新疆北部典型黄土沉积晚更新世以来古地磁漂移研究

A reasonable chronology of loess is prerequisite for paleoclimate study and thus has recently become a key issue to be solved urgently in the paleoclimate study of loess sections during last interglacial in northern Xinjiang. However, most of the chronological studies could hardly provide effective data that facilitate stratigraphic correlation and paleoclimate assessment among loess sections in this area. The geomagnetic excursions since late Pleistocene have been widely revealed in loess deposits, which are of great scientific value in chronological study of loess and can serve as an alternative solution for solving the current dating problem of the Xinjiang loess. The proposers plan to select two typical loess sections (Baiyanghe and Zeketai section, respectively) with continuous sequence and consolidated texture in northern Xinjiang. On the basis of systematic rock magnetism analysis, the thermal demagnetization and alternating field demagnetization will be conducted on two groups of parallel samples to study the characteristic Natural Remanence in the Xinjiang loess. Furthermore, the possible recording of the magnetic polarity anomalies will be assessed. Multi-parameters analysis will be applied to establish the geomagnetic relative paleointensity curves since late Pleistocene in the Xinjiang loess. The characteristics of possible geomagnetic excursions recorded in the Xinjiang Loess since late Pleistocene will be discussed from multiple aspects including reliability, detailed structures, global consistency and chronological significance. This study will further enrich the records of geomagnetic excursions since late Pleistocene. On the other hand, these geomagnetic markers are expected to provide the necessary reference for the chronological study in the Xinjiang loess.

年代序列的建立，近年来成为新疆北部晚更新世以来黄土古气候研究中亟待解决的科学问题，而目前多数测年结果的相对不一致性限制了高分辨率古气候研究的深入开展。晚更新世以来的地磁漂移广泛记录于黄土沉积中，在高分辨率黄土定年研究中具有重要的科学价值，可成为解决新疆黄土定年问题的新尝试。本研究拟选取新疆北部沉积连续、固结较好的典型黄土剖面（柏杨河剖面和则克台剖面）为研究对象，于系统的岩石磁学分析基础上，开展热退磁和交变退磁相结合的特征剩磁研究，据此分析特征剩磁记录的地磁方向变化，探索可能记录的地磁极性异常。采用多参数分析方法，尝试建立新疆黄土晚更新世以来的地磁相对古强度变化曲线。综合特征剩磁方向和相对古强度探究结果，多角度探讨新疆黄土晚更新世以来主要地磁漂移记录的可靠性、详细特征、全球对比及其年代学意义。该研究将进一步丰富晚更新世以来地磁漂移的研究内容，有望为新疆黄土定年问题的解决提供必要参考。

沉积物可靠年代标尺的建立是开展古环境演化研究的前提。本项目对亚洲中部干旱区典型黄土沉积剖面开展系统岩石磁学和古地磁漂移的研究，基于古地磁方法开展黄土沉积年代的综合探讨。从古地磁强度和方向两个方面，初步揭示该区域可能存在的古地磁漂移（包括Blake和Laschamp事件）记录特征，并与其它区域的古地磁记录较好对比，尽管其记录细节特征与其他区域记录存在一定差异。地磁场相对古强度记录特征与全球及区域相对古强度记录也能够很好对比，基于相对古强度年代控制的古环境变化与格陵兰冰芯及深海氧同位素的记录可以较好匹配，从一定程度上表明古地磁方法可以为区域晚更新世以来沉积物古环境演化研究提供替代性的年代标尺。同时，岩石磁学结果与其他非磁学指标揭示古环境记录可能受到成壤强度、风动力态势以及相对湿度变化影响。磁浓度指标在该区域无法有效指示成壤强度，可能一定程度上反映相对湿度的变化，磁粒度参数可能具有指示剖面中不同层位成壤强度的潜力。另外，本项目也对珠江三角洲附近区域的沉积钻孔高分辨率古地磁长期变开展了部分先行样品的测试分析工作，并结合其他测年方法对区域沉积钻孔年代序列进行了初步探讨。鉴于不同区域古地磁长期变记录的复杂性，多方法学的系统研究和综合分析仍然是沉积物年代确定的趋势和着眼点。