兰州九州台“上粉砂层”黄土L9的剩磁记录特征研究

THE PALEOMAGNETIC RECORDS OF THE UPPER SANDY LOESS L9 IN JIUZHOUTAI SECTION, LANZHOU

中国第四纪黄土-古土壤序列记录了距今2.6Ma以来丰富的古气候和地球磁场变化的信息。近年来的研究表明，松山反向极性期内沉积的“上粉砂层”黄土L9，在黄土高原多个剖面上记录的特征剩磁方向除了反向极性之外，还有多个正极性段。对于“上粉砂层”L9记录的正极性段究竟代表了短期地磁漂移事件还是重磁化的结果，目前还存在较大的争议。为了进一步查明黄土层L9古地磁记录的可靠性，本文对沉积速率高、成壤作用影响弱的兰州九州台黄土剖面（36.09°N，103.79°E）的L6~L10段地层（海拔1830~1916m）开展了详细的古地磁学和岩石磁学研究。研究结果显示，剖面黄土粒度与磁化率成正相关关系，磁化率主要由风力输送的粗粒磁性矿物颗粒含量所控制；磁化率各向异性（AMS）呈扁平状，磁化率最小轴与最大轴分布方向显示其为较强劲西北向冬季风作用下形成的原生磁组构；磁滞回线和Day图显示该剖面中主要的磁性矿物颗粒可能为低矫顽力的假单畴/多畴磁铁矿与磁赤铁矿以及高矫顽力的赤铁矿和针铁矿组合。特征剩磁（ChRM）结果表明松山/布容界限（MBB）位于黄土层L8底部，贾拉米洛顶界（UJ）位于古土壤层S10中。MBB与UJ界限之间记录的3段较稳定（最大角偏差MAD〈10°）的反极性段落分别对应L9和L10中粒度较粗的位置，其间还记录有两个可信度较差（MAD〉10°）的正极性段落。按MBB（0.78Ma）与UJ（0.99Ma）之间的平均沉积速率计算，这两段正极性中间时间点分别为0.888Ma和0.927Ma，可分别与Kamikatsura（0.850~0.899Ma）和Santa Rosa（0.922~0.936Ma）地磁漂移对应。九州台黄土L9中粒度较粗层位记录稳定反极性与黄土西北边缘古浪剖面一致，而与中部及东南部洛川、三门峡、宋家店剖面粒度较粗层位记录相反，反映了黄土剩磁记录的复杂性。黄土高原不同地区的黄土可能具有不同的剩磁记录机制，需要今后深入研究。

Chinese loess-paleosol sequences were accumulated continuously and recorded detailed information of paleoclimate and paleomagnetic changes since 2.6Ma.The upper sandy loess unit L9, which deposited in the end of Matuyama reverse polarity chron, was reported of having recorded positive polarity segments of different lengths in different locations on the Chinese Loess Plateau（CLP）.The question of whether these positive polarity intervals within L9 were reliable paleomagnetic records of geomagnetic excursions or results of remagnetizations is currently in debate.In order to clarify the exact paleomagnetic records in L9, we conduct a detailed paleomagnetic study of Jiuzhoutai section（36.09°N, 103.79°E）on the western margin of CLP.Jiuzhoutai section was reported to be one of the thickest loess section（about 300m）with high sedimentation rate and low degree of pedogenesis, and the upper sandy loess unit L9 alone was about 25m.According to the previous studies on Jiuzhoutai loess, we collected total 190 oriented samples from L6~L10 between altitude 1830~1916m with 30~50cm interval between each sample.Parallel cubic samples（2×2×2cm3）were cut for the measurements of anisotropy of magnetic susceptibility（AMS）and demagnetization analysis, while powder samples were applied for hysteresis loop and grain size measurements.This work found that the magnetic susceptibility have a positive correlation with grain size variations, giving evidence for dominant control of aeolian coarse magnetic grain on the magnetic susceptibility; AMS data indicate an oblate fabric formed under strong current of a north-west winter monsoon; Hysteresis loops and the Day-plot suggest that pseudo-single domain and multi-domain magnetite and maghemite grains were the main carriers; Combining thermal demagnetization and alternating field demagnetization, both demagnetization behavior and Characteristic Remanent Magnetization（ChRM）were analyzed.Results show that the Matuyama-Brunhes reversal Boundary（MBB）was recorded in the lower part of loess L8, and the Matuyama-Brunhes reversal precursor（MBpc） in S8, while the upper boundary of Jaramillo（UJ）in S10.Between MBB and UJ, there are 3 intervals of stable reversed polarity（Maximum angular deviation or MAD〈10°）corresponding with the coarse parts of L9 and L10.Besides, there are two intervals of normal polarity with less-reliability （MAD〉10°）.Calculating by the average sedimentation rate between MBB and UJ, the middle point timing of these two normal polarities are 0.888Ma and 0.927Ma, which can correspond to the Kamikatsura and Santa Rosa excursions respectively.Similar to the polarity records in Gulang loess L9, the coarsest parts of L9 in Jiuzhoutai section recorded stable reverse polarities, which is in the contrary to what were reported in Luochuan, Sanmenxia and Songjiadian, where the coarse parts of L9 recorded normal polarity.This contradictory polarity records in L9 may suggest a significantly different magnetization mechanism of loess in the west CLP compared to the central and south-east CLP.