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The lack of suitable indicators of changes in such as sea‐level and circulation has been a major limit to paleoenvironmental and paleoceanographic investigations in continental shelf regions. This paper presents an environmental magnetic study by comparing two late‐Quaternary sediment cores (DH02 and DH03) from the outer shelf of the East China Sea (ECS). Late and early Marine Isotope Stage (MIS) 3 sediments were deposited in a prodelta under cold coastal currents and an open‐shelf with the Taiwan Warm Current and upwelling. The dominant iron‐bearing minerals of the late and early MIS 3 sediments are authigenic greigite (Fe3S4) and pyrite (FeS2), respectively, which were assumed to be formed nearly syndepositionally. The overlying sediments, however, are magnetically dominated by detrital magnetite. This pattern corresponds well to the temporal changes in sea‐level over this period. The widespread occurrence of greigite in the late MIS 3 sediments can also be used for future stratigraphic division and correlation in the ECS. Additionally, compared to microfossil assemblages, rock magnetic parameters based on greigite may be more sensitive to environmental changes on continental shelves. Furthermore, the inter‐borehole spatial comparisons imply not only a sedimentary hiatus/erosion of at least 30‐m thickness in core DH02, most probably during the Last Glacial Maximum, but also that core DH02 was in a more reductive environment than core DH03 during late MIS 3. The findings highlight the potential of authigenic greigite as an indicator of spatiotemporal changes in paleoenvironmental and paleoceanographic conditions on the continental shelf at orbital or even suborbital timescales. Plain Language Summary: Environmental and oceanographic conditions on continental shelves are vital for sedimentation processes and human settlements, but the reconstruction of their evolution is hindered by the lack of effective indicators. Recent studies have indicated that syndepositional formation of greigite, a precursor to pyrite, is highly likely in shallow‐water environments with rapid sedimentation. Here, two sediment cores meridionally traversing the outer shelf of the East China Sea were investigated. The magnetic mineral assemblage and sedimentary environment exhibited almost identical change patterns, with pyrite, greigite, and magnetite as the major magnetic minerals of the early and late Marine Isotope Stage 3, and Last Glacial Maximum/last deglacial deposits, respectively. This agreeably reflects the temporal fluctuations in sea‐level and ocean circulation. By comparing the two cores, the spatial characteristics of stratigraphy and redox conditions could be determined clearly. This study thus provides a good reference for paleoenvironmental and paleoceanographic research based on authigenic greigite in continental shelf sediments. Key Points: Greigite was further confirmed as the major magnetic mineral of the late Marine Isotope Stage 3 deposits widespread on the outer shelf of the East China SeaSpatiotemporal changes in environmental and oceanographic conditions on the continental shelf are well revealed by greigite‐based parametersAuthigenic greigite can serve as a sensitive indicator for paleoenvironmental and paleoceanographic studies in continental shelf regions [ABSTRACT FROM AUTHOR]

The anhysteretic remanent magnetization (ARM) is an important magnetic parameter for quantifying the concentration of "fine", mostly sub‐micron ferrimagnetic particles in rocks and sediments. A sound understanding of ARM carriers is needed to interpret magnetic and environmental information in sedimentary archives. This is often not achieved for marine sediments, which routinely contain several magnetic mineral components. Here, we analyze marine sediment records over different timescales for four sediment cores from the Eastern Pacific Ocean (Hole 1218A), the Antarctic margin (core P4‐1), the Arctic Ocean (core ARC5‐ICE4), and the South China Sea (core L07), using coercivity spectra analyses of ARM, first‐order reversal curve principal component analysis (FORC‐PCA), transmission electron microscopy (TEM), and unmixing of isothermal remanent magnetization (IRM) curves. By combining rock magnetic and TEM results, we find that the ARM is mainly carried by non‐interacting single domain (SD) biogenic magnetite (magnetofossils) in the studied sediments over all timescales. Low‐coercivity magnetic particles (detrital and fine‐grained extracellular magnetite) also contribute to the bulk ARM. Variable magnetofossil chain structures are detected and have a significant influence on ARM data interpretation. Quantitative FORC‐PCA endmember analyses based on quantile contours and coercivity distributions provide valuable information on the nature of endmembers and improve ARM data interpretation. We provide an integrated strategy to help reduce interpretational ambiguities related to ARM in future studies. Plain Language Summary: The anhysteretic remanent magnetization (ARM) is an important parameter that is used to assess information about Earth's magnetic field variations and environmental changes. To interpret ARM signals robustly, we must understand the sedimentary magnetic minerals that control ARM changes. However, the origin of ARM changes in marine sediments is not well understood. Here, we analyze marine sediment records from the Eastern Pacific Ocean, the Antarctic margin, the Arctic Ocean, and the South China Sea to identify the carriers of ARM in the respective sediments. Our results indicate that all of the sediments contain abundant remains of magnetotactic bacteria (known as magnetofossils). Magnetofossil abundance variations dominate ARM on both 10,000 to million‐year timescales. Part of the bulk ARM is also contributed by other low‐coercivity magnetite particles. Furthermore, ARM is influenced significantly by magnetofossil chain structure variations. Our results have important implications for future studies involving use of ARM in magnetofossil‐rich sediments. Key Points: Integrated component analyses are presented for four sediment cores to identify magnetic particle sourcesMagnetofossils are the predominant anhysteretic remanent magnetization (ARM) carrier with detrital and extracellular magnetite also contributing to the ARMQuantitative analyses provide valuable insights into the nature of components to improve ARM data interpretation of marine sediments [ABSTRACT FROM AUTHOR]

The relationships among the abundance of magnetofossils, the ensuing magnetic properties, and the controlling paleoenvironmental factors in marine sediments remain broadly unexplored. Here, we identify magnetofossils in core XB1 from the northwestern South China Sea (SCS) since end of the Last Glacial. Using rock magnetic and electron microscopic data, we propose a model that links the anisotropy of magnetic susceptibility fabric and the abundance of magnetofossils. The magnetofossil concentration in sediments increases significantly during the 14.7–4.7 ka period, which in turns leads to inverse magnetic fabrics and near‐horizontal of the minimum magnetic susceptibility axes. Further, we show that the abundance of magnetofossils is linked to paleoenvironmental changes in the northwestern SCS. The production and preservation of magnetofossils during the 14.7–4.7 ka period are promoted by an intensified East Asian summer monsoon and sluggish deep‐water ventilation, while the paucity of magnetofossils after 4.7 ka is attributed to high oxygen content. Plain Language Summary: The anisotropy of magnetic susceptibility (AMS) is a useful magnetic property of rocks for a wide range of scientific questions. Typically, the sedimentary AMS fabric carried by coarser‐grained magnetic particles (i.e., multi‐domain, MD) exhibits an oblate shape with a horizontal maximum magnetic susceptibility axis. However, single‐domain (SD) magnetic particles (e.g., nano‐sized magnetofossil magnetites) complicate the AMS fabric, through their intrinsic inverse magnetic fabric with the minimum magnetic susceptibility axis aligned along the horizontal plane. For natural samples, quantification of the contributions of SD and MD particles to AMS is essential for accurate interpretation of the AMS. This paper puts forward a novel approach that links the AMS and the abundance of magnetofossils. By applying this new method to sediments from the South China Sea, we found that the inverse magnetic fabrics in the studied sediments are associated with high magnetofossils concentration, which can be affected by various factors including the East Asian monsoon, nutrient supply or deep‐water ventilation. Key Points: Values of the minimum susceptibility axis inclination are associated with the abundance of magnetofossilsThe magnetofossils can be quantified by the proportion of inverse component in sedimentsThe strong deep‐water ventilation hinders the production and preservation of magnetofossils in the South China Sea [ABSTRACT FROM AUTHOR]

Magnetotactic bacteria (MTB) require iron and organic carbon for biomineralization, which can be supplied by dust inputs that stimulate primary productivity in iron‐limited oceans. However, MTB activity and its relationship to Asian dust inputs and primary productivity are not well known in the North Pacific Ocean. Here, we present an eight‐million‐year record of magnetofossil abundance, Asian dust flux, and primary productivity from North Pacific sediments (Ocean Drilling Program Hole 885A). Our results demonstrate that Asian dust delivered iron to stimulate primary productivity and then boost MTB biomineralization in the late Miocene. Since the late Pliocene, dust inputs and primary productivity appear to have been decoupled, with MTB activity controlled mainly by primary productivity rather than dust inputs. Equant magnetofossil abundances co‐vary with eolian flux, and may provide here a proxy for dust inputs. This work provides perspectives on relationship between MTB activity, dust inputs, and primary productivity on million‐year timescales. Plain Language Summary: Magnetotactic bacteria (MTB) biomineralize chains of characteristic magnetically single domain magnetite (or greigite) crystals in their cells and can thrive in deep‐sea ecosystems and absorb iron for biomineralization. Magnetic particles that are preserved in sediments after MTB death are known as magnetofossils. Eolian dust supplied from continents is a major iron source in pelagic environments. Hence, magnetofossils can be important indicators of the impacts of dust‐supplied iron on deep‐sea ecosystems. A better understanding of the relationship between MTB activity, dust inputs, and primary productivity is needed for reliable magnetofossil interpretation. However, this relationship is not well determined in the North Pacific Ocean. By integrating rock magnetic, transmission electron microscope, and diffuse reflectance spectroscopy observations, we present an eight‐million‐year record of magnetofossil variations, Asian eolian inputs, and primary productivity from the North Pacific Ocean. Our results demonstrate that Asian dust inputs appear to have promoted MTB activity by stimulating surface primary productivity in the late Miocene. Since the late Pliocene, MTB activity was controlled mainly by primary productivity, with decoupling of the relationship between dust inputs and primary productivity. Moreover, equant magnetofossil abundances vary with eolian flux and provide a proxy for dust inputs in these marine sediments. Key Points: Asian dust inputs promoted magnetotactic bacterial activity by providing iron to stimulate primary productivity in late MioceneMagnetofossil production was controlled mainly by primary productivity changes rather than dust inputs since late PlioceneEquant magnetofossil abundances co‐vary with dust inputs and may be regarded here as a dust indicator [ABSTRACT FROM AUTHOR]

Abstract: To accurately decode the paleoclimatic and paleoenvironmental significance of magnetic properties in Chinese loess/paleosols, neoformation of magnetic iron oxides via pedogenesis as well as the relationship between ferrimagnetic (maghemite and magnetite) and antiferromagnetic minerals (hematite and goethite) of both detrital and pedogenic origins should be well determined. To resolve this problem, magnetic iron oxides from a contiguous loess–paleosol sequence (lower part of the loess unit L1, paleosol unit S1, and the upper part of L2) at Luochuan, Center of the Chinese Loess Plateau, were well characterized and quantified by using an integrated approach including chemical dissolution, diffuse reflectance spectroscopy and rock magnetism methods. Results showed that these magnetic minerals have distinct coercivity and dissolution behavior, which is highly grain size-dependent. More specifically, the paleosol is enriched with nano-sized pedogenic hematites with a medium coercivity of ~130mT, which are also preferentially dissolved in a weak reductive environment. In contrast, the coarse-grained micro-sized detrital hematites with a medium coercivity of ~1T are dominant in loess. In addition, these coarse-grained detrital hematites are more stoichiometric than their pedogenic counterparts as evidenced by the higher band position on the diffuse reflectance spectrum. Goethite can be well distinguished from hematite by its higher coercivity (4–7T) at low temperature (10K). The consistent dissolution behavior of goethite for both loess and paleosol samples in citrate–bicarbonate–dithionite solution indicates that goethite could be less relevant to the pedogenic processes. The reductive dissolution of pedogenic maghemite lags the dissolution of pedogenic hematite and fine-grained goethite, which suggests that average grain size of pedogenic maghemite is relatively larger than those of pedogenic hematite and fine-grained goethite. Detrital magnetite is hard to dissolve and ~50% of its initial remanence survived after dissolution. On the basis of these contrasting behaviors, pedogenic hematite, fine grained goethite and maghemite content was further quantified. Our results suggest that single domain pedogenic hematite was produced after initial formation of pedogenic maghemite. This provides direct evidence for the recently proposed pathway in which ferrihydrite is transformed into a transient maghemite-like phase before its final transformation into hematite. [Copyright &y& Elsevier]

Abstract: Parallel studies on rock magnetic properties, petrology and mineralogy were conducted on 16 eclogite samples from the ZK703 hole and magnetic susceptibilities and densities of 41 eclogite samples with different degree of retrograde metamorphism (from fresh eclogite to fully-retrograded eclogite) from the Chinese Continental Scientific Drilling (CCSD) near the ZK703 hole, located at Donghai, southern Sulu ultrahigh-pressure metamorphic belt, eastern China. Results show: (1) that the high-field slopes obtained from the hysteresis loops (the paramagnetic fraction χ para) and density have a positive correlation with the volume concentration of garnet+omphacite, a typical mineral assemblage used to semi-quantify the degree of retrograde metamorphism. The low-field slopes obtained from hysteresis loops (the ferrimagnetic susceptibility fraction χ ferri), saturation isothermal remanent magnetization M rs and saturation magnetization M s have a positive correlation with the volume concentration of symplectite, a mineral related to retrograded metamorphism. Therefore they could be potential indicators for quickly semi-quantifying the degree of retrograde metamorphism of the eclogite units. (2) The dominant magnetic carriers in retrograded eclogites are magnetite particles in pseudo-single domain grain size region. (3) The P–T conditions during the retrograde (decompressional) process could first increase the concentration of magnetite, which can reach up to 3% for extensively retrograded eclogite and then was dissolved for fully-retrograded eclogite. Therefore, change in the magnetite contents during the retrograde process is the major factor controlling the magnetism of retrograde eclogites. [Copyright &y& Elsevier]

Abstract: Magnetic parameters (volume-specific susceptibility κ, and hysteresis parameters and ratios) of 47 samples, collected from an oil-producing well (M36) and a dry well (M46) from the oil-bearing II-You Formation of Paleogene Xingouzui Group in the Mawangmiao Oil Field in China, were measured to address the secondary alteration of iron-bearing minerals associated with hydrocarbon migration. Our results indicated that both κ and magnetization (saturation magnetization J s and saturation isothermal remanent magnetization J rs) of oil-bearing formation have been dramatically enhanced. Further grain size estimation reveals that the background samples (samples both in M46 and outside the oil-bearing formation in M36) contain coarser-grained magnetic particles (∼30μm) of detrital origin. In contrast, the alteration of hydrocarbon produces finer-grained (∼25nm) magnetic particles. The new constraints on grain sizes and its origin of the hydrocarbon-related magnetic particles improve our understanding of the mechanism of formation of these secondary finer-grained particles, even though the precise nature of this process is still unknown. [Copyright &y& Elsevier]

In order to isolate magnetic signals carried by single-domain (SD) ferrimagnetic (FM) minerals from multi-domain (MD) FM minerals, we developed a few parameters using partial anhysteretic remanent magnetization (pARM). Because MD fraction contains only soft (easy to be demagnetized) coercivity spectrum, pARM(>20 mT) was sensitive in eliminating MD contributions. Ratio of pARM(5 mT, 10 mT)/pARM(0, 5 mT) is useful in quantifying a relative abundance of mass ratio between SD and MD fractions. These new proxies can quickly characterize the details of grain size distribution of magnetic minerals in paleoclimatic and paleomagnetic studies. [ABSTRACT FROM AUTHOR]

Abstract: The hominoid fossils found in the Yuanmou Basin, southwestern China, are among the key fossils for understanding the evolution of early hominoids in eastern Asia and their relationship with coeval hominoids in Europe and Africa. However, their exact ages have not yet been well determined. We provide a new high-resolution magnetostratigraphy for the Zhupeng profile, which is one of the fossil-bearing type sections in the region. Based on magnetostratigraphy of 227 remanent directions, together with the age of micromammalian fauna, indicate that the hominoid-bearing layer is dated within polarity chron 3Br 2r or 3Br 1r, i.e. within the interval 7.43–7.38 Ma or 7.34–7.17 Ma. This unambiguously indicates that the Yuanmou hominoid has a late Miocene age, which therefore makes it the youngest hominoid found in Eurasia. It is possible that the Yuanmou Basin provided a refuge for hominoids during a time of major environmental changes. [Copyright &y& Elsevier]

It has been proposed that paleoclimatic changes and tectonic events strongly affect hominoid evolution. The Zhupeng section at Yuanmou Basin, southwestern China, with the hominoid-bearing fluvial-lacustrine sediments, is an ideal natural laboratory to test this hypothesis. This study provided an integrated magnetic study, including low-frequency susceptibility, the anisotropy of magnetic susceptibility, temperature-dependent susceptibility, hysteresis loops, isothermal remanent magnetization, and anhysteretic remanent magnetization, on samples from the Baozidongqing section at Zhupeng. Results show that the dominant magnetic carrier is hematite, with minor amount of magnetite. Both the composition and concentration of magnetic minerals strongly correlate with the lithostratigraphy. At least eight short-term events defined by higher concentrations of magnetite were identified. These short events reflect that the subtropical dry-hot climate is similar to today's climate. Basing on the lithostratigraphic and rock magnetic results, we suggest that Yuanmou Basin was in a steady deposition environment from about 11 to 7 Ma, which is characterized by alternating of long-term torrid-humid climate and short-term dry-hot climate. Our study provides invaluable environment information for understanding the climate shift and the relationship between paleoenvironment and hominoid evolution in southwestern China during late Miocene. [ABSTRACT FROM AUTHOR]

Magnetic properties of rocks are crucial for paleo-, rock-, environmental-magnetism, and magnetic material sciences. Conventional rock magnetometers deal with bulk properties of samples, whereas scanning microscope can map the distribution of remanent magnetization. In this study, a new scanning microscope based on a low-temperature DC superconducting quantum interference device (SQUID) equipped with an in-situ magnetization/demagnetization device was developed. To realize the combination of sensitive instrument as SQUID with high magnetizing/demagnetizing fields, the pick-up coil, the magnetization/demagnetization coils and the measurement mode of the system were optimized. The new microscope has a field sensitivity of 250 pT/√Hz at a coil-to-sample spacing of ∼350 µm, and high magnetization (0–1 T)/ demagnetization (0–300 mT, 400 Hz) functions. With this microscope, isothermal remanent magnetization (IRM) acquisition and the according alternating field (AF) demagnetization curves can be obtained for each point without transferring samples between different procedures, which could result in position deviation, waste of time, and other interferences. The newly-designed SQUID microscope, thus, can be used to investigate the rock magnetic properties of samples at a micro-area scale, and has a great potential to be an efficient tool in paleomagnetism, rock magnetism, and magnetic material studies. [ABSTRACT FROM AUTHOR]

Abstract: The sediments of Lake Chiemsee, located in the Alpine foreland in Southern Germany, host a variety of magnetotactic bacteria (MTB), which contain intracellular crystals of magnetite arranged in linear chains. To detect bacterial magnetite in the carbonate-dominated surface sediments and further quantify its contribution to the magnetic signal of the sediments, we conducted detailed rock magnetic measurements as well as complimentary non-magnetic analyses (electron microscopy, powder X-ray diffraction, and sediment pore-water analysis). Our results demonstrate that biogenic single-domain magnetite (characterized by bullet- and truncated hexagonal prismatic shapes) is the dominant ferrimagnetic component in the topmost few centimetres of the sediment. The changes of magnetic properties with depth are due to the occurrence of live MTB and the downward increasing dissolution of biogenic magnetite. Moreover, the ratios of remanence loss on warming through the Verwey transition after field cooling and zero-field cooling of saturation isothermal remanence (δ FC/δ ZFC) were determined as 1.47 and 1.25 for freeze-dried and air-dried sediment samples containing MTB, respectively. These low ratios suggest that the bacterial magnetite chains were disrupted to a large extent and/or that the bacterial magnetosomes might have undergone partial low-temperature oxidation. It is proposed that although rock magnetic measurements are suitable for quantifying the contribution of fine-grained particles to the overall magnetic signal of sediments, complementary non-magnetic methods are essential to unambiguously identify its bacterial origin. [Copyright &y& Elsevier]