(3) Paleoclimate reflected by loess

Loess is generally yellow, from light yellow to brown yellow, even red and yellow, which are all included in the general "yellow". Geologically, the reddish-yellow to grayish-red soil sandwiched in grayish-yellow loess is called paleosol, which is 0.5 ~ 3.5 meters thick and gradually transits with the lower loess. Generally, it is only seen on the ridge, and it is often distributed slowly and obliquely, and the length can reach hundreds of meters. Through research, this kind of red soil was transformed from ancient soil sample layer by plant pedogenesis in that year.

There is often a difference between raw soil and cooked soil in agricultural production. Ordinary loess is hard and massive, which needs a hammer or hoe to break, that is, raw soil. You can't grow crops without destroying them. The soil that can be caught with one hand on the ground can be counted as cooked soil. One of the biggest differences between them is porosity, which is actually the porosity of soil.

Dry powdery loose soil is short of water, so no plant can grow, so mature soil should also contain appropriate amount of water. With a certain amount of water, you have the necessary conditions to grow crops. Real soil should also contain a certain amount of organic matter and elements such as nitrogen, phosphorus and potassium. In the best soil, the particles should also be aggregate structure, gathering into small pellets with a certain particle size. The difference between the soil we see in the forest and the soil in the desert is the difference between raw soil and cooked soil. "Grab a handful of oil and flow" is the standard soil. After tens of thousands of years of compaction, dehydration and consolidation, raw soil and cooked soil reflect that "raw soil" is grayish yellow and ancient cooked soil is grayish red, which is called ancient soil.

Through chemical analysis, the content of alumina and iron oxide in paleosol is higher than that in loess (10%), and the content of silica and potassium oxide is slightly higher (5%). Through particle size analysis, the mineral particles in paleosol are finer than loess. Loess belongs to clayey silt, and paleosol belongs to loam. Before the popularization of gas honeycomb coal, people need to add burning soil (30% is better) to household coal. This burnt soil belongs to sub-clay, which has good adhesion to coal particles. After burning, it becomes a block, but the loess has poor adhesion, so it is difficult to burn when it is pulverized with tongs.

The loess was blown away by the strong wind. In sandstorms, dust with a thickness of 0.5 ~ 3 mm can be accumulated on the ground. If they are compacted, loess with a thickness of 0. 1 ~ 0.3 mm can be formed. Assuming that sandstorms blow 3 ~ 5 times a year, loess with a thickness of 0.3 ~ 1.5 mm can be accumulated in a year. It is known that loess has been formed for 2.5 million years. If calculated by the annual average 1mm, 2500m thick loess will be accumulated. At present, the maximum thickness of loess in the Loess Plateau is only about 400 meters, which is less than one sixth of the theoretical value. It can be seen that sandstorms are less than once a year in history. Based on the loess thickness of 200 meters in the west of Shanxi, the annual loess accumulation is less than 0. 1 mm, and it is concluded that the weather in Shanxi for 2.5 million years should be sunny, with blue sky and white clouds and few sandstorms (at most once a year).

According to the analysis of loess with ancient soil, only one-fifth of the 250-meter-thick loess has ancient soil layer. In other words, only one fifth of the weather is suitable for the large-scale growth and reproduction of plants and the formation of soil. Only when the weather is good and sunny will plants flourish, and the rest of the weather is too cold and dry for plants to multiply in large numbers. A large part of this cold and dry weather belongs to the ice age in geological history, and the other part is hot and dry with little rain. Therefore, in the 2.5 million years since the emergence of mankind, only one-fifth of the weather is really good, and the rest of the days are freezing, dry and hot. Of course, there are many days in the freezing period, and there should be fewer days in the dry period, because when the wind blows in dry heat, the dust will cover the sun, and the thickness of loess accumulation should exceed today's thickness.

The above inference does not take into account that loess will be washed away in rainy days. If the topsoil loses 0.5 mm a year (this is the statistical value of soil erosion), then 0.6 mm of soil will be accumulated every year. Considering that the amount of soil erosion during the ice age should be close to zero, because there is little rainfall, the soil layer is frozen and cannot be eroded. Considering that the topsoil loss will increase greatly during the high temperature and drought period (but the accumulation will also increase greatly at that time), it can only be assumed that they are balanced when the proportion of the above weather in history is still difficult to count. This is equal to positive and negative cancellation, ignoring, and returning to the inference before considering these two situations.

Fossils were discovered in the Quaternary (2.5 million years) since the birth of mankind, compared with similar creatures that still exist today. Fossils include rhinoceros and elephants in Africa today, ostriches in Australia, and these tropical and subtropical steppe fauna; There are also cold zone fauna such as hairy rhinoceros, mammoth and bighorn deer; Of course, there are also cattle, horses, pigs, dogs, badgers and other temperate animals. Their appearance reflects the variability of Quaternary climate.

Similarly, plant spores and pollen become fossils, and through the analysis of ancient sporopollen, we can also get the ancient climate. Conifers, such as pine, fir and cypress, are widely used to represent the cold environment, while broadleaf, locust and poplar represent the warm climate. Gramineae pollen represents grassland environment. This can also be inferred from the changeable climate in Quaternary.