摘要
详细的古地磁及岩石磁学研究表明磁铁矿和赤铁矿是宝鸡古土壤层S7和黄土层L8的特征剩磁的主要载磁矿物，在稳定的正反极性期间，这两种磁性矿物携带的剩磁方向完全一致。然而，在地磁漂移事件或者松山-布容（M—B）极性转换的过程中，这两种载磁矿物有时携带的剩磁方向不同。高分辨率的磁性地层结果证实宝鸡古土壤层S7和黄土层L8能记录300—500年的短期地球磁场极性倒转事件。经与日本大阪湾高分辨率的海洋沉积物中记录的同期短期极性倒转事件相比，估算宝鸡剖面土壤层S7和黄土层L8的天然剩磁锁定深度小于5—8cm。
Detailed paleomagnetic and rock-magnetic investigations have been carried out in paleosol S7 and loess Ls from Baoji section, southern Loess Plateau. Mass-specific low-filed susceptibility revealed that the degree of pedogenesis of these two units is rather weak. Acquisition of isothermal remanent magnetization （IRM） , thermal demagnetization of three-axis IRM, and thermomagnetic analyses showed that detrital magnetite and hematite are dominant magnetic carriers in the studied loess-paleosol sediment. Measuring results of magnetic susceptibilities after each progressive thermal demagnetization step displayed no significant change. Progressive thermal demagnetization results showed that a few specimens from the geomagnetic excursion and the Matuyama-Brunhes transition have two clearly high temperature components（ HTC ）above 250℃ , while those from the fully normal and reverse polarity intervals have only a single HTC. High-resolution magnetostratigraphy indicated a geomagnetic excursion, which is mainly characterized by short-lived departures in declination, in the paleosol S7, and the Matuyama-Brunhes（M-B）transition, which is characterized by 14 short-lived reversal episodes, in the loess Ls. Multiple rock-magnetic experiments demonstrated that the specimens from the excursion and M-B transition intervals have uniform magnetic properties as those from the Brunhes normal or Matuyama reversed polarity intervals. By extrapolating a mean accumulation rate of the late Matuyama chron, the geomagnetic excursion in the paleosol S7 is estimated to occur from 759ka to 751ka, and the M-B transition in loess Ls is dated to occur from 785ka to 775ka. Comparison with a high-resolution paleomagnetic record from the Osaka core sediment revealed that the paleosol S7 and loess Ls have potential to record the short-lived reversal episodes lasting for 300 - 500 years, which, combined with many reversal episodes occurring during the M-B transition, implies that their lock-in depth is less than 5 - 8cm.
出处
《第四纪研究》
CAS
CSCD
北大核心
2007年第6期972-982,共11页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40602028)
中国科学院知识创新工程项目(批准号:KZCX3-SW-143)资助
关键词
古土壤
黄土
地磁极性转换
地磁漂移
天然剩磁锁定
paleosol, loess, geomagnetic polarity transition, geomagnetic excursion, lock-in
