Sedimentary phosphorus species and sedimentation flux in the East China Sea

Abstract

Core sediment samples were collected from the middle shelf of the East China Sea (ECS) to study the phosphorus forms, P accumulation rate (PAR), P burial efficiency and the burial flux in the ECS. The sediment samples were sequentially extracted and directly extracted to analyze different forms of sedimentary P: lossely sorbed P and iron-bound P (P CDB); inorganic P associated with francolite (carbonate fluorapatite, CFA), biogenic hydroxyapatite, smecite, and CaCO 3 (P CFA); detrital P (P Detrital); organic P (P Organic); and total P. In addition, the Fe contents in the citrate-dithionite-bicarbonate (CDB) extracted solution were also measured. The total concentrations of P in the surface sediments in the study area ranged from 13.5 to 22.3 μmol g −1. Inorganic P was the major form and accounted for 72–93% of the total P pool. The average percentage of each fraction of P followed the sequence: P Detrital (70%)>P Organic (15.5%)>P CDB (8.4%)>P CFA (5.8%). The distribution pattern of total P in the surface sediment was similar to that of P Detrital and P Organic, but different from that of P CDB and P CFA. The profile variation of P Organic was the most significant among the phosphorus forms at the study stations. The concentrations of P CDB and P CFA showed minor variation with depth. These results may suggest that transformations of P Organic, P CFA and P CDB occurred at the study stations during sedimentary P burial. Based on the concentrations of total P, P CDB and Fe CDB obtained in the present study and the mass accumulation rate (MAR) reported in the literature, the values of the PAR, the P diffusion flux ( J PFe) supplied by reducible iron hydroxides and the P burial efficiency were calculated. The calculated results for PAR and J PFe in the study area ranged from 1.02 to 24.23 μmol cm −2 yr −1 and from 0.1 to 2.11 μmol cm −2 yr −1, respectively. The phosphorus burial efficiency (PBE) was approximately 90%. The ECS is a phosphorus sink, and the average annual P burial flux has been reasonably estimated to be in the range of 20–25×10 9 mol yr −1.