【摘要】 文章对西宁第四纪黄土-古土壤序列L9以来的黄土和古土壤层进行了土壤化学分析,揭示出剖面中含有大量的可溶盐。古土壤层中的可溶盐含量远高于黄土层,且二者所含可溶盐类型的相对比例存在明显差异。黄土层中Na2CO3和NaHCO3的相对比例较高,表现出更多的碱化特性,而古土壤中含有较多的NaC l和Na2SO4,表现出更多的盐化特性。从剖面所处的地貌-水文条件及土壤微形态特征来看,这些可溶盐主要来自大气降尘。研究揭示,约900ka以来,该区周边地区一直存在较多的干盐湖,是形成粉尘物质的重要源区之一,且间冰期粉尘比冰期粉尘可溶盐含量高得多。这些粉尘中含有较多的、包括对气候有影响的硫酸盐在内的可溶盐类。更多还原

【Abstract】 Dust composition is reflective to environmental conditions of the source areas,the transportation dynamics,and the post-depositional environments.In this study,soil chemical analysis and micromorphological approaches were conducted on a loess-soil sequence of the last 900ka near Xining at the western most part of the Loess Plateau.Thirty representative samples were selected to determine the pH,exchangeable sodium percentage（ESP）,cation exchangeable capacity（CEC）,and the exchangeable cations.Soil solutions of sixty representative samples were analyzed to determine the composition of soluble salts.All the analyzed samples show alkaline properties with the pH values varing from 8.5 to 10.0.pH values in loess layers（generally >9.0） are higher than in soil layers（generally <9.0）.Except the Holocene soil S₀ with weak alkaline property,ESP in loess varies from 15%～25% while it ranges from 20%～40% in soils.Both loess and soil layers contain a significant amount of soluble salts,including abundant sulfates that are thought to have significant climate impacts.Na⁺ represents a proportion of more than 70% among cations.Among anions,CO^2-₃ and HCO^-₃ are more abundant in loess layers than in soil layers while SO^2-₄ and Cl^- are more abundant in soils.These indicate that loess layers contain more（Na₂CO₃） and NaHCO₃,while soil layers mainly contain NaCl and(Na₂SO₄.) Overall,soluble salts are much more abundant in soils than in loess layers.Micromorphological investigations show a weak weathering intensity of the studied soils,characterized by relatively fresh feldspars and biotite grains.All soil samples still contain detrital carbonates,suggesting their weakly developed steppe condition.These do not support a chemical weathering origin of the soluble salts.Because the studied section is located on high-terraces and no hydromorphic features were observed in either loess or soil layers,these soluble salts were unlikely to be originated from groundwater.Consequently,we interpret that these soluble salts were derived from eolian dust during the loess deposition. Salt-bearing dust should have been originated from the surrounding alkaline and saline lakes through eolian erosion.Our results therefore revealed a detrital origin for the abundant soluble salts. These indicate that alkaline/saline lakes have been constantly developed in the surrounding areas over the last 900ka.These lakes were probably seasonal and have experienced wind erosion during dry seasons.A most startling feature is the much higher content of soluble salts in loess layers than in soil layers,indicating that the interglacial dusts contain more soluble salts than glacial dusts. These might be attributable to the more abundant rainfalls associated with the higher temperatures and stronger evaporation during the interglacial times.更多还原