[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-07] Unmixing complex sedimentary magnetic mineral assemblages

★Invited Papers

*Andrew P Roberts¹, Jinhua Li^2,3,4,5, Yan Liu^2,3,4,5, Shuangchi Liu^2,5, Hongmiao Pan^4,6, Tian Xiao^4,6, Yongxin Pan^2,3,4,5 (1.Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia, 2.Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Innovation Academy for Earth Sciences, Chinese Academy of Sciences, Beijing 100029, China, 3.Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China, 4.International Associated Laboratory of Evolution and Development of Magnetotactic Multicellular Organisms (LIA-MagMC), CNRS-CAS, Beijing 100029, China, 5.College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China, 6.Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China)

Keywords:Magnetic unmixing, Environmental magnetism, Paleomagnetism

Magnetic unmixing is an important part of environmental magnetic and sedimentary paleomagnetic studies because unambiguous magnetic mineral identification is usually a prerequisite for reconstructing magnetic signals of interest. Unmixing is an inverse problem that can have an infinite number of solutions, so magnetic studies tend to focus on identifying constituent magnetic minerals and their relative grain size variations through a series of magnetic measurements to minimize ambiguities by “supervised” unmixing. Supervised unmixing is normally achieved using bulk magnetic measurements that are used to indirectly interpret magnetic mineral components. Direct magnetic mineral observation using electron microscopy is a valuable addition to such studies, although it often raises questions about the representativeness of observed particles in relation to measured bulk magnetic properties. Challenges associated with magnetic unmixing are illustrated here from a detailed magnetic and electron microscopy study of a deep-sea surface sediment sample from the central Pacific Ocean. Magnetic analyses are interpreted to reveal the presence of 2-4 magnetic components depending on the type of mathematical functions used for fitting and whether the unmixing is unsupervised or supervised, while comprehensive electron microscope observations indicate the presence of 8 (titano-)magnetite particle types. Magnetite magnetofossils are also identified with extensive searching by transmission electron microscopy, but their contribution to the sediment magnetic properties is small. Combined results indicate that two particle types dominate the magnetic properties. This work illustrates the potential magnetic complexity of sediments and the need for detailed supervision of unmixing. Our results demonstrate the value of electron microscope observations in sedimentary magnetic studies and illustrate the potential magnetic complexity of sediments, which needs to be recognized to arrive at meaningful interpretations of conventional bulk magnetic data.