| Based on water samples collected from three cruises in spring (7th to 19th May 2013), summer (1st to 30th August 2013) and autumn (8th October to 7th November 2013), in the present study, the disequilibrium of 210Pb/210Po was used to study the residence time and fractionation factors of 210Pb and 210Po in the upper surface water of the East China Sea (ECS). And then, the particulate organic carbon (POC) export flux from the euphotic layer in the ECS was also estimated. The main results are as follows:1) The average activities of dissolved (D), particulate (P) and total (T) 210Po in the surface water of the ECS in spring were 0.34±0.07,0.42±0.19 and 0.76±0.18 (n=34) Bq m-3,respectively. The dissolved 210Po increased with the distance offshore, and particulate and total 210Po showed high values at the southeast of the study area. While the average activities of D210Pb、 P210Pb and T210Pb in the surface water of the ECS in spring were 0.77±0.06,0.79±0.02 and 1.57±0.07 (n=34) Bq m-3, respectively. The activities of dissolved 210Pb in the surface water increased with distance from the shore, while the particulate and total 210Pb were on the contrary.210Po/210Pb activity ratio (A.R.) in both dissolved phase and particulate phase was blow 1, indicating that 210Po was deficient relative to 210Pb in both phases. There was no obvious spatial distribution pattern for 210Po/210Pd)A.R. because of the impacts of riverine discharge, ocean currents and biological activity, etc.2) The average residence time of D210Po、 P210Po and T210Po in the surface water of the ECS in spring were 0.49,0.36 and 0.65 a, respectively. The residence time of dissolved 210Po increased with the distance from the shoreline, while the particulate and total 210Po were just on the contrary. The average residence time of D210Pb、P210Pb and T210Pb in the surface water of the ECS in spring were 0.46,0.48 and 0.94 a, respectively, and distributed uniformly within the whole study area. The average residence time of dissolved 210Po and 210Pb were quite similar, while particulate and total 210Pb were longer than that of 210Po, since the scavenging effect of particulate 210Pb. The Kd (the distribution coefficients in the solid-liquid phase) of 210Po and 210Pb averaged 4.72×105 and 3.10×105 L mg-1, respectively. The negative correlation between Kd and the total suspended matter (TSM, mg L-1) was found.3) By the steady model, the average scavenging rate and removal rate of 210Po in the ECS in summer were calculated to be 1.52 and 2.92 Bq m-3 a-1, respectively, decreasing as the water depth increased except for individual water layer. Total 210Po/210Pb)A.R. went near to 1 with the further increase of the water depth.4) The export flux of 210Po from the euphotic layer (60-80 m) in the ECS in summer ranged 185-321 Bq m-2 a-1, with an average of 214 Bq m-2 a-1. And the export flux of POC ranged 18.1-105 mg C m-2 d-1, with an average of 39.7 mg C m-2 d-1. The export flux of PON ranged 15.6-46.7 mg N m-2 d-1, with an average of 26.1 mg N m-2 d-1. The positive correlation between the export flux of 210Po and the water dpeth was found.5) The average activities of D210Po、 P210Po and T210Po in the PN section of the ECS in fall were 1.05±0.13,0.70±0.09 and 1.75±0.16 Bq m-3. Dissolved 210Po increased with the distance offshore at the near-bottom layer, while particulate and total 210Po increased with water depth. The average activities of D21oPb、P210Pb and T210Pb were 2.01±0.08,1.08±0.06 and 3.09±0.10 Bq m-3, and they showed a silimar distribution which was that the activities increased with the distance from the shore, and were higher in the surface water than in the bottom water for the impact of the atmospheric deposition and riverine discharge. Dissolved 210Po/210Pb)A.R. increased with water depth while particulate and total 210Po/210Pb)A.R showed higher values in the middle of the study area than in the near shore and open sea.6) The average residence time of D2l0Po、P210Po and T210Po in the PN section of ECS in fall was 0.52,0.32 and 0.85 a, respectively. All of them had the silimar spatial distribution which was that the residence time increased with the water depth and with distance from the shoreline. The average export fluxes of 210Po, POC and PON were 149 Bq m-2 a-1,24.6 mg C m-2 d-1 and 26.7 mg N m-2 d-1, respectively. Generally, the export flux was higher at the nearshore than the open sea for all of them.7) The activities of 210Po and 210Pb showed an obvious seasonal variation. In general, the activities of 210Po in both dissolved and particulate phases in fall were slightly higher than that in summer, but low activities was clearly found in spring. The activities of total 210Pb was similar in fall and summer, which were higher than that in spring. The average time of 210Po were higher in dissolved phase than that in particulate phase. Because the sampling time and stations were different, the export flux of POC were also different in different seasons, which were slightly higher in summer than in fall. The estimated export fluxes of POC (39.7 and 24.6 mg C m-2 d-1) in the ECS in this study were just within the range of the reported data (8.4-354 mg C m-2 d-1) in other sea area around the world. |
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