Magnetic unmixing of first-order reversal curve diagrams using principal component analysis

• Published in: Geochemistry, Geophysics, Geosystems
• Main Authors: Lascu, Ioan, Harrison, Richard J., Li, Yuting, Muraszko, Joy R., Channell, James E. T., Piotrowski, Alexander M., Hodell, David A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: AGU 2015
• Subjects: 03 - Mineral Sciences; Rockall Trough; NADW; North Atlantic Deep Water; North Atlantic
• Online Access: http://eprints.esc.cam.ac.uk/3486/; http://eprints.esc.cam.ac.uk/3486/1/Lascu_et_al-2015-Geochemistry,_Geophysics,_Geosystems.pdf; http://eprints.esc.cam.ac.uk/3486/2/Lascu_et_al-2015-Geochemistry,_Geophysics,_Geosystems.sup-1.pdf; http://eprints.esc.cam.ac.uk/3486/3/ggge20812-sup-0002-2015GC005909-SupInfo.zip; http://eprints.esc.cam.ac.uk/3486/4/ggge20812-sup-0003-2015GC005909-S1.mov; http://eprints.esc.cam.ac.uk/3486/5/ggge20812-sup-0004-2015GC005909-S2.mov; https://doi.org/10.1002/2015GC005909

We describe a quantitative magnetic unmixing method based on principal component analysis (PCA) of first-order reversal curve (FORC) diagrams. For PCA we resample FORC distributions on grids that capture diagnostic signatures of single-domain (SD), pseudo-single-domain (PSD), and multi-domain (MD) magnetite, as well as of minerals such as hematite. Individual FORC diagrams are recast as linear combinations of end-member (EM) FORC diagrams, located at user-defined positions in PCA space. The EM selection is guided by constraints derived from physical modeling and imposed by data scatter. We investigate temporal variations of two EMs in bulk North Atlantic sediment cores collected from the Rockall Trough and the Iberian Continental Margin. Sediments from each site contain a mixture of magnetosomes and granulometrically distinct detrital magnetite. We also quantify the spatial variation of three EM components (a coarse silt-sized MD component, a fine silt-sized PSD component, and a mixed clay-sized component containing both SD magnetite and hematite) in surficial sediments along the flow path of the North Atlantic Deep Water (NADW). These samples were separated into granulometric fractions, which helped constrain EM definition. PCA-based unmixing reveals systematic variations in EM relative abundance as a function of distance along NADW flow. Finally, we apply PCA to the combined dataset of Rockall Trough and NADW sediments, which can be recast as a four-EM mixture, providing enhanced discrimination between components. Our method forms the foundation of a general solution to the problem of unmixing multi-component magnetic mixtures, a fundamental task of rock magnetic studies. This article is protected by copyright. All rights reserved.