| The arid central Asia region, as the most widely distributed arid area in the world, becomes an important part of global environment system. Based on its significant influence on regional and global climate and environment, the arid central Asia region plays a vital role in the global environmental change research. It is urgent to probe into the process and mechanism of the climatic evolution in the arid central Asia region. The arid areas in Xinjiang have been observed as one of the most sensitive regions to response the climate change. A wide range of research has been conducted in this region, on the information carrier of lake sediments, loess and stalagmites, and significant results have been achieved. However, some controversy still exists. Peat is a great geological archive on the study of past climate change, especially showing its significant advantages on the reconstruction of high-resolution palaeoclimatic and palaoenvironmentental evolution during the Holocene, howerver, which has never been carried out in northwest arid area. The high concentration of dust in peat in arid area lead to that it can be carried out better work by environmental magnetic and rock magnetic method, but related application is just very little, the understanding of magnetic minerals and its variable mechanism is not clear so far. Consequently, ATM10-C5 and ATM10-C7 peat cores located in the west and middle part of the Altai Mountains are selected for our study, to reveal the characteristics of magnetic minerals and their variable mechanism, under systematic environmental magnetism and geochemical elements analysis. On this basis, we pick up effective proxies’ from magnetic parameters to discuss climate and dust changes of this region during the Holocene, and present our preliminarily understanding of their variable mechanism. The main results and finding are as follows:1) The concentration of magnetic minerals has a low enrichment degree; the major ferrimagnetic and antiferromagnetic minerals are magnetite, hematite, goethite. In addition, there is no existence of magnetotactic bacteria (MTB) in peat sediments.The dominant magnetic mineral is hematite and the major remnant carrier is perhaps magnetite. The parameters indicate that grain sizes are mainly distributed in the fine single domain (SD) particles and small multidomain (MD)-like grains appear to play another major role in the magnetic domain.2) The source of magnetic particles is relatively stable, magnetic characteristics are mainly controlled by the soft coarse-grained minerals from origin areas, ii) the value of χlf and SIRM are notable positive correlation with magnetic grains. The high concentration of magnetic minerals corresponds with coarse-grained particles while the low concentration of magnetic minerals corresponds with fine-grained particles. In contrast with the desert zones, surface soils in other landscape zones are more strongly affected by the late reformation, leading to the changes of mineral concentration and grain size. As the altitude increases to 1300 m, the significant variation of the magnetic characteristics of surface soils in Altay area may be mainly the result of combined action due to regional climate environmental factors (such as temperature, precipitation and evaporation), which result in the destroy of the original magnetic minerals. Howerver, SP (super-particles) material producing from pedogenesis process are relatively less.3) The combined mechanism of partial dissolution and refinement is proposed to illustrate the variations of magnetic minerals in Altay peat. It is argued that the pseudo-single domain (PSD) and MD magnetite on the peatland surface are dissolved and refined in the acid waterlogged sub-oxic conditions during the relatively short time of deposition, which results in low concentration and grain-size reduction. We also speculate that no long-term burial-diagenetic dissolution processes take place or little magnetite dissolution happens under the reductive and alkaline conditions of deeply buried peat.4) The magnetic parameters of χ、IRM1T and χarm/IRM1T in peat sediments can reflect the warming and cooling of climate. In early Holocene(11～1.5 cal kyr BP), the climate in Altay region was cold, but showed a gradual trend of warmer. In middle Holocene(9.5～4.0 cal kyr BP), the climate was warm. In late Holocene(4.0 cal kyr BP～), the climate turns relatively cold again. The climate change in this region has a certain similarity with the warming and cooling of climate recorded by climate proxies of Ulungur lake in north Xinjiang, the warming and cooling of climate is closely related with 60°N solar radiation.5) The concentration change range of Pb element in ATM10-C5 and ATM10-C7 is between 20～35 mg/kg and 20-40 mg/kg, the high value of which occurred during 11～7.5 cal kyr BP in both two cores. During 7.5-4.0 cal kyr the concentration of Pb was significantly decreasing, the change range is between 10～20 mg/kg and 10-25 mg/kg. After 4.0 cal kyr BP the concentration of Pb was significantly higher than last stage, the change range is between 20-30 mg/kg and 20～30 mg/kg. Meanwhile, the concentration of Pb has three peak values, which happened in 1.6～2.1 cal kyr BP,3.5～4.1 cal kyr BP and 7.9～8.5 cal kyr BP, respectively.6) During the Holocene dust change in Altay region recorded by HIRM and Ti can be divided into three stages:In early Holocene(11～9.5 cal kyr BP) the concentration was high, in middle Holocene(9.5～4.0 cal kyr BP) the concentration was the lowest, in late Holocene(4.0 cal kyr BP～) the concentration rose again. The influence factors of dust change are complex, which is closely related to the warming and cooling of climate and is probably influenced by winter monsoon and west wind. |
PDF Full Text Request