HR: 08:30h
AN: GP41B-01 INVITED     [Abstracts]
TI: A review of component analysis based on magnetization curves: state-of-the art and future developments.
AU: * Egli, R
EM: eglix007@umn.edu
AF: Institute for Rock Magnetism, Newton Horace Winchell School of Earth Sciences, 219 Shepard Laboratories 100 Union Street S.E., Minneapolis, MN 55455-0128 United States
AB: Rocks and sediments inevitably contain mixtures of magnetic minerals, grain sizes, and weathering states. Most rock magnetic interpretation techniques rely on a set of value parameters, such as susceptibility and isothermal/anhysteretic remanent magnetization (ARM or IRM). These parameters are usually interpreted in terms of mineralogy and domain state of the magnetic particles. In some cases, such interpretation of natural samples can be misleading or inconclusive. A less constrained approach to magnetic mineralogy models is based on the analysis of magnetization curves, which are decomposed into a set of elementary contributions. Each contribution is called a magnetic component, and characterizes a specific set of magnetic grains with a unimodal distribution of physical and chemical properties. Magnetic components are related to specific biogeochemical signatures rather than representing traditional categories, such as SD magnetite. This unconventional approach gives a direct link to the interpretation of natural processes on a multidisciplinary level. Despite the aforementioned advantages, component analysis is not yet come into wide use for three reasons: 1) the lack of quantitative magnetic models for natural, non-ideal magnetic grains and/or the statistical distribution of their properties, 2) the intrinsic mathematical complexity of unmixing problems, and 3) the need of accurate measurements that are beyond the usual standards. Since magnetic components rarely occur alone in natural samples, unmixing techniques and rock magnetic models are interdependent. A big effort has been recently undertaken to verify the basic properties of magnetization curves and obtain useful and reliable solutions of the unmixing problem. The result of this experience is a collection of a few hundred magnetic components identified in various natural environments. The properties of these components are controlled by their biogeochemical history, regardless of the provenance of the hosting sediment. For example, the coercivity of all detrital magnetites is tuned by the transport mechanism (air/water), and the ARM of biogenic magnetites is controlled by the (paleo)redox conditions of the sediment. The consistency of these results supports the linear additivity principle upon which all current magnetic unmixing methods are based. Once the rock magnetic properties of individual components and their statistical distribution is known, the solution of unmixing problems provides important benefits including a great simplification which makes it accessible to non-specialized users. Simplified unmixing algorithms are robust and deliver reliable results based on relatively fast measurements. Two key examples will be presented. In the first example, the magnetic composition of lake sediments is used to develop a model that describes the nonlinear response of a lake to environmental changes. The response function can be used to deconvolute magnetic measurements for paleoclimatology reconstructions. The second example deals with an application of component analysis to obtain a low-cost and fast assessment of the air quality in urban areas. A community effort in setting up a database of magnetic components occurring in the most varied environments will provide us with a new, powerful tool for rock- paleo- and environmental magnetism research.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly