基于EMD分解的甘孜黄土磁化率记录的西南季风演化周期性

The Periodicity of the Southwest Monsoon Revealed by the Magnetic Susceptibility of the GarzêA Loess-Paleosol Sequence Using EMD Method

黄土磁化率可用以指示夏季风强度,并已在中国风尘堆积的古气候演化周期研究中得到较好应用,但迄今多集中于东南季风区.以青藏高原东南缘甘孜A黄土剖面磁性地层学研究为基础,对1.16Ma以来的磁化率记录进行经验模态分解(EMD),探讨西南季风演化的多时间尺度周期性.结果表明:偏心率、地轴倾斜、岁差的准周期成分在西南季风区黄土沉积中有明显反映,指示西南季风的演化受天文因素控制;出现岁差的拍频周期,指示印度季风系统具有热带特征;准200ka周期是真实存在的,但其成因机制需要进一步研究;约400ka周期波动,可能对应理论值为准400ka的偏心率长周期.西南季风演化的主要驱动因素可能是低纬太阳辐射.对于沉积记录的周期性研究来讲,经验模态分解更切合实际,它比传统谱分析方法有明显的优势和应用潜力.

The magnetic susceptibility of loess as a proxy for the intensity of summer monsoon has be used to investigate the periodicity of paleoclimatic changes of aeolian deposits in China. However, previous studies focused most on the southeast monsoon areas. On the basis of magnetostratigraphic study of the Garzfi A section on the southeastern margin of the Qinghai-Tibet Plateau, the magnetic susceptibility records during the past 1.16 Ma were subject to an empirical mode decomposition （EMD） to discuss the periodicity of the southwest monsoon evolution at various time scales. The results show that： （1） the significant orbital periodicities do exist including the eccentricity, obliquity and precession periods in our records, which indicates that the southwest monsoon is also controlled by the astronomical factors; （2） The beat frequency periods of the precession are detected, which shows the tropical climatic characteristics of the southwest monsoon; （3） the quasi-200 ka cycle is confirmed in existence but its forming mechanism needs further study; （4） the appearance of quasi-400 ka cycle fluctuation may correspond to quasi-400 ka eccentricity cycle. Therefore, the main forcing of Indian monsoon system may be the solar insolation at low latitude. As for the studies of paleoenvironment evolution of sedimentary records, EMD is superior and has more potentials than the traditional method of spectral analysis.