HR: 11:55h
AN: GP22A-06    [Abstracts]
TI: Magnetic Mineralogy as Indicator of dry Conditions in Lacustrine Sediments From Santa María del Oro, Nayarit, Central Mexico
AU: * Ortega, B
EM: bortega@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica, Cd. Universitaria, Mexico, D.F 04510, Mexico
AU: Vazquez, G
EM: gvazquez@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Posgrado en Ciencias de la Tierra, Instituto de Geofisica, Cd. Universitaria, Mexico, D.F 04510, Mexico
AU: Rodriguez, A
EM: alerdz@geofisica.unam.mx
AF: Posgrado en Ciencias del Mar y L. Universidad Nacional Autonoma de Mexico., Instituto de Geofisica. Cd. Universitaria., Mexico, D.F 04510, Mexico
AB: Combined magnetic and geochemical analysis were conducted on laminated sediments from Santa Maria del Oro, a crater lake in Nayarit (Mexico), to build up a model of paleoenvironmental conditions for the late Holocene. The occurrence of a severe drought at the end of the archeological Classic period (100 - 900 AD) has been documented in sites of central Mexico (Zirahuen lake and Lerma basin), the Gulf of Mexico coast (Los Tuxtlas) and the Yucatan peninsula. The effects of this climatic event are considered to have stressed the social and political situation in the Yucatan area and other sites in Mesoamerica, and resulted in the "collapse" of the Maya civilization. Santa Maria del Oro sediments between ca. 600 - 1140 AD are characterized by repeated sequences of ocher silt laminae with high inorganic carbon content, authigenic siderite, and low concentration of SD magnetic minerals, followed upward by an increase of concentrations of fine grained SD and SP ferrimagnetic minerals in brown silt laminae. This sequence is considered to represent dissolution-precipitation cycles of magnetic minerals in low erosion, concentrated waters and anoxic water-sediment interface environments. Dissolution of magnetite occurs in reductive conditions, which are considered as warmer and dryer periods. Above the ocher silt, precipitation of fine grained magnetite occurs when conditions change to oxic environments. Ostracode C and O isotopy document a negative precipitation/evaporation balance during this time period.
DE: 1512 Environmental magnetism
DE: 3344 Paleoclimatology (0473, 4900)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly