Mineralogia e química de solo e fragmentos cerâmicos de sítios arqueológicos em Salobo, Carajás-PA

Abstract

Many archaeological sites in the Amazon are characterized by presenting Dark Earth soils type commonly called Dark Earth Archaeological (ADE), in which the main records of prehistoric human occupation are represented by the most common materials of everyday life such as ceramic fragments (CFs), lithic artifacts and charcoal. The soils are characterized by high ADE fertility conferred by high levels of nutrients such as Ca, Mg, Na, K, Mn, Zn and P, a standout compared to predominantly poor soils of the Amazon. Among the several regions rich in archaeological sites with ADE is the Carajás Mineral Province, more specifically the area of the field of copper mine Solobo, which are situated on the slopes of valleys and even on top of the mountain. In this region, field studies were conducted by researchers at the Goeldi Museum, which delimit two archaeological sites, one called Cachorro Cego containing ADE, and the Orlando site, although rich in CFs is presented devoid of ADE in its composition. Of the two sites, the Cachorro Cego was selected to evaluate the contribution of CFs on soils, especially the ADE on the chemistry of ceramic fragments, particularly regarding the content of phosphorus. For this purpose 14 samples were selected CFs collected at the two sites investigated, as well as six soil samples restricted to Cachorro Cego from soil point where the ceramics were extracted. The ceramics were collected previously cleaned, sprayed and subsequently subjected to mineralogical analysis by XRD and optical microscopy, total chemical analysis by ICP-MS and ICP-OES via alkaline fusion, optical microscopy and scanning fertility analysis. These same analytical techniques were employed in soil samples, except for light and electron microscopy. The results indicate similarities in both chemical composition and mineralogy of the two sites in the CFs investigated. CFs are composed of quartz, albite and metakaolin as major minerals, a background matrix represented by amorphous material, equivalent to metakaolin, and microcline and muscovite as accessory minerals. The analysis emphasize the high content of SiO2, Al2O3 and Na2O, and MgO, CaO and K2O in smaller proportions. The contents of Fe2O3, 5.26% in 5.8% and Cachorro Cego in Orlando showed no correspondence with oxide-hydroxides (goethite and hematite, perhaps maghemite), CFs found in other sites. The concentration of P2O5 is less than 0.5% also at the two sites, contrasting with values observed in most sites ADE. Among the trace elements analyzed, only Y, Zr, Th, Cu, Ga and U stand out with above average crustal values in the two sites studied. The rare earth elements when normalized to chondrites showed enrichment in LREE with subtle positive Ce anomalies and negative. In terms of fertility CFs of Cachorro Cego stand out by high levels of available Ca and available P corresponding to (57.02 mg / dm3 and 2.44 cmol/dm3) against Orlando (13.47 and 1.79 mg/dm3 cmol/dm3). However Orlando stands out in available K (0.77 cmol/dm3) and Mg available (cmol/dm3 1.05) vs 0.42 and 0.63 cmol/dm3 cmol/dm3 respectively. The soils vary in texture Cachorro Cego franco silty to sandy loam, consisting predominantly of quartz and kaolinite, anatase and hematite with a lesser proportion, mirroring themselves in the abundance of SiO2 and Al2O3, with lower contents of Fe2O3 and TiO2. The content of K2O, Na2O, CaO and MgO is very low. The average of the nutrients provided by the soil was 33.49 mg.Kg-1 to P to 55.88 mg.Kg Na+, 59.12 mg. Kg-1 K+, 75.9 mg.Kg-1 for Mg2+ and 835.7 mg.Kg-1 for Ca2+, confirming the signature data ADE. CFs of the two sites are both so similar in their chemical composition and mineralogy, the levels of nutrients such as Ca, Mg, K, Na content presented next except P, higher in TPA. The Cachorro Cego soils are comparable to other Amazon sites, both in mineralogy, chemistry, particle size and in fertility. Apparently CFs contributed to soil fertility TPA, like Cachorro Cego.