Origem biomolecular de resíduos orgânicos em fragmentos de cerâmicas arqueológicas da Amazônia e sua relação com o fósforo contido

Abstract

The high and perennial fertility of the Archaeological Dark Earth (ADE) soils in Amazon has been the subject of research since its recognition and dissemination and generates interest to the present day. Interconnected to these soils are potsherds of old ceramic objects corresponding to records of the material culture of ancient societies, and that are there fruit of the discard and consequent incorporation to the processes of formation of tropical soils. Chemical, mineral, and fertility investigations have identified the potsherds as potential collaborators in the resilient fertility of ADE soils. Moreover, phosphorus has recently been shown to be a geochemical signature of the potsherds contained in sites with ADE and also responsible for the chemical signature in phosphorus of these soils. These researches have also shown that phosphorus is incorporated into unglazed ceramic vessels through the contact of the fired clay matrix with nutrient-rich aqueous solutions, such as in the cooking of food. The use vessel or primary function has been determined for at least half a century in Europe and Asia at molecular levels by means of solvent extraction and the use of a mass spectrometer coupled to the chromatograph. Still not explored in potsherds of the Amazon and assuming that phosphorus and organic matter come from the same source, products processed during the lifetime of use in the vessels, the present work aims to characterize the possible organic compounds preserved confronting these data to the typological characteristics of the potsherds and to the chemical-mineralogical information, especially to the contents of phosphorus. For this, potsherds of four archaeological sites located near the channel of the great Amazon river, therefore in the east-west direction in the Amazon region were selected: Quebrada Tacana (Letícia-Colômbia) to the west, Juruti (Juruti - PA), Boa Vista (Porto Trombetas - PA) e Raimundo (Caxiuanã - PA) to the eastern. Initially, the potsherds were described mesoscopically and selected for fertility analyzes, desorption assays, XRD, SEM-EDS, FT-IR, optical microscopy and GC-MS. The aim of this study was to evaluate the ability of potsherds inserted in the Amazonian context to preserve organic compounds directly linked with vessel use in antiquity, by comparing them with morphology, chemical composition, mineralogical and fertility, on the lifetime of use of the vessel represented by their potsherds and their potential contribution to soil fertility TPA. Potsherds have been shown to be fertile in nutrients and micronutrients, whose sources are derived from the raw material, with the exception of phosphorus, a nutrient of greater prominence in terms of concentration and with desorption capacity. The estimated firing temperature was different from the sites, based on the identification of kaolinite and metakaolinite by IR. In the Quebrada Tacana and Juruti sites, the potsherds showed a temperature above 600 ºC and below 800 ºC, while the potsherds of the Boa Vista site did not exceed 600 ºC. Extracts from all fragments were dominated by short and medium chain (C12-C20) n-alkanoic acid series, another dominant component was C18:1 (oleic acid), and in only one fragment belonging to the Quebrada Tacana site were the sesquiterpenes. The distribution pattern among all the potsherds showed that the most abundant was C16:0, followed by C18:0 and C18:1 and small amounts of n-alkanes and n-alkanols of short chains. The predominance of free fatty acids indicates that lipids are highly degraded, a typical feature of earlier organic residue. The lipid extract of the QT-07 sample (Quebrada Tacana site) composed entirely of sesquiterpenes, evidences the presence of plant resin. In the same way, even if insignificant register of nalkanes and n-alkanols whose average does not exceed 1.3% are also indicative of plant contribution, for this case determined in potsherds of the Raimundo site. Mineralogical data ensure that the lipids preserved in the potsherds contain information on past activities, provided that the indicated firing temperature is above 550 °C, sufficient to remove oxygenated compounds from the raw material, clay and non-plastics. The presence of diagnostic biomarkers of aquatic organisms was identified in only two potsherds of the Quebrada Tacana site, 4,8,12-trimethyldecanoic (TMTD) and in addition the identification of odd-chain acids, C15:0 and C17:0 together with their respective C15:1 and C17:1 branched acids are strong indications of animal contribution. Therefore, they may suggest that these potsherds represent vessels used in the prolonged preparation of foods, and consequently because they have had contact with primary sources of nutrients, in the stage of use of the ceramic cycle (nutrient adsorption), the discard stage (current stage) would represent a potential source of nutrients caused by the degradation itself, mainly to the most exposed potsherds of the surface and sub-surface.