Isothermal Remanent Magnetization Acquisition Research Articles

Assessing lunar paleointensity variability during the 3.9 - 3.5 Ga high field epoch

Numerous paleomagnetic studies suggest that a lunar dynamo, with surface field intensities potentially as high as 40-100 µT, existed between ∼3.9 Ga and ∼3.5 Ga. This period is referred to as the High Field Epoch (HFE). However, the debate over the origin of magnetization recorded in lunar rocks still persists. In addition, whether the Moon could have sustained a continuously strong dynamo during the HFE remains unclear. To unravel the origin of magnetization preserved in lunar rocks and to better characterize the evolution of the ancient lunar dynamo, we conducted a comprehensive set of experiments including rock magnetic tests, electron microscopy, and paleomagnetic investigations on four HFE-aged Apollo 11 mare basalt samples: 10003, 10044, 10069, and 10071. Rock magnetic experiments and electron microscopy indicate that the remanence carriers are kamacite grains of varying sizes and domain states. Sample 10003 recorded a paleointensity of 54.10 ± 4.66 µT. Sample 10044, which was shocked (peak pressure >5 GPa), did not preserve a stable high coercivity remanent magnetization. Samples 10069 and 10071 recorded paleointensities of 61.46 ± 26.09 µT and 10.69 ± 2.87 µT, respectively. A series of hydrostatic pressure experiments, isothermal remanent magnetization (IRM) acquisition experiments, and viscous remanent magnetization (VRM) tests preclude the possibility of our samples containing shock remanent magnetization from transient impact-generated fields, IRM acquisition from exposure to spacecraft fields, and VRM acquisition from exposure to the Earth magnetic field. Overall, our study suggests that the source of these magnetizations was likely the lunar dynamo and may indicate nearly order-of-magnitude magnetic field fluctuations during the HFE.

Palaeosecular variation in Northern Patagonia recorded by 0–5 Ma Caviahue–Copahue lava flows

SUMMARY Palaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (<3 per cent) superimposed on the GAD term, as reported by South American studies at mid southern latitudes.

Rockmagnetic Characterization of the Quaternary Fluviolacustrine Sediments of the Thimi Section, Kathmandu Valley, Nepal

The fine-grained silt/clay sediments sampled at 38 levels of a 21 m thick segment at the Thimi outcrop in the Kathmandu valley have been studied for their rockmagnetic properties. These sediments have yielded consistently normal magnetic polarity and 14C ages determined by various workers between 20 and 45 kyrs or even more. They were measured for a set of rock magnetic parameters: low-field magnetic susceptibility, parameters derived from isothermal remanent magnetization (IRM) acquisition and demagnetisation (IRM0.3T, IRM3T, MDFIRM), anhysteretic remanent magnetization (ARM) (ARMtot, ARM(30 mT)/ARMtot, χARM), hysteresis parameters, etc. Several relationships between these parameters have been suggested for the Kathmandu valley sediments. Four rockmagnetic zones, based on the minor variations in magnetic mineral types and grain size parameters, are proposed and their possible relationship to the published pollen data has been discussed.

Occurrence and Distribution Patterns of Magnetic Particles Within Stalagmite Growth Laminae

AbstractThe concentrations of magnetic particles in stalagmites depend greatly on the regional hydrologic condition, and thus can act as an important proxy for paleoclimate reconstruction. The central axis region of a stalagmite is usually selected for such magnetism study. However, little is known about the distribution and influence factors of exogenetic magnetic particles within the growth laminae. In this study, the occurrence and variations of magnetic particles along the growth laminae of three shape stalagmites were investigated by a set of magnetic measurements, including isothermal remanent magnetization (IRM) acquisition curves and low‐temperature magnetic analysis. The results show the main magnetic remanence carrier is detrital magnetite/maghemite, coexisting with small amounts of hematite and goethite. The distribution of soft magnetic particles along the growth laminae showed different patterns among the three shape stalagmites (dome, cone, and candle). For the dome shape stalagmite, the soft magnetic particle concentrations in all studied growth layers showed an increasing trend from center to edge. Similar patterns were also appeared in the bottom growth layers of the cone and candle shape stalagmites. This edge enrichment of magnetic particles is caused by drip water washing effect. However, such enrichment pattern became weaker or even inverse in the top growth layers of the cone and candle shape stalagmites, indicating that the shapes of stalagmites could also affect the surface distribution of soft magnetic minerals. These uneven distributions of magnetic particles imply that the central axis region of a stalagmite may not always be the optimal specimen for environmental magnetism study.

Identification of magnetic coercivity components in natural substances using Max Unmix web-application

Identification of magnetic minerals in rocks and other natural substances (sediments, ashes, soils, etc.) is important in Earth sciences. Such identification often uses a measurement of the IRM (isothermal remanent magnetization) acquisition curve that in turn provides the coercivity spectrum of the sample. In this study, MAX Unmix web-based application was used to unmix the coercivity spectrum of 13 natural samples to obtain their coercivity components. Digitized IRM acquisition data were inputted into the application, then adjusted specific parameters to obtain a best-fit coercivity spectrum revealing coercivity components. As a result, two to three components are generally identified for each sample. MAX Unmix application is highly recommended in future studies requiring the identification of magnetic minerals in natural substances.

Rock Magnetism of Lapilli and Lava Flows from Cumbre Vieja Volcano, 2021 Eruption (La Palma, Canary Islands): Initial Reports

We present initial rock magnetic results for both lava flows and lapilli produced by the 2021 eruption of the Cumbre Vieja, La Palma (Canary Islands). Samples were taken during the eruption to minimize early alteration and weathering of the rocks and tephra. Standard procedures included progressive alternating field and thermal demagnetization, hysteresis curves, thermomagnetic experiments, progressive acquisition of isothermal remanent magnetization (IRM), and First-Order Reversal Curves (FORCs). Overall, our observations, including low to medium unblocking temperatures, isothermal remanent magnetization to 1 Tesla, and the abundance of wasp-waist hysteresis loops, strongly suggest the presence of Ti-rich titanomagnetites as the main remanence carriers in both lava flows and lapilli, in addition to some hematite as well. Whereas the former has been directly seen (SEM), hematite is elusive with nonmagnetic-based methods. Rock magnetic data, on a Day plot, also reveal that the magnetic grain size tends to be larger in the lava flows than in the lapilli.

Variation of sediment magnetic properties and their paleoenvironmental implications in Yangtze River Delta during 30–50 ka B.P.

Identifying the variability in and driving forces of relative sea-level (RSL) change in the Yangtze River Delta during 30–50 ka B.P. is vital for understanding global climatic and RSL changes. In this study, the environmental magnetism of sediments in the Yangtze River Delta was systematically studied for the period 30–50 ka B.P. based on the optically stimulated luminescence (OSL) dates. Detailed magnetic measurements, including high-temperature κ-T, magnetic hysteresis loop, isothermal remanent magnetization acquisition, and demagnetization curve measurements illustrate that the predominant magnetic contribution of core YZ07 is from magnetite, with a small contribution from hematite. Detrital input may be the primary source of magnetic minerals according to the χ/χfd% and χARM/χ correlations. In addition, the positive relationship between χ and the RSL curve indicates that χ is a valid indicator of RSL change. Additionally, comparing χ, RSL curves, and δ18O dates of the stalagmites demonstrated that climatic change influenced the RSL change, which mainly affected the concentration of magnetite. The results are conducive for understanding the response mechanism of magnetic indicators to relative sea-level change, and provide a scientific basis for encouraging for future work in understanding climate variability.

Archaeomagnetic evidence of a likely earlier occupation of “El Caracol” lava flow (Zacapu Malpaís, Western Mesoamerica)

A comprehensive archaeomagnetic investigation was carried out on seven ceramic fragments recovered at the "El Caracol" lava flow in the Zacapu Malpaís –Michoacan state, Mexico –. A full set of magnetic-mineralogy experiments (encompassing thermomagnetic curves, hysteresis loops, backfield curves, and isothermal remanent magnetization acquisition measurements) were carried out. The experimental methodology also considers magneto-chemical alteration detection, cooling rate, and remanence anisotropy effects to ensure reliable archaeointensity determinations. Five of the seven ceramic fragments – a pipe (potsherd I), vessel's supports (potsherd II and V), a vessel's wall-fragment (potsherd III), and a pipe's nozzle (potsherd VII) – yielded archaeointensity results within the established acceptance criteria to guarantee quality data. Thermal demagnetization of potsherds I, II, V, and VII shows between two and three magnetization components: one associated with potsherd's fabrication, the second with sample's reheating, and the third one, when present, with a viscous component. For each of the first two components, an intensity value was calculated. Potsherds I, II, III, and VII yielded archaeointensity values between 30 and 40 µT, while potsherd V had a much lower value <20 µT. Although secular variation models do not predict such low intensities, similar values have been reported for the center and south of Mexico. Archaeomagnetic dating was carried out with two regional Paleosecular Variation Curves. The "El Caracol" lava flow is an area with human occupation reported from AD 1200. The results obtained in this work, on the contrary, envision the possibility of an early occupation of the site.

Magnetic Properties of a Holocene Sediment Core from the Yeongsan Estuary, Southwest Korea: Implications for Diagenetic Effects and Availability as Paleoenvironmental Proxies

The sensitivity of magnetic properties, which characterize the mineralogy, concentration, and grain size distribution of magnetic minerals, to environmental processes may provide useful information on paleoenvironmental changes in estuarine environments. Magnetic property studies of estuaries are less common than other environments and, due to the west coast of South Korea having an abundance of estuaries, it provides a good place to study these processes. In this study, we analyzed a variety of magnetic properties based on magnetic susceptibility, hysteresis parameters, progressive acquisition of isothermal remanent magnetization and first-order reversal curve data from a Holocene muddy sediment core recovered from the Yeongsan Estuary on the west coast of South Korea. We examined diagenetic effects on magnetic properties and tested their availability as proxies of paleoenvironmental change. The presence of generally low magnetic susceptibility, ubiquitous greigite-like authigenic magnetic component, and very fine magnetic particle occurrence suggested that the analyzed sediments had undergone considerable early diagenetic alteration. Electron microscopic observations of magnetic minerals support this suggestion. Our results confirm that the use of initial bulk susceptibility as a stand-alone environmental change proxy is not recommended unless it is supported by additional magnetic analyses. We recognized the existence of ferromagnetic-based variabilities related to something besides the adverse diagenetic effects, and have examined possible relationships with sea-level and major climate changes during the Holocene. The most remarkable finding of this study is the two distinct intervals with high values in magnetic coercivity (Bc), coercivity of remanence (Bcr), and ratio of remanent saturation moment to saturation moment (Mrs/Ms) that were well coincident with the respective abrupt decelerations in the rate of sea-level rise occurred at around 8.2 and 7 thousand years ago. It is then inferred that such condition with abrupt drop in sea-level rise rate would be favorable for the abrupt modification of grain size distribution toward more single-domain-like content. We modestly propose consideration of the Bc, Bcr, and Mrs/Ms variability as a potential indicator for the initiation/occurrence of sea-level stillstand/slowstand or highstand during the Holocence, at least at estuarine environments in and around the studied area.

Magnetic signature of sewage polluted river sediments

The present study was designed to characterize the magnetic proxy for sewage-related pollution from river sediments. A suite of mineral magnetic analyses including low-field magnetic susceptibility, isothermal remanent magnetization (IRM) acquisition and demagnetization, back-field IRM treatment, and thermal demagnetization of composite IRMs are carried out for topsoil samples collected from river sediments in Daejeon, South Korea. A 350-m long in situ profile of magnetic susceptibility shows a background mean magnetic susceptibility of 22.4 ± 5.6 × 10−5 SI. The coercivity spectra display dual peaks of lower- and higher-coercivity fractions. The predominance of magnetic signal with maximum unblocking temperatures of 580 °C points to magnetite as the sole magnetic mineral. Hence the lower- and higher-coercivity fractions correspond to coarse-grained and fine-grained magnetite, respectively. Extraordinarily high magnetic susceptibility was observed from BTC02, near to the sewage discharge point. However, its mineral magnetic properties and particle morphologies are not different to those of other soil samples. Then, it is apparent that a high magnetic susceptibility simply reflects the high concentration of magnetites in BTC02. It is likely that a continuous discharge of heavy metal enriched water at the discharging point concentrated significant amount of magnetic particles. It is fortunate that anomalously high magnetic susceptibility (by implication heavy metal concentrations) diminished only several meters away from the discharge point. Perhaps, influences of anthropogenic magnetite were diluted as the high magnetic phases were washed away from the water cycles along the river. In practice, it can be proposed that a periodic replacement of soils near the sewage discharge point is efficient to reduce the heavy metal concentrations.

Assessment of Magnetic Techniques for Understanding Complex Mixtures of Magnetite and Hematite: The Inuyama Red Chert

AbstractMagnetite and hematite mixtures occur widely in nature. Magnetic unmixing of the signals recorded by these minerals can be important for assessing the origin of their respective paleomagnetic remanences and for extracting geological and paleoenvironmental information. However, unmixing magnetic signals from complex magnetite and hematite mixtures is difficult because of the weak magnetization and high coercivity of hematite. We assess here the relative effectiveness of first‐order reversal curve (FORC) and extended FORC‐type diagrams, FORC‐principal component analysis (PCA), isothermal remanent magnetization (IRM) curve decomposition, and PCA of remanent hysteretic curves for unmixing magnetic components in samples from the magnetically complex Inuyama red chert, Japan. We also further characterize the domain state and coercivity distributions of both magnetite and hematite with FORC‐PCA and IRM acquisition analysis in the red chert. We show that IRM curve decomposition can provide valuable component‐specific information linked to coercivity, while FORC‐PCA enables effective magnetic domain state identification. PCA of remanent hysteretic curves provides useful information about the most significant factors influencing remanence variations and subtle coercivity changes. To identify components in complex magnetite and hematite mixtures, we recommend PCA analysis of remanent hysteretic curves combined with FORC analysis of representative samples to identify domain states and coercivity distributions.

Integrated Magnetic Analyses for the Discrimination of Urban and Industrial Dusts

Industrial and urban dusts were characterized by investigating their magnetic properties. Topsoil composed of technogenic magnetic particles (TMP) originating from areas affected by three ironworks, street dust mainly composed of traffic-related pollution, and particulate matter (PM) from urban agglomeration in Warsaw, Poland were investigated. Several magnetic methods, namely magnetic susceptibility, thermomagnetic curves, hysteresis loops, decomposition of isothermal remanent magnetization acquisition curves, and first-order reversal curves, were performed to evaluate the magnetic fraction of dust. Magnetite was the main magnetic phase in all types of samples, with a small amount of high-coercive hematite within ironworks and street dust samples. Significant differences were observed in the domain structure (grain size) of industrial and traffic-related magnetic particles. The grain size of TMP obtained from steel production was in the range of 5–20 µm and was predominated by a mixture of single-domain (SD) and multidomain (MD) grains, with the prevalence of SD grains in the topsoil affected by Třinec ironwork. The traffic-related dust contained finer grains with a size of about 0.1 µm, which is characteristic of the pseudo-single-domain (PSD)/SD threshold. Street dusts were composed of a slightly higher proportion of MD grains, while PM also revealed the typical behavior of superparamagnetic particles.

Tracing of traffic-related pollution using magnetic properties of topsoils in Daejeon, Korea

The present study was designed to explore the possibility of roadside pollution screening using magnetic properties of topsoil samples in Daejeon, South Korea. Low-field magnetic susceptibility, frequency dependence of magnetic susceptibility, susceptibility of anhysteretic remanent magnetization, isothermal remanent magnetization (IRM) acquisition and demagnetization, back-field IRM treatment, and thermal demagnetization of composite IRM were determined for roadside topsoil samples. Magnetic susceptibility measured on 238 samples from the upper 5 cm of the topsoils ranged from 8.6 to 82.5 × 10–5 SI with a mean of 28.3 ± 10.8 × 10–5 SI. The proximal zone, 55 m wide area situated on either side of the main street, exhibited an enhancement of magnetic susceptibility. In areas distant from the main street, low magnetic susceptibility (< 50 × 10–5 SI) was observed. The topsoil samples exhibited significant susceptibility contrasts, suggesting that two dimensional magnetic mapping was effective in identifying traffic-related pollution. A few magnetic hotspots with intensities of magnetic susceptibility near or over 50 × 10–5 SI might reflect the difference in topographic elevation and surface morphology. Among various IRM-related parameters, remanence of coercivity was most significant statistically. In most samples, IRM component analysis provided dual coercivity components. Thermal demagnetization of composite IRM and morphological observation of magnetic separates suggest angular magnetite produced by vehicle non-exhaust emissions spherical magnetite derived from exhaust emission to be the dominant contributors to the magnetic signal. It is likely that lower- and higher-coercivity components represent the presence of coarse-grained angular magnetite and fine-grained spherical magnetite, respectively.

Benchmarking Component Analysis of Remanent Magnetization Curves With a Synthetic Mixture Series: Insight Into the Reliability of Unmixing Natural Samples

AbstractGeological samples often contain several magnetic components associated with different geological processes. Component analysis of remanent magnetization curves has been widely applied to decompose convoluted information. However, the reliability of commonly used methods is poorly assessed as independent verification is rarely available. For this purpose, we designed an experiment using a series of mixtures of two endmembers to benchmark unmixing methods for isothermal remanent magnetization (IRM) acquisition curves. First‐order reversal curves (FORC) diagrams were analyzed for comparison. It is demonstrated that the parametric method, which unmixes samples using specific probability distributions, may result in biased estimates. In contrast, an endmember‐based IRM unmixing approach yielded better quantitative results, which are comparable to the results obtained by FORC analysis based on principle component analysis (FORC‐PCA). We demonstrate that endmember‐based methods are in principle more suitable for unmixing a collection of samples with common endmembers; however, the level of decomposition will vary depending on the difference between the true endmembers that are associated with distinctive processes and the empirical endmembers used for unmixing. When it is desired to further decompose endmembers, the parametric unmixing approach remains a valuable means of inferring their underlying components. We illustrate that the results obtained by endmember‐based and parametric methods can be quantitatively combined to provide improved unmixing results at the level of parametric model distributions.

Permian remagnetization of the Early Cambrian Guachos Formation, Eastern Cordillera, Argentina

The Permian remagnetization belt that affects the southwestern margin of Gondwana has an important geographic extension reaching several South American cratons and terranes. A paleomagnetic pole for the Early Cambrian Guachos Formation in the Eastern Cordillera of Argentina was computed (LG: Lat: 58.1°S, Long: 20.2°E, A95: 6.4°, n: 30), that likely corresponds to a Permian remagnetization and suggests that such belt might extend further north than previously envisaged. We performed several rock-magnetic studies on this succession of laminated fine grain sandstones and siltstones in order to determine the magnetic phases that carry the remanence. Thermomagnetic (k vs. T) curves, field and frequency dependence of bulk susceptibility, hysteresis loops, isothermal remanent magnetization acquisition and back-field curves, three-axis IRM thermal demagnetization, among others, permitted to establish magnetite as the main mineral carrying the remanence. Microscopic observations confirmed a secondary chemical origin for magnetite. Magnetic fabric studies indicate a tectonic fabric, consistent with cleavage developed on these rocks together with low-grade metamorphism. However, a Permian remagnetization, as suggested by the paleomagnetic pole position, cannot be related to Cambrian deformation and metamorphism and might be related to hydrothermal processes.

Last 3000 years of geomagnetic field intensity from India: New reference palaeointensity data from two east coast archaeological sites and archaeomagnetic dating insights

A comprehensive rock magnetic and absolute archaeointensity (AI) investigation was carried out on baked bricks from the east coast of India with an aim to obtain a set of well-defined palaeointensity (PI) data. The new PI data from India comprise a significant first step to improve the regional intensity reference curve, which can be used as reliable dating tool mainly for archaeomagnetic material. Standard Thellier-Coe experiments were applied on 12 independent brick samples collected from a Buddhist monument at Adurru in the Godavari delta and a Medieval Fort at Kolleti Kota site situated on an island in the Kolleru Lake. The samples thus collected represent two different archaeological periods, one around the 3rd century BCE and the other between the 4th and 7th centuries CE respectively. The data obtained from thermal demagnetization of Saturation Isothermal Remanent Magnetization (SIRM), temperature dependent magnetic susceptibility (χ–T), Isothermal Remanent Magnetization (IRM) acquisition curves, and hysteresis measurements showed that the primary carriers of Natural Remanent Magnetization (NRM) are fine-grained Pseudo-Single Domain (PSD) (Stable Single Domain (SSD) + Superparamagnetic (SP)) titanomagnetite with various titanium contents. This implies that the samples had significant stable Thermoremanent Magnetization (TRM), and, therefore, good candidates for PI analysis. Strict selection and quality-control criteria for detecting magnetomineralogical alteration and intensity data corrected for anisotropy and the cooling rate effects on TRM acquisition, offer high quality PI determinations with respect to the previous available data that missed such corrections.A total 33 specimen level PI determinations that correspond to linear NRM–TRM plots, were made using the Thellier GUI software. The corrected data yielded mean PI values of 44.6 ± 1.4 μT for the Buddhist Monument, and 36.6 ± 2.9 μT for the Fort site. By comparison and evaluation of new PI data with Global Geomagnetic Field Model predictions, the sampled bricks were archaeomagnetically dated using the Matlab tool developed by Pavón-Carrasco et al. (2011). Both ARCH3K.1 and SHA.DIF.14 K global models were used to evaluate the age estimates. The SHA.DIF.14 K model provided the most accepted age estimates that varied between 305 and 182 BCE for the early historic Buddhist Structure at Adurru, and 516–889 CE for the Medieval Fort at Kolleti Kota. These new reference archaeointensity data highlight the potential of archaeomagnetic dating to improve the archaeologically determined time periods of bricks, and also show the need for new high quality reference data to improve the geomagnetic field models from India.

Contribution of magnetic measurement methods to the analysis of iron sulfides in archaeological waterlogged wood‑iron assemblies

Pyrite and greigite were identified in the wood of two ancient shipwrecks using an original multi-technique analytical approach. Structural characterization methods such as environmental scanning electron microscopy, micro-Raman spectroscopy and X-ray diffraction were combined with magnetic measurement methods, such as magnetic susceptibility measurements and isothermal remanent magnetization acquisition curves. This is the first time that magnetic measurement methods are used in the field of cultural heritage to study wet organic archaeological materials. They proved to be particularly suitable to detect with a very high sensitivity ferromagnetic s.l. mineral phases inside waterlogged wooden samples, i.e. in the bulk. The occurrence of iron sulfides in archaeological shipwrecks extracted from waterlogged environments is usually attributed to microbiologically influenced corrosion of iron fasteners. This study demonstrates that the nature of the identified iron sulfides is consistent with a step-by-step in situ anoxic oxidation process of mackinawite.

Relative paleointensity (RPI) and age control in Quaternary sediment drifts off the Antarctic Peninsula

Lack of foraminiferal carbonate in marine sediments deposited at high latitudes results in traditional oxygen isotope stratigraphy not playing a central role in Quaternary age control for a large portion of the globe. This limitation has affected the interpretation of Quaternary sediment drifts off the Antarctic Peninsula in a region critical for documenting past instability of the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula Ice Sheet (APIS). Here we use piston cores recovered from these sediment drifts in 2015 during cruise JR298 of the RRS James Clark Ross to test the usefulness for age control of relative paleointensity (RPI) data augmented by scant δ18O data. Thermomagnetic and magnetic hysteresis data, as well as isothermal remanent magnetization (IRM) acquisition curves, indicate the presence of prevalent magnetite and subordinate oxidized magnetite (“maghemite”) in the cored sediments. The magnetite is likely detrital. Maghemite is an authigenic mineral, associated with surface oxidation of magnetite grains, which occurs preferentially in the oxic zone of the uppermost sediments, and buried oxic zones deposited during prior interglacial climate stages. Low concentrations of labile organic matter apparently led to arrested pore-water sulfate reduction explaining oxic zone burial and downcore survival of the reactive maghemite coatings. At some sites, maghemitization has a debilitating effect on RPI proxies whereas at other sites maghemite is less evident and RPI proxies can be adequately matched to the RPI reference template. Published RPI data at ODP Site 1101, located on Drift 4, can be adequately correlated to contemporary RPI templates, probably as a result of disappearance (dissolution) of maghemite at sediment depths >∼10 m.

Identifying pedogenic magnetic minerals in loess from China and Siberia using isothermal remanent magnetization acquisition curves

Aeolian loess is carried by wind and undergoes pedogenesis after deposition. Therefore, both detrital components from the source region and soil pedogenic components contribute to the magnetic properties of the loess. The pedogenic component can be identified by analyzing the coercivity spectra of loess with different degrees of pedogenesis. We used isothermal remanent magnetization acquisition curves to analyze the coercivity spectra of loess in China and Siberia and defined the low ( 100 mT) coercivity components, i.e., components 1, 2, and 3, respectively. In the arid region of Xinjiang, Northwest China, the Bole section, with negligible soil development has loess with only component 2 centered at ∼80 mT. In semiarid central China, the Xifeng section has both loess and paleosols with three coercivity components centered at ∼26 mT, ∼82 mT, and ∼960 mT. Component 1 has a pedogenic origin, and the remanence contribution increases in a positive linear relationship with the intensity of pedogenesis. In the humid region of Siberia, the Kurtak section has three coercivity components are centered at ∼23 mT, ∼78 mT, and ∼1014 mT. The remanence contribution of component 1 shows a low correlation with the intensity of pedogenesis. Component 3 is characterized by high-coercivity minerals and its remanence contribution is related to the intensity of pedogenesis. The soil development in the semi-arid Xifeng section tends to produce massive fine-grained ferromagnetic minerals, so the remanence contribution of component 1 is positively correlated with the intensity of pedogenesis. The humid pedogenic environment of the Kurtak section is prone to form high-coercivity minerals and destroys fine-grained ferromagnetic minerals, so the remanence contribution of component 3 increases with the degree of pedogenesis.

Paleomagnetic results from Lower Devonian sandstones of the Niqiuhe Formation in the Duobaoshan area and its constraints on paleoposition of the Xing&amp;rsquo;an block

Two common but contradictory views toward the timing of the final closure of the eastern Paleo-Asian Ocean (PAO) are either the Pre-Late Devonian or the Late Permian to Early Triassic. Previous paleomagnetic studies support a consolidation of the unified North China and Mongolia by the Late Permian. However, Pre-Late Permian paleogeography of the eastern Central Asian Orogenic Belt (CAOB) is not well understood due to the scarcity of reliable paleomagnetic records. The Xing’an block plays a key role in the evolution of the CAOB, however, no paleomagnetic results are available so far to provide constraints on its Late Paleozoic paleo-position. We performed a systematic paleomagnetic study on a purple siltstone section of the Lower Devonian Niqiuhe Formation near the Duobaoshan County, Heilongjiang Province. Representative fresh end samples were subjected to rock magnetic analysis to identify their dominant magnetic carriers and particle size distribution. Rock magnetic experiments include measurements of magnetic susceptibility variation dependence of temperature ( κ - T ) curves, hysteresis loops, acquisition of isothermal remanent magnetization (IRM) and back-field demagnetization of saturated IRM (SIRM) curves. The results indicate that the magnetic remanence carriers are dominant by hematite with minor contributions of magnetite. 112 cylindrical specimens were thermally demagnetized using an ASC TD-48S furnace and measured in a 2G-755-4K SQUID system in the Magnetotectonics Lab of Peking University. A total of 11 sites revealed identical bi-components character. The low temperature component resembles the present geomagnetic field in geographic coordinates, while the high temperature characteristic remanence (ChRM) component was reversed polarity both before and after tilt correction. Optimal concentration is achieved at 35.7% unfolding percentage, which suggests the magnetization was acquired during the late stage of a folding process. A previous structural study indicated that stress field of the research area switched from NW-SE to N-S orientation during the Late Permian, which is supported by the predominant NE-SW oriented Ordovician to Devonian strata as well as the angular unconformity between the Devonian and Upper Permian strata. The folding time of the Niqiuhe Formation is likely Middle Devonian to Middle Permian. Supported by evidence from a suite of granites near our sampling section (less than 3 km) dating at 309−299 Ma along with the universal reversed polarity of all 11 sites, we argue that the ChRM component was acquired during the Permo-Carboniferous Reversed Superchron (~318−262 Ma), when the study area was affected by a granitic intrusion thermal event (309−299 Ma). The ChRM component ( D / I =28.6°/−33.2°, k =18.6, α 95=10.9°) corresponds to a paleomagnetic pole of λ p/ φ p=17.3°S/97.1°E ( dp / dm =7.0°/12.4°). For a common reference site (43°N/114°E), our results suggest the Xing’an block was situated at 27.7°±12.4°N during the Late Carboniferous. Coeval results obtained from rocks of different lithology suggest 28.5°±9.7°N and 19.9°±14.8°N for the Uliastai continental margin and the Central Mongolia-Euruna block. After applying a blanket flattening factor f =0.6 for results obtained from clastic sedimentary rocks, the paleolatitudinal difference between the Xing’an block, Uliastai continental margin, and the Central Mongolia-Erguna block is less than 3.2°±12.8°. Late Carboniferous paleomagnetic poles from those three blocks are distributed along a small circle. Significant post-Permian local rotations are likely attributed to extensive Mesozoic strike-slip faults among the eastern CAOB. Those three blocks were probably consolidated into a unified block, situated at ~28°−30°N during the Late Carboniferous and dominant by the cold water Borel realm fauna. By contrast, the North China and Songliao-Xilinhot blocks was situated at an equatorial region (4.3°S−12°N) and dominant by the warm water Tethyan realm fauna, while Siberia was located at ~45°−50°N. The Paleo-Asian Ocean therefore remained open and the main basin was the Hegenshan ocean rather than the Solonker ocean.