中国黄尘初探

A PRELIMINARY STUDY ON THE LOESS DUST IN CHINA

黄土降尘以单颗粒自重降落为主,凝聚降落和雨淋降落为次。单颗粒自重降落虽以粉砂和细砂为主,但也杂有明显的细团粒。细粒楔入粗粒均曾见于自重降落和凝聚降落的黄土中。凝聚降落是典型的“雨土”,多呈珠球状,着陆后常变形和破碎。雨淋降落可直接渗入地下。各期黄土层中均见有完整细粒团保存,古土壤层的细粒团除明显铁染外,一般未见细粒位移和重新排列等现象。

Eolian dust of recent years in Guanzhong area (southern Shaanxi) has been observed, collected, and then studied under electron-microscope (SEM), polarizing microscope, and stereo-microscope. A comparison has been made among the layers from the bottom of Wucheng Loess up to the various Holocene loess-paleosol. The following points may be shown: 1) Although the 'fall by coagulation'; and 'fall by rain-out' can be seen, the 'fall by selfweight' dominates over the dust falls. 2) The fall-by-self-weight dusts are mainly composed of silts and very fine sands with some aggregate of fine grained materials. Besides, there is the phenonmenon of adherence of the fines to the coarses. 3) The fine grain (probably quartz) can be seen wedged into the coarse grains including both of the individual grains and the fine-grain coagulations. 4) The fall-by-coagulation grains are formed by the collision and coagulation of the moisture-absorbed dust particles. They occur as bead-like fine-earth lumps generally of 100—500 μm in diameter with a minimum of 50 μm and a maximum of about 2 mm. 5) The lumps might embrace some coarse particles during they were falling, and then deformed or broken into fragments due to impacting when landing. Sometimes one can see a ring of coarse particles around a lump, or coarse particles being pushed aside. Sometimes the lump is brocken into smaller earth lumps and coarses of various shapes. 6) The rainy dust or rain-out dust process refers to the dust being washed down onto the land by rain. The landing dust either penetrates into the underground accompanying with the rain water, or sometimes adheres to the plant leaves and building surface and then usually is washed down a slope and deposited along its way to the slope end forming some concentrations. 7) The fine-grained earth lumps can be seen throughout the above-mentioned loess (including the silt) layers indicating how important the conteraction between the dry-cold west moonson and the east warm-humid moonson during the spring-summer junction is in the formation of the loess plateau. 8) The soil-structure and assembling materials of the modern soil and the pasleosols are formed mainly by biological process especially by the plant-root system. Except for the soluable salt such as Na, K, Mg and Ca, the weathering and the leaching are rather weak. Even the fine-grained earth aggregates formed during the deposition are mottled by iron only. 9) Soil forming process, particularly biological selective absorption and feedback effect are very significant. 10) The feedback effect of the soil in loess area takes place mainly among the plant roots. The soil forming process in loess mainly characterized by perennial corrosion of the plantroot system, alternative regeneration and ultimate mineralization. It follows that the biological origin of adherent granis in the soil is worth further stadying.