FOCUS: Calcite Crystals Inside Archaeological Plant Tissues

Abstract

Calcite crystals (CaCO3) have been observed in the reserve tissues of maize (Zea mays mays) seeds and fragments of cassava (Manihot esculenta) roots, both found in indigenous subterranean storage silos, discovered in archaeological sites at Januária, in the Peruaçu Valley, state of Minas Gerais, Brazil.The silos were buried in calcareos shelters used by prehistorical populations more than 11,000 years ago. The age of the analysed samples were estimated by C14dating techniques, and varied from 560 years, for the more recent, to 2040 years for the oldest samples. The cassava roots had an estimated age of 870 years. These dates establish these crop samples as among the oldest discovered in the tropical areas of South America, and the cassava root sample as the oldest. The calcite may have contributed to preservation of the archaeological samples by increasing the pH of the microenvironment.The analyses were carried out by SEM, observing the shape and size of the crystals, followed by X-ray analysis to determine the elementar composition of the minerals in the samples. Calcite present in the shelters was probably solubilised and infiltrated into the tissues of stored plant material, where it recrystallised.Another SEM observation was the presence of aggregates of starch grains in both the maize and cassava archaeological samples, and not on fresh samples. This is suggestive of the calcite acting as a cementing agent on starch grains.