Reconstructing tropical nature: Prehistoric and modern anthrosols (terra preta) in the Amazon rainforest,Upper Xingu River,Brazil.
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文摘
This project investigates the formation of prehistoric anthrosols known as terra preta in Brazil. Terra preta soils were formed by human activities and mark the locations of ancient settlements. Two complementary hypotheses are advocated to explain the formation of darkened anthrosols in the Amazon. In the "midden model" the darkest terra preta with abundant ceramics was formed mainly from refuse disposal at habitation sites while in the "cultivation model" agricultural practices introduced ash, charcoal, and organic material into the soil forming brown soils with few ceramics on the periphery of settlements. The significance of this research is the ethnoarchaeological context. It was carried out in a contemporary Xinguano Amerindian) community, the Kuikuro, with historical links to nearby abandoned settlements that have large expanses of terra preta. Working with the community allowed direct observations of activities and soil sample collection from specific activity areas. This doctoral research is the first to carry out comprehensive soil testing in an ethnoarchaeological context with the objective of documenting terra preta formation processes and characteristics. Several thousand soil samples were collected from the current Kuikuro village, two historic villages, and three large prehistoric sites. Areas that were defined and sampled included the plaza, house floors, backyards, fire hearths, manioc processing areas, trash middens, fields, fallows, and forest. Soil analyses included particle size, Munsell color, pH, organic carbon OC), available nutrients, and total elements. The groups were shown to have distinctive "soil signatures" in the current village context. Middens had a high pH, high OC content, the highest enrichments of the greatest range of plant nutrients Ba, Ca, Cu, K, Mg, Mn, Na, P, Sr, and Zn), and especially low Al, Fe, Ti, and V. Fallow areas on the periphery of the village showed enrichment similar to middens but at much lower magnitudes. Fire hearths had an elevated pH, low OC content, and exceptionally elevated K content. By comparing middens of different ages it was shown that Mn, Ba, and Sr are among the elements, in addition to pH, that stay most fixed in the soil and are therefore especially useful in pedo-archaeological studies. Many of the variables that are typically elevated from human activities were shown to significantly decrease over time in middens from the current and historic villages including pH, OC, Ca, K, Mg, Na, Sr, P, and Zn. The research documented the extreme heterogeneity of the soil in all areas. Discriminant analyses found that most of the variables were useful in separating groups of samples from different areas but several of them consistently performed better while others added relatively little to the separating power of the discriminant functions. The results of soil analyses and ethnographic observations support the hypotheses set forth by previous researchers and offer further details. Bulky organic refuse, namely waste from manioc processing, is preferentially deposited further away from the house while more diverse and durable refuse is deposited closer to the house. In addition to cultivation, the sorting of refuse by type could contribute to the formation of lighter brown soils on the periphery of settlements. I conclude that the bulk of terra preta was formed in middens. Refuse disposal in middens forms a distinctive pattern of linear mounds on the edges of backyards and trails. It is hypothesized that this process formed the linear and ring-shaped mounds observed not only in Upper Xingu prehistoric sites but also in sites in the Central Amazon and Trombetas regions. Farmers were observed planting a diverse range of crops directly in the middens. They were also observed fertilizing land on the periphery of the village with organic refuse and mulching crops with weeds cleared from within the village. It is clear that they know what they are doing when they enrich soil in the refuse disposal areas and then use it for planting crops. In addition to refuse disposal, there is a significant anthropic impact on the soil in domestic and public areas. Activity areas are extremely dynamic, shifting over time and forming a palimpsest of human impact on the soil. When the village expands, activity areas shift on top of the middens and middens are sometimes leveled to create a pad for building new houses. These processes were likely responsible for the deep stratified deposits of terra preta found in many prehistoric sites throughout the Amazon. Abstract shortened by UMI.)