Phosphorus and iron cycles during early diagenesis of Lake Kai-ike sediments, Kami-koshiki Island, southwest Japan

A meromictic Lake Kai-ike is located on the northeastern margin of Kami-koshiki island, Japan. Permanent density stratification develops due to seawater infiltration through a gravel bar separating the lake from the ocean. The oxygenated surface water overlays a stagnant, saline, and anoxic deep water containing hydrogen sulfide. Purple sulfur bacteria (Chromatium sp.) inhabit the chemocline at 4.5m depth. At the lake bottom, green sulfur bacteria form microbial mat-like structures (Nakajima et al., 2003; Environ. Microbiol.). Such environment can be treated as a model for the past anoxic ocean, such as during Cretaceous OAEs (Oguri et al., 2003; Frontier Res. on Earth Evol.).

A 25 cm-long KAI4 sediment core (Yamaguchi et al., 2010; Palaeo3) was used for two sequential extraction methods. SEDEX method (Ruttenberg, 1992; Limnol. Oceanogr.) was used for partitioning phosphorus-bearing species into P_abs (absorbed), P_Fe (Fe-bound), P_auth (authigenic), P_det (detrital), and P_org (organic). Iron-bearing species were also divided into Fe_HCl (HCl-soluble), Fe_carb (carbonate), Fe_ox (oxide), Fe_mag (magnetite), and Fe_resi (residue), following the method of Poulton et al. (2005; Chem. Geol.).

At the uppermost part of KAI4 core, P_org was the most abundant P-bearing species (~90% of total P). The P_org content sharply decreased with increasing depth to 5cm. The second most abundant species was P_Fe; however, P_Fe and Fe_ox contents remained constant throughout the whole depth.

At sediment surface in present-day oxygenated ocean, Fe³⁺-(oxy)hydroxides trap phosphate diffusing from deeper-anoxic sediment, and the phosphate concentration in pore water becomes high enough to precipitate authigenic apatite (Slomp et al., 1996; J. Mar. Res.). In case of Lake Kai-ike, however, the amount of Fe³⁺-(oxy)hydroxides was small relative to that of P_org (P_Fe/P_org = ~0.1). We suggest that the excess phosphate not adsorbed on Fe³⁺-(oxy)hydroxides was diffused out to the overlying water mass, and the fraction enhanced the microbial activity. This positive feedback mechanism plays a significant role in maintaining anoxic environment in the bottom water.