Research to date has resulted in a number of significant findings. (1) By means of a modified selective extraction technique, separate solid forms of phosphorus have been distinguished, and their concentrations determined, in sediments from a number of locations, with emphasis on the Amazon Delta. Results indicate that previously unrecognized mechanisms for the removal of P from seawater (as dispersed authigenic apatite and P associated with hydrous ferric oxides) may be important on a global basis. Also, differences in relative proportions of P- containing phases from place to place agree with independent geological reasoning which serves as a check on the method. (2) Organic phosphorus in marine sediments has been investigated in terms of C/P ratios and its chemical nature using 31P NMR. Principal forms are phosphate monoesters and diesters, and phosphonate compounds, the latter having been identified in marine sediments for the first time. Phosphonate, where C is directly bonded to P, is especially important because of its resistance to bacterial decay. (3) An experimental study of the kinetics of nucleation and crystal growth of fluorapatite from seawater-like solution has resulted in the hypothesis that phosphorite in upwelling areas forms as the consequence of the buildup of very high concentrations (>400 uM) of interstitial dissolved phosphate at shallow depths. The high concentrations enable the precipitation of metastable Ca-phosphate phases which serve as necessary nucleating agents for fluorapatite precipitation. This study will be continued by (1) analyzing in detail for different forms of solid phosphorus in sediments from three major rivers and their marine deltas: the Amazon, the Yangtze (Chang- jiang) and the Yellow (Huang-he). (2) Extending laboratory precipitation studies to include the role of specific nucleating agents, such as collagen, and the effect of changing dissolved carbonate levels on apatite precipitation.
