Magnetism of a speleothem from Midwest Brazil and paleoclimatic implications

• Autor(a) principal: Jaqueto, Plinio Francisco
• Data de Publicação: 2016
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: http://www.teses.usp.br/teses/disponiveis/14/14132/tde-31052017-165938/
• topic: environmental magnetism.; magnetismo ambiental.; magnetismo de espeleotema; magnetismo de rochas; paleoclima; paleoclimate; rock magnetism; soil; solo; speleothem magnetism

This thesis provides a detailed study of environmental magnetism in a speleothem. It focuses on a stalagmite from Pau DAlho cave (15°1220S, 56°4841W) located in Rosário dOeste, Mato Grosso State, Brazil. This speleothem grew during the past 1355 years, with average growth rate of ~168 mm/ka and encompasses two key events in the South American Summer Monsoon (SASM), the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA), major dry and wet, respectively. The rock magnetic experiments conducted, include isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) acquisition curves, thermal demagnetization of three-axis IRM acquisition, hysteresis loops, first order reversal curves (FORC) and low-temperature SQUID magnetometry experiments. The main magnetic remanence carriers in the stalagmite are magnetite and goethite, with a nearly constant relative proportion. Magnetite has remanent coercivities between 14-17 mT, and its magnetic properties are similar to those produced by pedogenic processes. Magnetic remanence is broadly correlated with carbon and oxygen isotope data throughout most of the speleothem, suggesting that precipitation and soil dynamics above the cave exert a strong control on the input of magnetic minerals into the Pau dAlho cave system. Dry periods like the MCA are associated with less stable soils that result in higher mineral fluxes carried into karst systems via groundwater, while conversely, colder and wetter periods like the LIA are associated with soils topped by denser vegetation that are more capable of retaining micrometer-scale pedogenic minerals, and thus reduce mineral fluxes into karst environments.