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The observed variations in intense geomagnetic flux lobes have a significant impact on regional geomagnetic fields, and produce unique geomagnetic characteristics. To investigate the time‐dependent effect of the Siberian flux lobe on the geomagnetic field in East Asia, we reconstructed a stacked full‐vector paleomagnetic secular variation record (since 2000 BCE) from three sediment cores collected in the Bohai Sea, China. Age models of the studied cores were established through a combination of radiocarbon 14C dating and inter‐profile correlation of mass‐normalized magnetic susceptibility. Rock magnetic results indicate that fine‐grained magnetite is the primary remanent carrier. We found that the paleomagnetic field records in the Bohai Sea correspond more closely with those from the middle latitudes, rather than the low latitudes. This was primarily attributed to the variation of the Siberian flux lobe. This study provides a better understanding of the contrasting patterns of regional geomagnetic fields in East Asia, and emphasizes the significant role played by the Siberian flux intensity lobe in shaping their formation. Plain Language Summary: The Earth's magnetic field is in a constant state of flux, both temporally and spatially, and plays a crucial role in protecting our planet from solar wind and cosmic rays. The long‐term variation of the Earth's magnetic field is closely linked with the magnetic flux patch situated at the core‐mantle boundary. However, our understanding of the evolution of magnetic flux patches has been limited by the use of short‐term instrumental data and discrete archeomagnetic data. In this paper, we reconstruct a ∼4,000‐year high‐resolution full‐vector paleomagnetic record from sediments in the Bohai Sea, China. Our findings reveal a clear contrast in the paleomagnetic variations between the middle and low latitudes of East Asia. We proposed that the Siberian flux intensity lobe likely played an important role in the observed paleomagnetic behavior discrepancy between the low‐ and mid‐latitudes of East Asia. Key Points: A multi‐decadal resolution paleomagnetic record spanning approximately 4,000 years has been derived from sediments in the Bohai Sea, ChinaThe paleomagnetic behavior differs between middle and low latitudes across East Asia, indicating the variation of the Siberian flux lobeThe disparity in inclination between the Chinese and Levantine spikes implies that these spikes arise from separate magnetic flux patches [ABSTRACT FROM AUTHOR]

Continuous paleosecular variations reconstructed from sedimentary archives have remarkably deepened our insights into the dynamics of the Earth's magnetic field as well as the chronological purpose. Nevertheless, to construct reliable sedimentary paleomagnetic records in diagenetic reducing sediments is challenging generally due to the pervasive magnetic mineral diagenesis. The relict magnetic minerals are residuals after diagenesis and probably able to record a depositional remanent magnetization, thus it is worthy to explore their paleomagnetic potentials. For this study, two Black Sea sediment cores covering the past 8 ka were subjected to mineralogical and paleo‐ and rock magnetic analyses. Paramagnetic pyrite framboids are pervasive in the studied sediments deposited under anoxic bottom water conditions in the Black Sea. In addition, relict magnetic minerals of ferrous hemoilmenite, Fe‐Mn and Fe‐Cr spinels, and magnetite inclusions are also present in the studied cores. Compared to the previous published paleomagnetic results from the same cores over the last glacial period (20–30 ka) which are dominated by detrital (titano‐)magnetite particles, the studied relict magnetic mineral samples exhibit a similar behavior in recording the depositional remanent magnetization. Furthermore, the obtained paleosecular variations spanning the past 8 ka reproduce the high intensity patterns observed in the regional archeomagnetic and volcanic datasets. Thus, the successful reconstruction of paleomagnetic secular variations from the anoxic Black Sea sediments greatly extend the application of paleomagnetism in sediments deposited in water with a reducing sub‐surface environments from where paleomagnetic data are generally sparse. Plain Language Summary: The Earth's magnetic field can be recorded by magnetic minerals during their deposition within a water column, and are later preserved in sediments. The geomagnetic field signal may be biased if the magnetic minerals are subsequently altered because of changes in bottom water redox conditions. However, relict magnetic minerals are resistant to ambient environmental changes and thus have great potentials to record reliable paleomagnetic signals. To attest this novel idea, we systematically investigated sediments form the Black Sea under strong anoxic bottom water conditions. Our results demonstrate that the relict magnetic minerals survived in the Black Sea sediments covering the past eight thousand years. By comparison with detrital (titano‐)magnetite samples, we found that relict magnetic mineral samples have similar behavior in recording the geomagnetic field. Moreover, the geomagnetic field variations reconstructed from the Black Sea sediments are comparable with other validated regional datasets for the past eight thousand years. Therefore, our new study significantly extends the application of paleomagnetic methods in a wider range of sedimentary redox conditions. Key Points: Relict minerals of hemoilmenite, Fe‐Mn and Fe‐Cr spinels, and magnetic inclusions are magnetic carriers in studied Black Sea sedimentsThe ability in recording the DRM by relict magnetic minerals are comparable with detrital (titano‐)magnetiteThe obtained paleosecular variations resemble reginal field patterns observed in archeomagnetic and volcanic datasets for the past 8 ka [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]

Thellier-type paleointensity experiments associated with partial thermal remanent magnetization checks have been widely used to determine paleointensity values from volcanic and archaeological media. However, previous studies have revealed that a substantial portion of paleointensity results with positive checks for historical lava samples largely fails to predict known Earth magnetic field intensity values. To determine the fidelity of paleointensity values, conventional Thellier-type paleointensity experiments were performed on Kilauea lava flows that erupted in 1960. The positive partial thermal remanent magnetization checks for our results range from 30.28 ± 1.38 µT to 52.94 ± 1.89 µT. This strongly indicates that conventional paleointensity checks cannot guarantee the fidelity of paleointensity results, especially when the unblocking temperatures for the newly formed magnetic particles are higher than the treatment temperature. Therefore, in this study, to check for thermal alteration during heating, the temperature dependence of the hysteresis parameter measured at room temperature for the thermally treated samples was also measured. Our new results show that nearly all biased paleointensity values correspond to a ratio of the coercivity of remanence to the magnetic coercivity of > 3 and a chemical alteration index > ~ 10%, which indicates the strong effect of the domain state and thermal alteration on the fidelity of the paleointensity results. Our study provides feasible criteria to further improve the fidelity of paleointensity estimations. [ABSTRACT FROM AUTHOR]

North Pacific Ocean sediments are important archives of geomagnetic field and paleoenvironmental evolutions, yet late Quaternary sedimentary sequences in the North Pacific Ocean are less explored because of generally low sedimentation rates and the challenge of dating sediments deposited below the calcite compensation depth. Core NP02 sediments from the North Pacific Ocean contain three visible tephra layers, which are identified as the To‐Of, Spfa‐1, and Kt‐3 tephra, respectively. An age model for core NP02 is established based on tephra correlation and one radiocarbon date, which gives a mean sedimentation rate of ∼11.6 cm/kyr. Magnetic analyses suggest the dominance of two remanence carriers; detrital vortex state and biogenic single domain magnetite. Variable depth offsets between the relative paleointensity (RPI) signals carried by the two remanence carriers adds a negligible age error (<0.5 kyr) to the core NP02 RPI record. Pronounced RPI amplitude variations are observed, and the Rockall and Laschamp excursions are identified in the core NP02 RPI record based on the age model. The fidelity of the core NP02 RPI record is further verified by comparison with other RPI stacks and records, and we demonstrate that the RPI variations can assist in chronology refinement. Plain Language Summary: The North Pacific Ocean is too deep to preserve materials effectively for the oxygen isotope chronology, and sediments deposit slowly in this region. Therefore, dating is a big problem for late Quaternary sedimentary cores. In this study, core NP02 sediments from the North Pacific Ocean are studied. The To‐Of, Spfa‐1, and Kt‐3 tephra layers are identified in core NP02 sediments based on the geochemical features. An age model for this core is reconstructed with the aid of tephra ages and one radiocarbon date. Magnetic analyses suggest that both small biogenic and relatively large detrital magnetite record geomagnetic information, and the two groups of magnetite deposit at about the same time. Based on the age model, core NP02 RPI record preserves the Rockall and Laschamp excursions, and the RPI pattern is consistent with other RPI stacks and records, which verifies core NP02 RPI fidelity. Therefore, we demonstrate that RPI variations is a promising dating tool for North Pacific Ocean sediments. Key Points: We use an age model based on tephra correlation and one radiocarbon date to assess the fidelity of relative paleointensity variationsDetrital vortex state and biogenic single domain magnetite are the dominant remanence carriers, and they introduced <0.5 kyr age errorAssessment of component‐specific magnetic properties should be considered to ensure the reliability of relative paleointensity variations [ABSTRACT FROM AUTHOR]

There are two distinctly different views on Quaternary sedimentation rates in the central Arctic Ocean, namely, that they were on cm/kyr or on mm/kyr scales, largely as a result of divergent interpretations of magnetic reversal stratigraphy. This study provides new evidence to help resolve this controversy in the form of an almost 1‐m‐thick negative polarity interval located at ~1.15 m below the seafloor in a 4.15‐m‐long sediment core from the Lomonosov Ridge, central Arctic Ocean. This thick polarity anomaly was first revealed through alternating field demagnetization of natural remanent magnetization on u‐channel samples. It was subsequently confirmed to be an authentic signal of a syndepositional geomagnetic field by integrating detailed rock magnetic studies, transmission electron microscopy with energy‐dispersive X‐ray spectroscopy analyses of magnetic extracts, and both alternating field and thermal demagnetization of discrete paleomagnetic samples. Here we conclude that it is more reasonable to interpret the reversed interval as a record of the Matuyama Chron rather than a Brunhes‐aged excursion, which yields mean sedimentation rates of ~1.5 mm/kyr and ~0.5 mm/kyr in the Brunhes and Matuyama Chrons, respectively. These estimates are remarkably lower than assumed in most recent studies, but in good agreement with both previous magnetostratigraphic and recent geochemical studies relative to the decay of Be and U‐series isotopes, which might indicate "sediment‐starved" environments at least in some parts of this region. Overall, our results not only imply that chronostratigraphy in the central Arctic Ocean remains open to debate but also warrant further investigations in this microfossil‐barren region. Key Points: The effects of self‐reversed CRM of diagenetic titanomaghemite are probably only regional in the central Arctic sedimentsThe "reversal" interpretation supports "sediment starved" environments at least in some regions of the central Arctic OceanThe chronostratigraphy and usability of paleomagnetism in the central Arctic Ocean remains open to debate and further studies are warranted [ABSTRACT FROM AUTHOR]

Highlights • Palaeomagnetic relative intensity for Chinese loess <130 ka has been constructed at Luochuan and Xifeng sections, respectively. • The PDRM lock-in depths were quantitatively estimated for loess <130 ka and for the middle Pleistocene loess. • A new palaeoclimatic correlation between Chinese loess and deep-sea δ 18 O since the middle Pleistocene was proposed. Abstract The Quaternary Chinese loess is one of the most continuous terrestrial sediments, and preserves excellent palaeoclimate records at different timescales. The conventional land-ocean palaeoclimatic comparison has been established by correlation between magnetic susceptibility of the Chinese loess and deep-sea δ 18 O records during the 1980s. However, such a correlation results in long-standing debates on the lock-in depth of the palaeomagnetic polarity boundaries in Chinese loess. In this study, the lock-in depth of palaeomagnetic signals for loess <130 ka were estimated through correlation between palaeomagnetic relative palaeointensity (RPI) and 10Be RPI curves, and are linearly related with the calculated loess dust accumulation rate. Palaeomagnetic displacement for the geomagnetic polarity boundaries and excursions in the middle Pleistocene loess at Luochuan, Zhaojiachuan and Lingtai sections could be estimated using the linear relation only in decimeter scale (generally <50 cm) which indicates paleomagnetic boundaries in Chinese loess can be used as reliable age control points after moderate adjustments. Based on this as well as a high-resolution loess magnetostratigraphy and grain-size sequence, we propose a new scheme for palaeoclimatic correlation between Chinese loess and marine sediments since the middle Pleistocene. [ABSTRACT FROM AUTHOR]

We investigated sea‐surface magnetic anomaly data for the past 5 Myr in the slow‐to‐intermediate spreading South Atlantic Ridge. After error analysis and signal processes, the sea surface magnetic anomaly profiles were downward continued to 1.5 km below sea level, and then the distance from spreading axis was converted to time by matching the observed anomalies with forward models of known polarity reversals. The stacked anomaly profiles from South Atlantic Ridge exhibit short‐wavelength wiggles superimposed on the first‐order variations of magnetic anomalies. The stacked anomaly profiles were first compared with commonly used geomagnetic palaeointensity data synthesized from sedimentary relative intensity records in the past and were further correlated with other available magnetic profiles derived from Central Indian Ridge, East Pacific Rise, and Pacific‐Antarctic Ridge. We found that short wavelength variations in the South Atlantic Ridge can be ascribed to geomagnetic intensity variations from excursions or short polarity events. Spectral analysis of marine magnetic anomalies records reveals that there is no obvious dominant periodicity for the geomagnetic field behavior during the past 5 Myr. Key Points: A long‐term paleointensity record was constructed from sea surface magnetic anomaly data at a slow‐to‐intermediate spreading SAR for the first timeThe newly stacked paleointensity curves agree well with other coeval relative paleointensity dataWe proposed a time scale based on the Earth's paleointensity from marine composite profile for the past 5 Myr [ABSTRACT FROM AUTHOR]

High-resolution paleomagnetic secular variation (PSV) records bear great information of dynamics processes of the Earth’s geomagnetic field, and can be further used for inter-profile correlation and for dating sediments. However, effects of changes in the depositional environment on PSV records have not been fully determined. This study constructed Holocene PSV records for the gravity piston core (ZSQD34) obtained from the northern South China Sea. Rock magnetic and Transmission Electron Microscope (TEM) results indicate that single (SD) and PSD domain magnetites are the main carrier of the natural remanent magnetization. Comparable to the records derived from the freshwater lakes and the modeling results, we observed that direction curves from these two environments of contrasting salinity content are rather consistent. The direction curves are independent of the constructed salinity. However, the gradual increasing trend of relative intensity since about 5 kyr might be related to the decreasing sea surface salinity. Furthermore, on the centennial and millennial time scale, the relative intensity and salinity show some positive relation, suggesting a potential contribution of salinity to the paleomagnetic relative intensity recording processes. [ABSTRACT FROM AUTHOR]

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 μ T ). 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 μ T , the CRM records the field direction faithfully. However, for field intensities ≲ 40 μ T , 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 ∼ 600 to 680 ° 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. [ABSTRACT FROM AUTHOR]

Abstract: The Huguangyan (HGY) lake, a typical maar lake in the southeast of China, is sensitive to the evolution of the Asian monsoon system, including the East Asian summer monsoon (EASM) and the Indian summer monsoon (ISM). So far, the mechanism of how magnetic properties of the HGY lake sediments respond to climate changes has not been well determined. In this study, we performed a detailed mineral-magnetic analysis on the high-resolution HGY sediments. Results revealed that finer-grained magnetite (superparamagnetic and single domain magnetite mixed with a small amount of goethite and hematite) is the dominant magnetic mineral, which is consistent with the magnetic properties of the red weathered soils collected from the catchments, but differs greatly from that of the aeolian inputs. Therefore, magnetic minerals of the HGY sediments were originated mainly from the lake catchment, rather than the wind-blown dust inputs. Considering the high tephra wall (relative elevation of 10–20m) and great precipitation, surface runoff by rainfall is interpreted as the main process to transport red soils into the lake. The magnetic records of HGY sediments revealed that the summer monsoon was enhanced at the early Holocene and then declined abruptly at the mid-late Holocene (~5.5kyr cal. BP). This pattern strongly mimics the evolution pattern of ISM, but is inconsistent with the EASM. Thus, we suggest that the magnetic records from HGY have the great potential to investigate the evolution of ISM. [Copyright &y& Elsevier]

SUMMARY Widely distributed haematite-bearing red beds are an important source of palaeomagnetic field records. However, unresolved issues regarding remagnetization and inclination shallowing in red beds have questioned the reliability of the palaeomagnetic results obtained from such materials. In this study, we investigated the remagnetization mechanism in red beds from Lower Triassic sandstones in Yunnan Province, southwestern China. Our results indicate that the characteristic remanent magnetizations (ChRMs) of most samples (112/125) are dominated by only one component at temperatures 80-660 °C, with a mean direction of D/ I= 0.9/46.6° ( k= 440.2, α95= 2.1°). The corresponding geomagnetic pole is 89.1°N, 331.7°E ( k= 338.2, A95= 2.4°). This coincides with the present geomagnetic field, which is a strong indication that these ChRMs are remagnetized. Combined rock magnetic and microscope investigations reveal that the remagnetization is due to the acquisition of a chemical remanent magnetization (CRM) carried by authigenic maghemite and haematite. Despite the widespread remagnetization, about 7 per cent of the studied samples still record a magnetization that we consider to be primary that was isolated at high temperatures 610-660 °C, with a mean direction of D/ I= 213.3/18.6° ( k= 16.0, α95= 11.7°). We propose that the CRM overprinting is controlled by the overlapping degree of the unblocking temperature between the CRM carried by the authigenic haematite and the primary remanent magnetization carried by the detrital haematite. Our results further suggest that microscope investigation, rock magnetic proxies for the haematite concentration and susceptibility-temperature curves are useful methods for pre-selecting samples suitable for isolating the primary remanence at this region. The linkage among the palaeomagnetic results, rock magnetic proxies and CRM remagnetization mechanism could be extended to other studies, although the detailed proxy would be different due to specific overprint process. [ABSTRACT FROM AUTHOR]

Abstract: It is commonly accepted that the climate records in Chinese loess can be correlated well with that in marine sediments. However, discrepancies for the stratigraphic position of paleomagnetic polarity reversal boundary seriously restrict an accurate teleconnection between these two archives. For example, the exact stratigraphic position of the whole Matuyama–Brunhes (MB) transitional polarity zone remains uncertain in Chinese loess. In this study, an accurate transitional zone of the MB reversal from the Mangshan profile, southeast margin of the Chinese Loess Plateau (CLP), was statistically determined using multiple subsets of parallel samples. By integrating results from the central CLP, the whole MB transitional zone is consistently located across the pedostratigraphic and climatostratigraphic transitional zones between S8 and L8 over a wide region of the CLP. This conclusion further supports that the paleosol unit S8 should be teleconnected to the marine oxygen isotope stage 19 rather than 21, and thus unambiguously supplies an accurate age control in constructing a new loess time scale. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic enhancement of A and B horizons during soil development is a common phenomenon. To better understand the exact mechanism for the magnetic enhancement, especially the mineral transformation pathways, systematic rock magnetic studies were conducted on a Xeralf chronosequence in northwestern Spain. A positive correlation was found between the average grain size and the concentration of pedogenic maghemite particles with the exception of some A horizons, in which multiple factors seem to influence the nature and concentration of neoformed ferrimagnets. It is argued that the interaction between the positively charged iron oxides and the anionic ligands present in the solution of these acidic soils plays an important role in this respect. The time trends of the average grain size of pedogenic ferrimagnets and the strong correlation between the concentrations of hematite and maghemite in the magnetically enhanced horizons (A and B) are consistent with the hypothesis of a gradual formation of maghemite (later converted into hematite) via a precursor (most likely ferrihydrite). Therefore, this study provides strong evidence for the dynamic evolution of magnetic minerals upon pedogenesis. [Copyright &y& Elsevier]

Abstract: Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama–Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted ≤20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit S8, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences. [Copyright &y& Elsevier]

Magnetosomes of magnetotactic bacteria are of great interest in understanding biomineralization and possible links between organisms and geomagnetic field. Fossil magnetosomes are ubiquitous in marine and lake sediments and may significantly contribute to magnetic signals. In this review, we firstly introduce some characteristics of magneto- tactic bacteria, followed by considering recent progress in magnetosome formation, magnetic measurements, and identification of bacterial magnetites in bulk sediments as well as their paleoenvironmental implications. Finally, we briefly discuss potential future breakthroughs in magnetosome studies and its applications. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution proxy records from Tibetan Plateau are essential to understand the past climatic and environmental changes. In this work, we conducted systematic environmental magnetic studies on lacustrine sediments from Lake Pumoyum Co, southern Tibet, spanning from the last deglaciation (~19ka) to present. We correlated the magnetic proxies in the lake core with environmental changes. The magnetic proxies from the 3units were studied. Both Unit 3 (19–13.2ka) and Unit 2 (13.2–8.4ka) contain evidences for an anoxic or sub-anoxic environment. Fine-grained greigite dominates the bulk magnetic properties of Unit 3, while both magnetite and greigite coexist in Unit 2. Overall, this reveals a transition from an anoxic environment to an oxic environment. In contrast, Unit 1 (8.4ka–present) spans the Holocene when the lake environment was oxic. There is an increasing trend in the concentration of magnetic minerals, which reflects an increasing transport of magnetic minerals from the catchment into the lake due to the increased post-glacial melt-water flow. Although different mechanisms affect the magnetic assemblage and associated magnetic properties in the different units, our results suggest that the magnetic proxies of the local lake environment on southern Tibet are sensitive to environmental changes. [Copyright &y& Elsevier]

Abstract: Magnetosomes produced by magnetotactic bacteria are of great interest for understanding bacterial biomineralization along with sedimentary magnetism and environmental magnetism. One of the most intriguing species, Magnetobacterium bavaricum can synthesize hundreds of bullet-shaped magnetite magnetosomes per cell, which contribute significantly to magnetic properties of sediments. However, the biomineralization mechanism and magnetic properties of such magnetosomes remain unknown. In this paper, we have conducted a comprehensive study of the crystallography and magnetic properties of bullet-shaped magnetosomes formed by uncultivated giant rod magnetotactic bacteria (referred to as MYR-1), recently discovered in Lake Miyun (Beijing, China). Transmission electron microscopy observations reveal that each MYR-1 cell contains hundreds of bullet-shaped magnetite magnetosomes, which are arranged into 3 - 5 braid-like bundles of chains. The formation of the bullet-shaped magnetosomes can be divided into two stages: initial isotropic growth (up to ∼ 20nm) followed by elongation along the [100] direction, which is unusual compared with the expected [111] magnetic easy axis. Although the [100] orientation is the hard axis of the face-centered cubic magnetite, the shape anisotropy of bullet-shaped magnetosomes and intra-bundle magnetostatic interactions confine the magnetization direction of the chain along the long axis of the cell/bundle. Due to each bundle of magnetosome chains effectively behaving as an elongated single domain particle, the MYR-1 cells show high coercivity, weak intra-bundle magnetostatic interaction, and high δ-ratio. These results provide new insights into the biomineralization process and magnetic properties of bullet-shaped magnetite magnetosomes. [Copyright &y& Elsevier]

Do non-dipole geomagnetic field behaviors persistently exist in the subarctic Pacific Ocean over the past 140 ka? Paleomagnetic records from globally distributed locations are essential for fully understanding geomagnetic field variations, particularly non-dipole field fluctuations [1]. The weak field intensities during ~64-70 and ~15-17 ka indicate decay of the dipole field and probably growing dominance of non-dipole fields [13]. Dipole field dominance, extremely high- and low- latitude flux patches, and weak Pacific hemisphere variations are robust field characteristics that have occurred over the past few hundred years [15]. [Extracted from the article]

Abstract: Previous rock magnetic investigations of oceanic basement samples have been mainly restricted by single drill holes or dredge collections. In this study, we present new and published data on the rock magnetic properties of igneous rock samples recovered during seven Ocean Drilling Program (ODP) legs in the Atlantic, Pacific, and the southern Indian oceans. The recovered igneous rocks from these legs are mainly basaltic flows (both submarine and subaerial), diabase sills, and serpentinized peridotites with approximate ages of 140,000 years, 10–12, 34, 69, 100–108, and 110–121Ma, which offers an opportunity to investigate magnetic properties and mineral changes attending alteration of basement rocks over a wide range of ages from variable tectonic settings. Titanomagnetite and titanomaghemite are commonly present in igneous rock samples of the Ontong Java Plateau, the Kerguelen Plateau/Broken Ridge, the eastern Equatorial Pacific, the middle Atlantic ridge (Trans-Atlantic Geotraverse area), and the Newfoundland-Iberia rifted margins. For basaltic rock samples, three general groups (A, B, and C, respectively) can be divided in terms of rock magnetic properties. Samples from group A have a single phase of Ti-poor titanomagnetite with Curie temperatures ranging between 480 and 580°C and exhibit a strong Verwey transition in the vicinity of 110K. Basalts from this group are most likely good paleomagnetic recorders and probably have preserved original and stable magnetic remanences. Group B is mainly observed in pillow lavas and is characterized by a Curie temperature of 260–280°C, which is typical of low-temperature oxidized titanomaghemite or titanium rich titanomagnetite. The low-temperature curves for group B do not show the Verwey transition. Group C has more than one Curie temperature, which suggests the presence of multiple magnetic phases. The thermomagnetic signature indicates the inversion of titanomaghemite to a strongly magnetized magnetite, as shown by the irreversible cooling curves. For the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rifted margins, results from low-temperature measurements show that (titano)magnetites are present in the dark-colored peridotites, with a strong Verwey transition in the vicinity of 110K, and with field- and frequency-dependent susceptibility curves that resemble those of synthetic TM0. In contrast to the magnetic properties observed from the dark-colored peridotites, the low-temperature curves for the yellow-brown-colored peridotites did not show any Verwey transition. Thermomagnetic analysis also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at about 420°C, probably maghemite. The magnetic signatures of the serpentinized peridotites recovered from both sides of the Newfoundland-Iberia rift appear not in conflict with the notion that conjugate margins will have generally similar crustal structure and evolution history. Our rock magnetic data indicate that the differences in the rock magnetic properties of basaltic rocks are mainly a function of mineralogy and alteration. There is no apparent coincidence between the age of the rocks and the degree of low-temperature alteration, as suggested by the presence of nearly unoxidized titanomagnetite in the Cretaceous aged basalts and by an examination of Curie temperature versus sample age that does not unambiguously show a positive relationship between the degree of low-temperature oxidation and crust ages. [Copyright &y& Elsevier]

Continental shelf is a globally important reservoir for terrigenous sediments, and provides sediment transportation pathways that extend from the shoreline to abyssal basins. Due to the strong land-sea interaction in response to frequent sea level change during the late Quaternary and methods' constraint, it is a big challenge to establish a reliable chronostratigraphy on the continental margin with terrigenous sediments dominance. Relative geomagnetic paleointensity (RPI) is quite useful for high-resolution chronostratigraphy in Brunhes epoch. In this study, relative paleointensity record for a sediment core (SFK-1, 82.9 m) from the outer East China Sea (ECS) shelf was studied. Rock magnetic results show that coarse vortex state magnetite is the dominant magnetic carrier throughout the core and that these sediments generally meet the accepted criteria for constructing relative paleointensity (RPI) record. RPI was established by normalizing the natural remanent magnetization (NRM) with isothermal remanent magnetization (IRM), both after 30 mT alternating demagnetization. Further, our RPI record was correlated with PISO-1500 and 7 age tie points were obtained. An age model is established for SFK-1 core with a basal age of 165 ka by combining the radiocarbon dates, relative geomagnetic paleointensity, and tandem correlation of environmental magnetic parameters to foraminiferal stable oxygen isotopes, which provides a reliable chronological framework for the later paleoenvironmental studies of ECS. The consistency of RPI record of SFK-1 and other worldwide observations suggest that global-scale pattern of geomagnetic paleointensity behavior is also documented in the sediments of the East China Sea, and the RPI of SFK-1 can be used as a reference curve for the RPI studies of other cores of ECS area, and even the whole west Pacific margin seas. • An age model with a basal age of ~165 ka was constructed by combining RPI and environmental magnetic methods. • The global-scale pattern of geomagnetic paleointensity behavior is also documented in the sediments of the East China Sea. • Environmental magnetic variations are consistent with the warm/cold cycle. [ABSTRACT FROM AUTHOR]