失水对海洋沉积物磁学性质的影响

为了探究沉积物的干燥过程及方式对其磁学性质的影响，选取含水率高的南海沉积物样品，对其中685个样品在自然条件下逐步晾干，并每间隔40 d测量1次低频磁化率（κlf）、高频磁化率（κhf）、饱和等温剩磁（SIRM）、S-300和频率磁化率系数（κfd），共测量5次；对另外60个样品进行冷冻干燥，并在干燥前后测量相同的磁学参数。结果表明：1）自然晾干过程中，除少数异常点（约7%）外，大多数样品的κlf和κhf呈减小趋势，少数样品呈增大趋势，S-300和κfd均呈减小趋势，SIRM先减后增，整体上呈增大趋势；2）冷冻干燥过程中，样品的κlf、κhf、S-300和κfd呈增大趋势，SIRM呈减小趋势；3）2种干燥方法均不影响κlf、κhf、SIRM和S-300的剖面变化趋势，但对κfd的影响较大，这在磁学指标的古环境研究中具有关键的指示意义。磁学参数的变化，可能是由于失水过程发生的微弱氧化作用、失水导致水的抗磁性“负贡献”降低、细粒磁铁矿和磁赤铁矿在湿润状态下转变成顺磁性和不完全反铁磁性等综合因素造成的。建议对同一批次的样品采取相同的干燥方法。

Effect of Dehydration of Marine Sediments on Their Magnetic Properties

The technology of environmental magnetism has been increasingly widely applied in marine sediment research. Marine sediment contains a lot of water which is a kind of antimagnetic material. However, it’s not clear that loss of water during sample preparation and preservation is influential to its magnetic properties or not. Magnetic parameters were measured before and after drying for hundreds of marine sediment samples collected from the South China Sea. 685 samples were dried at room temperature, and their magnetic parameters κlf, κhf, SIRM, S-300 and κfd were measured every 40 days, totally 5 measurements. The other sixty samples were freeze dried and the same magnetic parameters were measured before and after drying. The results are shown as follows : 1) For samples dried at room temperature, κ decreased for most samples and increased for a few of samples while both S-300 and κfd decreased except the outliers; SIRM decreased at first 40 days and then increased, and it showed an increasing trend on the whole. 2) Magnetic parameters κ, S-300 and κfd showed an increasing trend and SIRM showed a trend of decrease for samples dried under freeze condition. 3) The variation trends of κ, SIRM and S-300 in profiles didn’t change during drying process. However, dehydration played a significant influence on profile variation of κfd. The variations of magnetic parameters may be due to comprehensive factors such as a weak oxidation during drying process, the reducing of “negative contribution” of water caused by water loss, the transition when fine magnetite and maghemite in wet state changed into paramagnetism and incomplete antiferromagnetism. Therefore, the same drying method must be used for the same batch of samples.