Diagenesis of magnetic minerals in the intertidal sediments of the YangtzeEstuary, China, and its environmental significance

In this study, diagenesis of iron oxides in intertidal sediments of the Yan gtze Estuary, China, has been investigated by combined environmental magnet ic and geochemical methods. The results indicated that the magnetic propert ies of the sediments were dominated by ferrimagnetic magnetite. The content of Fe, DCB- and AOD-extractable iron oxides correlated positively with the concentration of fine grained magnetite near the superparamagnetic/stable single domain (SP/SSD, similar to 0.03 mum) boundary, and with anti-ferroma gnetic minerals (hematite/goethite). The magnetic parameters for core SDK i ndicated a substantial decrease in magnetite concentration from a depth of approximately 20 cm toward the surface, together with a shift in the grain- size distribution of magnetic minerals toward the coarse end, suggesting se lective dissolution of fine grained magnetite under reducing conditions. Th e reduction of iron oxides inferred from magnetic measurements was supporte d by the similar decrease in the concentration of Fe and Mn and a lower rat io of Mn/Fe. Magnetic measurements on another core from elsewhere also indi cated substantial reductive dissolution of iron oxides. In conjunction with the results of heavy metal analysis, it was suggested that the dissolution of iron oxides had a direct effect on the cycling of heavy metals. Therefo re, magnetic measurements may provide useful information as to early diagen esis within intertidal sediments, which greatly influences the behavior of heavy metals in coastal environments. (C) 2001 Elsevier Science B.V. All ri ghts reserved.