Diagenetic modification of magnetic properties observed in a piston core (MD01–2407) from the Oki Ridge, Japan Sea
Diagenetic modification of magnetic properties observed in a piston core (MD01–2407) from the Oki Ridge, Japan Sea
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DOI：
10.1016/j.palaeo.2006.11.022
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发表时间：
2007-04
期刊：
影响因子：
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通讯作者：
A. Hayashida;Sosuke Hattori;H. Oda
中科院分区：
文献类型：
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作者：
A. Hayashida;Sosuke Hattori;H. Oda
为了获得日本海过去地磁场和古环境变化的高分辨率记录，我们研究了来自巨型活塞芯(MD01-2407)的U形通道样品的磁特性。自然剩余磁化强度(NRM)的测量表明，在几个层位上出现了异常的磁方向，这可能代表了布朗日的地磁极性漂移。然而，它们大多发生在包括特弗拉层在内的岩性边界附近，表明这些特征可能是由岩心沉积物的变形造成的。由人工诱导剩磁归一化的NRM强度与标准的古强度记录没有明显的相关性。这可能是由于包括火山灰层在内的岩性变化和磁性矿物成岩作用的影响所致。顶端岩心磁性的向下变化详细描述了还原成岩作用中磁性矿物的溶解和蚀变过程，这对于具有高沉积速率和高有机质含量的半远洋沉积物来说是可以预见的。磁选参数包括低场磁化率、磁滞剩余磁化强度和等温剩余磁化强度(IRM)，表明细粒磁铁矿的损失范围为0.6~1.2 Mbsf。虽然早成岩作用水平以下的剩磁强度比最上面的低一到两个数量级，但大多数薄层(TL)暗层的磁场强度都有较大的增强。通常，TL2在顶部显示一个明显的峰值，并逐渐向底部下降。反映饱和磁化率中低矫顽力成分比例的S比的变化表明，低矫顽力和细粒铁磁性矿物占主导地位。这些数据表明，在TL层的沉积过程中形成了新的磁性矿物，这可能类似于已知的地中海腐蚀物中的成岩磁性增强。
We investigated the magnetic properties of u-channel samples from a giant piston core (IMAGES MD01–2407), in order to obtain high-resolution records of past geomagnetic field and paleoenvironmental changes in the Japan Sea. Measurements of natural remanent magnetization (NRM) show occurrence of anomalous magnetic directions at several horizons, which may represent geomagnetic polarity excursions in the Brunhes Chron. However, most of them occur close to the lithological boundary including tephra layers, suggesting that these features might have been caused by deformation of core sediments. Intensity of NRM normalized by artificially induced remanence shows no significant correlation with a standard paleointensity record. This is probably caused by a variation of lithology including volcanic ash layers and effect of the diagenetic changes of magnetic minerals. Down-core variations of magnetic properties in the topmost part depict in detail the process of dissolution and alteration of magnetic minerals during reductive diagenesis, which is to be expected for hemipelagic sediments characterized by a high sedimentation rate and a high organic matter content. Magnetic concentration parameters including low-field susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetization (IRM) indicate that a loss of fine-grained magnetite occurs from 0.6 to 1.2 mbsf. While remanence intensities below the level of the early diagenesis are one or two orders of magnitude lower than those in the topmost part, most of the thinly-laminated (TL) dark layers showed rather enhanced magnetic intensities. Typically, TL2 shows a marked peak at the top with a gradual decrease towards the bottom. Variation of S-ratio, which gives the proportion of a low-coercivity component in saturation IRM, suggests that low-coercivity and fine-grained ferromagnetic minerals are dominant. These data suggest new magnetic minerals formed during deposition of the TL layers, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea.