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Hematite-bearing red beds are renowned for their chemical remanent magnetization (CRM). If the CRM was acquired substantially later than the sediment was formed, this severely compromises paleomagnetic records. To improve our interpretation of the natural remanent magnetization, the intricacies of the CRM acquisition process must be understood. Here, we contribute to this issue by synthesizing hematite under controlled 'Earth-like' field conditions (≲100 μT). CRM was imparted in 90 oriented samples with varying inclinations. The final synthesis product appeared to be dominated by hematite with traces of ferrimagnetic iron oxides. When the magnetic field intensity is ≳40 μT, the CRM records the field direction faithfully. However, for field intensities ≲40 μT, the CRM direction may deviate considerably from that of the applied field during synthesis. The CRM intensity normalized by the isothermal remanent magnetization (CRM/IRM@2.5 T) increases linearly with the intensity of growth field, implying that CRM could potentially be useful for relative paleointensity studies if hematite particles of chemical origins have consistent properties. CRM in hematite has a distributed unblocking temperature spectrum from ~200 to ~650 °C, while hematite with a depositional remanent magnetization (DRM) has a more confined spectrum from ~600to680°C because it is usually coarser-grained and more stoichiometric. Therefore, the thermal decay curves of CRM with their concave shape are notably different from their DRM counterparts which are convex. These differences together are suggested to be a potential discriminator of CRM from DRM carried by hematite in natural red beds, and of significance for the interpretation of paleomagnetic studies on red beds.

Hematite concentration is an important climatic proxy for environmental (climatic) studies of soils and sediments. However, the accurate quantification of naturally occurring hematite has always been a difficult question, especially for those areas with lower hematite concentrations. Diffuse reflectance spectroscopy (DRS) is an effective method for hematite identification and quantification with lower detection limits. In this study, we synthesized a set of samples with well-determined concentrations to explore the exact detectable range of hematite and propose the most effective transfer function between the DRS proxy and hematite concentration. In addition, natural sediments from Inland Asia and the Western Pacific Ocean were used to further test the feasibility of the new transfer function. Results show that the lowest DRS detection limit for hematite could reach ~0.00078%, but is affected by the natural matrix. We also find that the second derivative of the Kubelka–Munk (K–M) function is monotonically correlated with the hematite concentration (0.00078%–100%), but ambiguities exist for the first derivative. Therefore, the second derivative of the K–M function is highly suggested for the hematite quantification, especially when concentration exhibits a wide range of variations. This study provides important references for the application of hematite proxy and promotes the popularization and development of the DRS method. [ABSTRACT FROM AUTHOR]

Hematite carries magnetic signals of interest in tectonic, paleoclimatic, paleomagnetic, and planetary studies. First‐order reversal curve (FORC) diagrams have become an important tool for assessing the domain state of, and magnetostatic interactions among, magnetic particles in such studies. We present here FORC diagrams for diverse hematite samples, which provide a catalog for comparison with other studies and explain key features observed for hematite. Ridge‐type signatures typical of uniaxial single‐domain particle assemblages and "kidney‐shaped" FORC signatures, and combinations of these responses, occur commonly in natural and synthetic hematite. Asymmetric features that arise from the triaxial basal plane anisotropy of hematite contribute to vertical spreading in kidney‐shaped FORC distributions and are intrinsic responses even for magnetostatically noninteracting particles. The dominant FORC distribution type in a sample (ridge, kidney‐shaped, or mixture) depends on the balance between uniaxial/triaxial switching. The identified signals explain magnetization switching and anisotropy features that are intrinsic to the magnetic properties of hematite and other materials with multiaxial magnetic anisotropy. Plain Language Summary: First‐order reversal curve (FORC) diagrams have become a standard method for identifying the domain state of magnetic materials. It has recently been demonstrated that the magnetic anisotropy type can also be identified using FORC diagrams. The magnetic domain state and anisotropy type of a magnetic mineral control the fidelity of recording of magnetic information, so identifying these features is fundamental to paleomagnetic, rock magnetic, and environmental magnetic studies. Hematite is a naturally abundant magnetic mineral that is encountered commonly in such studies, so it is important to have extensive FORC reference data for this mineral. We present the most comprehensive available FORC study of diverse natural and synthetic hematite samples, including numerical simulation results, to seek to explain key domain state and anisotropy signals for hematite. Key Points: FORC diagrams are presented for diverse natural and synthetic hematite samples to demonstrate domain state and anisotropy signaturesKidney‐shaped FORC distributions are indicative of multiaxial anisotropy and central ridge‐type distributions reflect uniaxial anisotropyThe dominant FORC distribution type depends on the balance between uniaxial and triaxial switching mechanisms [ABSTRACT FROM AUTHOR]

The North Pacific Ocean (NPO) has received abundant aeolian dust transported by westerlies from the Asian inland. The aeolian components preserved in NPO sediments record information on palaeoclimatic and palaeoenvironmental changes in Asian source areas at different timescales. Previous studies have systematically investigated the source–sink effect of aeolian dust using the sedimentology, geochemistry, isotope and magnetic methods. In this study, we focus more on recent developments of aeolian signals in NPO sediments obtained by magnetic approaches. Generally, aeolian components contain a mixture of magnetite, maghemite, hematite and goethite of different origins. Magnetic properties (mineral category, concentration and particle size) of these minerals are modulated primarily by climatic/environmental conditions in source areas and sorting effects during the transportation process. Compared with the other methods, magnetic measurements have the advantages of non-sample destruction, high sensitivity and high efficiency. Finally, future studies are also discussed to address the importance of magnetism for tracing the dynamic transportation processes of the aeolian dust. [ABSTRACT FROM AUTHOR]

Abstract: Eastern Mediterranean sapropels are organic-rich layers whose formation was triggered during boreal summer insolation maxima (precession minima). Sapropel formation is associated with increased precipitation and runoff from Europe and North Africa, although the relative influence of high- and low-latitude climate mechanisms is not fully resolved. Likewise, it remains uncertain how centennial-scale interruptions in conditions that give rise to sapropels may reflect atmospheric circulation changes in low latitudes. We present magnetic, geochemical, and diffuse reflectance spectroscopy data from three sapropels that are representative of glacial (S6) and interglacial (S1, S5) conditions in the eastern Mediterranean Sea to assess environmental changes associated with sapropel formation. The extent of diagenetic magnetite dissolution and authigenic formation of fine-grained greigite in these sapropels and their underlying dissolution intervals are linked to enhanced sulphidic conditions during sapropel formation. Aeolian hematite and goethite are largely unaffected by this reductive dissolution except within the interglacial sapropel S5 and its underlying dissolution interval, which formed under relatively stronger sulphidic conditions. Nevertheless, low hematite contents indicate that the three studied sapropels accumulated under reduced aeolian dust inputs in response to an intensified African monsoon, which resulted in expansion of savannah landscapes into NE Saharan dust source areas. Small variations in goethite contents across sapropels indicate additional aeolian entrainment of goethite that formed under previous wet phases in the NE Sahara or in subtropical savannahs located further south. We link short-lived dust abundance peaks within sapropels S1 and S6 to centennial-scale periods of enhanced bottom-water circulation reported previously for these sapropels. Although these sapropel interruptions are driven by high-latitude cooling events, our results indicate that such centennial-scale episodes of atmospheric reorganization affected not only the eastern Mediterranean northern borderlands, but also subtropical North Africa. Overall, our results point to a dominant low-latitude forcing on sapropel formation via boreal summer insolation maxima and intensification of the African monsoon. [Copyright &y& Elsevier]

Abstract: Several sets of micro-sized synthetic hematite samples were systematically studied by infrared absorption spectroscopy, X-ray diffraction, thermogravimetric and low-temperature magnetic analysis. These hematite samples could be divided into three distinctive morphologies of pseudocubic (G ⊥ >0.17, the shape factor), platy, and rhombohedral (both G ⊥ <0.14). Pseudocubic hematite contains more vacancies than other morphologies due to the presence of OH in the crystal. Accordingly, the pseudocubic hematite exhibits a wider Morin transition and higher magnetic coercivity. Both the low temperature changes in coercivity and first-order reversal curve indicate that the pseudocubic hematite is dominated by a combination of magnetocrystalline and magnetoelastic anisotropies. In contrast, because of the low vacancy content, hematite with non-pseudocubic morphologies is governed primarily by the magnetocrystalline anisotropy. Such a simple morphology dependence successfully links microstructure to the bulk magnetic properties of micron-sized hematite and hence improve our understanding of the magnetism of hematite and its possible application in elucidating the origin of hematite in natural environments. [ABSTRACT FROM AUTHOR]

Variations in the low-field magnetic susceptibility of the wind-blown Chinese Loess Plateau (CLP) loess-paleosol sequences reflect changes in the global paleoclimate on different time scales. Magnetic enhancement in paleosols has been ascribed to the neoformation of fine-grained maghemite; however, little is known about the pathway through which this mineral was formed in the CLP paleosols, its relationships with the other pedogenic Fe oxides (viz. hematite and goethite), and the pedoclimatic significance of such relationships. In this work, we characterized various magnetic, chemical, and mineralogical properties of the loess-paleosol units at depths from about 23 to 55 m in the Upper Luochuan section, central CLP. The concentration of pedogenic hematite (ΔHm) and the frequency-dependent magnetic susceptibility (χFD), which is used as a proxy for the concentration of fine-grained pedogenic maghemite, were found to be linearly correlated (R² = 0.825, P < 0.001). This supports the idea that these two minerals were formed concomitantly during pedogenesis, which is consistent with the results of previous in vitro experiments showing that the ferrihydrite → maghemite → hematite transformation takes place under aerobic conditions. By contrast, the concentration of pedogenic goethite (ΔGt) was only weakly correlated with either χFD or ΔHm, which suggests that goethite formed through an alternative pathway. The paleosols above 40 m (S4, S5, corresponding to marine isotope stages 9 and 11, respectively) exhibit a higher degree of weathering and higher ΔHm/(ΔHm + ΔGt) ratio than those below such a depth (S6-S8). This was ascribed to differences in paleoclimatic conditions, which are moister and warmer in the former paleosols than in the latter, rather than to differences in pedogenesis duration. [ABSTRACT FROM AUTHOR]

The evolution of aridity and its forcing mechanisms in the Arabian Peninsula are of significance for better understanding the environmental response of the region to global change and the impact of past climate shifts in human evolution. Paleoclimatic reconstructions for the region are predominantly derived from fragmentary terrestrial records. Here, we present a detailed environmental magnetic record for marine sediments from the western equatorial Indian Ocean, which constitute the terminal sink for eolian material originated from the Arabian Peninsula. The dominant magnetic minerals identified in the studied sediments include magnetite, maghemite, and hematite. The hard isothermal remanent magnetization (HIRM), widely used as a proxy for the abundances of the high coercivity mineral hematite, has been used to track variations in the concentration of Arabian dust over the last 200 kyr. Dust concentrations are lowest during interglacial periods of maximum boreal summer isolation, and coincide with intervals during which lake and fluvial systems developed and speleothems formed across most of southern and central Arabia. Our results point to a sharp decrease in the production of dust due to the expansion of vegetation cover up to 30°N during these periods. West African Monsoon appears to be the major source of increased rainfall across Arabia, while the Indian Summer monsoon likely contributed to enhance rainfall in the south and southeastern parts of the peninsula. Variation in the supply of Arabian dust were also driven by changes in high-latitude ice volume and atmospheric CO 2 , as indicated by highest dust concentration found during glacial periods. We interpret that the expansion of high-latitude ice sheets increased the global meridional temperature gradient, weakened the monsoon system, and resulted in a more southerly, colder and descending subtropical westerly jet, leading to the retreat of vegetation cover and the enhanced production of dust from the Arabian deserts. Our results highlight a link between the production of Arabian dust and orbital-scale climate variability, including both monsoon and northern hemisphere ice sheets dynamics. • A 200-ka record of Arabian dust supply into the equatorial Indian Ocean was reconstructed based on hematite content. • Dust concentrations are lowest at the peak of the interglacial periods, and highest during glacial periods. • The aridity in Arabia was controlled by summer monsoon variability and northern hemisphere ice-sheet dynamics. [ABSTRACT FROM AUTHOR]

Highlights: [•] The effect of hydrostatic pressure on the magnetic remanence was studied on SD Al-hematites. [•] A pressure of 1.44GPa resulted in 50% reduction of the isothermal remanent magnetization. [•] Demagnetization degree is controlled by B cr, crystal structure and domain state of minerals. [•] Al substitution influences the resistance to pressure by generating defects in magnetic lattices. [Copyright &y& Elsevier]

Abstract: Vermiculated red soil, the main pedostratigraphical unit of the red soil sequences widespread in subtropical and tropical China, has experienced a higher degree of pedogenesis than loess/paleosols and the underlying red clays in the Loess Plateau of northern China. Previous studies suggested that magnetic properties of the vermiculated units of the red soils are significantly degraded compared to those of the non-vermiculated counterparts. The mechanism for this magnetic decrease, however, has not been well determined. In order to resolve this problem, this study has investigated two red soil sequences of different origin in southeast China by integrating environmental and mineral magnetic research. The results show that the magnetic depletion in the vermiculated units was probably caused by both dissolution of magnetic minerals in the white veins and transformation of highly magnetic maghemite to weakly magnetic hematite. It appears that stronger pedogenesis favors the transformation of more maghemite to hematite and magnetic properties associated with the relative concentration of maghemite to hematite can be used as climate indicator in the vermiculated red soils. The variation trends of the magnetic parameters of the vermiculated red soils prior to 0.85–0.9Ma suggest a strengthening of the East Asian summer monsoon. The reverse trend of these parameters after 0.85–0.9Ma may correlate with a significant strengthening of the East Asian winter monsoon since 0.85Ma. [Copyright &y& Elsevier]