Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment: Implications for Magnetofossil Abundance and Alignment
Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment: Implications for Magnetofossil Abundance and Alignment
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DOI：
10.1029/2019jb018128
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发表时间：
2019-11
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影响因子：
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通讯作者：
Y. Usui;T. Yamazaki;T. Oka;Y. Kumagai
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作者：
Y. Usui;T. Yamazaki;T. Oka;Y. Kumagai
单畴磁铁矿颗粒沿其伸长率表现出最小磁化率，从而产生所谓的磁化率各向异性（AMS）的逆结构。我们报告了从北太平洋西部约 12 m 长的活塞核心回收的远洋粘土中发现了反 AMS 织物。之前的一项研究发现，趋磁细菌（磁化石）产生的单域磁铁矿化石是沉积物中的主要亚铁磁性矿物。在约 2 m 的区域中发现了反 AMS 织物。该区域上方和下方 ∼6 和 ∼4 m 的沉积物显示出正常的水平 AMS 结构。岩石磁力数据和铁磁共振波谱表明，无论正常或反 AMS，磁化石占平均磁化率的大部分。这是通过混合模型来解释的，其中磁化石的逆结构几乎与陆源矿物的正常结构平衡。沉积物中磁化石携带的 AMS 校正度估计为 ∼1.01，与浅层典型中上层沉积物的校正度相当。另一方面，沉积物中的陆源矿物估计具有较高程度的各向异性，可能反映了> 80 m沉积物的埋藏和随后的侵蚀，海底声学地层学也表明了这一点。这表明磁化石引起的反 AMS 组构可能广泛存在于没有强压实作用的远洋粘土中。
Single‐domain magnetite particles exhibit minimum susceptibility along their elongation, resulting in so‐called inverse fabric of the anisotropy of magnetic susceptibility (AMS). We report the discovery of inverse AMS fabrics from pelagic clay recovered by a ∼12 m long piston core from the western North Pacific. A previous study identified fossil single‐domain magnetite produced by magnetotactic bacteria (magnetofossils) as the dominant ferrimagnetic mineral in the sediment. The inverse AMS fabrics were found in a ∼2 m zone. The ∼6 and ∼4 m of sediment above and below this zone showed normal, horizontal AMS fabrics. Rock magnetic data and ferromagnetic resonance spectroscopy indicated that magnetofossils account for most of the mean susceptibility regardless of normal or inverse AMS. This was explained by the mixing models where the inverse fabric from magnetofossils is nearly balanced by the normal fabrics of terrigenous minerals. The corrected degree of AMS carried by magnetofossils in the sediment was estimated to be ∼1.01, which is comparable to that of typical pelagic sediment at shallow depth. On the other hand, terrigenous minerals in the sediment were estimated to have higher degree of anisotropy, possibly reflecting burial and subsequent erosion of >80 m of sediment, which was also suggested by a subbottom acoustic stratigraphy. This suggests that inverse AMS fabrics due to magnetofossils may be widespread in pelagic clay without strong compaction.