P_Fe/Fe _Ox2（iron of crystalline Fe-oxides）indicate the formation of vivianite in sediments.The formation of vivianite in sediments thus may serves as a marker for the occurrence of iron-driven anaerobic oxidation of methane,implying that this process may occur not only in methane-producing zonenon-sulfidic conditions,but also under sulfidic conditions with high phosphate and low sulfate concentrations,which is conducive to the formation of vivianite in microenviroments.The pore-water related parameters and different iron components and phosphorus forms of core sediments from the cold seep area of the R1 site in the southeast of the Qiongdongnan Basin in the northern South China Sea were analyzed.It was found that the concentration of phosphate in the pore-water and the content of iron oxides in the sediment decreased in gas hydrate bearing sediments adjacent to the sulfate-methane transition zone,while the content of iron-bound phosphate and authigenic phosphate increased,indicating the formation of vivianite and authigenic apatite in the sediment.The presence of gas hydrates indicates a state of methane oversaturation,suggesting that the environment can provide sufficient methane to react with iron oxides in the sediment,promoting the formation of vivianite.A fossil sulfate-methane transition zone was recorded about 6 to 7 meters away from the current sulfate-methane transition zone,where the concentration of phosphate in the pore-water was high,but decreased rapidly,and the content of Fe _Ox and P_Auth in the sediment reached the highest level,but the P_Fecontent and the ratio of organic phosphorus to total organic carbon(P_Org/TOC)were the lowest,indicating the formation of authigenic apatite in the fossil sulfate-methane transition zone.The high concentration of phosphate in the pore-water suggests the possibility of high activity and degradation of organic matter that derived from methane sources.The low P_Org/TOC ratio observed in the sediment is due to the preferential decomposition of phosphorus-containing components of organic matter.This study shows that different phosphorus components in sediments from methane-rich environments on the seafloor can provide important information on the formation of authigenic phosphorus minerals near the sulfate-methane transition zone and their involvement in the carbon cycling.In summary,the formation of vivianite in the sulfide environment suggests the possible existence of microenvironments with localized enrichment of divalent iron ion and phosphate,which may be related to microbial activity.However,due to the lack of analysis of mineralogical parameters in this study,the specific formation mechanism of vivianite in sulfide environments is not yet clear and requires further analysis.Nevertheless,geochemical evidence from sulfide environments in deep-sea cold seep areas on the continental margin in this study indicates the formation of vivianite,suggesting that it may be widespread in continental margin seafloor environments,affecting the phosphorus cycle in these extensively distributed environments.The simultaneous occurrence and coupling relationship between gas hydrates and vivianite in anoxia sediments on the continental margin suggest that the iron-driven anaerobic oxidation of methane may be the key to establishing such a coupling relationship.This coupling relationship and the carbon,iron,and phosphorus cycles involved may be widely present in hydrate-bearing sediments at the continental margin,affecting the key element cycling processes of the deep biosphere.