洛川黄土地层学

黄土和古土壤的磁化率与所含磁性矿物的浓度和颗粒大小密切相关。磁化率信号的主要载体是非常细的磁铁矿和(或者)磁赤铁矿。洛川黄土序列的磁化率曲线变化与成壤作用和粉尘堆积作用强弱有关,反映了气候的变化。磁化率值为识别黄土和古土壤层提供了一个工具。文中建议了黄土岩石地层单位命名的原则。洛川黄土-古土壤序列的磁化率曲线记录了最近2.5Ma气候变化的历史,与深海沉积的气候记录可以对比。

LOESS STRATIGRAPHY IN LUOCHUAN OF CHINA

Loess sequences in Luochuan of China provide a detailed record of climate changes during the last 2.5 million years. At least 44 major shifts from glacial to interglacial conditions occurred during this time in China. Prolonged loess deposition lasting more than 40 millenia occurred eleven times with only four of these intervals in Brunhes. The climate oscillations took place with increased frequency between 2.4 and 0.5 Ma B.P. compared to the last half million years. Major shifts toward colder and drier climates occurred at about 2.4 Ma B. P., 1.2 Ma B. P. and 0.5 Ma B. P., while loess pronounced shifts are dated to 1.65 Ma B.P., 0. 8Ma B. P. and 0.2 Ma B. P. The loess and soil units can be differentiated based on color, carbonate content, the degree of bioturbation, or the humus content. However, the most accurate, objective, and easily reproducible delimitation of a soil unit is based on the low field magnetic susceptibility. The low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and the accumulation of dust, and serves as a proxy measure of climate. This parameter is a measure of magnetization temporarily induced by an artificially applied low amplitude magnetic field. The susceptibility signal is approximately proportional to the concentration and the grain size of ferrimagnetic minerals. The susceptibility of Luochuan loess sequence is high in soils and low in the loess, It is lowest in the carbonate horizons. In part, this is due to the negative susceptibility of carbonates. The chief carrier of the magnetic susceptibility signal in the Chinese loess is the ultrafine magnetite and/or maghemite. Its submicron single domain particles are superparamagnetic, displaying several times higher susceptibility than the same volume of coarse grained minerals. The soil/loess sequences of the Chinese Loess Plateau contain a detailed 2. 5Ma long proxy record of the changing density of regional vegetation, closely controlled by paleoprecipitation. It correlates well with the oxygen isotopic stratigraphy of the deep sea sediments. The paleoclimatic environment of the polarity reversals of the Brunhes and Matuyama Chrons was established by repetitive sampling at Luochuan section. Three of the four studied reversals of Brunhes and Matuyama age were found in the sediments of a glacial climate. The so-called 100,000a cycle, was, with a single exception, restricted in China to the last 1.2Ma.