| The reduction of Fe(Ⅲ)through biological and abiotic pathways speed up the dissolution and recrystallization of iron-bearing minerals in marine sediments,The biological pathways include the dissimilatory reduction of Fe(Ⅲ)coupled to the organic matter mineralization(OM)and anaerobic methane oxidation(AOM).The abiotic pathway is the chemical reduction by H2S,production of sulfate reduction which also were reported to couple with OM and AOM.So both OM and AOM are the main factor that control the type,content and morphology of iron-minerals in marine sediments.Meanwhile they are also the important biogeochemical processes to control the emission of methane from marine sediments into the ocean even atmosphere.Especially in continental margin sediments,AOM consumed about 90%methane that product in the terminal step of OM or seep from the methane hydrates which effectively prevent climate change and ocean acidification caused by methane.Meanwhile the geochemical and mineralogical anomalies formed during AOM in sediments and pore water are important scientific bases for tracing methane hydrates reservoirs.So,the signatures of iron-bearing minerals within different geochemical zones responded to these two diagenetic mechanisms are good bases to retrieval the biological and nonbiological geochemical reaction in early diagenesis and explain the event of climate jump and sea level changes in geological time.But little was known for now.Aiming at that,this paper chooses the sediment cores collected from Shenhu area and Taiwan basin in the northern South China Sea,the most potential area with methane hydrate bearing.Combined the geochemistry,mineralogy and microbiology methods,we investigated the mechanisms of OM and AOM in iron-bearing minerals transformation.First,we investigated the sedimentary history in study area,evaluated the potential of methane hydrate bearing in study sites and divided the geochemical zone through all the geological and geochemical data that we tested.Then characteristics of ironminerals and microbial community in different geochemical zones were analyzed by mineralogical and molecular biotechnology methods.Meanwhile the cultivation experiments were carried out to clarify the biological and abiotic geochemical reactions and to assessment the methane production and oxidation potential in each geochemical zone.Finally,we analyzed the variation of microbial community,the major dissolved ions and other surroundings after incubation.So far,we have gotten the following insights:1.Since sediments in two sites were buried,no structural changes or depositional hiatus occurred.The calculated the bottom boundary of methane hydrate zone depths at A27 and SH1 sites were 84.45～136.46 mbsf(meters blow the sea floor)and 193.11～301.42 mbsf,respectively,which was similar to the actual bearing depth of methane hydrate that drilled in shenhu sea.Meanwhile,the average content of total organic carbon in sediments were>1%in both cores which could provide sufficient biological gas sources for the methane hydrate reservoirs.The methane fluxes could up to 18.87～24.77 mmol/m2/yr(SH1)、15.33～19.80mmol/m2/yr(A27)also suggested the burial potential of methane hydrate below the two study sites.The above results indicated that the sediments have experienced steady early diagenesis since burial and affected by methane seeps,which is suitable for the study of the two metallogenic mechanisms of OM and AOM,and the distribution and morphology characteristics of the iron-bearing minerals in the sediments controlled by these two metallogenic mechanisms.2.The sulfate-methane transition(SMT)in A27 and SH1 sites were calculated at depths of 4.0-10.76 mbsf and 4.0-9.63 mbsf,respectively,based on the linear decline of sulfate in porewater from 4 mbsf.The incubations of sediments at different depths confirmed that only in sediments within the sulfate linear decreasing region(SLD region)the injected 13CH4 were converted to 13CO2.There was also an interesting phenomenon that we test a certain amount of methane only in headspace of incubations with SLD region sediments,treated with N2 instead of 13CH4,which indicated that sediments within SLD region also had the potential of methanogenesis.Therefore,we suggested that the up boundary of SMTZ should extend to the depth that sulfate started to linear decrease,the layers from 4 mbsf to the bottom in two study cores were located in the extending SMTZ.Then geochemical data showed that there were iron reductions in layers at depths of 1.6-2.0 mbsf(A27)、1.2-2.4 mbsf(SH1)and the extending SMTZ because the Fe2+ and Mn2+concentration were accumulated at these layers,which could be the results of dissimilatory reduction by microorganisms and chemical reduction by H2S,respectively.3.The characteristics of iron-bearing minerals varied in different geochemical zones.Magnetite was almost completely dissolved and remineralization to be siderite in dissimilatory reduction of iron zone.And in extending SMTZ the magnetite stayed the original character and there were poor crystallization pyrites formed on the surface which were mainly due to the chemical reduction by H2S.There were also a lot of framboidal pyrite aggregations exhibited as rod,tube and irregular blocks-like shapes,only enriched in 3.3-3.7 mbsf&4.4-5.2mbsf in A27 and 4.7-4.8 mbsf&5.1-5.2 mbsf in SH1.Furthermore,two episodes characteristics of framboidal pyrite aggregations were found in bottom layers.We roughly calculated the buried time of the very top enriched framboidal pyrite aggregations sediments(3.3 mbsf)based on the deposition rate in A27 was 15 ka BP.Considering the methane seep would enhance the formation of pyrite and the characters of framboidal pyrite aggregations,we speculated that there were at least three times methane emission from methane hydrates since 15 ka BP.4.The high-throughput sequencing analysis showed that the diversity of bacteria decreased with the increasing burial depth,while the diversity of archaea was unrelated to the burial depth.The dominant microorganisms in extending SMTZ at different sites were the same,but the abundance was different.We speculated that the difference of terrigenous input between two study sites might be the main factor.After incubation we found some bacteria,such as genera Pseudomonas,Halomonas,Marinobacter,Sva 1033 and Desulfobulbaceae,were maintained or even enriched during the incubation compared to the environmental samples.These bacteria were reported having metabolisms of OM,sulfate reduction and iron reduction and so on which implied that OM,SR and IR might be conducted and these enriched bacteria had played a direct or indirect role during the incubation.In contrast,archaeal diversity was mainly reduced,and only some uncultured archaeal belonging to phylum Bathyarchaeota were enriched after treatment with high sulfate concentration.In addition,the abundance of methanogens,such as genera Methanosarcina,Mehanococcoide,increased a little after the incubation only in some of the incubation samples but were still low(<0.15%).We observed little suspected anaerobic methanotrophic archaea(ANME)/sulfate reduction bacteria(SRB)aggregates through FISH.But,the sequences of ANME were not detected.Meanwhile,the proportion of SEEP-SRB1,which is in syntrophy with ANME,was found to decrease to almost zero,suggesting that other microorganisms except ANME might involved in the AOM during the incubation and the enriched/maintained Bathyarchaeota after incubation might be the best candidates.6.Multiple biogeochemical reactions leaded to the dissolution of silicate minerals,iron-bearing minerals and degradation of organic matters which caused the concentration of PO43-,SiO42-,Mg2+,Ca2+,K+ and Fe2+increasing at the early stage of cultivation.As time goes on Fe2+ and some of the anions surpassed saturation and formed pyrite and silicate minerals which lead to these ions concentration decrease.The iron-reduction rates were discordance with variation of AOM rates also suggested that iron reduced not only by the H2S,production of sulfate reduction and AOM,fitted the observation of enriched of iron dissimilation reduction bacteria.So,the iron could be the best candidate of additional electron acceptors except the sulfate in AOM during incubations.At the end of incubation,we examined other variations through SEM observation.We found abundant extracellular polymeric substance(EPS)distributed on the surface of some minerals,especially the on the surface of single crystal pyrites,which contributed to the assemble of minerals,nucleation sites and further developed into mineral crystal.Based on all the results we get during incubation,we concluded that in extending SMTZ the iron reduction through chemical pathway by H2S and dissimilatory reduction by bacteria were both caused the dissolution of iron oxide/hydroxide and transformed to pyrite.These two mechanisms via iron reduction reaction supplied material foundation(Fe2+)of pyrite formation.And the EPS that produced by the microbial community provides effective sites and forming conditions to further growth of single crystal pyrite into pyrite aggregate. |
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