西峰黄土-红粘土序列有机质记录及其对磁化率古气候意义启示

ORGANIC MATTER RECORD OF XIFENG EOLIAN DEPOSITS AND ITS DECIPHERMENT FOR THE PALEOCLIMATIC PROXY OF MAGNETIC SUSCEPTIBILITY OF THE LOESS-RED CLAY SEQUENCES IN CHINESE LOESS PLATEAU

大量研究证实成壤磁铁矿和磁赤铁矿是黄土-红粘土磁化率变化主要原因,但是对这些磁性矿物起源于无机沉淀还是生物作用尚需深入研究。为此,本文选择黄土高原200余米厚的西峰黄土-红粘土序列为主要研究对象,采用化学分析和透射电镜手段,对其中有机质含量和磁性矿物成因进行研究,结果表明:S_5以来的沉积层有机质含量相对较高,集中在0.25%～0.50%之间,有机质含量变化与黄土和古土壤层磁化率变化基本一致,即有机质高值对应磁化率高值,有机质与磁化率值成正相关;S_5以下至午城黄土底部不仅有机质含量降低而且变化幅度减小;红粘土沉积层中,虽然不能完全遵循磁化率高值对应有机质高值,但基本符合峰-峰和谷-谷对应规律。有机质和磁化率的相关性以及透射电镜研究表明,生物成因磁性矿物是风尘序列磁化率变化的主要原因,有机质含量高低反映风尘序列成壤过程中生物地球化学强度变化,生物将磁化率和古气候密切联系起来。黄土和红粘土磁化率古气候意义相似,但是由于黄土和红粘土形成于不同气候背景下,不能仅仅利用二者磁化率大小对比反演古气候,利用表土磁化率重建古气候要考虑气候载体形成的气候背景差异。

The magnetic susceptibility of the Chinese loess-paleosol sequence in the Northwest China represents a proxy climate index related to the East Asian summer monsoon intensity. In contrast to the loess deposits, the magnetic susceptibility is almost independent of each other in the red clay, thus challenging the validity of the readily measurable magnetic susceptibility in describing the monsoon evolution history recorded by the red clay. Concentration variation of pedogenic magnetite and maghemite produced during pedogenesis is considered as the main reason for variation of magnetic susceptibility of the loess-red clay sediments. However no identical view on formation of magnetite and maghemite via inorganic precipitation or organic process was proved,the latter including the mediation of Fe-reducing bacteria which may be termed a ＇ Biologically Induced Mineralisation＇ （BIM） process and magnetotactic bacteria process called a ＇ Biologically Organised Mineralisation＇ （BOM） process. The organic matter content and morphological characteristics of magnetic mineral in the eolian sediments have great potential for the study of magnetic minerals genesis and variation of magnetic susceptibility. In this paper,the Zhaojiachuan section （35~45＇N, 107~49＇E） in Xifeng town is selected for this study. This section is located in the centre of Dongzhiyuan which is the largest Yuan in the Chinese Loess Plateau, Gansu Province. The present day mean annual temperature is about 8.7℃ with a range of -22. g -55. I℃ ,and the mean annual precipitation is 555mm in this region. The profile contains an upper about 170m alternating loess-paleosol sequence and a lower red clay with a thickness of about 55m. The loess-paleosol sequence is traditionally subdivided into five major stratigraphic units. From the surface downward these are： Holocene Black Loam （So ）, late Pleistocene Malan Loess（L1 ）,Middle Pleistocene Upper Lishi Loess（ S1 -L9 ）and Lower Lishi Loess（ S9 -L15 ）, and early Pleistocene Wucheng Loess（ S,5 - Lz3）. The Late Miocene red clay lies on a base of lacustrine sediments. Magnetostratigraphic measurements have dated the lower boundary of the red clay deposit at 7.6Ma which indicates aeolian dust accumulation began at least 7.6Ma ago. In this study, about 2240 dry mass samples were taken at 10cm intervals for organic matter measurements that were quantified by Potassium bichromate-Ferrous ammonium sulfate- heating method. In addition,and magnetic minerals from the paleosol units S1 and Ss together with the loess layers L1 were characterized using transmission electron microscopy （TEM）. The result indicates that the loess-paleosol sediments above the paleosol unit Ss have higher values of organic matter greatly varying from 0.25% to 0.50% in the whole section, and variation of the organic matter concentration of the loess-red clay was positively related to the alternation of magnetic susceptibility of the loess-red clay horizon. The correlation indicates that nearly all the organic matter maxima corresponds with susceptibility peaks, and organic matter minima corresponds with susceptibility troughs,according with the loess and paleosol above the paleosol unit S5. The content from Ss to the bottom of the red clay correlates poorly with the signatures of the magnetic susceptibility compared with the records above the paleosol unit S5. All of these facts demonstrate that the magnetic minerals are mainly in pedogenic origin and the variation of organic matter content of the aeolian sequence may be regarded as an effective proxy of biogeochemical intensity during deposits weathering. The microbiology is a bridge between the magnetic susceptibility of aeolian sediments in North West China and paleoclimate. Thus it is suggested that the magnetic susceptibility of the Late Miocene red clay has same paleoelimatic implication as the Quaternary loess-paleosol sequence,which shows biogeochemical process intensity during aeolian sediment pedogenesis. In addition, the traditional viewpoint is that the Quaternary loess-paleosol sequence form under alternation of cold-dry and warm-wet climate while the red clay results from warm-dry climate which results in the variation of magnetic susceptibility. Thus climate setting should be taken into account for paleoclimatic reconstruction using environmental magnetism.