This study was carried out in order to understand the early diagenetic redistribution of phosphorus and relevant mass balance in the sediments of the East Sea. In two cruises during May 1993 and October 1995, 11 box cores were collected in the southwestern part of the East Sea. Dissolved phosphorus and iron were analyzed in the porewater from the cores. Sediment samples were analyzed for solid-phase P species and solid-phase Fe oxyhydroxide by sequential extraction.
Phosphorus speciation results show that organic P is the major chemical form of phosphorus in young sediments within the upper 50 cm of sediment. However, the authigenic fraction of total P increases with depth, indicating the precipitation of carbonate fluorapatite (CFA) in the sediments. The authigenic CFA (Ca5(PO4)2.6(CO3)0.4F) was formed and buried at rates of 11-110 μmol cm-2 kyr-1. The main source of dissolved phosphorus for the precipitation of CFA is organic P. Dissolved phosphorus, released from the decomposition of organic P, diffuses upward to return to bottom water, or is sorbed to iron oxides in the oxidized sediments. As sedimentation proceeds, the iron oxide-bound P is released in the reduced layer and enters the dissolved phase, which contributes P to the formation of CFA in addition to that contributed by the organic P.
The burial flux of reactive P (iron-oxide-bound P + authigenic P + organic P) is 0.09-0.53 g P m-2 yr-1 that accounts for 18-58％ of the reactive P arriving at the sediment/water interface. The burial flux of reactive P is high in the upper and lower continental margin sediment. The burial flux of reactive P in the Ulleung Basin sediment is less than those in the continental margin sites by a factor of 6, indicating that the reactive P burial flux is mainly dependent on sedimentation rate.
Keywords: phosphorus; early diagenesis; redistribution; Fe oxides; carbonate fluorapatite; burial flux; sedimentation rate』
2. Study area and methods
2.1. Geological setting
3.1. Total P concentrations
3.2. P concentrations in speciation
3.2.1. Dissolved P in porewater
3.2.2. Detrital apatite-bound P (Det-P)
3.2.3. Iron oxide-bound P (Fe-P)
3.3. Authigenic apatite-bound P (CFA-P)
3.4. Organic P (Org-P)
4.1. The role of redox cycling of Fe in sedimentary P geochemistry
4.2. Formation of authigenic CFA
4.3. The phosphorus cycle in sediments
Appendix A. Calculation of fluxes between P reservoirs
|Step||Extractant||P component extracted|
|Adsorbed + iron-bounda||0.3 M Na-citrate + 1 M NaHCO3 (pH 7.6) 0.5 g Na-dithionite in 20 ml Na-cttrate-bicarbonate solution (8h)||Exchangeable or loosely aorbed P + easily reducible or reactive ferric Fe-bound P|
|Authigenic||1 M Na-acetate buffered to pH 4 with acetic acid (6 h)||CFA + biogenic hydroxyapatite + CaCO3-bound P|
|Detrital||1 M HCl (16 h)||Detritalfluorapatite-bound P|
|Organic||Ash at 550℃ 1 M HCl||Organic P|
|a This step combines SEDEX steps I and II of Ruttenberg (1992) (after Rao, 1994).|
Fig. 6. The phosphorus cycle in the study area. Units in g P m-2 yr-1. The values in parentheses are benthic fluxes calculated to balance the fluxes between each reservoir in sediment. The solid and dotted line boxes represent P species in solid and dissolved phase, respectively.