11:15 AM - 11:30 AM
[MIS19-03] Phosphorus early diagenesis in surface sediments of cold methane seepage area off Sakata in the northeastern Japan Sea
Keywords:Phosphorus, Early diagenesis, Methane hydrate, Cold seep
Oceanic phosphorus (P), the major nutrient element, is ultimately removed to sediments. Assessing the details of early diagenesis of P in surface sediments are essential for understanding the long-term marine P biogeochemical cycle. In a knoll off Sakata (called Sakata Knoll) in the eastern margin of the Japan Sea, methane hydrates (MH) are present below the seafloor and a variety of seafloor environments are created depending on the flux of methane to the surface sediments. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction associated with large methane fluxes has a significant role in early diagenesis in the MH-bearing area. We have revealed the spatial variability of geochemical features, including cycles of major and trace-elements, in the surface sediments of Sakata Knoll. Samples were collected from the MH-bearing area including inside the microbial mats (IBM) and outside the microbial mats (OBM), and from outside the MH-bearing area (Ref Site). Here, further investigation was made for P biogeochemistry. Comparing the distribution and forms of P in sediments at different locations, including the microbial mats, within Sakata Knoll is expected to provide new insights into the early diagenesis of P.
Ref Site showed a vertical distribution of P typical of open ocean sediments. The maximum values of total P and inorganic P were found at the surface indicating that inorganic P is adsorbed and concentrated on iron oxides under aerobic condition. The concentrations decreased rapidly to about half within the top 5 cm and remained relatively constant below the subsurface. Organic P exhibited a gradual decrease trend throughout the entire depths. Labile fraction of organic P deposited is considered to have already been degraded before being incorporated into early diagenetic processes within the sediments. The observed gradual decrease with depth is likely due to the gradual remineralization of relatively refractory components.
At the surface sediment in IBM, no inorganic P enrichment was observed. Active AOM can promote reduction of iron oxides by producing large amounts of sulfides. Consequently, iron-oxide-bound P can be released into the porewater. In contrast, P enrichment was observed at the surface sediment in OBM, although the concentrations were not as high as that in Ref Site. In contrast to IBM where there is upward advection of methane-rich fluid, methane supply is likely limited only by diffusion fluxes in OBM. It is considered that the production of sulfides is not enough to reduce the iron oxides in the entire depths of sediments, and the remaining iron oxides, although small in quantity, adsorb inorganic P at the surface of OBM. Interestingly, in IBM and OBM, inorganic P was enriched in the deep layers along with the high concentrations of calcium carbonate. Carbonate rocks can be formed via AOM coupled to sulfate reduction. We found that the calcite-rich, nodule-like carbonate rocks collected within the microbial mats of Sakata Knoll contained substantially high concentrations of inorganic P. It is possible that calcium carbonate as it grows incorporates dissolved inorganic P from the ambient porewater.
The vertical profiles of organic P in IBM and OBM were also distinctive with respect to that in Ref Site. The organic P concentrations in the surface sediments were comparable between IBM, OBM and Ref Site, whereas they were significantly lower in IBM and OBM in the deep layers of sediments. The sediment with the lower concentrations of organic P also had significantly high values of organic C/P ratio. This result indicates that phosphorus is preferentially remineralized relative to carbon during degradation of organic matter. It is possible that AOM-driven processes strongly influence early diagenesis of both inorganic and organic P in sediments of MH-bearing areas.
This study was conducted as part of the Methane Hydrate Research Project funded by the Ministry of Economy, Trade and Industry, Japan.
Ref Site showed a vertical distribution of P typical of open ocean sediments. The maximum values of total P and inorganic P were found at the surface indicating that inorganic P is adsorbed and concentrated on iron oxides under aerobic condition. The concentrations decreased rapidly to about half within the top 5 cm and remained relatively constant below the subsurface. Organic P exhibited a gradual decrease trend throughout the entire depths. Labile fraction of organic P deposited is considered to have already been degraded before being incorporated into early diagenetic processes within the sediments. The observed gradual decrease with depth is likely due to the gradual remineralization of relatively refractory components.
At the surface sediment in IBM, no inorganic P enrichment was observed. Active AOM can promote reduction of iron oxides by producing large amounts of sulfides. Consequently, iron-oxide-bound P can be released into the porewater. In contrast, P enrichment was observed at the surface sediment in OBM, although the concentrations were not as high as that in Ref Site. In contrast to IBM where there is upward advection of methane-rich fluid, methane supply is likely limited only by diffusion fluxes in OBM. It is considered that the production of sulfides is not enough to reduce the iron oxides in the entire depths of sediments, and the remaining iron oxides, although small in quantity, adsorb inorganic P at the surface of OBM. Interestingly, in IBM and OBM, inorganic P was enriched in the deep layers along with the high concentrations of calcium carbonate. Carbonate rocks can be formed via AOM coupled to sulfate reduction. We found that the calcite-rich, nodule-like carbonate rocks collected within the microbial mats of Sakata Knoll contained substantially high concentrations of inorganic P. It is possible that calcium carbonate as it grows incorporates dissolved inorganic P from the ambient porewater.
The vertical profiles of organic P in IBM and OBM were also distinctive with respect to that in Ref Site. The organic P concentrations in the surface sediments were comparable between IBM, OBM and Ref Site, whereas they were significantly lower in IBM and OBM in the deep layers of sediments. The sediment with the lower concentrations of organic P also had significantly high values of organic C/P ratio. This result indicates that phosphorus is preferentially remineralized relative to carbon during degradation of organic matter. It is possible that AOM-driven processes strongly influence early diagenesis of both inorganic and organic P in sediments of MH-bearing areas.
This study was conducted as part of the Methane Hydrate Research Project funded by the Ministry of Economy, Trade and Industry, Japan.