How does Chinese loess become magnetized?

Abstract

Thick loess deposits on the Chinese Loess Plateau provide an outstanding archive of paleoclimate, geomagnetic field and paleoenvironmental changes over at least the last 2.6 Ma. Much work on magnetic polarity reversals, pedogenic magnetic enhancement and climatic change has been carried out over the past 30 years. However, questions about how Chinese loess becomes magnetized remain unanswered. In this thesis, I have sought to address key questions concerning magnetization processes in the Chinese loess. To understand magnetization processes, it is important to understand which magnetic minerals are present and the processes that caused them to be present. Magnetic and other analyses of both loess and paleosol samples indicate that pedogenic superparamagnetic and single domain magnetite and hematite can be present. The mass concentration of hematite can exceed 90% of the magnetic particle assemblage. An eolian dust deposition simulator designed for laboratory redeposition experiments was then used to demonstrate that both a depositional remanent magnetization (DRM) and a post-depositional remanent magnetization (PDRM) can be acquired by Chinese loess. Water content significantly affects the efficiency of PDRM acquisition. No significant PDRM was observed when the water content was below a critical threshold of 40%. Published paleomagnetic data from the Chinese loess indicate the widespread occurrence of inclination error. Detailed paleomagnetic studies of U-channel samples from the last loess/paleosol sequence at Yuanbao were made to assess whether the Chinese loess can accurately record geomagnetic relative paleointensity variations. Three normalized remanence records were determined using conventional normalization as well as the pseudo-Thellier method. Strong coherence among the normalized remanence records indicate that the whole range of magnetic minerals has been affected by pedogenesis. Poor similarity among published normalized remanence records from the Chinese loess, as well as with marine relative paleointensity records indicate that the Chinese loess is not suitable for relative paleointensity determinations. The range of magnetic minerals present and their varying distribution within loess and paleosol sediments indicate that the magnetization is due to a mixture of remanence acquisition mechanisms including DRM, PDRM, chemical remanent magnetization (CRM) and viscous remanent magnetizations, which also indicates complexities in remanence acquisition that will complicate attempts to extract relative paleointensity information. This thesis has provided an improved understanding of the mechanisms of Chinese loess magnetization, and re-emphasizes the importance of pedogenic CRM acquisition.

Magnetostratigraphy of Chinese loess–paleosol sequences

“…Jin and Liu (2011a) also proposed that coarse magnetite particles, e.g., in specific loess intervals such as in L9, can be remagnetized by VRM acquisition because of the weak coercivity of remanence of multi-domain magnetic particles. Zhao and Roberts (2010) proposed that water plays an important role in remanence acquisition in loess because it allows magnetic particles that are poorly oriented during deposition to align more efficiently with the geomagnetic field before being mechanically locked in place during subsequent drying. Zhao and Roberts (2010), therefore, proposed that the NRM of Chinese loess/paleosol sediments is a mixture of DRM, pDRM, CRM, and VRM.…”

“…Zhao and Roberts (2010) proposed that water plays an important role in remanence acquisition in loess because it allows magnetic particles that are poorly oriented during deposition to align more efficiently with the geomagnetic field before being mechanically locked in place during subsequent drying. Zhao and Roberts (2010), therefore, proposed that the NRM of Chinese loess/paleosol sediments is a mixture of DRM, pDRM, CRM, and VRM. For the former two mechanisms, water content provides the major control on which remanence acquisition mechanism is dominant.…”

“…Similar to lake and marine sediments, a depositional remanent magnetization (DRM) or a combination of DRM and a post-depositional remanent magnetization (pDRM) is acquired by Chinese loess (Zhao and Roberts, 2010), where pDRM acquisition involves a gradual lock-in process (Irving and Major, 1964;Kent, 1973;Hyodo, 1984;. Subsequently, magnetic particles with small energy barriers will acquire a secondary viscous remanent magnetization (VRM), which grows with the logarithm of time (Stacey and Banerjee, 1974) during longer-term exposure to the ambient geomagnetic field.…”

A sedimentary paleomagnetic record of the upper Jaramillo transition from the Lantian Basin in China

“…Regionally complex geological background and paleoenvironmental conditions may have played an important role in the remanence acquisition in loess during the postdepositional process. Additionally, the exact location of the reversal is important for future reliable age determinations of Paleolithic sites discovered in loess from the Lantian Basin (Zhang et al 1978;An and Ho 1989;Zhu et al 2015;Dennell, in press). These sites often lack a continuous and complete loesspaleosol sequence when referring to a classical frame sequence of loess (e.g., Ding et al 2002;Sun et al 2006).…”

Magnetic stratigraphy constraints on the Matuyama–Brunhes boundary recorded in a loess section at the southern margin of Chinese Loess Plateau

“…Variable annual precipitation across the CLP may play an important role in the variable lock-in depth of loess, because the pedogenesis-affected higher/lower coercivity components of the magnetic minerals in loess would acquire a remanent magnetization at different depths (Spassov et al 2003). Moreover, laboratory redeposition experiments proved that the water content in loess sediments is a key factor for the remanent magnetization lock in (Wang & Løvlie 2010;Zhao & Roberts 2010).…”