中国黄土天然剩磁锁定深度量化研究

Paleomagnetic events in Chinese loess-paleosol sequences are main age control points for loess astronomical timescale and correlations between paleoclimatic records in both marine sediments and Chinese loess. However, there is a longstanding controversial debate concerning the natural remanent magnetization (NRM) lock-in depth resulting from the stratigraphic position discrepancy of polarity reversal boundaries recorded in Chinese loess and marine sediments. Limiting to the geochronology, it is still a conundrum for the quantity of the lock-in depth in loess. In this study, loess sections, such as Weinan, Luochuan, and Huanxian, with different sediment rate distributed in different areas in the Chinese Loess Plateau will be selected to observe the NRM lock-in depth combining rock magnetism, paleogeomagnetism, geochemistry (10Be), paleoclimatology, and OSL dating. Loess sediments since the last interglacial period (S1-S0) in the three selected sections and loess layers around the Matuyama-Brunhes reversal (S8-L7) in Weinan and Luochuan sections will be sampled. Lock-in depth would be determined through correlations between stratigraphic positions of the same paleomagnetic anomalies, such as Laschamp, Mono Lake, Blake, MB reversal boundary, revealed by the relative paleointensity curves constructed by two independent proxies, paleomagnetism and 10Be, respectively, on the basis of high resolution magnetic stratigraphy. Linear fitting average sediment rates after OSL dating will be determined for L1 and S1 to calculate the time lag of the NRM recording after dust deposition, and to construct the numerical correlations between the sediment rate and lock-in depth through sections with different sediment rates. Quantity of the NRM lock-in depth in Chinese loess will be helpful for determining the actual polarity boundaries after adjustment of the measured boundaries according to the lock-in depth during the process of astronomical tuning. Therefore, it is significative for construction of a more accurate astronomical timescale and for refined paleoclimate correlations between Chinese and marine sediments.

中国黄土记录的古地磁极性事件是黄土天文调谐时间标尺和海陆古气候对比的主要年龄控制点。但是古地磁极性倒转界线海陆记录的差异导致对黄土剩磁锁定尺度的长久争议。受测年所限，量化黄土剩磁锁定深度成为研究难题。本项目拟选择不同气候区域、不同沉积速率的黄土剖面，突破单一古地磁学研究模式，综合多种指标开展多学科交叉研究。以渭南、洛川及环县剖面末次间冰期以来沉积物及记录松山－布容倒转的层位为研究对象，同时采集古地磁和10Be样品，基于高分辨率磁性地层，分别通过磁学参数和10Be示踪构建地磁场相对古强度曲线，并识别磁异常事件。通过两条独立手段构建的磁场强度曲线所示磁异常的地层位置差异，量化剩磁锁定深度，并估算剩磁获得滞后时间，建立沉积速率与剩磁锁定深度之间的数值关系。量化剩磁锁定尺度可为黄土天文调谐过程年龄控制点的选取，即极性界线的厘定提供依据，对建立更为精确的黄土时间标尺和精细的海陆对比方案具重要意义。

中国黄土记录的古地磁极性事件是黄土天文轨道调谐时间标尺和海陆古气候对比的主要年龄控制点。但是古地磁极性倒转界线海陆记录的差异导致对黄土天然剩磁锁定尺度的长久争议。受测年所限，量化黄土剩磁锁定深度成为研究难题。本项目选择不同气候区域、不同沉积速率的黄土剖面，突破单一古地磁学研究模式，综合多种指标开展多学科交叉研究。本项目以渭南、洛川及西峰黄土剖面末次间冰期以来的沉积物以及记录松山－布容倒转（MB）的层位为研究对象，对S0、L1、S1、S7-S8进行了连续的古地磁定向柱状样品采集，加工出5套古地磁定向平行样品。通过详细的岩石磁学研究了采样剖面的磁性矿物组合，通过系统的热退磁识别了剖面中的极性倒转、极性漂移等磁异常事件。.通过三个剖面MB倒转带的空间对比分析，获得了半定量化的黄土天然剩磁锁定深度。相对而言，邙山剖面的锁定尺度显著大于洛川和西峰剖面，洛川剖面的锁定尺度稍大于西峰剖面。通过降雨量、沉积速率、孔隙度替代性指标的初步空间对比分析，认为降雨量可能是影响剩磁锁定尺度的主要原因。降雨量适中的沉积环境，例如黄土高原腹地，适度的水汽更利于载磁颗粒的锁定，测得的极性倒转界线经适度上调后可以作为绝对年龄控制点。.将西峰和洛川剖面末次间冰期以来的10Be示踪地磁场相对古强度曲线与磁学指标建立的相对古强度曲线进行地层深度上的对比，观察到Blake、Laschamp甚至Mono Lake极性漂移事件在两条不同曲线上记录的地层位置差异，得到了量化的黄土天然剩磁锁定深度。根据地层的平均沉积速率估算了相应的剩磁记录滞后时间，发现末次间冰期以来的黄土沉积物中不同极性事件时期的剩磁锁定深度与黄土粉尘沉积速率线性正相关。.本项目量化黄土天然剩磁锁定尺度可为黄土天文轨道调谐过程中年龄控制点的选取，即极性界线地层位置的重新界定提供量化依据，对建立更为精确的黄土时间标尺和精细的海陆古气候对比方案具有重要意义。