Discussion on the Holocene Paleoclimate Environment at the Banpo Site in Xi'an
Bingcheng Li (Department of Geology, Northwest University)
Abstract: Objective To discuss the vegetation and environment in the past 1000 years, to formulate a strategic approach to the control of loess plateau and its soil erosion and predict future climate change on the loess plateau. Methods The succession of vegetation in this area was discussed by geological survey, intensive sampling and pollen analysis of the Daweigou section of Xi'an Banpo Site. The results demonstrate that the characteristics of the Holocene paleoclimate environment summarize the vegetation developed in the area in the early, middle and late Holocene and the climatic environment it represents. From 17 samples in this profile, a large amount of pollen was analyzed to make the pollen pattern of the loess layer. According to the analysis of the change of pollen combination on this profile, the temporal evolution and spatial distribution range of Holocene vegetation and corresponding climate were obtained. Conclusion The Holocene climate was warm and humid; the late Holocene was cool and arid.
In order to illustrate the Holocene paleoclimate environment of the Xi'an Banpo site, the range of 34° to 39° north latitude and 106° to 10° east longitude was investigated and studied. The area belongs to the middle of the Loess Plateau in the middle reaches of the Yellow River, and the Xi'an Banpo Site, which is located in the middle section of the Weihe Graben Basin, has been studied.
1 Geological overview
Holocene strata in the Weihe Valley are generally distributed on the surface of the first-class terraces and above the valleys and the loess strata on the surface of the loess layer on the moat, including thin layers of loess on black ridges.
New system: (1) gray, yellow, gray-brown tillage soil loose large pore thickness 0. 3m;(2) Gray-brown loess-like soil loose pottery pieces (1st cultural layer) thickness 0. 75m;(3) Yellow-brown loess-like soil containing charcoal chips, pottery pieces, bone pieces thick 0. 2m;(4) Yellow-brown loess-like soil contains a large amount of clay, a small amount of hard pores, a small amount of pottery pieces thick 0. 62m;(5) Brown-red loess-like soil containing charcoal chips, bone chips, pottery pieces and gravel (2nd cultural layer) thickness 0. 7m;(6) Brown-gray sub-clay containing pottery pieces, bone pieces, charcoal chips (3rd cultural layer) thickness 0. 62m;(7) Gray-yellow sub-clay dense, hard containing calcium nodules of different sizes, pottery flakes and carbon chips thick 1. 27m; Upper Pleistocene: (8) Typical loess (Late Pleistocene Maran loess) has not bottomed out.
2 Succession of vegetation
A large number of charcoal chips 14C age data ranged from 560±105 to 6640 ±105aB were found in the main cultural layers of the Banpo site (Figure 1 by the 5th layer above). P． between [12]. This indicates that the main period of activity of the Banpo people in this area lasted about 100 years. According to the stratigraphic situation and pollen pattern (Figure 1 and Figure 2), it can be seen that the previously described data only reflect the main activity period of the Banpo people is about equivalent to the II.III. cultural layer, and the lower layer 7 also has remnants of human activities (pottery flakes, charcoal chips) may be earlier than the above data. According to relevant experts, the dating results of a large number of Banpo cultural sites in Guanzhong area prove that the early culture of Banpo was in 750-500aB. P． Between the late half-slope cultural ceilings can reach 400aB. P． Therefore, the time limit of the large weigou profile strata is estimated to be about 850-300aB. P． between.
The pollen pattern of the Banpo site (Figure 2) is very clearly divided into two parts: the lower part of the tree pollen rich in upper tree pollen is poor From the above chronological data, the boundary between the two is about 500a B at the upper boundary of the ii. cultural layer. P． on the occasion. The lower part is therefore 850~500aB. P． The upper atlantic period belongs to 500~250aB. P． Sub-Northern Period. We use II. to represent the Pollen Belt of the Middle Holocene and further divide it into two sub-bands, 1 and 2
2.1II.1 Substrip
1) Mesophytic herbaceous pollen represented by X17 sample has an advantage of up to 9. 5% mainly cruciferae (Cruciferae) accounted for 50. 6% leguminosae (Leguminosae) 10. 2% Caryophyllaceae 7. 4% artemisia, quinoa (Chenopodiaceae), Convolvulaceae, and Ty-pha (Ty-pha) arborvitae account for only 0. 5％。 At that time, the vegetation was a meadow grassland temperature rise.
2) Including X15.X16 two samples, arbor pollen accounted for 16.8%, of which the cold-tolerant conifer pollen increased significantly up to 14. 8% are mainly pinus (Pinus) (Picea), fir (Abies) followed by 0. 3％～2． 3% broad-leaved trees. For example, drought-tolerant herbs such as Betula, Ailanthus, and Carpinus are dominant in pollen (67. 2％～91． 5％）。 Many of the aquatic cattails and Concentricystes in the combination were also slightly cooler in the grasslands of pine and artemisia forests at that time.
3) X14 sample as representative of arbor pollen accounted for 24. 2% of which pine 9. 7% hemlock (Tsuga) 6. 4% spruce 4. 8% fir 0. 5％。 In addition, there are broad-leaved birch, thrush, juglans, and the pollen of Ailanthus, and the pollen of a few herbaceous plants has Artemisia 41. 6% Quinoa 17. 6％。 Followed by the Family Compositae 2. 4% of grasses (Gramineae) 2. 9% of the vegetation at that time was warm and humid with temperate broad-leaved deciduous forest grasslands containing subtropical mountain tree species hemlock.
4) Include x12X13 two samples of arbor pollen up to 30. 6% conifer pollen significantly increased pine 20. 2% spruce, fir 13. 2% of broad-leaved tree and hemlock pollen decreased to 1.5% of broad-leaved tree pollen dominated by Quercus. 4％～2． 0% herbaceous pollen up to 68. 3% of the aquatic of artemisia, chrysanthemum and grass are mainly the peak of the profile (16. 1%)At that time, the vegetation was a coniferous forest dominated by pines.
5) Including X9X10X1 sample tree pollen accounted for 17. 7％～31． 1% of which coniferous pine 5. 2％～8． 7% spruce 2. 5％～3． 7% fir 0. 4％～1． 2% hemlock 4%~14. 2% broad-leaved tree pollen 2%~6. 3% of the species are more birch, hazel (Corylus), goose ear fir, alnus (Alnus), oak, chestnut (Castanea), walnut and other herbaceous pollen artemisia 31. 6％～43． 8% Quinoa 17. 7％～2． 3% etc. At that time, the vegetation was warm and humid in mixed coniferous and broad-leaved forests containing hemlocks or forest grasslands.
2． 2II.2 Substrip
This subsacon is characterized by a significant decrease in arborvitae pollen and a significant increase in the pollen of drought-tolerant herbs. Its lower part includes X4 ~ X8 and other 5 samples of tree pollen accounted for 3. 4％～8． 9% broad-leaved trees have more pollen than coniferous trees such as oak, ailanthus, walnut, elm (Ulmus) and so on. Coniferous trees dominated by pine have gradually disappeared and hemlocks have also seen a large increase in herbaceous pollen 89. 4％～96． 6% of the aquatic cattails based on artemisia, chrysanthemum and quinoa significantly reduced the vegetation and the temperature of the sparsely forested grassland was lower than that of the previous period. Its upper part includes X1 ~ X3 and other 3 kinds of arborvitae pollen accounted for 3. 3％～6． 3% broad-leaved trees have more birch, oak, elm, walnut and other coniferous trees with pine-based herbaceous pollen up to 89. 1％～96． The spores of the 2% warm-dried fern Salaginellasinensis increased to 3. 9% of the vegetation is sparsely forested grassland or the dry grassland climate tends to be dry and cool. The characteristics of the spore powder combination of the Banpo site in Xi'an show that the climate of the early Miocene in the Early Miocene of the Banpo people was first developed with a cool and dry forest grassland dominated by pine and artemisia. Although the sub-existing mesophytic grassland vegetation dominated by cruciferous and leguminous people in the lower part of the country indicates that the temperature once rose and warmed at that time. Later, when the arborvitae flourished, the herbaceous plants near Banpo Village flourished and the drought-tolerant Artemisia genus was spread throughout the mesophytic herbaceous mother-in-law, hairy hyacinth, umbrella-shaped, Tang pine grass, eggplant, legume, and aquatic plant cattail flourishing freshwater algae ringed algae also grew, indicating that the lake was developing at that time. Near the site, there are also a variety of deciduous broad-leaved tree species such as oak, ailanthus, walnut, elm, goose ear fir, park, hazel, birch, maple poplar, chestnut, lacquer and so on. In the mountains farther away from Banpo Village, there are more subtropical mountain needle tree species hemlocks that are now mainly distributed south of the Qinling Mountains, and there are pines, a small number of spruces and firs. During this period, when the number of warm deciduous broad-leaved tree species increased, the hemlock also correspondingly increased, reflecting the relatively good temperature and precipitation conditions at that time, and the forest or forest grassland vegetation climate was humid and suitable for the Atlantic period. Cooling climate fluctuations have also occurred in the form of conifer flourishing.
3 Paleoclimate environment
The II. band in the pollen schema (Figure 2) is the most striking zone of the Miocene. It is characterized by a surge in arbor pollen that shows that the scene of lush vegetation in the forest can be divided into two stages. In the early period (Sub-Belt II.1), in the southern Xi'an and Lantian areas, there were temperate broad-leaved deciduous forests or mixed coniferous and broad-leaved forests, and the area north to Fuping and Yulin became forest grasslands in the Ningxia Shuidonggou area, which was a shrub grassland or forest grassland. Among them, the broad-leaved deciduous forest is dominated by oak and the composition of the southern temperate deciduous broad-leaved forest where modern Guanzhong is located, but some of them can be found in the southern temperate zone and more in the northern subtropical tree species such as maple chestnut, lacquer incense, and also only found in the subtropical hickory and holly, as well as the subtropical mountain conifer hemlock. This proves that the climate belt at that time moved north, and the trees that were born south of the Qinling Mountains seeped northward into the climate warmer and wetter than modern times, and a large area of lakes and marshes developed around Lantian Fuping. Among them, the flourishing of aquatic cattail and ringed algae also undoubtedly proves the warmth and humidity of the climate.
Among the prey excavated at the Banpo site are roe deer (river chamois Hydropotesinermis) and bamboo rats (Rhiz-omyssinensis), which are subtropical animals that are no longer found in xi'an, especially when bamboo rats were based on bamboo roots, and there must have been dense bamboo forests growing in the Banpo area at that time. According to Zhu Kezhen's research[3] the distribution of modern bamboo retreated southward by 1 to 3 latitudes and the average annual temperature dropped by 2 °C compared with that time.
All of this suggests that this period was the warmest and wettest period in the Holocene in line with the characteristics of the global climatic Opti-mun and the Atlantic period.
However, pollen data also show some changes in the climate during this period. For example, in Xi'an Banpo can be divided into five stages: (1) cruciferous meadow grassland, the climate begins to warm; (2) pine and artemisia forest grassland is a slightly cool and dry climate fluctuation; (3) broad-leaved deciduous forest containing subtropical mountain conifer species hemlock and other temperature rise; (4) pine-dominated coniferous forests containing a considerable number of spruce and fir indicate a considerable drop in temperature; (5) pine and oak-based mixed coniferous and broad-leaved forests such as thrush, walnut, elm, etc.; (5) coniferous and broad-leaved mixed forests dominated by pine and oak, and many temperate broad-leaved deciduous trees such as thrush, walnut, elm, What is particularly striking is that the hemlock pollen reaches the highest peak of the full profile. This is the highest temperature period in the High Temperature Period (Hypsithermal), which is about 5000 to 6000a, or the peak of life of the Banpo people.
It is worth noting that the above stage (4), a cooling fluctuation in the Atlantic period, shows on each profile such as the pine of the Xinzhuang village profile [4] and the pine-dominated forest grassland of the Artemisia forest grassland Shuidonggou. This period dates back about 700aB. P． around.
The Shuidonggou profile can be used as a representative of the northern and southern desert areas of the Loess Plateau in the early and late Atlantic periods are shrub grasslands, but the growth of some pine and oak in the late Atlantic period indicates that the warmest period is consistent with the situation in the late half-slope and the middle period for the pine-dominated forest grasslands shows a cool fluctuation.
Although the Shuidonggou area was much more desolate than the dense forest landscape of Xi'an at that time, both the Qiao shrub grassland and the forest grassland were much more lush than the desert grassland vegetation that grows today with artemisia artemisia or-dosica) and Ephedrasinica communities[5]. This shows that the influence of warm climates during the Atlantic period is extremely common.
This period is generally in line with the period of Yangshao culture development 6750~4650a. Therefore, we followed the name of Pu Qingyu [6] and slightly modified it to call it "Yangshao's warmest period".
Late Miocene equivalent to spore powder subsistroad II.2 At this time in the Loess Plateau from south to north, there were temperate broad-leaved trees or loose grasslands or forest grasslands mixed with pines[78] Individual sites such as the Lantian Dongcheng Reservoir see subtropical pecan invasions indicate that the climate at this time is still quite warm but cooler, drier than in the previous period, but warmer and wetter than current. It is estimated that the average annual temperature is 1 ~ 2 ° C higher than the present precipitation of about 10 ~ to the summer, Shang, so it is called the Longshan - Summer - Shang warm period.
4) Late Holocene 250a B. P． Since then, temperatures have fluctuated to the point where the climate of the Loess Plateau is characteristic today.