Ratio Of Anhysteretic Remanent Magnetization Research Articles

Sedimentary Rock Magnetic Response to Holocene Environmental Instability in the Pearl River Delta

Located on the northern coast of the South China Sea, the densely populated Pearl River Delta has experienced the combined effects of sea-level change, monsoon-driven discharge, and especially human activity, since the late Holocene. However, how these factors have regulated the regional environmental and sedimentary evolution remains unclear. To better understand these processes, we conducted a high-resolution rock magnetic investigation of the Holocene sediments of core DS01, drilled in the vicinity of the West River channel in the head area of the Pearl River deltaic plain. The magnetic grain-size proxy of the ARM/κlf ratio (the ratio of anhysteretic remanent magnetization to low-field magnetic susceptibility) indicates a long-term fining trend of the magnetite grain size, which may be a response to an increase in the weathering intensity in the Asian monsoon region during the Holocene. An interval with an enhanced concentration of magnetic minerals (mainly magnetite and hematite) occurred during 7.7–4.8 kyr BP (calendar years before 1950), coinciding with a period of delta progradation. During the marine transgression in the early Holocene, two similar intervals of magnetic enrichment may reflect regional hydrodynamic shifts associated with cooling events at ∼9.5–9.3 kyr BP and 8.2 kyr BP. The subsequent 4.2 kyr BP cooling event possibly induced a cold and dry environment in the sediment source area. From ∼800 yr BP onward, there was a major increase in the sedimentary magnetic mineral content, likely in response to intensified agricultural and industrial activities.

Linkages between soil organic matter and magnetic mineral formation in agricultural fields in southeastern Minnesota, USA

Magnetic properties of soil are widely utilized to study soil development in a variety of settings due to the formation of strongly magnetic iron oxides during pedogenesis. Similarly, soil organic matter (SOM) is commonly measured in soil surveys conducted on agricultural lands due to the essential role SOM plays in the soil ecosystem. Here, we present data from two agricultural fields in southeastern Minnesota that demonstrate a relationship between soil magnetic properties and SOM. In each field, we collected 100 topsoil samples along a 40 m by 20 m grid to determine spatial variability in soil magnetic properties and SOM, as well as two soil cores to constrain variability with depth (∼0–60 cm). Magnetic susceptibility, low-field remanence, and hysteresis properties were used to characterize magnetic mineral abundance and grain-size in the soils. There are strong positive correlations between SOM and three magnetic properties: the frequency dependence of susceptibility (χfd), anhysteretic remanent magnetization (ARM), and the ratio of ARM to isothermal remanent magnetization (ARM/IRM). All three of these magnetic properties (χfd, ARM, and ARM/IRM) are sensitive to the concentration (or relative abundance) of fine-grained (<75 nm) magnetite/maghemite known to form in well-drained soils during pedogenesis. Correlation between SOM and magnetic properties persist in each field despite differences in the management strategy over the past three decades. Our results support a functional link between SOM and soil-formed magnetite/maghemite, where increasing SOM (up to a threshold) enhances the production and stability of soil-formed magnetite due to its role in soil redox processes and iron-organic complexes. Agricultural soils seem particularly well suited to demonstrate correlations between SOM and pedogenic magnetic minerals due to their relatively low SOM and typically well-drained environments, supporting the utility of soil magnetism in agricultural soil survey studies.

Rock-magnetic and archaeomagnetic investigations on archaeological artefacts from Maharashtra, India

AbstractArchaeointensity and rock-magnetic studies were undertaken on 49 baked clay artefacts from four archaeological sites (Ter, Junnar, Nalasopara and Kanheri) in Maharashtra, India. Rock-magnetic properties, including bulk magnetic susceptibility, magnetic remanence and thermomagnetic analysis, indicate the presence of a low-coercivity magnetite in fine (superparamagnetic, single domain) grain-sizes. The ratio of anhysteretic remanent magnetization to saturation isothermal remanent magnetization, the reversible high-temperature susceptibility curves and the 3-axes isothermal remanent magnetization tests also indicate that the artefacts dominantly possess fine-grained magnetic particles, carrying a stable thermoremanent magnetization (TRM). Archaeointensity was estimated using Coe's modified Thellier method corresponding to the linear behaviour of natural remanent magnetization loss and TRM gained plots, which were evaluated with ThellierTool4.0 software. Cooling rate and anisotropy of the TRM corrections were applied and the corrected intensities were used to calculate a mean archaeointensity value for each one of the four sites. The new archaeointensity values were plotted along the existing Indian archaeointensity values derived only from archaeological artefacts, and were compared with the SHA.DIF.14k and ARCH10k.1 global models’ predictions. The present study aims to improve the overall understanding of Indian geomagnetic field variation in the past by providing new high-quality archaeointensity results. However, still more archaeointensity values are required to develop a reliable secular variation curve for India.

A Geomagnetic Paleointensity Record of 0.6 to 3.2 Ma From Sediments in the Western Equatorial Pacific and Remanent Magnetization Lock‐In Depth

AbstractInformation of past geomagnetic intensity variations is important for better understanding of the geodynamo, and efforts to recover continuous paleointensity records continue, in particular for ages older than 2 Ma. In this study, a new relative paleointensity (RPI) record of 0.6 to 3.2 Ma was obtained from a sediment core in the western equatorial Pacific, which has good age control by the oxygen isotope stratigraphy from 2.0 to 3.2 Ma. The RPI record could well be correlated to existing RPI templates despite the low sedimentation rate of ~5 m/Ma and will be useful for future construction of a stacked paleointensity curve beyond 2 Ma. Magnetic minerals of the core consist mainly of biogenic and terrigenous oxidized magnetite. Some rock magnetic contamination to long‐term RPI changes is recognized; RPI inversely correlates with the ratio of anhysteretic remanent magnetization to saturation remanent magnetization, which may be caused by differences in remanence acquisition efficiency between the biogenic and terrigenous magnetic components. The lock‐in depth of the studied core is estimated to be approximately 0 from a line of evidence including no apparent offset between RPI and cosmogenic beryllium nuclide flux changes across some polarity boundaries. Lock‐in depths vary locally from core to core or even within a single core and have no obvious relation with sedimentation rates and carbonate contents. Lock‐in depths may be controlled by factors that are currently difficult to be evaluated, such as variations in sediment flocculation sizes and biogeochemical remanent magnetization dependent to chemical conditions of sediments.

Rock‐magnetic artifacts on long‐term relative paleointensity variations in sediments

Long‐term changes of geomagnetic field intensity, including possible dependence on lengths of polarity intervals, provide fundamentally important information for understanding the geodynamo. A positive correlation between paleointensity and polarity interval length was previously suggested from an Oligocene (ca. 23–34 Ma) relative paleointensity record at Deep Sea Drilling Program Site 522 in the Atlantic Ocean, which is the only continuous paleointensity data set published so far for this age interval. We have conducted a paleomagnetic study of Eocene to Oligocene sediments at three sites in the eastern equatorial Pacific Ocean. Our objectives include revisiting the issue of the paleointensity‐polarity length correlation. Magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation. Although short‐wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites, long‐term changes are inconsistent. Apparent positive correlation between normalized intensity and polarity length was observed, but the normalized intensity has an obvious anti‐correlation with the ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals. Furthermore, the normalized intensity correlates with sedimentation rate. These facts indicate a lithological contamination on the normalized intensity records. The dependence on ARM/IRM and sedimentation rate is also evident at Site 522. It is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization. It is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records.

Magnetic anisotropy, magnetostatic interactions and identification of magnetofossils

Single‐domain magnetite particles produced by magnetotactic bacteria (MTB) and aligned in chains, called magnetosomes, are potentially important recorders of paleomagnetic, paleoenvironmental and paleolife signals. Rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes (magnetofossils) preserved in geological materials. However, ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post‐depositional diagenesis. In this paper, magnetic properties of three sets of samples containing extracted magnetosomes of the culturedMagnetospirillum magneticumstrain AMB‐1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties. Intact MTB and well‐dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions, but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction. This results in a change of the magnetic signature toward properties typical of interacting, single‐domain particles, i.e., a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization, decreasing in the crossing point of the Wohlfarth‐Cisowski test and in the delta ratio between losses of field and zero‐field cooled remanent magnetization across the Verwey transition, as well as vertical broadening of the first‐order reversal curve distribution. We propose a new diagram that summarizes the Verwey transition properties, with diagnostic limits for intact and collapsed chains of magnetosomes. This diagram can be used, in conjunction with other parameters, to identify unoxidized magnetofossils in sediments and rocks.

Magnetic signature of different vegetation species in polluted environment

Detailed magnetic study on vegetation samples from several strongly polluted and clean sites in Bulgaria is carried out in order to evaluate suitability of different species as passive dust collectors in magnetometry. From each location, available species among lichens, mosses, poplar leaves, dandelion, needles have been sampled. Magnetic susceptibility calculated on mass-specific basis shows wide variability between diamagnetic signal up to 846 × 10−8 m3/kg. Lichens and mosses are found to be the species, showing magnetic signals with the strongest contrast between clean and polluted environment. The main magnetic phase is magnetite-like according to the results from thermomagnetic analysis of susceptibility on magnetic extracts. Scanning electron microscopy (SEM) microphotographs reveal the presence of abundant particulate matter on vegetation surface both with anthropogenic (spherules) and lithogenic origin. Magnetic grain size deduced by the ratio of saturation remanent magnetization (SIRM) and mass-specific magnetic susceptibility (χ) and coercivities (Bc and Bcr) suggest that different species accumulate preferentially small SD-like grains from pollution emissions. Contrasting relationship of the ratio of anhysteretic remanent magnetization (ARM) and χ for polluted vs clean sites deduced by needles and lichens may be related to transformation of the accumulated dust particles within lichens’ tissue. This finding indicates that the exact species used as biological dust collector is of importance when studying spatial grain size distribution of magnetic dust particles. Pilot study on polycyclic aromatic hydrocarbons (PAH) content and its relation to magnetic parameters shows good correspondence between high levels of PAHs and high SIRM values for locations affected by non-ferrous industrial production.

A relative paleointensity record of the geomagnetic field since 1.6 Ma from the North Pacific

Abstract A paleomagnetic study was conducted on a sediment core KR0310-PC1 taken from the central North Pacific in order to obtain a relative paleointensity record in the Matuyama chron from this region. The core reached to about 1.6 Ma. The age control is based on the correlation of the S ratio (S −0.1T) variations with a global oxygen-isotope stack. Isothermal remanent magnetization (IRM) was used as the normalizer of the relative paleointensity estimation; anhysteretic remanent magnetization (ARM) was not adopted because ARM is sensitive to magnetostatic interaction among magnetic particles, which is evidenced in these sediments by an inverse correlation between the ratio of ARM to saturation IRM (SIRM) and SIRM without significant magnetic grain-size changes. For the last 350 kyrs, the record of core NGC65, which was obtained at practically the same site as KR0310-PC1 and covers the Brunhes chron (Yamazaki, 1999), was incorporated because the upper part of KR0310-PC1 was physically disturbed. In the record of NGC65/KR0310-PC1, the average paleointensity in the late Matuyama chron is not lower than that during the Brunhes chron, which does not support the conclusion of Valet et al. (2005) based on their Sint-2000 stack. A spectral analysis on the NGC65/KR0310-PC1 paleointensity record shows a power at the ∼100 kyr eccentricity period. The relative paleointensity and magnetic properties of NGC65/KR0310-PC1 were compared with those of MD982185 from the western equatorial Pacific (Yamazaki and Oda, 2002, 2005). The two sites belong to different oceanographic regimes. Coherent variations in the relative paleointensity despite incoherent changes in the magnetic properties suggest that rock-magnetic contamination to the relative paleointensity is small, if any, and the ∼100 kyr period in the relative paleointensity records would reflect the geomagnetic field behavior.

Variations of the geomagnetic field during the Holocene: Relative paleointensity and inclination record from the West Pacific (ODP Hole 1202B)

We conducted detailed rock magnetic investigations on 36 m of drill core collected during Ocean Drilling Program Leg 195 at Hole 1202B (24°48.24′N, 122°30.00′E), in the Southern Okinawa Trough, with the goal of extracting a reliable paleointensity signal with centennial resolution. An age-depth model was established from a chronology obtained by accelerator mass spectromety 14C dating. The sedimentary section spans almost the entire Holocene (0–9.4 kyr) and exhibits sedimentation rates close to 400 cm/kyr. The magnetic properties are dominated by stable, pseudo-single domain magnetite. High-field hysteresis data and the grain-size sensitive ratio of anhysteretic remanent magnetization (ARM) to low field magnetic susceptibility indicate a narrow range of grain sizes and concentrations. Magnetic parameters vary by a factor of 4 thereby fulfilling the usual criteria for a relative paleointensity study. The relative geomagnetic paleointensity was obtained by normalizing the intensity of natural remanent magnetization (NRM) by the ARM and the low field magnetic susceptibility. Both normalizations yield nearly identical results ( r = 0.89). Spectral analysis indicates that the record is not significantly affected by local environmental conditions. Comparison of this West Pacific paleointensity curve with other curves suggests a geomagnetic origin for the signal. Millennial-scale features of our record correlate to variations of the archeomagnetic dipole moment. This suggest that the sediments at Hole1202B recorded changes of the geomagnetic field over the studied time interval.

Long-term secular variation in geomagnetic field inclination during Brunhes Chron recorded in sediment cores from Ontong-Java Plateau

A paleomagnetic study has been conducted on three gravity cores obtained from the Ontong-Java Plateau (OJP) in the western equatorial Pacific in order to investigate long-term secular variations during the Brunhes Chron. The cores consist of nanofossil foraminiferal ooze, and covers the last 500,000–800,000 years with average sedimentation rates of 6–9 m/m.y. Downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less, and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters, ratio of anhysteretic remanent magnetization (ARM) to saturation isothermal remanent magnetization (SIRM), and S ratio. These results indicate that the sediments are rock-magnetically homogeneous in general, and are hence suitable for relative paleointensity estimation. I chose SIRM as a normalizer for relative paleointensity. The three cores could be precisely correlated with each other using relative paleointensity variations. Depth-to-age conversion was carried out by tying to the Sint-800 paleointensity stack (Guyodo and Valet, 1999). The ages are consistent with the oxygen-isotope stratigraphy available for one of the three cores. Inclination variations were obtained after alternating-field demagnetization. Large inclination anomaly, about −7° in average, was observed as has been suggested to occur in the western equatorial Pacific by some studies. The inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity. Long-term inclination variations of the order of 10,000–100,000 years were recognized. The inclinations tend to deflect toward negative when relative paleointensity is low. The variations are similar in general to the record from the West Caroline Basin (Yamazaki and Oda, 2002), which supports their model that long-term inclination variation is controlled by relative strength of the geocentric axial dipole (GAD) and persistent non-dipole components.

AF demagnetization characteristics of NRM, compared with anhysteretic and saturation isothermal remanence: an aid in the interpretation of NRM

AF demagnetization characteristics and isothermal remanent magnetization (IRM) acquisition of a variety of rocks are analysed in the form of Cisowski plots and plots of progressive demagnetization of natural remanent magnetization (NRM) and anhysteretic remanent magnetization (ARM) against IRMs. The igneous rocks studied carrying a primary NRM acquired during initial cooling have a ratio of NRM:IRMs of parts in 100, whereas altered igneous rocks have much smaller ratios of parts in 1000 or 10,000. Conversely, the detrital sedimentary rocks carrying a primary NRM have a ratio of parts in 1000, but altered sedimentary rocks carrying secondary magnetization have ratios of parts in 100. Although not as discriminatory as the ratios of NRM:IRMs, altered igneous and sedimentary rocks frequently have strongly concave up curves, reflecting the common admixture of a small amount of hard material in samples carrying secondary magnetization, The ratio of ARM to IRMs serves as a proxy for weak field magnetization and is an indicator of grain size.

Relative geomagnetic paleointensity from the Jaramillo Subchron to the Matuyama/Brunhes boundary as recorded in a Mediterranean piston core

Piston core LC07, located west of the Sicily Strait in the Mediterranean Sea, unambiguously records the Matuyama/Brunhes (M/B) and the upper Jaramillo polarity reversals, with similar average sediment accumulation rates (SARs) for the Brunhes Chron (2.29 cm/kyr) and late Matuyama Chron C1r.1r (2.19 cm/kyr). We report a relative paleointensity record for the interval spanning the M/B boundary down into the Jaramillo Subchron, which is unique in the Mediterranean because existing records from this basin cover only the last 80 kyr. The average SAR in core LC07 is used to translate the depth-related paleointensity record to the time domain; the ratio of anhysteretic remanent magnetization to low-field magnetic susceptibility is climatically sensitive and is used to tune the age model. This correlation produces a good fit to the global ice volume model derived for summer insolation at 65°N. With this age model, a paleointensity minimum in association with the M/B boundary has a duration of about 4–5 kyr, while the directional change has a duration of <3 kyr. A second paleointensity minimum of similar duration is found about 16 kyr below the M/B boundary. This feature (precursor or ‘dip’ in the literature) has previously been recognized at the same time interval in many marine records, which reinforces the validity of our age model. Other relative paleointensity minima are found within chron C1r.1r, and, within the uncertainties of the respective age models, these minima coincide with those observed from the few published coeval paleointensity records. In particular, there is good correspondence between the ages of minima at about 0.92 and 0.89 Ma, which probably correlate with two geomagnetic excursions (Santa Rosa and Kamikatsura, respectively) that have been recorded in lava flows and dated using the 40Ar/ 39Ar technique. In contrast, a recently dated excursion at 0.83 Ma from La Palma seems to correspond to a paleointensity maximum. This observation is opposite to that expected and this excursion needs to be confirmed. In contrast to some recently published paleointensity records, spectral analysis of the LC07 record does not reveal identification of significant power at the orbital obliquity frequency.

Latest Quaternary rock magnetic record of climatic and oceanic change, Tanner Basin, California borderland

A high-resolution record of sediment magnetic properties and stable oxygen isotopes has been determined from marine isotope stage (MIS) 6 to the Holocene (170 ka to present day) for hemipelagic sediments (Site 1014 of Ocean Drilling Program Leg 167) from a subbasin west of the Tanner Basin, off the coast of southern California. The spacing between samples is 10 cm. The sedimentation rate in the top 20 m of the sequence was determined to be 12 cm/k.y. We have taken the intensity of anhysteretic remanent magnetization (ARM) as a measure of the concentration of magnetic minerals. The quantity of magnetic material increased during transitions from warm to cool periods; this change is inferred to have resulted from increased shelf erosion during lower sea levels. During interglacials and possibly interstadials, a reduction occurred in the quantity of magnetic material in the sediment, and magnetic susceptibility therefore dropped. This decrease seems to have resulted from dilution of the magnetic fraction due to increased biogenic deposition during these warm intervals. The ratio of ARM to ARM after 15 mT alternating-field demagnetization (ARM/ARM[15]) is used as a coercivity indicator. The magnetic properties of the sediments from the subbasin west of the Tanner Basin are largely determined by two climatically controlled fluxes of magnetic material. Large and magnetically softer titanohematite grains were deposited during times of decreasing sea level when the concentration of magnetic particles was elevated. Small magnetite grains (<2 μm) control the magnetic properties in sediments deposited during transgressions and times of high sea levels, when detrital titanohematite content was low. The ARM/ARM[15] ratio leads the δ18O curve by 0.3–0.8 m. This offset between the rock magnetic coercivity parameter and intervals of climatic change represents evidence for authigenic growth of magnetite grains near the Fe-redox boundary. The Fe-redox boundary appears to have been at 0.8 m depth during MIS 3 and at 0.3 m during MIS 5. Authigenic growth of magnetite is associated with rapid climate events as recorded by planktonic foraminiferal assemblage shifts. These relationships may have resulted from changes in oxygen concentrations in intermediate-depth waters associated with the climate shifts. Therefore the ratio of ARM/ARM[15] is suggested to be a new indicator (proxy) of past changes in oxygen levels of intermediate water.

Rock magnetism of sediments in the Angola-Namibia upwelling system with special reference to loss of magnetization after core recovery

A rock magnetic study was performed on sediment cores from four sites in the South Atlantic off the western coast of Africa, which were taken during the Ocean Drilling Program Leg 175 (Sites 1078, 1082, 1084, and 1085). The sites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentration of ferrimagnetic minerals at these sites is very low, and the magnetic susceptibility is dominated by paramagnetic and diamagnetic minerals. Severe and rapid loss of remanent magnetization occurred during storage of the cores, with less than 10% of the initial intensity remaining a few months after core recovery. The loss of magnetization may prevail in organic-rich sediments. Changes of magnetic properties with time were examined using samples that were kept frozen before the experiment. Hysteresis parameters and the ratio of ARM (anhysteretic remanent magnetization) to SIRM (saturation isothermal remanent magnetization) indicate increases in the average magnetic grain size with the decay of magnetization, which suggests preferential dissolution of finer magnetic minerals. Loss of low-coercivity magnetic minerals with time was estimated from the decrease of S ratios. Low-temperature magnetometry revealed the presence of magnetite in the sediments even after the completion of sulfate reduction. Magnetization attributable to magnetite decreased with the loss of magnetization. This suggests the transformation of magnetite into non-magnetic phases, which is consistent with the decrease of S ratios.

Cautionary note on magnetic grain‐size estimation using the ratio of ARM to magnetic susceptibility

The ratio of anhysteretic remanent magnetization (ARM) to magnetic susceptibility (χ) is widely used for rapid estimation of magnetic grain‐size variations. On interpreting ARM/χ variations, no significant influence of interactions among magnetic grains on ARM acquisition is usually assumed implicitly, although some previous experiments using artificial sediments and biogenic magnetites opposed this assumption. Our rock‐magnetic study of deep‐sea surface sediments from the Pacific ocean showed that ARM/χ depends strongly on χ, the concentration of magnetic minerals, and on the R value of Cisowski [1981], an index of the strength of magnetic interactions. This result demonstrates that ARM/χ can be influenced significantly by magnetic interactions in natural sediments. We corrected for the influence of the magnetic interactions using the observed concentration dependence of ARM/χ. Regional variations of ARM/χ values before the correction were geologically unnatural in terms of magnetic grain‐size variations. Previous interpretations of ARM/χ variations on magnetic grain‐size without consideration of magnetic interactions may not be valid.

Detection and correction of inclination shallowing in deep sea sediments using the anisotropy of anhysteretic remanence

Paleomagnetic data from recent Pleistocene to recent deep sea sediments from the continental rise of eastern North America exhibit a cyclical inclination shallowing, up to 30° with respect to the geocentric axial dipole value. This shallowing is strongly correlated with a ratio of anhysteretic remanent magnetization (ARM) anisotropy determined from a four position ARM anisotropy method. It is therefore proving that inclination variations in these cores are not due to paleosecular variation but in part to a bias in the remanence recording processes linked to depositional anisotropy. This study suggests that ARM anisotropy could provide a method to identify and correct for inclination shallowing in natural sediments.