Higher-Resolution Studies Of The Holocene Pedogenic Environmental Changes On The Fen-Wei Plains

Holocene environment evolution is an essential part of "PAST GLOBAL CHANGES"(PAGES). Many methods have been introduced into its studies, such as: ice-core, marine isotope records, loess-soil sequence, tree-ring, lacustrine sediments, stalagmite-stalactite, spore-pollen and charcoal etc. Beginning in 1985, high-resolution studies of the loess-palaeosol sequence at Luochuan, Xfan, Baoji, Xifeng, Lanzhou have established various proxy climatic records and which correlated well with marine isotope records. Based on the high-resolution studies on Holocene loess-palaeosol sequence, we studied the Holocene pedogenic environment evolution in Fen-wei plains of loess plateau from the angle of quaternary geology and pedology.In this research, three typical high-resolution Holocene soil profiles in Fen-wei plains were studied by proxy climatic data of magnetic susceptibility and geochemical methods including measurements of Fe, Cu, Cr, Rb, Sr, Ba, Rb/Sr and Rb/Ba. The results provide new insights into Holocene pedogenic environment evolution and human impacts on soil development in this climatically sensitive semi-arid zone.The results are as follows:1. It indicates that this loess-soil sequence has been developed by eolian dust deposition and the bio-pedogenesis controlled by monsoonal climatic changes. The Malan Loess was deposited by dust storms in a cold and dry semi-desert environment before 11500 a B.P.. The transitional layer between the Holocene soil and the Malan Loess was developed in the early Holocene between 11500-8500 a B.P. when a climatic amelioration occurred. Very strong bio-pedogenesis occurred during the mid-Holocene between 8500-3100 a B.P. when a fertile Cinnamon soil was developed with leaching and argillic processes. All of the climatic proxies indicate this soil was formed in response to the warmer and moister Holocene Megathermal. During the late Holocene from 3100 a B.P, the intensified Northwest monsoon and dust storms has caused a degradation of soil and land resources indicating that a climatic decline took place.2. The proxy data in JVC site shows that there was a climatic deterioration in the mid-Holocene, the processes of soil formation were interrupted by intensified dust-falland climatic aridity in 6000-5000 a B.P., but the interbedded loess didn't appear in DXF-S and SLC sites.the cause for it may be the real information was covered up by the soil-forming or soil erosion.3. These proxy indexes also reflect obvious regional difference of pedogenic environment and the bio-pedogenesis intensity caused by the micro-climate and micro-physiognomy. In SIC site, the change of precipitation seemed to have a lag to the variation in temperature, especially in the climatically transitional period, the cause for which is still need to be further probed into.4. In DXF-S site, there was a relatively weak soil-forming peoriod in the late Megathermal, which was caused by the accelerated soil erosion that was forced by human activities (esp.arable farming), in result, there lies a layer of rainwash between 220-180cm. And in the mean time, another three layers of rainwash had been found in DXF-S sequence, which were flushed from the mid-top pluvial fans by the past runoff yielding at the time when the rainfall intensity is greater than the infiltration intensity of the soil. These events may appear in the humid period or the period of rainstorm in high frequency, and all these are probably based on the transitional climate or short-scale abruptly climatic changes.5. The soil type, cause of soil formation and soil quality changes of Holocene loess-soil sequence have been clearly distinguished. The transitional loess is eolian dust accumulation forming in early Holocene, which had very low soil moisture, experiencing weak leaching and argillic processes. Although a climatic amelioration occurred in this period, the total rainfall was still very low. So water and soil resources had improved, but the soil quality in early Holocene was still very low. In response to the warmer and moister Holocen...