三门峡地区黄土L9的重磁化现象及原因探析

对黄河三门峡地区曹村黄土剖面L₁至L₁₃(年代持续约1.1Ma)的高分辨率古地磁研究发现:布容/松山界线位于第8层古土壤(S₈)的顶部,贾拉米洛正极性亚时的顶、底界分别位于S₁₀和L₁₃的顶部,上述古地磁极性转换界线与洛川标准剖面一致。“上粉砂层”(L₉)的绝大多数样品特征剩磁方向与现代地磁场方向一致,而其中的弱发育古土壤层(L₉SS₁)则呈预期的反极性特征。综合L₉的厚度、岩性和岩石磁学特征,认为这一巨厚的正极性带可能反映了完全由岩性所控制的重磁化特征。对于L₉重磁化的原因,似乎很难用后生的化学剩磁和粘滞剩磁来解释。文章尝试性地提出,可能在L₉沉积后的间冰期(对应S₈)和布容正极性时内气候较湿润的时期,由于雨水的向下渗透而诱导了其中强磁性矿物(磁铁矿)在沉积后随地磁场方向的变化而进行了重新排列。

REMAGNETIZATION OF L9 IN THE SANMENXIA AREA AND PRELIMINARY ANALYSIS OF REMAGNETIZATION MECHANISMS

Here we report a high-resolution paleomagnetic study of a loess section in the Sanmenxia area spanning the last 1.1Ma. The results demonstrate that the Matuyama/Brunhes Boundary (MBB) occurs at the top of S_(8,) and the upper and lower boundary of the Jaramillo Normal Subchron (JNS) are encountered at the top of S_(10) and L_(13) respectively, in agreement with the classic Luochuan section. The upper sand layer (L_(9)) is the coarsest loess unit over the last 1.1Ma and carries an imperfect normal polarity magnetization. Although it is fully separated by a short reversed polarity interval, this thick normal polarity zone is too extensive to represent excursions, and is concluded to represent an artifact rather than a record of geomagnetic field directions. The short reversed polarity interval within this thick normal polarity magnetization coincides with a magnetic susceptibility peak, which represents a weakly pedogenic interval. Detailed rock magnetic analyses show that there are no intrinsic differences in magnetomineralogical properties between this pedogenic interval and other parts of L_(9). Based on its exceptional thickness and coarse-grained characteristic of L_(9), we propose that the extensive remagnetization of L_(9) most likely reflects a lithology-dependent process, and the dramatic coarsening of particle-size across L_(9) is the fundamental reason for pervasive remagnetization. As for the origin of remagnetization, it appears that viscous remnant magnetization (VRM) and chemical remnant magnetization (CRM) are not the actual processes of the inferred remagnetization. We tentatively concluded that the extensive remagnetization of parts of L_(9) is probably due to postdepositional re-alignment by wetting (precipitation) during the Brunhes time, and then most samples retain a pDRM by perfect physical re-orientation of magnetic mineral grains parallel with the ambient geomagnetic field.