Time variations of phosphorus and sources of sediments beneath the Chang Jiang (Yangtze River)

Depth-dependent changes of different forms of sediment phosphorus in a 98-m drill core from near the mouth of the Chang Jiang (Yangtze River) have been determined using a modified chemical extraction method. Results indicate that the dominant portion of the phosphorus is in inorganic form, and that there is an upward increase in detrital apatite. The concentration of phosphorus associated with hydrous ferric oxides (Fe-P) exhibits an excellent positive correlation with the fine size fraction of the sediments, indicating the close association between ferric oxides and high-surface-area mineral grains. Most notable is a yellow clayey-silt layer of low sand content at 39.5-m depth which is very rich in Fe-P. Four possibilities are presented to explain the possible source of the yellow layer. One possibility is that the yellow layer may represent loess-rich Huang He (Yellow River) sediment transported far to the south due to a large former shift of the Huang He mouth. The visual characteristics and chemical observations for the above core, combined with some geological clues are in agreement with this hypothesis. Besides these geological observations, Chinese historical documents provide additional evidence for large former shifts of the mouth of the Huang He.