HR: 10:40h
AN: GP22A-02 INVITED     [Abstracts]
TI: Comparison of Magnetic, Geochemical and Biological Proxies Signals in a ca. 2,000 yr Record from the Tropical Lowlands of Eastern Mexico.
AU: * Caballero, M
EM: maga@geofisica.unam.mx
AF: Instituto de Geofísica, Universidad Nacional Autonoma de México, Circuito Exterior, Ciudad Universitaria, Del. Coyoacan, Mexico, DF 04510, Mexico
AU: Beatriz, O
EM: bortega@geofisica.unam.mx
AF: Instituto de Geofísica, Universidad Nacional Autonoma de México, Circuito Exterior, Ciudad Universitaria, Del. Coyoacan, Mexico, DF 04510, Mexico
AU: Ma. del Socorro, L
EM: mslozano@servidor.unam.mx
AF: Instituto de Geología, Universidad Nacional Autonoma de México, Circuito Exterior, Ciudad Universitaria, Del. Coyoacan, Mexico, DF 04510, Mexico
AU: Rodríguez, A
EM: alerdz@servidor.unam.mx
AF: Posgrado en Ciencias del Mar y Limnología, Universidad Nacional Autonoma de México, Circuito Exterior, Ciudad Universitaria, Del. Coyoacan, Mexico, DF 04510, Mexico
AB: Pollen, diatoms, geochemical, magnetic and non-magnetic mineral analyses were conducted on a lacustrine sequence from a maar lake on the tropical lowlands of eastern Mexico. Chronological framework for this lake is based on age determinations by 210-Pb, 137-Cs and 14-C. The studied sequence covers the last ca. 2000 yr, a time of important environmental transformations in the area due to climatic variability as well as human impact since the early Olmec societies until the recent forest clearance of the 20th century. Through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes, and compare it with the biological proxy records (diatoms and pollen) and the archeological record. Inferred climatic changes for this area are further compared with the documented climatic changes in the northern hemisphere of tropical America. Volcanic activity has played a major influence on sediment magnetic properties, as a purveyor of Ti-magnetites/Ti-maghemites, and as a factor of instability in the environment. Moisture availability has been determinant for the diatom and pollen records, and human impact is mostly reflected in the pollen and geochemical records. Direct observations of magnetic minerals and ratios of geochemical (Fe, Ti), and ferrimagnetic (χ f ) and paramagnetic (χ p) susceptibility (χ) data, are used as parameters for magnetite dissolution (χ p/χ, Fe/χ f ), and precipitation (χ f/Ti) of magnetic minerals. Evidence of agricultural practices associated with increased erosion, deforestation, higher evaporation rates, lower lake levels, anoxia and reductive diagenesis in non-sulphidic conditions are inferred for laminated sediments between A.D. 20-850. This deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the Maya civilization. Dissolution of magnetite, a high organic content, framboidal pyrite and a change in the diatom assemblage point to anoxic, sulphidic conditions and higher lake levels after A.D. 850. Higher lake levels in Lago Verde coincide with a recovery in the forest cover, the same lake and vegetation signals are present in nearby lake Pompal, allowing to infer increased precipitation. This signal is coeval with the increased moisture documented during the Medieval Warm Period (A.D. 950-1350) in the northern tropical and subtropical regions of the American continent. For the Little Ice Age (A.D. 1400-1800) data are suggestive of relatively mistier conditions, with a deeper lake and highest vegetation cover, in concordance with the glacial advances recorded in central Mexico and tropical Andes. Higher erosion rates reflect destruction of the rainforest over the last 40 years.
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly