Abstract:
Sediments in tidal deposits of the northern Hangzhou Bay are mainly supplied by fluvial discharge of the Yangtze River. In recent years, sediment discharge of the Yangtze River has declined dramatically. Therefore, the potential impact on tidal deposits of the Hangzhou Bay has attracted great attentions. Magnetic minerals, ubiquitous components of sediments, are sensitive to sediment source variations. As a result, magnetic methods have been widely used in provenance tracing of suspended matters and sediments in fluvial, marine, lacustrine and atmospheric environments. We have carried out magnetic measurements on surface tidal deposits collected daily from a fixed intertidal site at Luchaogang along the northern bank of the Hangzhou Bay during 2004 and 2005. It was found that particle size and magnetic properties exhibit significant seasonal changes. In general, during the accretion period of tidal flat, magnetic mineral concentrations are higher when sediments are finer, and vice versa. In this study, magnetic measurements and particle size analysis were carried out on tidal flat sediments, collected daily at the same site from 2007 to 2010. The purpose is to study the annual changes in sediment composition under a context of the declined fluvial sediment discharge of Yangtze River. The monthly and annual variation pattern of magnetic properties and its relationship with particle size were discussed. Magnetic parameters of sediments show similar monthly variations from 2007 to 2009. Magnetic susceptibility (χ), Susceptibility of anhystereric remnant magnetization (χARM), Saturation isothermal remnant magnetization (SIRM) and the ratio (χARM/ SIRM, show higher values in December to July and lower values in August to November, and S300 shows the opposite tend. Particle size analysis indicates that sediments are finer between December and July, and coarser between August and November in these three years, suggesting that ferrimagnetic minerals are enriched in finer sediments, which is similar to the previous results reported for samples collected in 2004 and 2005. Compared to samples collected in 20072009, it can be seen that magnetic mineral concentrations in sediments of 2010 decrease between December and July, and increase between August and November. As a result, the amplitude of monthly variation of magnetic properties becomes smaller. Meanwhile, sediments of 2010 become coarser between December and July, and finer between August and November. The relationship between magnetic properties and particle size is generally similar in the four years, which suggests that the monthly variation of particle size is the main factor for the monthly changes of magnetic properties in 2010. In general, χ, χARM, SIRM and χARM / SIRM are positively correlated with the <16 μm fraction and negatively with the >63 μm fraction. In contrast, S300 is negatively correlated with the <16 μm fraction and positively with the >63 μm fraction. It indicates that magnetic minerals are enriched in the <16 μm fraction, which is diluted by the >63 μm fraction. In 2010, the decreases of concentrationrelated magnetic parameters between December and July are caused by the decline of the <8 μm fraction. On the contrary, the increase of concentrationrelated magnetic parameters in the remaining months are caused by the increase of the <32 μm fraction. However, the above mentioned relationships between magnetic parameters and particle size weaken since 2009, which is especially significant in 2010. The factors for the observed relationship between magnetic properties and particle size in 2010, which is different from previous years, may include tides, winds, sediments flux, coast morphology and engineering works. Therefore, an integrated monitoring of hydrodynamics, meteorology, and topography is needed to address the mechanism. In addition, the potential impacts of decline sediment input of the Yangtze River on tidal flat sediment source, need to be assessed in future study. This study demonstrates that magnetic methods, with the virtue of being simple and sensitive, can provide valuable insights to environmental change through the analysis of sediment composition in a simple and nondestructive way.
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