Ferrimagnetic Iron Sulphide Research Articles

Diagenetic dissolution, maghemitization and sulphidization of magnetic minerals in rapidly deposited gas hydrate bearing sediments from the Bay of Bengal

We conducted a comprehensive rock magnetic investigation complemented by a scanning electron microscopy analyses on a 148.8 m sediment core (NGHP-01-20A) from the methane-hydrate bearing site in the Bay of Bengal to systematically evaluate how magnetic minerals selectively respond to a range of diagenetic processes in the sulfidic and methanic sediments. For diagenetic analysis, we defined five distinct sediment magnetic zones (I–V) based on the sediment magnetic signatures. Rock magnetic property is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, silicate hosted magnetic inclusions), altered (titanomaghemite, titanohematite) and fine-grained diagenetic (pyrite) and authigenic (greigite) iron sulfide minerals. Three distinct bands of enhanced magnetic susceptibility in Z-II, Z-III and Z-V mainly comprised of detrital magnetite-rich minerals delivered by the peninsular river systems as a result of higher sedimentation events in the Krishna-Godavari (K-G) basin. Progressive diagenetic dissolution of primary iron oxide and consequent formation of iron sulfide in the uppermost sulfidic zone (Z-I) is clearly mirrored in the magnetic susceptibility (χlf) profile. Presence of moderately altered coarse-grained detrital Fe–Ti particles in the deeper methanic and magnetically enhanced zones (Z-II, Z-III, Z-V) suggest that sediment diagenesis at this site was significantly constrained by the multiple events of rapid sedimentation which might have restricted the complete dissolution of the primary minerals. Decline in magnetic grain size diagnostic proxy (ARM/IRM2.5T) in the uppermost sulfidic zones (Z-I, Z-II) can be attributed to the preferential dissolution of fine-grained magnetite. Further, downcore increases could be linked with the authigenic formation of fine-grained magnetite in methanic zones (Z-III to Z-V). Ferrimagnetic iron sulfide (greigite) indicative (SIRM/χlf) parameter showed sudden rise in Z-II and remain fairly constant throughout, suggesting the presence of authigenic greigite. Scanning electron microscopy in-line with energy dispersive X-ray spectroscopy analyses of magnetic particles from different sediment magnetic zones revealed a range of ferrimagnetic (titanomagnetite, titanomaghemite), antiferromagnetic (titanohematite) iron oxides and iron sulfide (pyrite). Detrital titanomagnetite with angular, irregular, spherical and well-defined octahedral with micron and submicron sized particles mainly derived from Deccan Volcanic Province (DVP) dominates the rock magnetic signal of bulk sediments in the studied core. We noticed several evidence of diagenetic dissolution, sulphidization and maghemitization of magnetic particles in the studied sediment core. Few nanometre-sized detrital magnetic particles occur as inclusions within the silicate minerals and remains protected against the sulfidic dissolution fronts through geologic times. Optical microscopy revealed the dominance of diagenetic pyrite occurring as overgrowth on the ferrimagnetic particles. We demonstrate that the rockmagnetic methods in combination with focussed electron microscope analyses provide valuable insights into the mineralogy, genesis and state of preservation of magnetic particles in sulfidic and methanic environments and thereby enhances the interpretative value of rock magnetic proxies.

Authigenic Ferrimagnetic Iron Sulfide Preservation Due to Nonsteady State Diagenesis: A Perspective From Perseverance Drift, Northwestern Weddell Sea

AbstractWe document magnetic mineral diagenesis with high‐resolution magnetic susceptibility, hysteresis, isothermal remanent magnetization, and other rock magnetic measurements through a shallow sulfate‐methane transition (SMT) at Perseverance Drift—a high‐accumulation rate Holocene biosiliceous Antarctic marine sediment deposit. The structure of the SMT is defined with porewater measurements from the same core, allowing direct comparison. Dissolution of the detrital (titano)magnetite assemblage, with preferential dissolution of stochiometric magnetite, occurs in the upper SMT. Higher coercivity magnetic minerals dissolve more slowly, continuing to dissolve through the entire SMT and could be a source of ferric iron for microbial respiration following exhaustion of porewater sulfate, as suggested by accumulation of porewater ferrous iron below the SMT. Superparamagnetic ferrimagnetic mineral enrichment/depletion occurs in three phases through the SMT and is coupled tightly to the availability of dissolved ferrous iron relative to dissolved sulfide. High concentrations of authigenic remanence‐bearing iron sulfides, including greigite and hexagonal 3C pyrrhotite, which can be detected using remanence parameters but not in‐field concentration dependent parameters, accumulate in a transient horizon at the base of the SMT during this early diagenesis, where sulfide is present but limited relative to dissolved ferrous iron. Formation of this remanence‐bearing iron sulfide horizon is likely facilitated by continued iron reduction through the SMT. Nonsteady state perturbations that shift the porewater profile, such as changes in carbon flux or sedimentation rate, can lead to preservation of these transient horizons, much like well documented preservation of manganese oxide layers in marine sediments following similar shifts to porewater profiles.

Greigite formed in early Pleistocene lacustrine sediments from the Heqing Basin, southwest China, and its paleoenvironmental implications

The ferrimagnetic iron sulfide greigite (Fe3S4) occurs widely in sulfidic lacustrine and marine sedimentary environments. Knowledge of its formation and persistence is important for both magnetostratigraphic and paleoenvironmental studies. Although the formation mechanism of greigite has been widely demonstrated, the sedimentary environments associated with greigite formation in lakes, especially on relatively long timescales, are poorly understood. A long and continuous sequence of Pleistocene lacustrine sediments was recovered in the Heqing drill core from southwestern China, which provides an outstanding record of continental climate and environment. Integrated magnetic, geochemical, and paleoclimatic analysis of the lacustrine sequence provides an opportunity to improve our understanding of the environmental controls on greigite formation. Rock magnetic and scanning electron microscope analyses of selected samples from the core reveal that greigite is present in the lower part of the core (part 1, 665.8–372.5 m). Greigite occurs throughout this interval and is the dominant magnetic mineral, irrespective of the climatic state. The magnetic susceptibility (χ) record, which is mainly controlled by the concentration of greigite, matches well with variations in the Indian Summer Monsoon (ISM) index and total organic carbon (TOC) content, with no significant time lag. This indicates that the greigite formed during early diagenesis. In greigite-bearing intervals, with the χ increase, Bc value increase and tends to be stable at about 50 mT. Therefore, we suggest that χ values could estimate the variation of greigite concentration approximately in the Heqing core. Greigite favored more abundant in terrigenous-rich and organic-poor layers associated with weak summer monsoon which are characterized by high χ values, high Fe content, high Rb/Sr ratio and low TOC content. Greigite enhancement can be explained by variations in terrigenous inputs. Our studies demonstrate that, not only the greigite formation, but also its concentration changes could be useful for studying climatic and environmental variability in sulfidic environments.

Ferrimagnetic Iron Sulfide Formation and Methane Venting Across the Paleocene‐Eocene Thermal Maximum in Shallow Marine Sediments, Ancient West Siberian Sea

AbstractAuthigenesis of ferrimagnetic iron sulfide minerals (greigite and monoclinic pyrrhotite) occurred across the Paleocene‐Eocene Thermal Maximum (PETM) within the Bakchar oolitic ironstone in southeastern Western Siberia. Co‐occurrence of these minerals is associated with diagenetic environments that support anaerobic oxidation of methane, which has been validated by methane fluid inclusion analysis in the studied sediments. In modern settings, such ferrimagnetic iron sulfide formation is linked to upward methane diffusion in the presence of minor dissolved sulfide ions. The PETM was the most extreme Cenozoic global warming event and massive methane mobilization has been proposed as a major contributor to the globally observed warming and carbon isotope excursion associated with the PETM. The studied sediments provide rare direct evidence for methane mobilization during the PETM. Magnetic iron sulfide formation associated with methanogenesis in the studied sediments can be explained by enhanced local carbon burial across the PETM. While there is no strong evidence to link local methane venting with more widespread methane mobilization and global warming, the magnetic, petrographic, and geochemical approach used here is applicable to identifying authigenic minerals that provide telltale signatures of methane mobility that can be used to assess methane formation and mobilization through the PETM and other hyperthermal climatic events.

Natural synthesis of bioactive greigite by solid–gas reactions

Greigite, a ferrimagnetic iron sulfide Fe(II)Fe(III)2S4, is thought to have played an essential role in chemical evolution leading to the origin of life. Greigite contains a [4Fe–4S] cluster-like structure and has been synthesized in the laboratory by liquid-state reactions. However, it is unclear how greigite can be synthesized in nature. Herein, we show that greigite is synthesized by the solid–gas reaction of Fe(III)-oxide-hydroxides and H2S. We discovered that the hyperthermophilic hydrogenotrophic methanogen Methanocaldococcus jannaschii reduced elemental sulfur, and the resulting sulfide generated greigite from hematite. The time course and pH dependence of the reaction respectively indicated the involvement of amorphous FeS and H2S as reaction intermediates. An abiotic solid–gas reaction of hematite and H2S (g) under strictly anaerobic conditions was developed. The solid–gas reaction fully converted hematite to greigite/pyrite at 40–120°C within 12h and was unaffected by the bulk gas phase. Similar abiotic reactions occurred, but relatively slowly, with aqueous H2S in acidulous liquids using hematite, magnetite, or amorphous FeO(OH) as starting materials, suggesting that greigite was extensively produced in the Hadean Eon as these Fe(III)-oxide-hydroxides were shown to be present or routinely produced during that era. Surprisingly, the obtained greigite induced methanogenesis and growth of hydrogenotrophic methanogens, suggesting that the external greigite crystals enhanced reactions that would otherwise require enzymes, such as [4Fe–4S] cluster-harboring membrane-bound hydrogenases. These data suggested that the greigite produced by the solid–gas and solid–dissolved gas reactions was bioactive.

Greigite from carbonate concretions of the Ediacaran Doushantuo Formation in South China and its environmental implications

Greigite (Fe3S4) is a ferrimagnetic iron sulfide that commonly forms as a precursor of pyrite in anoxic environments where the supply of reactive Fe outpaces that of sulfide (H2S and HS−). Because of its metastability and sensitivity to redox changes during burial, greigite has been rarely documented in rocks older than the Cretaceous. Here we report well-preserved greigite in carbonate concretions of the upper Doushantuo Formation (ca. 551Ma) in the Yangtze Gorge area, South China. Greigite in the carbonate concretions coexists with anhedral and framboidal pyrite, and is distributed in clay-rich carbonates with card-house microtextures and dolomitic spherical structures indicative of early diagenetic formation during shallow burial. Preservation of greigite in carbonate concretions of the upper Doushantuo Formation implies that these concretions maintained a closed micro-system since their formation and that they provide information about the ancient depositional and early diagenetic environments.

Dissolution of titanomagnetite and sulphidization in sediments from Lake Kinneret, Israel

SUMMARY Magnetic extracts obtained from sediment cores recovered from Lake Kinneret, Israel, were subjected to comprehensive scanning electron microscope and in-line energy dispersive X-ray spectroscopy analyses. A variety of ferrimagnetic iron oxides and iron sulphide particles were identified, along with apparently non-magnetic iron sulphides. The majority of the iron oxides are titanomagnetites that have undergone pervasive dissolution, and in many cases were only preserved as skeletal, 3-D lattices of ilmenite lamellae. Often the spaces between the submicron ilmenite lamellae are empty. Also sponge-like dissolution features are observed in some residual titanomagnetite grains. Another group of titanomagnetite grains is characterized by superficial shrinkage-cracks, which indicate low-temperature maghemitization. In some cases, these cracks appear to be where dissolution started. Most titanomagnetites contain impurities of Al, Mg, Si and Mn on the order of 1 per cent. Chromium-magnetites, with minor Ti, Mg and Al content on the order of a few percent, but without Mn, were also found. Chromium-magnetites mostly occur as octahedra in a well-preserved state, which indicates that they are less prone to dissolution. Magnetite spherules of 40–60 μm in size, which were mostly hollow, were only found in material collected from the outer zone of the core section that was in contact with the steel core barrel. These spherules were not found in material collected from the centre of the core. They are therefore interpreted as contaminants that were produced during fabrication of the steel core barrel. Iron sulphides are present as pyrite, either as larger aggregates of cubic crystals, each of up to 20 μm in size, or as 5–10 μm framboidal aggregates, consisting mostly about 0.3 μm idiomorphic octahedra. The cubic pyrite crystals have heterogeneous compositions. Pure pyrite should be non-magnetic, but even aggregates of up to 150 μm in size were magnetically separated. They probably therefore contain a magnetic core of greigite or overgrown titanomagnetite. The latter possibility is supported by observations of residual titanomagnetites with individual pyrite crystals residing directly on their surfaces, or filling space between ilmenite lamellae. Irregular aggregates of flaky, submicron iron sulphides were also found, which are interpreted to be greigite. The greigite has a low degree of visible crystallization. Pyrite and/or greigite grains were found inside the aggregates, or in agglomerates with organic remnants. Together, visual observations and elemental analyses provide clear evidence of pervasive dissolution of primary titanomagnetite and concurrent sulphidization of sediments from Lake Kinneret.

Reductive diagenesis, magnetite dissolution, greigite growth and paleomagnetic smoothing in marine sediments: A new view

In many anoxic sedimentary environments, the onset of sulfate reduction, and pyritization of detrital iron-bearing minerals, leads to a precipitous decline in magnetic mineral concentration during early diagenesis. The usefulness of the surviving paleomagnetic record in such environments is usually argued to depend on how much of the primary detrital magnetic assemblage survives diagenetic dissolution. Detailed rock magnetic and electron microscope analyses of rapidly deposited (~ 7 cm/kyr) latest Pleistocene–Holocene sediments from the continental margins of Oman (22°22.4′N, 60°08.0′E) and northern California (38°24.8′N, 123°58.2′W) demonstrate that pyritization during early diagenesis also leads to the progressive down-core growth of the ferrimagnetic iron sulfide greigite. Greigite growth begins with nucleation of large concentrations of superparamagnetic (SP) nanoparticles at the inferred position of the sulfate–methane transition, which can explain the apparently paradoxical suggestion that diagenetically reduced sediments contain enhanced concentrations of SP particles. Looping of hysteresis parameters on a “Day” plot records the dissolution of single domain (SD) (titano-)magnetite and the formation of SP greigite, which then slowly and progressively grows through its SD blocking volume and acquires a stable paleomagnetic signal. This looping trend is also evident in data from several published records (Oregon margin, Korea Strait, Japan Sea, Niger Fan, Argentine margin, and the Ontong–Java Plateau), indicating that these processes may be widespread in reducing environments. Our observations have profound implications for paleomagnetic records from sulfate-reducing environments. The paleomagnetic signal recorded by greigite is offset from the age of the surrounding sediments by 10's of kyr, and ongoing growth of greigite at depth results in smoothing of the recorded signal over intervals of 10's to 100's of kyr. We therefore expect the presence of greigite to compromise paleomagnetic records in a wide range of settings that have undergone reductive diagenesis.

The magnetization of clay-rich rocks in sedimentary basins: low-temperature experimental formation of magnetic carriers in natural samples

Rocks are often magnetically altered during burial, thus providing a useful method to study their thermo-chemical history. In order to improve the understanding of burial diagenetic magnetization, we experimentally synthesized magnetite in natural samples. Lower Jurassic argillites from the Paris Basin were heated to 150 °C, under a confined atmosphere of argon. When a magnetic field was applied during the heating, a chemical remanent magnetization was acquired. Rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite. Using anhysteretic remanent magnetization as a measure of the quantity of magnetite, an increase of up to 150% after heating was observed. Ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3% calcite, and new magnetite is sometimes completely absent in samples containing less than 0.5% calcite. Hysteresis measurements suggest that the new magnetite is in the pseudo-single domain state. Scanning electron microscope studies indicate that the new magnetic carriers are associated with pre-existing pyrite. The magnetic transformations during the experiments involved a fluid which reacted with iron-bearing and sulfur-bearing minerals in ways depending on local redox conditions. This study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins, without any external supply from long-range fluid flow.

Magnetic mineral diagenesis in the post-glacial muddy sediments from the southeastern South Yellow Sea: Response to marine environmental changes

Core YSDPlO3 was retrieved in the muddy deposit under the cold eddy of the southeastern South Yellow Sea, and the uppermost 29.79 m core represents the muddy sediments formed in the shelf since about 13 ka BP. The lower part from 29.79 to 13.35 in, called Unit A2, was deposited during the period from the post-glacial transgression to the middle Holocene (at about 614c ka BP) when the rising sea level reached its maximum, while the upper part above 13.35 m (called Unit Al) was deposited in a cold eddy associated with the formation of the Yellow Sea Warm Current just after the peak of post-glacial sea level rise. Rock-magnetic properties of the uppermost 29.79 m core were investigated in detail. The experimental results indicate that the magnetic mineralogy of the core is dominated by magnetite, maghemite and hematite and that, except for the uppermast 2.35 m, the magnetic minerals were subject to reductive diagenesis leading to significant decline of magnetic mineral content and the proportion of low-coercivity component. More importantly, ferrimagnetic iron sulphide (greigite) is found in Unit A2 but absent in Unit Al, suggesting the control of marine environmental conditions on the magnetic mineral diagenesis. Magnetic parameters show abrupt changes across the boundary between Units A1 and A2, which reflects a co-effect of environmental conditions and primary magnetic components of the sediments on the diagenesis. Alternating zones of high and low magnetic parameters are observed in Unit A2, which is presumably due to periodic changes of the concentration and/or grain size of magnetic minerals carried into the study area.

Apparent magnetic polarity reversals due to remagnetization resulting from late diagenetic growth of greigite from siderite

A mixed-polarity zone, representing alternations between remagnetized and non-remagnetized strata, has been documented within the lower few metres of the CRP-1 core (Ross Sea, Antarctica). Detailed rock magnetic investigation of this interval indicates that the normal polarity remagnetization is carried by magnetostatically interacting single-domain particles of a ferrimagnetic iron sulphide mineral, while the reversed-polarity magnetization of non-remagnetized strata is carried by magnetite with a broad range of grain sizes and negligible magnetostatic interactions. Scanning electron microscope observations of polished sections indicate that the ferrimagnetic iron sulphide mineral is greigite (Fe3S4). Based on microtextural relationships, it is not possible to determine the relative timing of formation for much of the greigite. However, a significant proportion of the greigite has grown on the surface of authigenic siderite (FeCO3) grains that occur as microconcretions and as cement surrounding detrital matrix grains. In such cases, microtextural relationships indicate that siderite post-dates early diagenetic pyrite and that greigite post-dates the siderite. Siderite usually forms in environments with abundant dissolved iron and carbonate, but without dissolved pore water H2S. This set of geochemical conditions occurs in methanic settings below the sulphate reduction zone (in which early diagenetic pyrite forms). We interpret the observed remagnetization of the lower part of the CRP-1 core as due to a late diagenetic pore water migration event where abundant iron on the surface of siderite grains reacted with fluids containing limited dissolved sulphide, thereby causing precipitation of greigite. The distribution of siderite (and associated greigite) in the lower part of the CRP-1 core is patchy, which accounts for the apparent alternation of polarities. This study is part of a growing catalogue of remagnetizations involving greigite, which suggests that occurrences of greigite should be treated with caution in palaeomagnetic and environmental magnetic studies.

Mineral Magnetic Properties of Polluted Topsoils: A Case Study in Fujian Province, Southeast China

AbstractWe have conducted mineral magnetic investigations of polluted topsoil samples, which were collected in the vicinity of a steel plant and a coal‐burning power plant in Sanming City, Fujian Province, southeast China. The methods used include the temperature‐dependence of magnetic susceptibility, hysteresis loops and isothermal remanent magnetization acquisition curves. High‐temperature susceptibility measurements suggest that magnetite, partially oxidized magnetite, hematite and pyrrhotite exist in the polluted topsoil samples. The studied samples show mean magnetic grain‐sizes of large pseudo‐single domain, and multidomain‐like or multidomain. For the samples with significantly high susceptibility (x) and low frequency‐dependent susceptibility (x), x and xfd measurements can be used as an effective method for detecting the polluted topsoils. For the samples with relatively low x and high xfd, the presence of ferrimagnetic iron sulphide can serve as a potential pollution indicator in the case that an origin from bedrock weathering has been excluded. We further suggest that mineral magnetic measurements can provide a better and more complete understanding of the pollution processes forindustrially‐polluted topsoils.

Magnetic minerals as indicators of major environmental change in holocene black sea sediments: preliminary results

Four of the 38 sediment cores, collected from the northwestern part of the Black Sea (French–Romanian BLASON Programme, 1998), were analysed in order to study the major hydrological and environmental change of the sea during the Holocene. This change was caused by the inflow of the marine Mediterranean waters into the freshwater basin of the Black Sea. Two cores from the northwestern shelf and two from the deep part of the sea, representing distinct sedimentary environments along the same profile, were chosen. Lacustrine sediments from deep-water cores and uppermost marine sediments in all studied cores are characterised by the presence of pyrite (FeS 2), which indicates anoxic, sulphate reducing conditions in sediment and in water column (as it is observed now in the Black Sea). Pyrite formation in this anoxic environment takes place below the oxic/anoxic limit, by reduction and sulphidation of iron. The transition from freshwater to marine conditions is marked in all studied cores by the presence of ferrimagnetic iron sulphide––greigite (Fe 3S 4). Greigite is an intermediate mineral in the pyritisation reactions during early diagenesis, which can be preserved in the sediment if pyritisation cannot be accomplished, due to a high Fe/S ratio. In shallow water cores from the shelf, where marine transgression is marked by a discontinuity, greigite is present below and above it. This suggests that the anoxic, sulphate reducing conditions, allowing preservation of greigite, started in the sediment after marine transgression. In cores from the deep part of the sea, the transgression is represented by a level of sediments with high organic matter content (sapropel). The greigite is present only in the lower part of this level and its formation is related to the arrival of marine water. The presence of pyrite in the upper part of the lacustrine sediments indicates that anoxic conditions probably prevailed here during marine transgression. These results show the sensitivity of magnetic parameters to major environmental change in the Black Sea, and allow a correlation between cores from different depths, by recognition of greigite in the sediments marking the marine transgression.

Environmental mechanism of magnetic susceptibility changes of lacustrine sediments from Lake Hulun, China

The changes of magnetic susceptibility (κ) are correlated with those of corresponding sedimentological, geochemical, mineralogical and biological results, which verifies that κ can be taken as one of the environmental proxies. However, usually the exact origin of magnetic signal is poorly understood, and is difficult to relate with the environmental evolution. Magnetic properties of material derived from the catchment and sedimentary environment may affect the accumulation, preservation, or authigenesis and diagenesis of magnetic minerals. In the Lake Hulun region in Inner Mongolia, it is found that muddy sediments, deposited during high water level period (corresponding to humid climate), have comparatively high κ values. In contrast, the sandy sediments, deposited during low water level period (corresponding to arid climate), have low κ values. Detailed rock magnetic investigation confirms that detrital magnetite derived from volcanic rocks in the catchment exists in both muddy and sandy sediments. During high water level period, secondary ferrimagnetic iron sulphide was produced in muddy sediments under relatively reductive conditions. Ferrimagnetic iron sulphide, coexisting with detrital magnetite, predominates the magnetic properties of muddy sediments, resulting in increasing κ. This paper reveals the significance of authigenic ferrimagnetic iron sulphide produced after sediment deposition.

Incursion of sea water into Gucheng Lake detected by magnetic, biologic and chemical data

Magnetic, biological and chemical studies were performed on lacustrine deposits from Gucheng Lake, Jiangsu Province, China. Magnetite and haematite are identified along the entire profile. It is not clear whether magnetite or/and ferrimagnetic iron sulfide (likely greigite) predominates above a striking boundary at 12.0 m, expressed by higher susceptibility (χ) and remanence values. Low NRM intensities above 12.0 m correlate to lower P factors (χmaxχmin) and shallow inclinations of χmin, possibly indicating some disturbing remanence acquisition processes.Pollen, diatom, pigment and TOC analyses reveal that the early Holocene warming (started at the 13.7 m) led to a pulsative sea water incursion at 12.0 m. This incursion resulted in a period of brackish water, which favours the authigenesis of magnetite or ferrimagnetic iron sulphide, resulting in higher magnetic susceptibility and remanences values. Progressive freshing water occurred between 6.0 and 4.0 m. NRM changes correlate with these environmental trends. Magnetic changes above 2.5 m seems to be a result of human-driven deforestation and cultivation, as evidenced from pollen studies and lithological observations. Our results show that pollen and pigment assemblages directly reflect palaeoclimatic changes, whereas magnetic properties and diatoms respond primarily to the changes of hydrology and the limnology of Gucheng Lake.

Diagenetic magnetic enhancement of sapropels from the eastern Mediterranean Sea

Diagenetic dissolution of magnetic minerals has been widely observed in organic-rich sediments from many environments. Organic-rich sediments from the eastern Mediterranean Sea (sapropels), recovered during Leg 160 of the Ocean Drilling Program, reveal a surprising catalogue of magnetic properties. Sapropels, from all sites studied across the eastern Mediterranean Sea, are strongly magnetic and the magnetization is directly proportional to the organic carbon content. The magnetization of the sapropels is dominated by a low-coercivity, probably single domain magnetic mineral (with an inverse magnetic fabric) that exhibits a clear decay in magnetic properties when exposed to air. During heating, the magnetic particles irreversibly break down between 360 and 400°C. The contrast between the magnetic properties of sapropels and surrounding sediments is marked, with remanence intensities of sapropels often being more than three orders of magnitude higher than those of underlying sediments. The contrast between the magnetic properties of sapropels and the surrounding sediments is apparently controlled by non-steady-state diagenesis: sulphate-reducing conditions dominated during sapropel deposition, while overlying sediments were deposited under oxic conditions. The mineral responsible for the magnetic properties of sapropels is most likely to have formed under sulphate-reducing conditions that existed during times of sapropel formation. Attempts to identify this mineral have been unsuccessful, but several lines of evidence point toward an unknown ferrimagnetic iron sulphide phase. The influence of diagenesis on the magnetic properties of cyclically-deposited eastern Mediterranean sedimentary sequences suggests that magnetic parameters may be a useful proxy for diagenesis in these sediments.

Rock magnetism and palaeomagnetism of greigite-bearing mudstones in the Italian peninsula

Magnetic measurements were carried out on a collection of samples from different geological settings throughout the Italian peninsula. The samples display magnetic properties that indicate the presence of ferrimagnetic iron sulphide minerals. Paired samples were also investigated to compare the palaeomagnetic behaviour with respect to different demagnetization treatments (thermal, static and tumbling alternating field (AF) demagnetization). Greigite (Fe 3S 4) was positively identified in most samples. In agreement with previous studies of greigite, the sediments display: thermal decomposition of the magnetic carriers at temperatures above ca. 230°C, resulting in decreased magnetic susceptibility values and maximum unblocking temperatures ( T ub) mostly in the range 320–350°C; high saturation isothermal remanent magnetization (SIRM) to low-field magnetic susceptibility ( k) ratios (SIRM/ k > 10 kA/m); hysteresis ratios that are typical for single domain (SD) grains ( M rs/ M s>0.5 and ( B 0) cr/( B 0) c<1.5, where M rs is the saturation remanence, M s is the saturation magnetization, ( B 0) c is the coercive force and ( B 0) cr is the coercivity of remanence); moderate coercivity, with ( B 0) cr values that range between 52 and 81 mT and a tendency to acquire a significant rotational remanent magnetization (RRM). We also found that greigite-bearing sediments are particularly sensitive to field impressed anisotropy. We propose a new magnetic parameter that can be used as a rapid mean to screen a rock sample for the presence of greigite, which relies on its SD behaviour in field impressed anisotropy experiments. In greigite-bearing sediments, the magnetic susceptibility measured in a given direction is increased by the application of a relatively high magnetic field (0.9 T in our experiments) at right angles to this direction. The susceptibility increase is directly correlated to the concentration of greigite in the sediment. The comparative palaeomagnetic analysis of standard demagnetization techniques indicates that thermal demagnetization, in close steps up to 350–380°C, is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite, whereas AF demagnetization treatments display significant acquisition of unwanted magnetic remanences (gyroremanent remanent magnetization (GRM) and RRM) at fields above ca. 40 mT.

Magnetic properties of sedimentary greigite (Fe 3S 4)

New data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite (Fe 3S 4). Several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite. Such discriminants are necessary because greigite displays a similar (or higher) range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals. Sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility ( M rs/χ), which are accompanied by hysteresis ratios of M rsM s≈ 0.5 and B cr/ B c≈ 1.5 ( M s is the saturation magnetization, B c is the coercive force and B cr is the coercivity of remanence). Greigite has no low-temperature phase transition, whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 K, and unoxidized magnetite displays the Verwey transition at 118 K. Low-temperature measurements may therefore prove valuable in distinguishing between magnetite, pyrrhotite and greigite, although the absence of a low-temperature transition cannot be taken as definitive evidence for the presence of greigite. Greigite also displays characteristic high-temperature behaviour, with a major drop in magnetization between 270 and 350°C. Determination of the M rs/χ ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite. Subsequent low-and high-temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals.

Diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments, South Island, New Zealand

Detailed magnetostratigraphic studies of Late Neogene siliciclastic sediments of the Awatere Group, South Island, New Zealand (41°45′S, 174°05′E) have revealed a wide range of palaeomagnetic behaviour. Examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic, X-ray diffraction and electron microprobe analyses. These analyses indicate that the ferrimagnetic iron sulphide minerals, greigite and pyrrhotite, are responsible for a stable and intense magnetic remanence in fine-grained sediments, whereas titanomagnetite is the only remanence-bearing mineral identified in coarser-grained sediments, which are less strongly and less stably magnetised than the fine-grained sediments. Detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation, which occurs commonly in rapidly deposited, anoxic sediments that support active sulphate reduction and H 2S formation. Preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process, probably due to the low permeability of the fine-grained sediments and consumption of available H 2S before full reaction to pyrite occurred. Relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution.

Greigite (Fe 3S 4): magnetic properties and first domain observations

Recently the ferrimagnetic iron sulphide greigite (Fe 3S 4) has been recognized as an important carrier of magnetic remanence in young sediments. New magnetic measurements and properties of this rare mineral are presented. Magnetic domain observations were made using the Bitter method and the magneto-optical Kerr effect. In most cases the domain structures observed are very complicated and their interpretation is rather difficult. The cause of this domain structure is very likely high magnetostriction. A first attempt to establish a relationship between grain size and domain state is made.