Vertical distributions of interstitial phosphate and fluoride in anoxic sediment: Insight into the formation of an authigenic fluoro-phosphorus compound

In order to investigate the chemical form of authigenic phosphate compounds being precipitated in coastal sediments, the vertical distributions of dissolved phosphate and fluoride in pore water from the sediments of Funka Bay (Japan) were determined for the period from April 1997 to July 1998. The phosphate concentration in surface sediment pore water was typically 0–5 μM and increased up to 40–60 μM at 70 cm depth below the surface. In spring and summer, small maximums higher than 100 μM were found at a depth of around 20 cm, probably resulting from decomposition of organic matter and/or desorption of phosphate from the ambient sediment particles under anoxic conditions. Seasonal and vertical variations in the fluoride concentration of the pore water were in the range 5–74 μM. We found an inverse relationship between interstitial phosphate and fluoride concentrations, suggesting precipitation of an authigenic fluoro-phosphorus compound with a P/F atomic ratio of around 1/3 in the sediment. Formation of such a compound will greatly contribute to the global sink of phosphate from the ocean.