5:15 PM - 5:30 PM
[BAO01-06] Phosphorus and iron cycles during early diagenesis of sediments under anoxic water mass in a meromictic Lake Kai-ike
Keywords:phosphorus, iron, early diagenesis, photic zone anoxia, anoxic water mass
We focused on the early diagenesis of phosphorus and iron in the lake. Phosphorus is a bio-essential element and a limiting nutrient for primary production in the ocean on geological time scales. Biogeochemical processes during early diagenesis in sediments play important roles in controlling oceanic P budget, because P fluxes to and from the sediments depend on redox state of the overlying water. Therefore, it is of great importance to understand early diagenetic geochemical behaviors of P in sediments overlain by, for example, oxic, anoxic, and sulfidic (euxinic) water. In this study, we applied sequential extraction procedures to investigate geochemical behaviors of P and Fe in the surface sediments overlain by sulfidic water of the Lake Kai-ike.
A 25 cm-long KAI4 sediment core (Yamaguchi et al., 2010) was used for two sequential extraction methods. SEDEX method (Ruttenberg, 1992) was used for partitioning phosphorus-bearing species into Pabs (absorbed), PFe (iron-bound), Pauth (authigenic), Pdet (detrital), and Porg (organic). Iron-bearing species were also divided into FeHCl (dissolved in HCl), Fecarb (carbonate), Feox (oxide), Femag (magnetite), Fepy(pyrite), and Feresi (residue), following Poulton et al. (2005).
Main phosphorus-bearing form was found to be Porg. Its content sharply decreases with increasing depth near the sediment surface. Little precipitation of Fe-hydroxides (no significant peak for PFe or Feox) reflects anoxic water condition. Absence of Fepy at the sediment surface implies that syngenetic pyrite did not form, The Fepy content showed downcore increase as the most abundant Fe-bearing phase.
Transformation of Porg and PFe to Pauth is called "sink-switching". At sediment surface in an oxygenated ocean, ferric (oxy)hydroxides precipitate to trap phosphate diffusing from deeper-anoxic sediment, and the phosphate concentration in pore water becomes high enough to precipitate authigenic apatite, meaning effective sink-switching (Slomp et al., 1996).
We compared our results with the previous work focusing on early diagenesis under oxic water mass (Ruttenberg and Berner, 1993). Two differences were clearly observed; the abundant Porg with sharp downcore decrease just below the sediment surface, and the similar downcore decrease in Ptot. We suggest that the amount of ferric (oxy)hydroxides relative to that of Porg (PFe/Porg) is a key factor for the retention of phosphorus in the sediments. Further, we quantified the degree of sink-switching, which appears to be related to PFe/Porg ratio. These results suggest the importance of PFe/Porg for sink-switching as an useful proxy to estimate the extent of benthic phosphorus regeneration.
Very low PFe/Porg ratio or absence of PFe and Feox at the sediment surface reflect anoxic water condition, and further, absence of Fepy and very limited abundance of Fecarb indicate ferruginous condition. Anoxic conditions are also indicated by downcore increase in the Fepy content, FeHR/Fetot ratios, and DOP (degree of pyritization; Fepy/(Fepy+FeHCl)) values.
This study provides useful clues for modeling geochemical cycles of phosphorus and iron in the sediments overlain by photic zone anoxia, and for understanding how Fe- and P-related redox proxies recorded early diagenetic processes of the sediments.