Geoquímica e pedogeoquímica de de sítios arqueológicos com terra preta na região de Caxiuanã-PARÁ

Abstract

The main objective of the present work is to identify the geochemical and pedological distribution patterns in archeological black earth (ABE) sites and adjacent areas, as well as, the anthropogenic action on the chemical elements responsible for the typical ABE geochemical associations. Three archeological sites were selected in the Caxiuanã region, Portel Municipality, Pará State. Trenches were excavated in these sites and in adjacent areas as well. At Manduquinha cite, the soil-horizon sampling have been carried out by collecting at each 5 m along N-S trending traverses. The archeological-material sampling has been based on an Al-horizon P2O5-distribution map on which even-spaced points have been plotted from places bearing the lowest and the highest P2O5 concentrations. For each of these points, chemical and mineralogical analyses have been made aiming at the following determinations: a) mineralogical composition by X-ray diffraction; b) SiO2, TiO2, Fe2O3, P2O5 (Al2B4O7), Al2O3, Na2O, K2O, CaO, MgO, Cd, Co, Cu, Mn, Pb, and Zn (HF + HC1PO4 total dissolution), and C (modified Walkley-Black method), by atomic absorption, colorimetry, and titration; c) As, Se (hydrides), Hg (vapor), Zn, Mn, and Fe (NH4 oxalate + oxalic acid), sorptive ion-complexes (NH4 acetate at pH 7.0), by atomic absorption; d) Cr, B, V, Sc, by optical-ernission spectrochemical analysis, U, by fluorometry, and F by specific-ion electrode analysis; e) Ba, Cl, Ga, Nb, Sr, Y, and Zr by X-ray fluorescence; f) qualitative chemical composition of ceramic and sou by means of scanning electron microscopy; and, g) radiocarbon (C") dating of ceramic artifacts. From a geological point of view, the Caxiuanã region is made up of sediments belonging to Alter do Chão Formation which have been laterized in Tertiary. Ironstones also occur in that region, outcropping at the bay water level and, sometimes, they were used as abrasive material by the prehistoric man. The main drainage system is constituted by the Caxiuanã bay and the Anapu river. Yellow latosol predominates compared with lateritic profiles; hydromorphic soils also occur at the lower portions. The ABE occurrences, known as archeological sites, are found in the higher portions of the latosol domain and are characterized by a dark-colored soil and by the presence of numerous ceramic fragments and lithitic artifacts. In comparison with the adjacent areas, the ABE's A-horizons, besides exhibiting a dark color, are sandier and structurally better organized. Their bases exchange_sum, cation exchange capacity, base saturation, and pH values are higher than those of the adjacent soils, being thus more fertile than the other ones are. The B horizons do not show significant changes regarding their morphological, physical and chemical aspects both in ABE's and adjacent arcas. The chemical composition essentially constituted by SiO2, Al2O3, Fe2O3, and TiO2 is consistent with the described mineral assemblage which comprises quartz, kaolinite, ghoetite, and anatase, The ABE's A-horizon, compared with the B horizon, presents higher contents of SiO2, MgO, CaO, P2O5, C, Ba, Cl, Cu, K, Mn, Sr, and Zn; and lower values of Al2O3, Fe2O3, Na2O, As, Cd, Co, Ga, Cr, F, Pb, Se, and V, that is the first group got enriched and the second one depleted in these elements. Some elements such as Nb, Hg, and Zr do not show significant relative variations from ABE to adjacent areas. In Manduquinha site, soil morphologic features allowed the separation of the ABE's into N and S domains. Due to the fact of being at a slightly lower topographic level, the S-dornain generates an anaerobic microenvironment during the rainier season. The geochemical distribution of the elements in the A, horizon reflects the environmental changes which took place in ABE's. The S-domain is enriched in Si, Mg, P, Ba, Cl, Cu, K, Mn, Sr, and Zn; and presents lower concentrations of the remaining elements, The geochemical data treated through R-type cluster analysis, isocon maps, and factor analysis allowed the identification of three major geochemical associations and their respective concentration areas. 1) P2O5, MgO, CaO, Ba, Cu, Cl, Mn, Sr, and Zn represent the elements, closely related to the ABE's, which have been added to soil; 2) Fe2O3, Na2O, As, Cd, Co, Cr, F, Ga, Pb, and V correspond to the regional latosol geochemical signature which has indirectly been modified by the prehistoric human activity in the site; e, 3) B, Hg, Nb, Sc, Y, and Zr also constitute the regional latosol geochemical signature although they have not undergone significant changes in consequence of the past human occupation. Manduquinha site has been occupied by the prehistoric man during a period of at least 300 years (1280 to 1600 AD). Characteristics of their ceramic manufacture such as the use of antiplastic materials as well as burning and ornamentation techniques corroborate the hypothesis of cultural continuity. The occupation by the people which inhabited that place may have been contínuos or not. In both cases, the small ABE's surface extension (0.5 ha), the shallow depth of the anthropogenic-modified bed, and the small amount of archeological material suggest an occupation with low population density. The cultural habit of disposing organic leavings at separated places led to a significant increase in P, Ba, Ca, Cl, Cu, K, Mg, Mn, Sr, and Zn in the ABE's. The elevated Ca contents at the southwestern limit may be accounted for remains of shells, commonly found in the form of pockets in other archeological cites in the region. In the west- and northwestern ABE's parts there has been a greater addition of materials Mg- and P-rich in such as bones, feces, urine etc., while in the southeast- and northeastern portions it has been observed predominance of Zn-, Mn-, and Cu-rich materials. In the northern part, where the soil is more compact, and at the central portion as well, the occurrence of lower contents of the above-mentioned elements is interpreted to be a consequence of the localization of circulation areas of the group. The pedogenetic processes, which took place after the prehistoric human occupation, related to the own ground surface conformation favored a stronger leaching of P, Ba, B, Ca, Cl, Cu, K, Mg, Mn, Sr, and Zn which concentrate at the extreme-south, in the adjacent area Fe2O3 and associated elements are found in greater concentrations in the northern part with clear dispersion southward.