Determination And Biogeochemical Behaviors Of Dissolved Iron In The Western North Pacific

Iron(Fe)is an essential nutrient element for the growth of marine phytoplankton.As the active center or auxiliary factor of redox proteins and many enzymes,Fe plays a central role in many biochemical processes of marine phytoplankton,such as nitrogen fixation,photosynthesis,respiration and chlorophyll synthesis,and therefore regulates the primary productivity in the ocean and global climate change.The western North Pacific is a typical oligotrophic ocean.Despite its very low biological productivity,it still has an extensive contribution to the global marine primary productivity as part of North Pacific Subtropical Gyre.Until now,the investigations on the macronutrients(nitrogen,phosphorus,silicon)and Fe in the western North Pacific are very scarce,which seriously limit the understanding of the biogeochemical cycle of Fe and its controls on the primary productivity in the western North Pacific.The accurate analysis of the concentration of Fe in seawater is critical for studying the biogeochemical cycle of Fe in the ocean.This study establishs a flow injection-solid phase extraction-chemiluminescence method(FI-SPE-CL)suitable for the analysis of dFe in the open ocean.Then the FI-SPE-CL method is applied to measure the concentration of dissolved iron(dFe)in the surface and profile samples collected from the western North Pacific(10-21°N,118.5-155°E)during the GEOTRACES-CHINA GP09(KK1903)cruise from 25th April to 13th June,2019.The sources of dFe at different depths as long as the diapycnal diffusion fluxes of Fe in the euphotic layer are investigated based on the distribution patterns of dFe in the western North Pacific.The nutrient limiting factors in the tropical western North Pacific are further discussed in conjunction with the distribution of N and P.The main results of this study are as follows:1.The FI-SPE-CL protocol is established for the fast and accurate analysis of Fe in the open ocean seawater samples with the commercial Nobias Chelate-PA1 chelating resin as the preconcentration column.The experimental parameters,such as the length of sampling loop,flow rate,preparation method of Luminol reagent,concentration and reduction time of Na2SO3,pH of samples through the column and concentration of eluent are optimized to obtain the high precision and low detection limit.The detection limit of this method was 0.020 nmol/L,and the precision was less than 4%with an enrichment factor of 6.3.The sample output is satisfactory with the measurement of triplicates in an interval of 18 minutes and about 12 mL sample consumption.The analysis of consensus reference seawater materials(SAFe S,SAFe D1 and SAFe D2)by the FI-SPE-CL generates consistent results to the consensus values with 0.6-3.0%deviation,which validates the application of the established FI-SPE-CL method to the open ocean.2.The concentration of dFe in the surface water of the western North Pacific ranges from 0.17-0.54 nmol/L with an average of 0.24 ± 0.05 nmol/L.The concentration of dFe in the northwest part is generally higher than dFe in the southeast part of the investigated area.Influenced by the terrigenous input,the surface waters of the Taiwan Strait have a high dFe concentration of 0.32 ± 0.10 nmol/L.The Kuroshio surface water had a dFe concentration of 0.29±0.04 nmol/L,which may be contributed by the input of terrigenous materials from the Philippine islands to the west.The concentration of dFe in the surface waters of the open ocean(0.23±0.03 nmol/L)demonstrates an excellent linear relationship with the aerosol optical thickness(AOT),indicating the source of surface dFe in the open ocean from the atmospheric deposition.The surface dFe concentration in the western North Pacific is comparable to those in the South China Sea and HOTS stations,but is lower than that in the North Atlantic Ocean extensively affected by the Sahara dust and slightly higher than those in high nutrient and low chlorophyll(HNLC)oceans,such as the subarctic Pacific Ocean,the Southern Ocean(section of the Pacific Ocean)and the tropical southeast Pacific Ocean.3.The dFe profiles in the western North Pacific show minimum dFe concentrations in the surface water(0.21-0.31 nmol/L)and gradually increase with depth with a maximum(0.68-0.88 nmol/L)around 1000 m.Then the dFe concentration decreases continuously in deep water(0.4-0.5 nmol/L).The linear relationship between the concentration of dFe and apparent oxygen utilization(AOU)in intermediate water(200-1000 m)suggestes that remineralization is the main source of dFe in the intermediate waters.At the same time,the input of the Luzon Island-derived materials significant contributes a dFe concentration of about 0.34 nmol/L in the intermediate waters in the western part of the investigation area,including Stations K1,K2,K13,and K14.The dFe concentration in the deep waters of the Station K6 is as high as 2.63 nmol/L,likely affected by the hydrothermal input.The stations near the seamounts(K10,K11,K12)also present high dFe concentrations in the intermediate and deep waters,which are possibly related to the seamount input.The downward diapycnal diffusion fluxes of dFe in the surface water are 2.5 ± 4.7 mg m-2 yr-1,which are conincident with the atmospheric deposition dFe flux of dFe(0.5-3.0 mg m-2 yr-1).Compared with atmospheric input,the upward supply of dFe from deep water to the euphotic layer due to vertical mixing was negligible,further confirming that the dFe in surface water in the western North Pacific is mainly from atmospheric deposition.4.The ratios of N:P and Fe:N in the surface water of the western North Pacific were 0.05-1.51 and 0.007-0.074,respectively,showing a decreasing trend from the northwest to the southeast.It suggests that the limiting factors of the primary productivity gradually transit from the N limitation in the northwest to N-Fe seriallimitation in the southeast.Meanwhile,the concentration of Chl a in the surface waters shows a significant positive correlation with the dFe concentration and AOT,but a significant negative correlation with the concentration of PO43-.Moreover,the ratio of Fe:P in the euphotic layer shows an excellent linear relationship with N*in the intermediate waters.These results suggest that the dFe from atmospheric deposition,terrigenous sediment and islands influences the primary productivity to a great extent by limiting the nitrogen fixation in the western North Pacific.