杭州湾北岸芦潮港潮滩沉积物磁性特征的年际变化及其粒度控制

对杭州湾北岸芦潮港潮滩2007-2010年固定点沉积物进行了逐月的磁性测量和粒度分析，探讨了磁性特征的月际、年际变化及其与粒度的关系。结果显示，2007-2009年，芦潮港潮滩沉积物的磁性特征呈现出相似的季节性变化特征，磁性参数χ、χARM、SIRM以及比值χARM/SIRM总体上在12月次年7至月表现为高值，8-11月为低值，S-300变化趋势与之相反。粒度分析显示，每年12月至次年7月沉积物粒度总体较细，8-11月较粗，这说明潮滩较细沉积物中含有较多的亚铁磁性矿物，与已报道的2004-2005年研究结果相似。相比前三年，2010年1-7月及12月沉积物磁性矿物含量相对下降，8-11月份磁性矿物含量相对上升，导致2010年磁性参数月际变化幅度减小。同时，2010年沉积物粒度月际变化特征也发生了改变，即1-7月及12月沉积物粒度变粗，8-11月沉积物变细。相关分析表明，磁性参数与粒度之间的关系在2007-2010年中没有发生明显的变化，说明2010年沉积物粒度月际变化模式的改变是磁性特征月际变化改变的主导因素。尽管如此，长江流域来沙下降对河口潮滩沉积物物源的可能影响，仍需予以长期关注。

Annual Variations in Magnetic Properties of Tidal Flat Sediments from Luchaogang along the Northern Bank of the Hangzhou Bay and Its Response to Particle Size Change

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 S-300 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 2007-2009,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,S-300 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 concentration-related magnetic parameters between December and July are caused by the decline of the <8 μm fraction.On the contrary,the increase of concentration-related 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 non-destructive way.