泥河湾盆地磁性地层学研究——以东谷索和岑家湾剖面为例

The Nihewan Basin is a key area for research into human occupation at high northern latitudes in northeast Asia after the initial expansion of early humans out of Africa. Well-developed late Cenozoic lacustrine deposit sequence in this basin offers a unique opportunity to address this issue. In this thesis, detailed magnetostratigraphic investigation coupled with mineral magnetism was conducted on the Donggutuo and Cenjiawan sections in the eastern basin, where lacustrine deposits sequences containing the Donggutuo, Maliang and Cenjiawan Paleolithic sites are well developed. The sequences are mainly composed of grayish-white clays, grayish-green clayey silts, grayish-yellow silts and fine-grained brown sands, which have recorded reliable polarity variations of geomagnetic field.Characteristics of the anisotropy of magnetic susceptibility show that the sediments have preserved typical original magnetic fabric for sediments, indicating that the strata were developed in a low-energy lake environment and were never perturbed by tectonic stress since deposition. High-temperature magnetic susceptibility measurements (x-T) of representative specimens and demagnetization experiments indicate that the dominant magnetic mineral and remanent carrier at the two sections is magnetite. In addition, hematite and possibly minor maghemite exists in some portions of the sequences. The majority of the samples have relatively simple demagnetization behaviors. After removal of soft magnetic components, the stable characteristic remanent magnetizations (ChRMs) are isolated, which can represent the original remanences.The Donggutuo section mainly records the Brunhes chron, the Matuyama chron and the Jaramillo subchron. The Maliang stone artifact layer occurs just below the Brunhes/Matuyama boundary; and the Donggutuo artifact layer, just below the Jaramillo onset. Accordingly, the Maliang and Donggutuo sites can be dated at about 0.78 Ma and 1.1 Ma, respectively. The Cenjiawan section has recorded a portion of the Matuyama chron. After correlations with the magnetic polarity sequence of the Majuangou section adjacent to this section, the Cenjiawan stone artifact layer is determined below the Jaramillo onset, with an estimated age of 1.1 Ma.To establish the magnetic stratigraphy framework for the lacustrine sediments in the eastern Nihewan Basin, this thesis draws on the magnetic polarity sequences of the Donggutuo and Cenjiawan sections as well as previously obtained results from the Majuangou, Haojiatai, Xiaochangliang and Donggou sections for magnetostratigraphic correlations. The accumulation of the lacustrine sequences at the east margin of the basin commenced from about 2.0 Ma. These sequences record not only the coarse magnetostratigraphy of the Brunhes normal chron and the middle to late Matuyama reverse chron (that is, the Jaramillo and Olduvai subchrons) but also some of the fine structure (that is, the Kamikatsura, Santa Rosa, Punaruu and Cobb Mountain geomagnetic events). The development of the Nihewan paleolake experienced at least twice large expansion periods, split by a large-scale shrinking event in the middle period of the paleolake development. The accumulation of the lacustrine strata was controlled by fault activities.After temporal control for the Donggutuo, Maliang and Cenjiawan Paleolithic sites were established, the three sites along with other well-dated Paleolithic/hominin sites of the Early Pleistocene in North China were combined to construct a chronological sequence of early human occupation in northeastern Asia. Furthermore, after incorporation of paleoclimate changes retrieved from Chinese loess/paleosol sequences and marine sediments, it could be possibly proposed that human groups of the Early Pleistocene in North China might have survived repeated warm/humid interglacials and cold/dry glacials, which were paced by earth orbital variations of the Eastern paleomonsoon.