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Title: Magnetic properties of Mexican soils and lake sediments
Author: Braisby, John David James
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2001
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This study addresses two topics in environmental magnetism, i) the magnetic unmixing of lake sediments in terms of catchment samples and, ii) a connection between soil magnetic properties and annual rainfall. Soil and lake sediment cores from Mexico provided the material used. Palaeolimnology facilities the understanding of past environments by the analysis of lake sediments. Many palaeolimnological techniques are both time consuming and destructive. Analysis of pollen and diatoms require an expert to identify and count hundreds of pollen grains or diatoms, for each core horizon to be characterised. In contrast, magnetic remanence and susceptibility measurements can be obtained rapidly and without changing the physical properties of the sample that is being measured. The problem with magnetic measurements on lake sediment is the interpretation of the results. Previous workers have analysed trends in the magnetic properties of sediments. Linking high susceptibility to soil erosion, and low susceptibility to reduced sedimentation through, for example, the accumulation of organic material, low in magnetic content. In order to analyse environmental magnetic results I have developed a new magnetic unmixing technique. It uses non-negative least squares to unmix the magnetic properties of a target sample in terms of the magnetic properties of potential end-member samples. A stepwise technique is used to select appropriate end-members and estimate the errors involved. The newly developed magnetic unmixing procedure has been applied to the sediments of two lakes in Mexico, Lake Pátzcuaro, in the central Mexican highlands, and Lake Babicora in the northern Mexican highlands. The results for Lake Pátzcuaro indicate changes in the lake sediments’ magnetic properties that can be ascribed to climate changes and farming. The main climatic change is a reduction in lake level at approximately 11,0001 4C yr B.P. Human arrival, at approximately 3,5001 4C yr B.P. coincides with a dramatic increase in topsoil erosion. At Lake Babicora the results indicate that a reduction in rainfall led to a dramatic reduction in catchment material delivered to the core site. This change in the depositional regime occurred at approximately 17,000 1 4C yr B.P.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available