土耳其Nallihan黄土-古土壤岩石磁学特征及其环境意义

黄土广泛分布在东欧和西亚地区, 但位于欧亚大陆连接地带的土耳其未见风积黄土的报道, 本文对位于土耳其首都安卡拉西北的Nallihan新发现的黄土-古土壤剖面进行系统的环境磁学研究, 结合粒度参数和稀土元素指标, 讨论土耳其黄土的成因和磁化率变化机制。研究结果显示: 土耳其Nallihan S0~L3黄土剖面与典型风积的西伯利亚和中国西峰黄土的粒度特征图形态相似, 暗示它们的成因相似性; 稀土分配模式与黄土高原黄土基本一致, 为上地壳平均值, 指示了风积黄土在搬运过程中经过充分混合, 整个剖面上下稀土模式分配高度吻合, 说明研究区黄土堆积期间物源区相对稳定。热磁和磁滞分析表明样品的主要载磁矿物是亚铁磁性矿物磁铁矿/磁赤铁矿。该剖面磁化率在古土壤获得高值, 在黄土层获得低值, 磁化率与成土作用和指示细颗粒磁性矿物贡献的χfd % 、χARM正相关, 说明成土作用生成的细颗粒亚铁磁性矿物是磁化率增强的主导因素, 土耳其黄土剖面磁化率总体上低于中国黄土高原地区黄土。该黄土剖面中有一层古土壤层S2, 其顶部覆盖一层砾石层, 说明在该间冰期发生过洪水作用, 将高坡风化的砾石层冲到土层中堆积下来, 在该砾石层之下的古土壤层, 磁化率降低, 可能与该时期土壤层湿度大, 形成湿润氧化环境, 导致部分强磁性的超顺畅/稳定单畴(SP/SSD)颗粒磁性矿物转化为弱磁性矿物, 使得磁化率降低。

ROCK MAGNETIC PROPERTIES OF LOESS IN NALLIHAN,TURKEY AND ITS ENVIROMENTAL SIGNIFICANCES

Loess deposition is widely distributed in the Eastern Europe and Western Asia. But few loess sections have been reported in Turkey, which links the Europe and Asia continent. This loess-paleosol sequence was found in Nallihan(40°14.088'N, 31°18.211'E, altitude 764m), northeast of Ankara. As Nallihan is situated in the centre of Anatolia peninsula and adjoined the Mediterranean, the climate here is semi-arid with features of Mediterranean climate, with hot, dry summers and humid, cold winters. The thickness of the section is over 5m and 28 samples were totally collected from the top to 4.1m of the section, which the intervals of samples is 20cm in S0~L2 and 10cm in S2~L3. Therefore, we attempt to understand the origin, magnetic properties and environmental significance of Turkey loess.We have measured grain size, Rare Earth Elements(REE), and rock magnetic parameters including low-field and high-field frequency magnetic susceptibility(χlf and χhf), saturation isothermal remanent magnetization(SIRM), anhysteretic remanent magnetization(ARM), hysteresis loop and thermomagnetic curves(J-T and κ-T). Results show that grain size properties of Turkey loess-paleosol sequence(include C-M, A-M, and L-M)is similar to that of typical eolian loess deposition, but obviously distinguishing with that of river sediments. Moreover, distribution patterns of REEs in Nallihan section are similar to the typical loess profiles. The main characteristics were similar to and characterized by LREE-enrichment and HREE-depletion, in which the distribution curves are negative oblique lines, and Eu performed a negative anomaly. Therefore, samples of Turkey loess should be eolian origin, and originated from same sources. The rock magnetic properties of Turkey Loess can be summarized as: (1)The predominant magnetic minerals are magnetite and maghemite, coupled with paramagneticmineral and hematite, (2)the average grain size of magnetic minerals in Nallihan section is multi-domain (MD) or single-domain (SD), (3) excluding the S2 paleosol, the value of magnetic susceptibility in paleosol (36.1×10-8m3/kg) is higher than that in loess(22.3×10-8m3/kg). In addition, the χlf is significant correlation with χARM(R2=0.85)and χfd % (R2=0.60). Meanwhile, the magnetic susceptibility has a positive correlation with the pedogenic process, signifying that the paleosol layers have more superparamagnetic(SP)and SD grain size magnetic minerals. These characteristics of Turkey loess are similar to the typical loess in Chinese Loess Plateau(CLP), indicating that the magnetic susceptibility enhancement mechanism of Turkey loess is as same as that in CLP, which is the formation of SP/SD particles ferrimagnetism minerals(magnetite/maghemite)during the pedogenic process.The value of magnetic susceptibility of Turkey loess is lower than that of in CLP, and two main factors are responsible for that: First, the magnetic susceptibility of original materials properties(χ0) of Turkey(9.53×10-8m3/kg) is lower than that of CLP (15×10-8~20×10-8m3/kg). Second, all the magnetic properties imply that the intensities of pedogenic process in Turkey is weaker than CLP loess, so that the fewer strong magnetic minerals were created. Meanwhile, affected by the fluviation, the instable SP/SSD magnetite and maghemite converted into gothite which are steady in the humid pedogenesis condition, leading to the decrease of χlf in S2 paleosol.