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Ferromagnetic resonance (FMR) spectroscopy has become an increasingly useful tool for studying the magnetic properties of natural samples. Magnetite (Fe3O4) is the only magnetic mineral that has been well characterized using FMR. This limits the wider use of FMR in rock magnetism and paleomagnetism. In this study, we applied FMR analysis to a range of magnetic minerals, including greigite (Fe3S4), monoclinic pyrrhotite (Fe7S8), magnetically non-interacting titanomagnetite (Fe3-xTixO4), and synthetic magnetite chains to constrain interpretation of FMR analysis of natural samples and to explore applications of FMR spectroscopy. We measured the FMR signatures of a wide range of well-characterized samples at the X- and Q-bands. FMR spectra were also simulated numerically to compare with experimental results. The effects of magnetic anisotropy, mineralogy, domain state, and magnetostatic interactions on the FMR spectra are discussed for all studied minerals. Our experimental and theoretical analyses of magnetically non-interacting tuff samples and magnetically interacting chains enable quantitative assessment of contributions of magnetostatic interactions and magnetic anisotropy to the FMR spectra. Our results also indicate that intact magnetosomes are a unique system with distinct FMR signatures. While FMR analysis is useful for characterizing magnetic properties of natural samples, care is needed when making interpretations because of overlaps in a range of FMR signatures of different magnetic minerals with different magnetic properties. Our analyses will help to constrain such interpretations in rock magnetic studies.

Different lithologies between Taiwan and southeastern China lead to diverse mineralogical composition for weathering products derived from the two shores of the Taiwan Strait. Pyrrhotite and magnetite are respectively the dominant magnetic minerals associated with fluvial sediments from western Taiwan and southeastern China. While magnetite commonly co-exists with pyrrhotite in sediments sourced from Taiwan, pyrrhotite has not been found in sediments sourced from mainland China. Associated with such a distinction are vast differences in magnetic properties, including magnetic susceptibility (χ), SIRM, HIRM and the S-ratio, which can be used to study the provenances of sediments in the Taiwan Strait and adjoining marginal seas. Based on any two of these parameters, the magnetic characteristics of much of the Taiwan Strait sediment can be explained using a two-endmember mixing model. Source-to-sink dispersal of sediments in the Taiwan Strait can then be traced from the distribution of these parameters. The results not only corroborate an earlier study based on radionuclides and particle size distribution ( Huh et al., 2011 ) but reveal more diagnostic details. Besides spatial distribution based on a large number (216) of surface sediments, we also analyzed temporal variation of magnetic properties in six well-dated cores collected at key sites along the sediment source-to-sink pathways. From profiles of these parameters in cores from the middle of the northern Taiwan Strait, it is calculated that sediment supply from Taiwan has increased substantially in the past five decades, which may very well be related to accelerated land use and increased frequency of intense rainfalls in Taiwan during the same period. The approach described in this work may be extended to other source-to-sink systems around the world and through time, especially the mountainous islands fringing the Pacific and Indian Oceans in southeastern Asia. As with Taiwan, these islands have high denudation rates and pyrrhotite is in all likelihood a mineral characteristic of their metamorphic terrains.

Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low-field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low-temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo-single-domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.

Although magnetic noise fields generated by heater currents in thermal demagnetizers have been noticed for a long time, no satisfactory tests have been conducted to quantify their effects. Toward this end, we have developed a new high-precision thermal demagnetizer that greatly reduces the magnetic noise field. We show the data quality generated by the new oven and the comparative results on several real samples that demonstrate the effects of the magnetic noise field due to heater currents. The properties of the spurious magnetization emanating from the heater currents critically depend on the decay rate of amplitude and its waveform of electric power which is delivered to oven coils at the end of the heating stage of thermal demagnetization. These results also illustrate the potential applications of this new instrument in paleomagnetism and paleointensity studies.

We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

[1] We provide comprehensive low-temperature magnetic results for greigite (Fe3S4) across the spectrum from superparamagnetic (SP) to multidomain (MD) behavior. It is well known that greigite has no low-temperature magnetic transitions, but we also document that it has strong domain-state dependence of magnetic properties at low temperatures. Blocking of SP grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements. Thermally stable single-domain greigite undergoes little change in magnetic properties below room temperature. For pseudo-single-domain (PSD)/MD greigite, hysteresis properties and first-order reversal curve diagrams exhibit minor changes at low temperatures, while remanence continuously demagnetizes because of progressive domain wall unpinning. The low-temperature demagnetization is grain size dependent for PSD/MD greigite, with coarser grains undergoing larger remanence loss. AC susceptibility measurements indicate consistent blocking temperatures (TB) for all synthetic and natural greigite samples, which are probably associated with surficial oxidation. Low-temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages. Low-temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of greigite-bearing sediments.

The “hard” isothermal remanent magnetization (HIRM) and the S-ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. We demonstrate that synthetic Al-substituted hematite and goethite exhibit a wide range of coercivities, which significantly influences the HIRM and S-ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L-ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT: IRMAF@300mT/IRMAF@100mT. These parameters are easily measured using modern vibrating sample or alternating gradient magnetometers. Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L-ratio is relatively constant. Conversely, L-ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S-ratio variations should be interpreted with caution in such cases. The L-ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols. For example, the HIRM is equivalent to IRMAF@300mT. Likewise, 0.5*(SIRM + IRM−100mT), where IRM−100mT represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back-field of −100 mT, is equivalent to IRMAF@100mT. The HIRM/[0.5*(SIRM + IRM−100mT)] ratio is therefore a suitable substitute for the L-ratio when measurements are made with a long-core magnetometer. The newly proposed L-ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism, the HIRM and S-ratio.

[1] First-order reversal curve (FORC) diagrams have become a standard tool in rock magnetism, yet magnetite is the only magnetic mineral that is well characterized using FORC diagrams. We present FORC diagrams for predominantly single-domain (SD) synthetic aluminous hematite (a-Fe2-xAlxO3) and goethite (a-(FeAl)OOH) and natural greigite (Fe3S4) and pyrrhotite (Fe7S8) to constrain interpretation of FORC diagrams from natural samples. Hematite and goethite have low spontaneous magnetizations and negligible magnetic interactions, while greigite and pyrrhotite have higher spontaneous magnetizations and can have strong magnetic interactions. The coercivity of hematite systematically increases with Al content only for samples produced using the same synthesis method, but it is variable for samples produced with different methods even for similar Al content. This precludes use of magnetic coercivity alone to quantify the Al content of natural hematites. Goethite has much higher coercivity than hematite for all measured samples. SD and superparamagnetic (SP) behavior is common in natural greigite samples, with peak coercivities ranging from

[1] High-resolution biogeochemical and magnetic property analyses of a 20-kyr sediment core in the Okinawa Trough revealed a significant upward decrease in total sulfur content (TS) at around 10 cal ka BP, which coincides with a transformation from foliation to anomalous sedimentary magnetic fabric, which indicates an enhancement of deepwater circulation. Synchronous change in biogeochemical and magnetic properties in sediments was likely triggered by the intensified Kuroshio Current, which strengthened surface water productivity and the relative input of marine organics to the sediments. However, this event did not promote sedimentary sulfur burial; instead, the synchronously enhanced deepwater circulation switched the organic carbon oxidation pathway from sulfate reduction to aerobic respiration which resulted in TS-depleted sediments.

International audience; The Crostolo River section in Italy yielded a detailed record of the Upper Olduvai geomagnetic polarity transition that has been used to support the hypothesis of deep mantle control on the transitional geomagnetic field. The paleomagnetic record is carried by the authigenic iron sulphide, greigite, which was interpreted to have formed shortly after deposition. Our detailed scanning electron microscope investigations indicate the presence of at least 3 generations of pyrite, which usually forms with greigite as a precursor. This suggests that the total magnetization is a complex composite that produced a smoothed record of transitional field behaviour.

The continental slope off southwestern Taiwan is characterized by rapid sedimentation as a result of high denudation rates in southern Taiwan. Analyses of total organic carbon (TOC), total sulfur (TS), total iron (FeT) and cold acid extractable iron (FeA) in sediment samples from piston and box cores show unusually low sulfur contents (0.01–0.12%) in sediments against a background of moderate organic carbon contents (0.5–1.3%) and rich reactive iron (1.2–2.0%). Consequently, the degrees of sulfidation (less than 0.1) are very low for iron. Porewater sulfate concentration dropped rapidly in the top 30 cm of the piston core but decreased very gradually below. Below the top layer of relatively rapid sulfate depletion, only 5 mM of porewater sulfate was further consumed as the depth reaching 3 m below the sediment surface. The low content of reduced sulfur and low porewater sulfate depletion in the sediment column suggest very slow net sulfate reduction, which may result from competition for organic matter degradation or re-oxidation of reduced sulfur during microbial reduction of Fe(III) or Mn(IV) oxides. This may explain the exceptionally low S/C ratios (~ 1/30) and low degrees of sulfidation (mostly below 0.06) found in sediments off southwestern Taiwan. In light of the high fraction of fossil carbon in river-borne sediments observed in northern Taiwan, the highly refractory terrigenous material, which probably constitutes a major fraction of sedimentary organic matter in the study area, may also limit sulfate reduction.

[1] We present a 2.14-Myr astronomically tuned relative geomagnetic paleointensity record from the western Philippine Sea. Pseudosingle-domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations. No significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization. This suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at Earth orbital periods. The paleointensity record is highly coherent with the Sint-800 global paleointensity stack for the last 800 kyr and with a coeval record from the West Caroline Basin. Our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals, with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction. We do not observe an asymmetrical sawtooth form in our paleointensity record, which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts. Also, we do not observe a persistent 100-kyr inclination periodicity, in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity. Good agreement between our paleointensity record and the coeval West Caroline Basin record provides the beginning of a detailed view of geomagnetic field behavior between 0.8 and 2.14 Ma.

We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine delta(18)O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama./Brunhes boundary, 781 +/- 3 ka (slightly above delta(18)O Stage 19.3); top of the Santa Rosa polarity interval, 920 +/- 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 +/- 1 ka (Stage 24); top of the Jaramillo subchron, 988 +/- 3 ka (Stage 27); base of the Jaramillo subchron, 1072 +/- 2 ka (Stage 3 1); top of the Cobb Mountain subchron, 1173 +/- 4 ka (Stage 35/36); base of the Cobb Mountain subehron, 1185 +/- 5 ka (Stage 36); top of the Olduvai subchron, 1778 +/- 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 +/- 4 ka (Stage 71/72); top of the Reunion II subchron, 2118 +/-3 ka. (Stage 80/81); and base of the Reunion II subchron, 2133 +/- 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.

Over 2100 samples were collected from 130 sedimentary sites of early Pliocene to Pleistocene age in the Coastal Range of Eastern Taiwan. Their lithology is mainly unmetamorphosed marls, siltstones and sandstones and the formations are folded and faulted at different scales. Different shapes of the susceptibility ellipsoid were observed from an oblate form, with K min closely perpendicular to the bedding plane, to a “pencil” structure, with K min a K int forming a girdle around K max , reflecting an increasing tectonic contribution to the magnetic fabric. At sites where fault tectonic analysis is available, the magnetic lineation is perpendicular to the main compression. These results have a twofold significance: first, confirming previous preliminary results, they show that in such visually undeformed sediments the magnetic fabric may be, at least in part, of tectonic origin. Second, when considered together with recent paleomagnetic and tectonic data, they suggest that in the Plio-Pleistocene sediments of the Coastal Range of Taiwan the magnetic lineation arises from the compression due to the collision of the Philippine Sea and the Eurasian plates. In our interpretation, the stress pattern has rotated clockwise along with the structures during this collision.

In this paper, we present the results of down core variations in magnetic susceptibility and magnetic mineral composition of fifteen piston cores retrieved from the active continental margin offshore of southwestern Taiwan, where a wide distribution of bottom simulating reflectors (BSRs) related to gas hydrate layers has been detected. X-ray analysis on magnetic extracts from the cores indicates that detrital magnetite and authigenic greigite, in various proportions, are the dominant magnetic minerals in sediments. Non-magnetic, authigenic pyrite is generally associated with greigite, but it may co-exist with magnetite or may solely occur in sediments. Consequently, the sediment sequences of the fifteen piston cores have complicated magnetic mineral assemblages that result in various magnetic susceptibility profiles. For core segments containing detrital magnetite as the only magnetic mineral, values of magnetic susceptibility are moderate with small variations (8 - 15 Ă 10-6 SI), which can be regarded as the susceptibility backgrounds for the initially deposited sediments. However, high magnetic susceptibilities relative to the backgrounds were found in core segments where magnetic mineral is enriched in greigite. Magnetic susceptibilities lower than the backgrounds were found in segments where neither magnetite nor greigite were detected. The complicated occurrence of magnetic minerals reveals that their host sediments at different levels have suffered various degrees of early diagenetic process ranging from oxic to anoxic conditions. Due to very low sedimentary organic matter content in the study area, detrital magnetite persisted in sediments that were subjected to initially oxic (or sub-oxic) conditions. However, the sedimentary organic matter is apparently insufficient for providing reducing environments for the formation or enrichment of authigenic iron sulfides. Instead, we propose that gaseous methane derived from gas hydrates in deeper sedimentary layers should be the most likely source of extra organic matter. The different flux intensities of this gas either through slow diffusion or rapid venting resulted in various anoxic conditions, which caused the dissolution or survival of magnetite and the formation or enrichment of greigite and pyrite.

Four basic magnetic parameters (κ, SIRM, Hcr, MDF(subscript SIRM))and two thermal magnetic analyses were investigated in this study to characterize the magnetic properties of magnetite-, pyrrhotite- and greigite-bearing rocks of the Tsengwen-chi and Erhjen-chi sections. These rocks have very similar behaviors in the acquisition of isothermal remanent magnetization (IRM) in spite of bearing with different magnetic minerals. Because of the difference in magnetic mineral concentration and grain size, greigite-, pyrrhotite-, and magnetite-bearing rocks have relatively high, intermediate and low parameter values (κ, SIRM, Hcr, MDF(subscript SIRM)), respectively. Thermomagnetic analysis in a nitrogen atmosphere has further revealed that each magnetic mineral has its own diagnostic magnetization curves and that greigite could have transformed to pyrrhotite and magnetite, pyrrhotite to magnetite, and magnetite to hematite. Bulk magnetic susceptibility has also shown remarkable drop in greigite-bearing rocks below 320℃, while there are minor and almost noneffective susceptibility changes in pyrrhotite- and magnetite-bearing rocks below 400℃, respectively.