Zhaoxia Jiang

Magnetic discrimination between Al-substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance

Received 22 June 2011; revised 20 December 2011; accepted 24 December 2011; published 23 February 2012. [1] Hematite, a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas, is weakly magnetic. However, it carries a stable natural remanent magnetization, and thus can reflect paleoenvironment changes. To quantify the influence of Al content in hematite on its magnetic properties, two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension (HFh* series) and by thermal dehydration of goethite (HG* series). Crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively. More specifically, the HFh* series samples display oblate (plate-like) morphologies, while the HG* series samples are prolate (highly acicular). HFh* series samples display higher saturation magnetization but lower magnetic coercivity than that of the HG* series. It is tenable that a better lattice ordering of Al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite, resulting in weaker saturation magnetization for HG* series samples. The origin of single domain (SD) hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity: a positive correlation indicates the presence of pure (Al-free) SD hematite, while a negative correlation indicates a chemical origin of SD Al-substituted hematite. These results bear new information on decoding the complex magnetic properties of SD Al-hematite in nature environments, and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by Al-hematite.

Ferro and antiferromagnetism of ultrafine‐grained hematite

Aluminum-substituted hematite (here referred to as Al-Hm) is an important magnetic mineral for paleo and environmental magnetism. However, the magnetic properties of nanosized Al-Hm are poorly known. In this study, a series of Al-Hm samples (fourteen samples) and their nonsubstituted counterparts (referred to as pure-Hm) (seven samples) were synthesized with particle sizes ranging from 14 to 124 nm. With decreasing particle size, coercive force (Bc), magnetic remanence (Mr) acquired in a 5 T field at room temperature, and the maximum blocking temperature (Tb) of Al-Hm descend gradually; Tb drops abruptly from 800 to 200 K at 17.2 ± 3.7 nm. This size defines the superparamagnetic (SP)/single-domain (SD) threshold of Al-Hm which is significantly lower than that of pure-Hm (27.5 ± 1.5 nm). The antiferromagnetic high-field susceptibility (χanti) and the saturation magnetization (Ms) for the weak ferromagnetic component in hematite are both correlated negatively with temperature between 20 and 300 K, and grain size in the vicinity of the SP threshold. For very small grains (below 16–17 nm), Tb increases with increasing particle size. However, beyond this size, it descends with increasing particle size up to 19 nm before increasing with grain size again. These trends are attributed to the competition of surface anisotropy and bulk anisotropy (elastic anisotropy). This new study substantially improves our understanding of the complex magnetic properties of fine-grained Al-Hm and pure-Hm in natural sample, which is significant for the paleoenvironmental and climatic studies of natural samples, e.g., red beds/soils.

Magnetostratigraphy of a greigite-bearing core from the South Yellow Sea: Implications for remagnetization and sedimentation

Sediments from the continental shelf are sensitive to sea level, climatic changes, and local tectonic history. In this study, we carried out a high-resolution magnetostratigraphic investigation on the longest core (NHH01, 125.64 m) recovered from the South Yellow Sea (SYS). An abnormal interval dominated by negative inclinations was discovered by applying alternating field demagnetization (AFD) on samples from a greigite-bearing layer (44.90–51.80 m). In contrast, the inclinations of most greigite-bearing samples changed from negative to positive when heated to ~360°C. This strongly indicates that this inclination anomaly revealed by the AFD alone is not a true negative subchron. After neglecting the effects of greigite-bearing layers, the straightforward correlation of the interpreted magnetostratigraphy defines the Matuyama-Brunhes boundary (781 ka) and the Jaramillo top (990 ka) at 68.64 m and 101.54 m, respectively. The linearly extrapolated basal age of the core is ~1.10 Ma. In addition, several short-lived inclination anomalies can be tentatively assigned to magnetic excursions, which indicates that the sedimentation could be continuous even at the millennial time scale at depth intervals bracketing these fast geomagnetic events. Moreover, the excellent correspondence between clay content variations of the core and the marine oxygen isotope record indicates the potential of clay content as a paleoclimatic proxy in the studied region in the past ~1 Ma. In brief, our study provides not only a robust age model in the SYS but also a methodological guide for paleomagnetic studies in continental shelf region.

Characterizing magnetic mineral assemblages of surface sediments from major Asian dust sources and implications for the Chinese loess magnetism

Eolian dust plays an important role in the Earth’s climate system. Environmental magnetism has been widely used to trace dust variations at different spatial and temporal scales. However, the magnetic properties of sediments from key dust sources have not been well determined. In this study, surface samples from potential dust sources in inner Eastern Asia were systematically investigated. Our results indicate that ferrimagnetic and antiferromagnetic minerals are both present in surface sediments and that they have broad grain size distributions. Ferrimagnetic components are dominated by partially oxidized coarse-grained (pseudo-single domain and multi-domain) lithogenic magnetite particles with minor contributions from pedogenic fine-grained (single domain and superparamagnetic) particles. Antiferromagnetic hematite can be classified into three groups in terms of diffuse reflectance spectroscopy (DRS) band positions (P560 nm, P545 nm, and P535 nm, where numbers indicate the DRS band wavelength for hematite). The first group (P560 nm) is the coarse-grained hematite of lithogenic origin and is mostly confined to western China. The P535 nm group is of pedogenic origin. The P545 nm group is an intermediate phase that is present both in surface samples from the source regions and in loess. Therefore, the P545 nm and P535nm groups are related to eolian inputs to the Chinese Loess Plateau and pedogenic processes, respectively. In addition, significant differences exist between the magnetic properties of eolian material from sources and depositional regions due to gravitational sorting. These insights provide strong constraints on interpretation of dust signals recorded by the Chinese loess and marine sediments from the North Pacific Ocean.

Magnetism of Al‐substituted magnetite reduced from Al‐hematite

This study was supported by National Program on Global Change and Air-Sea Interaction (GASI-04-01-02), the National Natural Science Foundation of China (grants 41504055, 41430962, 41374073, and 41025013), and Chinese Continental Shelf Deep Drilling Program (GZH201100202). Z.X.J. further acknowledges support from the China Postdoctoral Science Foundation. A.P.R. and D.H. acknowledge support from the Australian Research Council (grants DP110105419 and DP120103952).

Estimating the concentration of aluminum-substituted hematite and goethite using diffuse reflectance spectrometry and rock magnetism: Feasibility and limitations

This study was supported by the National Natural Science Foundation of China (41374073 and 41430962), the National Program on Global Changes and Air-Sea Interaction (GASI-04-01-02), and the Chinese Continental Shelf Deep Drilling Program (GZH201100202). Pengxiang Hu was further supported by the China Scholarship Council ([2013] 3009). David Heslop and Andrew Roberts were supported by Australian Research Council Discovery Project DP110105419.

Magnetism of a red soil core derived from basalt, northern Hainan Island, China: Volcanic ash versus pedogenesis

Similar to loess‐paleosol sequences in northwestern China, terrestrial sedimentary sequences (red soils) in southern China also provide sensitive Quaternary records of subtropical/tropical paleoclimate and paleoenvironment. Compared with red clay sequences originated from eolian dust, red soils derived from bedrock have received little attention. In this study, a long core of red soil derived from weathered basalt in northern Hainan Island, China, was systematically investigated by using detailed magnetic measurements and rare earth element analyses. The results show that an extremely strong magnetic zone with a maximum magnetic susceptibility (>10 × 10−5 m3 kg−1) is interbedded in the middle of the core profile. This layer contains a significant amount of superparamagnetic magnetite/maghemite particles that primarily originated from volcanic ash, with secondary contributions from pedogenesis. The former has an average grain size of ~19 nm with a normal distribution of volume, and the latter has a much wider grain size distribution. The presence of volcanic ash within the red soil indicates that these Quaternary basalts were not formed by continuous volcanic eruptions. Moreover, the magnetic enhancement patterns differ between the upper and lower zones. The upper zone is more magnetically enhanced and experienced higher precipitation and temperature than the lower zone. Discrimination of superparamagnetic particles originating from pedogenic processes and volcanic ash thus provides a sound theoretical base for accurate interpretation of magnetism in red soils in this region.

Control of Earth-like magnetic fields on the transformation of ferrihydrite to hematite and goethite

Hematite and goethite are the two most abundant iron oxides in natural environments. Their formation is controlled by multiple environmental factors; therefore, their relative concentration has been used widely to indicate climatic variations. In this study, we aimed to test whether hematite and goethite growth is influenced by ambient magnetic fields of Earth-like values. Ferrihydrite was aged at 95 °C in magnetic fields ranging from ~0 to ~100 μT. Our results indicate a large influence of the applied magnetic field on hematite and goethite growth from ferrihydrite. The synthesized products are a mixture of hematite and goethite for field intensities <~60 μT. Higher fields favour hematite formation by accelerating ferrimagnetic ferrihydrite aggregation. Additionally, hematite particles growing in a controlled magnetic field of ~100 μT appear to be arranged in chains, which may be reduced to magnetite keeping its original configuration, therefore, the presence of magnetic particles in chains in natural sediments cannot be used as an exclusive indicator of biogenic magnetite. Hematite vs. goethite formation in our experiments is influenced by field intensity values within the range of geomagnetic field variability. Thus, geomagnetic field intensity could be a source of variation when using iron (oxyhydr-)oxide concentrations in environmental magnetism.

Origin of Magnetism in Hydrothermally Aged 2-Line Ferrihydrite Suspensions

As an iron oxyhydroxide, nanosized ferrihydrite (Fh) is important in Earth science, biology, and industrial applications. However, its basic structure and origin of its magnetism have long been debated. We integrate synchrotron-based techniques to explore the chemical structures of 2-line ferrihydrite and to determine the origin of its magnetism during hydrothermal aging in air. Our results demonstrate that both the magnetism and X-ray magnetic circular dichroism (XMCD) signal of 2-line ferrihydrite are enhanced with aging time, and that XMCD spectral patterns resemble that of maghemite (γ-Fe2O3) rather than magnetite (Fe3O4). Fe L-edge and K-edge X-ray absorption spectroscopy (XAS) further indicate formation of both maghemite and hematite (α-Fe2O3) with increasing concentrations with longer hydrothermal aging time. Thus, magnetic enhancement with longer hydrothermal aging time is attributed to increasing maghemite concentration instead of a magnetically ordered ferrihydrite as previously reported. Moreover, L-edge and K-edge XAS spectra with different probing depths yield different ratios of these Fe oxides, which suggest the formation of a core (ferrihydrite-rich)-shell (with a mixture of both allotropes; α-Fe2O3 and γ-Fe2O3) structure during hydrothermal aging. Our results provide insights into the chemical evolution of 2-line ferrihydrite that reveal unambiguously the origin of its magnetism.

Factors Controlling Magnetism of Reddish Brown Soil Profiles from Calcarenites in Southern Spain: Dust Input or In-situ Pedogenesis?

Under aerobic conditions, the A and B horizons of soils are magnetically enhanced due to neoformation of ferrimagnets through pedogenesis. This study systematically investigated soils developed on calcarenites of Neogene age in southern Spain to determine the dominant factors controlling the soil magnetism. Geochemical and clay mineral analyses indicate that aeolian dust significantly contribute to the A and B horizon material of the Spanish soil. Nevertheless, the magnetic enhancement of soils can be simply attributed to the pedogenically produced ferrimagnets in-situ. Therefore, the magnetism of Spanish soils is still linked to paleoclimatic variations regardless of the complexities of aeolian inputs from the Northwestern Africa.