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

Abstract

Laboratory of Marine and Atmospheric Geochemistry, Graduate School of Environmental Earth Science,Hokkaido University, Sapporo 060-0810, Japan(Received September 4, 2000; Accepted July 16, 2001)In order to investigate the chemical form of authigenic phosphate compounds being precipitated incoastal sediments, the vertical distributions of dissolved phosphate and fluoride in pore water from thesediments of Funka Bay (Japan) were determined for the period from April 1997 to July 1998. The phos-phate concentration in surface sediment pore water was typically 0–5 µM and increased up to 40–60 µM at70 cm depth below the surface. In spring and summer, small maximums higher than 100 µM were found ata depth of around 20 cm, probably resulting from decomposition of organic matter and/or desorption ofphosphate from the ambient sediment particles under anoxic conditions. Seasonal and vertical variationsin the fluoride concentration of the pore water were in the range 53–74 µM. We found an inverse relation-ship between interstitial phosphate and fluoride concentrations, suggesting precipitation of an authigenicfluoro-phosphorus compound with a P/F atomic ratio of around 1/3 in the sediment. Formation of such acompound will greatly contribute to the global sink of phosphate from the ocean.