Loess Deposition Rate Under Influence Of Climate In The Holocene In Loess Plateau In Northern Shaanxi

Soil loss in loess plateau in northern Shaanxi is the most serious all over the world. The loess deposition rate in this region is the base for determination of soil loss tolerance. Unstable climate in the Holocene affects on loess deposition and formation. Meanwhile, the loess formed in this period has the closest relationship with human. Thus, it is necessary to research the loess deposition rate under influence of climate in the Holocene. This study could provide theoretical basis and data supporting for determination of soil loss tolerance and assessment of soil erosion risk.Four soil profiles A, B, C and D were located in three locations in north, middle and south of loess plateau in northern Shaanxi. The loess deposition rates were researched by analysis of soil ¹⁴C age in every layer of the profiles. The main results were that: (1) Particle size distribution, TOC, pH and CaCO₃ content of A, B, D profiles on tableland were analyzed. The variation of particle size distribution and CaCO₃ content showed that the climate affected on soil formation. The changing of TOC and pH with latitude indicated that the degree of pedogenesis decreased from south to north in this region. These results laid a foundation for researches on climate changes and tempo-spatial variations of loess deposition rate.(2) The Hurst index and fractal dimension of seven climatic indexes were calculated, and correlation analysis was done between all the climatic indexes. The indexical relation between the climatic indexes and climatic elements was discussed. Finally, four indexes (χIf, CaCO₃ content, TOC and the content of 0.01⁰.05 mm particles) were selected, and wavelet analysis was used to analyze variations of paleoclimate. In total, in A and B profile, 10000 a B.P.⁸500 a B.P. was warming period, 8500 a B.P.³000 a B.P. was warm period, 3000 a B.P. present was cooling period; in D profile, 8000 a B.P.⁴000 a B.P. was warm period, 4000 a B.P. present was cooling period. The differences between them may be caused by the older age of the soil deposited at the same period in D profile than in A and B profile.(3) The loess deposition rate in different climate period was calculated by depth and age of soil in every layer. The average loess deposition rate of A, B and D profile in the Holocene was 0.0179 cm/a,0.0153 cm/a,0.0213 cm/a, respectively. The distribution of loess depostion rate in this region showed that, it decreased from northwest to southeast, and the minimum was in the middle of region. The soil erosion in the Holocene was analyzed by the annual rainfall which was calculated by magnetic susceptibility. The influences of soil erosion on soil deposition rate were discussed. The minimum in the middle of region was caused by soil erosion. Therefore, the loess deposition rate was comprehensive results of deposition, erosion and other pedogenesis. The loess depotition rate derived from soil depth and age could show the comprehensive result quantitatively.(4) Soil age, loess accumulation rate and climate indexes of B and C profile were used to reconstruct the landforms on the slope where the C profile on before 12000 a B.P.. The soil eosion rate was lager than loess deposition rate, and it was proved that this method could be used to research on soil erosion and reconstruct the landforms in geological period.(5) Soil erosion risk of the research region and a watershed was assessed by the ratio of soil loss to soil deposition. The soil erosion risk in Luochuan, Yanchang and Heangshan was 14.1,43.3 and 24.8, much larger than critical value 1, so this region should be an important protection and control area. With the growth of management degree, the soil erosion risk dropped from 28.8 in 1997 to 0.2 in 2007 in Yan'gou watershed. The soil erosion was under control.This study could provide theoretical basis for determination of soil loss tolerance, soil genesis and development. It would provide new idea and method for quantificationally research on reconstruct the landforms in geological period. The application of loess deposition rate on assessment of soil erosion risk could provide theoretical basis for environmental treatment decision.