The Temporal Variation And Its Controlling Factors Of Different Forms Of Nitrogen In The Sediments Of South China Sea And The Okinawa Through

Sedimentary total nitrogen content(TN)and its isotope composition(δ15NTN)are important tools for reconstruct the historic variation of oceanic nitrate pools.There are two nitrogen forms of TN in the sediments,organic nitrogen(ON)and inorganic nitrogen(IN).The sedimentary ON is mainly contributed from biological debris in euphotic zone,so its content and isotope composition record the nitrogen cycle in the overlying water.On the other hand,sedimentary IN mainly exists in the form of ammonium fixed in the lattice of 2:1 type clay mineral.Since the content of IN was gengerally low in open ocean sediments and often considered niglible,most studies use TN instead of ON to investigate nitrogen cycle paleoceanography.However,several recent studies have reported a significant contribution of IN to the TN,and theδ15NIN usually has a lighter value than that of ON,which would potentially bias the signal of targeted δ15NON in the δ15N measurements of TN at this case.Here,we firstly utilize the denitrifier method to directly measure the concentration and nitrogen isotope composition of ON.Meanwhile,we obtained the temporal variation of three different species of nitrogen(TN,ON,IN)and their isotope composition in two sediment cores(MD972142 in South China Sea,and MD012403 in Okinawa Through).Via comparing δ15NON and foraminifera shell-bound N isotopes(FB-δ15N)in the core MD972142,the study systematically discussed the possible mechanism for resulting in differences between δ15NON and FB-δ15N.On the other hand,the relationship between δ15NON of MD012403 and the intensity of denitrification in the Eastern Equatorial North Pacific(ETNP)was also discussed.Additionally,the study also shed light on the potential for δ15NIN as a proxy for reconstructing climate changes on land.The main findings are as follows:The result shows that there are significant occupations of IN in these two sediment cores,the contribution of IN to TN in core MD972142 is up to 68%(mean 51%);whereas IN constituted up 21%to 62%of TN in core MD012403,with mean of 38%.Although the variation patterns of δ15NTN and δ15NON are similar and have not been significantly biased by the influence of a high proportion of IN,the variation amplitude of δ15NON is higher than δ15NTN.In South China Sea,previous studies show that the FB-δ15N record the δ15N of the thermocline nitrate and is similar to the δ15N of sinking N,which represent productivity export from the euphotic zone.Through the protection of the shell,the FB-δ15N is less affected by diagenesis during sinking and burial,while sedimentaryδ15NON records the signal of early diagenesis alteration.Therefore,by comparing the difference of FB-δ15N and δ15NON,we can evaluate the reox changes of bottom water and sediments in different periods.Overall,the δ15NON shows a reverse temporal pattern with FB-δ15N in core MD972142,with δ15NON values higher than FB-δ15N during interglacial,while lower than FB-δ15N in the glacial.And Δδ15N(ON-FB)is consistent with the sedimentary redox state proxy Mn,which further supports the augument that the differences between δ15NON and FB-δ15N are related to early diagenesis.Specifically,the peaks of Mn contents normally display during interglacials,implying sedimentary oxic condition.At this period,organic matter experienced early diagenesis which leaded to an increase in δ15NON,so thatΔδ15N(ON-FB)shows positive values.By contrast,in the glacial sedimentary suboxic condition,the δ15NON decreased as anoxic decay,which caused negative values ofΔδ15N(ON-FB).Via comparing the δ15NON in core MD012403,and sedimentary δ15NTN records of ETNP、CTNP and WTNP,we found that all these records reveals largely similar variations from the last glacial maximum to Holocene,but the variation amplitude is decrease along NPIW path(from ETNP to WTNP).The results suggest that the variation of sedimentary δ15NON in the Okinawa Trough recorded the changes of global nitrate pools,and was controlled by global water column denitrification singals advected form the ETNP,rather than local denitrification.Further more,the δ15NON shows the same pattern with solar insolation,and correspond to the δ18O records of Dongge cave and Hulu cave,which indicate that the δ15NON were associated with climate events.On the other hand,δ15NIN is found to correlate with solar insolation,and reflect the climatic control on lithogenic materials sourced from land.In MD972142,IN%was generally high during glacial period when the sea level was low,implying IN was related to terrestrial input.The consistent patterns between IN%and illite contents,suggested that illite was the dominant carrier of IN in core MD012403.Meanwhile,we also found that δ15NIN was positively correlated with solar insolation,with highδ15NIN values corresponded to high insolation period and vice versa.Similar relationship was also found in core MD972142、core MD012403 and core St.3,which from SCS、OT and WSAP respectively,which suggest the δ15NIN variation might reflect the global solar changes.However,regional difference of δ15NIN values was also found,for example,the δ15NIN values in core MD972142 was higher than that of core MD012403 and core St.3,which might attribute to the reginal differnence of isotope ratio of NH4+ in soil solutions,and the fractionation factor between NH4+ in soil solutions and IN in clay minerals.The exact mechanism for the regional difference of δ15NIN needs to be confirmed by further study.