Distribution Of Terrestrial Organic Matter And Potential Carbon Sink Effect Of Reactive Iron In Sediments Of Different Typical Systems

Rivers play important roles in linking terrestrial and oceanic carbon reservoirs.The land transports a large amount of terrestrial organic carbon(OC)to the ocean through the rivers,most of the terrestrial OC deposited within rivers or converted into CO₂ and released into the atmosphere;the remaining terrestrial OC is transported to the ocean,and most of them is buried in the marginal sea sediments.The biogeochemical cycles of reactive Fe and OC are closely linked,and reactive iron was proved to be a key factor for the long-term preservation of OC in sediments.Therefore,the study of the preservation of OC by different forms of reactive Fe in river-estuary-margin sea sediments is important for the long preservation of terrestrial OC in river-estuary-margin sea sediments,and is important for the study of the global carbon cycle process.In this paper,we take the Changjiang Estuary with its adjacent sea as well as the small rivers and estuaries in Malaysia,which were strongly affected by peatland leaching,as the study areas.In the two study areas,we used lignin as the tracer of terrestrial organic matter,and combined with OC content(OC%),total nitrogen content(TN%),stable carbon isotope and grain size,to analyze the source,distribution,degradation state and influencing factors of terrigenous organic matter in the surface sediments,and we also compared the seasonal variations in the Changjiang Estuary.We used different extractants(sodium pyrophosphate(PP),hydroxylamine hydrochloride(HH)and sodium dithionite(DH))to extract the different forms of iron oxide selectively.With the selective extraction method,we studied and compared the concentration,composition ratio and the OC content that combined with the different iron oxides(Fe-OC)in the surface sediments in different study areas,we also discussed the influencing factors of Fe-OC bounding,and studied the relationship between different forms of Fe and lignin.The main conclusions are as follows.In the sediments of the Changjiang Estuary and its adjacent sea,grain size is the dominant factor that determines OC%,TN%,lignin concentration,Total Fe and different forms of Fe concentration,however,the impact of grain size on lignin degradation degree is limited.The overall distribution characteristics of lignin are as follows:the lignin content in near shore is high,while in offshore the lignin content is low,the lignin content decreases with the increase of offshore distances.The lignin parameters change with variation of the diffusion direction of the Changjiang dilute water.In the flood season,the terrestrial organic matter transported by the Changjiang is mainly deposited near the Changjiang Estuary and is transported northward,which result in higher?8 and Pn/P values in the sediments that near the Changjiang Estuary.However,in the dry season,most of the sediments are mainly transported along the Zhe-Min coast and to the inner shelf of East China Sea,which leads to the higher?8 and Pn/P values in the muddy area near the Zhe-Min coast.The overall distribution trends of Total Fe,chelated Fe(Fe_PP),low crystallinity Fe(Fe_HH)and high crystallinity Fe(Fe_DH)in sediments are as follows:the contents in the near shore are higher,and they decrease with the increase of the offshore distance;the proportions of the three different forms of Fe decrease with the increase of the offshore distance,which indicate the proportions of reactive Fe that participate in stabilizing OC decrease,the Fe phase is more‘aged';f_Fe-OC(PP),the proportion of Fe_PP bonded OC to the bulk sediment OC,and f_Fe-OC(HH)decrease simultaneously,which indicate that the role of reactive Fe in OC preservation will become more limited with the increase of the offshore distance.In the nearshore muddy area,the reactive Fe in the sediments is dominated by Fe_DH(Fe_HH/Fe_DH<1),with the increase of the offshore distance,Fe_HH is more dominant(Fe_HH/Fe_DH>1).It may due to the frequent resuspension processes in the muddy area,which lead to rapid redox cycle of iron(reduction and reoxidation)and increase the crystallinity of Fe.In addition,Fe_PP shows significant positive correlation with?8,Fe_HH also shows positive correlation with?8 except for some outliers.Both Fe_PP and Fe_HH tend to combine with lignin compounds,while the combining ability of Fe_PP with lignin compounds are stronger.Due to the intensive peatland leaching,the concentrations of OC%,TN%and lignin are much higher and the degradation states of lignin is lower in Malaysia riverine and estuarine sediments than in Changjiang Estuary sediments.Dissolved oxygen(DO)and pH in water have important effects on the concentrations of OC,TN and lignin in sediments,while grain size has relatively limited effects on them.The proportions of Fe_PP,Fe_HH,Fe_DH and the Fe-bounded OC are regulated by DO and pH:with the increase of DO and pH,the proportions of Fe_PP and Fe_HH decrease gradually,while the proportion of Fe_DH increases gradually.The bounding capacity of different Fe phases with OC are varied.Although Fe_PP/Total Fe%is only 5.51±4.93%,which makes the proportion of Fe_PP lowest,the f_Fe-OC(PP)is the highest,which is approximately 21.38±6.24%.Overall,f_Fe-OC(PP)>f_Fe-OC(DH)>f_Fe-OC(HH).In this study,for Fe_PP and Fe_DH phases,the bounding capacity of Fe with OC is much greater than Fe with TN,the two forms of Fe-TN only account for about 60%of the Fe-OC,which indicating the reactive Fe are more susceptible to bound with carbon-rich compounds and resulting in a higher C/N compared with the bulk sediment.In addition,compared with Fe_HH-OC,the Fe_PP-OC are more aromatic and the molecular weights are higher.The sum of the three forms of reactive Fe which can stabilize organic matter in sediments is higher(close to 100%)in Malaysia,which is approximately 100%.While in the Changjiang Estuary,only 40%of Fe is reactive and most of them are relatively‘aged'Fe,only a small part of Fe is involved in the preservation of organic matter.Besides,the f_Fe-OC(PP)and f_Fe-OC(HH)in Malaysia sediments are around 20%and11%,respectively,while in the sediments of Changjiang Estuary the proportions are about 10%and 6%,respectively.The high proportion of reactive Fe and high f_Fe-OC in Malaysian sediments may result from the several reasons:1)Malaysia is located in the tropical region,and the relative high degree of primary minerals weathering causes the relative accumulation of Fe minerals;2)The low DO and low pH conditions lead to the reduction and dissolution of Fe,and promote the activation of Fe;3)Due to the intensive leaching of peatlands,high concentration of dissolved Fe acts as a source to the sediment Fe;4)The high organic matter and high humus concentrations in the sediment may promote the inter-preservation of reactive Fe and OC.While in the Changjiang Estuary,these factors are not existed,in addition,the long-term hydrodynamic sorption and frequent physical modification in the Changjiang Estuary led to a rapid Fe-redox cycle,which may increase the crystallinity of iron oxides and accelerate the‘aging'of reactive Fe.