The Impact Of Methane Seepages On The Compositions Of Different Iron And Phosphorus Fractions In The Dongsha Area Of The South China Sea

Extensive methane seepages on the seafloor of the continental margins can significantly change the regional and even global ocean carbon and sulfur cycles,and potentially affect the iron and phosphorus cycles in the deep biosphere.However,the current understanding on the coupling between iron and phosphorus and phosphorus cycling influenced by methane seepages in marine sediments is limited,especially iron-phosphorus coupling characteristics and phosphorus cycling under different methane seepage intensities.Here we analyzed different iron speciation and phosphorus fractions and other basic geochemical parameters of sediment cores collected at a week seep-impacted site（D-7）and a nearby background site without the influence of seepages（D-8）,and chimney seep carbonates formed from strong methane seepages from the Dongsha area of the South China Sea.The purpose is to investigate the effect of methane seepages on the composition of different forms of iron and phosphorus,to reveal the effect of methane seepages on the burial pathways and characteristics of iron and phosphorus in marine sediments and to improve the understanding of the phosphorus cycling and iron-phosphorus coupling in methane seepage environments.Different iron speciation and phosphorus fractions of the sediments at the two sites show that the contents of carbonate-associated iron(Fe _Carb),iron of amorphous iron-oxides(Fe _Ox1),iron of magnetite(Fe _Mag)remain nearly unchanged with depth in sediment profiles.Fe _Carb,Fe _Ox1 and Fe _Mag contents were not significantly different between sites D-8 and D-7.The distribution of amorphous Fe-oxides(Fe _Ox2)contents in the vicinity of the sulfate methane reduction zone（SMTZ,about 1.5-2.0 m）of D-7comparing with the corresponding interval of D-8 is completely opposite.With these intervals,Fe _Ox2 contents averaging 4.1 mg/g at site D-8 was significantly lower than that at site D-7,which showed an average content of 7.6 mg/g.In terms of phosphorus components,the content of exchangeable P(P_Ex),detrital P(P_Detr),and organic P(P_Org)at the two sites are generally the same in the depth profile,and are close to within the SMTZ of D-7 compared with D-8,suggesting that D-8 and D-7 have the same depositional background,and D-8 as a background site can provide a good reference for D-7.The content of Fe-bound P(P_Fe)of D-7 at the SMTZ compared with D-8,is obviously higher,showing the average value of 2.3μmol/g and 2.9μmol/g,respectively.The two sites presented a large difference in contents of authigenic phosphorus(P_Auth).The P_Authcontent of D-7 near SMTZ increased significantly with an average value of6.0μmol/g,exceeding 20%of the background site D-8.The comparison results of iron components and phosphorus components of the two sites suggest that methane seepages promoted the reduction of easily reducible iron oxides and magnetite,resulting in an increase in reducible iron oxides,and a significant increase in authigenic P and phosphorus adsorbed by iron oxides.Therefore,methane seepages will significantly affect the formation of iron oxides in sediments,further affect the abundance of phosphates bounded in iron oxide,and facilitate the precipitation of authigenic phosphate such as apatite.According to the sampling location and mineral compositions,the subsamples of cold seep carbonate chimneys in the Dongsha Sea can be divided into two types:the outer layer and inner layer.The mineral compositions of the chimney carbonates are mainly quartz,feldspar,calcite,and aragonite.The carbonate minerals of the inner layer are mainly aragonite and/or calcite,and the outer layer is mainly composed of calcite.The appearance of aragonite in the inner layer indicates that it may be exposed to the surface of the seafloor due to hydrogeological activities in the later stage during the formation of the carbonates.At the same time,due to the high content of terrigenous materials such as quartz and feldspar in the study samples,it indicates that its formation process is greatly affected by background sedimentation.The carbon isotope values of the samples are generally negative and all subsamples are less than-55.0‰,and the average value is about-58.8‰,These results indicate that the source of methane in the seep fluid in the diagenetic environment is biogenic methane,which comprises theδ¹³C compositions of the seep carbonate chimney.The oxygen isotope composition of the sample was generally positive and generally higher than 5‰,and the average value was about 5.6‰,indicating that the chimney sample was affected by the decomposition of gas hydrate during formation.The composition of carbon isotopes are generally high the outer layer and low in the inner layer of the seep carbonates chimney.The characteristics of oxygen isotope composition are roughly opposite to the changes of the carbon isotope composition.The carbon-oxygen isotope compositions of seep carbonates chimney show a coupling relationship,suggesting that it is formed by the diffusion effect that occurs due to the movement of the seepages fluid in the carbonate chimney in a single channel.The carbon content of the chimney samples is very high,and overall there is a slight decrease from the outer layer to the inner layer.The total sulfur content nearly keeps unchanged and the peak appears at the center,and the carbonate and pyrite content calculated from the carbon and sulfur contents suggests that the chimney seep carbonate formed in an intensive seepage environment.According to the results of the analysis of the iron component content of the chimney sample,the overall change in Fe _Carb was not significant,while Fe _Carb content of the inner layer was slightly lower.Fe _Mag content increased gradually from the inner layer to the outer layer of the chimney,while Fe _Ox content also showed a trend of gradual decrease in the inner layer of the chimney.From the inner layer of the chimney to the outer layer,the total iron(Fe _Total)showed a gradual increase trend.According to the analysis of the phosphorus of the sample,there was no obvious difference between the inner and outer layers in P_Org,and P_Ex at inner layer was slightly lower than that of the outer layer.These results show that under the condition of strong methane seepage,the more intensive methane seepage can significantly increase the P_Fe and authigenic phosphate in the sample,indicating that this condition is conducive to the enrichment of P_Fe and P_Auth and the formation of vivianite.In summary,in the weak seepage environment,methane seepage can affect the formation of iron oxides and the distribution of different iron oxides,thereby affecting the abundance of phosphate adsorbed by iron oxides,and also favor the formation of authigenic apatite and the burial of authigenic phosphate.In the intensive seepage environment,higher seepage intensity r promotes the formation of vivianite and authigenic apatite.These patterns of phosphorus and iron in response to different intensities of methane seepages may be prevalent in numerous sediments or authigenic carbonate affected by methane seepage on continental margins,affecting the biogeochemical cycles of iron and phosphorus in the environment.