Resumo

VAZQUEZ-CASTRO, Gabriel et al. Magnetic mineralogy as drought indicator in lacustrine sediments of the last ca. 2,600 years, Santa María del Oro, western Mexico. Rev. mex. cienc. geol [online]. 2008, vol.25, n.1, pp.21-38. ISSN 2007-2902.

Combined magnetic mineral, organic and inorganic carbon content and geochemical analysis were conducted on laminated sediments from Santa Maria del Oro, a crater lake in Nayarit (-western Mexico), to build up a model of the paleoenvironmental conditions for the last 2,600 years. This sequence is of particular importance as it constitutes a high resolution record of late Holocene climatic and environmental change in west-central Mexico. The analyses attained in this study allowed to recognize abrupt changes, to identify dry periods, and to compare these variations with available records in central Mexico. Laminations are caused in general by alternations of their main components: lithogenic detritus, biogenic and authigenic carbonates, and amorphous material from biological and volcanic remains. The volcaniclastic accumulations include two tephras, one of them, the Toba Jala, produced by the Ceboruco volcano. Six fades of silt, sand and peat were recognized. Horizons characterized by high inorganic carbon content, authigenic siderite, and the dissolution of the finest ferrimagnetic mineralogy (magnetite) in reductive conditions, are upward followed by an increase in the concentrations of fine grained ferrimagnetic minerals. This sequence represents dissolution-precipitation cycles of magnetic minerals by anoxic/oxic variations in the water-sediment interface during warmer and dryer periods. These environmental conditions are especially present around 600 - 1140 A.D., and 1410- 1830 A.D., which coincide with the archeological Classic period (300 - 900 A.D.) drought, the Little Ice Age (1350 -1800 A.D.) and the droughts of the last 700 years. The effects of climatic variations such as the drought occurred in the archeological Classic period, the Medieval Warm Period, the Little Ice Age, and the droughts over the last 700 years, have been documented in sites along central Mexico.

Palavras-chave : environmental magnetism; paleolimnology; drought; Holocene; Santa Maria del Oro; Mexico.

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