Magnetic Mineral Diagenesis Research Articles

Holocene Paleosecular Variations Recorded by Relict Magnetic Minerals in the Anoxic Black Sea Sediments

Continuous paleosecular variations reconstructed from sedimentary archives have remarkably deepened our insights into the dynamics of the Earth's magnetic field as well as the chronological purpose. Nevertheless, to construct reliable sedimentary paleomagnetic records in diagenetic reducing sediments is challenging generally due to the pervasive magnetic mineral diagenesis. The relict magnetic minerals are residuals after diagenesis and probably able to record a depositional remanent magnetization, thus it is worthy to explore their paleomagnetic potentials. For this study, two Black Sea sediment cores covering the past 8 ka were subjected to mineralogical and paleo- and rock magnetic analyses. Paramagnetic pyrite framboids are pervasive in the studied sediments deposited under anoxic bottom water conditions in the Black Sea. In addition, relict magnetic minerals of ferrous hemoilmenite, Fe-Mn and Fe-Cr spinels, and magnetite inclusions are also present in the studied cores. Compared to the previous published paleomagnetic results from the same cores over the last glacial period (20-30 ka) which are dominated by detrital (titano-)magnetite particles, the studied relict magnetic mineral samples exhibit a similar behavior in recording the depositional remanent magnetization. Furthermore, the obtained paleosecular variations spanning the past 8 ka reproduce the high intensity patterns observed in the regional archeomagnetic and volcanic datasets. Thus, the successful reconstruction of paleomagnetic secular variations from the anoxic Black Sea sediments greatly extend the application of paleomagnetism in sediments deposited in water with a reducing sub-surface environments from where paleomagnetic data are generally sparse.

Effect of Hydrocarbon Presence and Properties on the Magnetic Signature of the Reservoir Sediments of the Catcher Area Development Region, UK North Sea

This paper presents a detailed study investigating the effect of hydrocarbon presence on magnetic mineral diagenesis in sediments from the Catcher Area Development (CAD) region, UK North Sea, between 1,000 and 1,500 m (True Vertical Depth Sub-Sea). Magnetic analysis of core samples from hydrocarbon fields of the region and nearby dry-well sandstones (background) was carried out to determine if their signatures can serve as a proxy for understanding petroleum reservoir systems. From the background samples, nanometric and micron-sized magnetite, hematite and titano-iron oxides, were identified. Hydrocarbon presence in the reservoir sediments was found to diminish the iron-oxide signature and favour the precipitation of hexagonal pyrrhotite, siderite and potentially vivianite, lepidocrocite, greigite and paramagnetic iron sulphides. Hexagonal pyrrhotite was found at the oil-water transition zones. This relationship is possibly related to biodegradation at this interface. Siderite was found in increased abundance at shallower depths within the reservoir, which we attribute to hydrocarbon vertical migration and biodegradation. The interbedded shales also experienced significant magnetic mineral diagenesis that depended on its proximity to the hydrocarbon plume. These findings suggest that mineral magnetism can be applied to the identification of oil-water transition zones, reserve estimation, production planning and the determination of hydrocarbon migration pathways. It also suggests that mineral magnetic methods can be used to estimate the timing of hydrocarbon migration.

Diagenesis of Magnetic Minerals in Active/Relict Methane Seep: Constraints From Rock Magnetism and Mineralogical Records From Bay of Bengal

In this study, we conducted a comprehensive investigation of rock magnetic, mineralogical, and sedimentological records of sediment cores supplemented by a high resolution seismic data to elucidate the controls of structural and diagenetic (early vs. late) processes on the sediment magnetism in active and relict cold seep sites in the Bay of Bengal. Two distinct sediment magnetic zones (Z-I and Z-II) are defined based on the down-core variations in rock magnetic properties. The sediment magnetism is carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and authigenic (fine-grained greigite) minerals. Overall, the magnetic susceptibility varies over one order of magnitude with highest values found in relict core. Uppermost sediment magnetic zone (Z-I) is characterized by higher concentration of magnetite as seen through elevated values of magnetic susceptibility (χlf) and saturation isothermal remanent magnetization (SIRM). A systematic gradual decrease of χlf and IRM1T in Z-I is attributed to the progressive diagenetic dissolution of iron oxides and subsequent precipitation of iron sulfides. Magnetic grain size diagnostic (ARM/IRM1T) parameter decreases initially due to the preferential dissolution of fine-grained magnetite in the sulfidic zone (Z-I), and increases later in response to the authigenic formation of magnetite and greigite in methanic zone (Z-II). Distinct low S-ratio and χlf values in methanic zone of relict core is due to increased relative contribution from highly preserved coercive magnetic (titanohematite) grains of detrital origin which survived in the diagenetic processes. A strong linkage between occurrence of authigenic carbonates and greigite formation is observed. Two plausible mechanisms are proposed to explain the formation and preservation of greigite in Z-I and Z-II: 1) decline in methane flux due to massive hydrate accumulation within the active fault system and formation of authigenic carbonate crust in the sub-surface sediments hindered the supply of upward migrating fluid/gas; thereby limiting the sulfide production which preferentially enhanced greigite formation in Z-I and 2) restricted supply of downward diffusing sulfide by the carbonate layers in the uppermost sediments created a sulfide deficient zone which inhibited the pyritization and favoured the formation of greigite in the methanic zone (Z-II).

Controls of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record from the Bay of Bengal

We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sediment core (MD161/Stn-11) retrieved from a complex marine sedimentary system of Krishna-Godavari (K-G) basin to delineate the control of mass transport deposits (MTD's) and methane-induced diagenesis on the sediment magnetic record. Four sediment magnetic zones (Z-I, Z-II, Z-III, Z-IV) were defined based on rock magnetic signatures. The sediment magnetic signal is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and diagenetic (pyrite) minerals. Changes in rock magnetic properties are mainly controlled by fluctuations in supply of detrital magnetic particles, onset of MTD's and differential rate of methane-influenced magnetic minerals diagenesis in the studied sediment core. Downcore reduction in magnetic susceptibility followed by subsequent precipitation of iron sulfides within sediment magnetic zone (Z-I) representing the period of normal sedimentation can be attributed to diagenetic dissolution caused by anaerobic oxidation of methane coupled to sulfate reduction. Decline in magnetic susceptibility and increase in sediment grain size within MTD-rich sediment intervals (Z-II, Z-III, Z-IV) is linked to loss of finer magnetic grains due to diagenetic dissolution and dilution caused by increase in concentration of diamagnetic minerals. Lower values of magnetic grain size diagnostic (ARM/SIRM) parameter indicate loss of finer and selective retention of coarser magnetic particles due to diagenetic dissolution beyond 12 mbsf. Elevated content of total organic carbon (TOC) content in Z-III and Z-IV can be attributed to the efficient preservation of labile organic matter due to rapid sediment deposition. A conceptual model is presented to explain the control of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record.

Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

AbstractUnderstanding the locus of fluid flow along thrust and splay faults is important to understand the hydraulic properties of accretionary systems and fault mechanics. Here, we use rock magnetic techniques in combination with backscattered electron imaging to depict the locus of enhanced magnetic mineral alteration within the Pāpaku fault, an active splay fault of the subduction interface at the northern Hikurangi Margin. The Pāpaku fault was cored at Site U1518 during Expedition 375 of the International Ocean Discovery Program and we report room temperature magnetic parameters, complemented by first‐order reversal and thermomagnetic curves in the depth interval 250–400 m below seafloor (mbsf). The ∼60‐m wide Pāpaku fault zone comprises two main slip zones, referred to as the upper main brittle (304–321 mbsf) and lower subsidiary (351–361 mbsf) fault zones, and an intervening zoned, termed the lower ductile deformation zone. Two narrow zones, at the top of the main brittle fault zone, and one in a sand‐rich interval above the subsidiary fault zone, experienced enhanced magnetic mineral diagenesis, which resulted in the recrystallization of ferrimagnetic greigite to paramagnetic pyrite. We propose that secondary magnetic mineral diagenesis was driven by anaerobic methane oxidation within these intervals, which occurs in the presence of methane and sulfate. We relate the observed changes to the fault parallel transport of fluids which is restricted to two damage zones. Overlying compacted and clay‐rich sediments likely act as a barrier to upward advective flow through the fault zone and into the hanging wall.

Magnetic Mineral Diagenesis in a Newly Discovered Active Cold Seep Site in the Bay of Bengal

Diagenetically formed magnetic minerals at marine methane seep sites are potential archive of past fluid flow and could provide important constraints on the evolution of past methane seepage dynamics and gas hydrate formation over geologic time. In this study, we carried out integrated rock magnetic, and mineralogical analyses, supported by electron microscope observations, on a seep impacted sediment core to unravel the linkage between greigite magnetism, methane seepage dynamics, and evolution of shallow gas hydrate system in the K-G basin. Three sediment magnetic zones (MZ-1, MZ-2, and MZ-3) have been identified based on the down-core variations in rock magnetic properties. Two events of intense methane seepage are identified. Repeated occurences of authigenic carbonates throughout the core indicate the episodic intensification of anaerobic oxidation of methane (AOM) at the studied site. Marked depletion in magnetic susceptibility manifested by the presence of chemosynthetic shells (Calyptogena Sp.), methane-derived authigenic carbonates, and abundant pyrite grains provide evidences on intense methane seepage events at this site. Fracture-controlled fluid transport supported the formation of gas hydrates (distributed and massive) at this site. Three greigite bearing sediment intervals (G1, G2, G3) within the magnetically depleted zone (MZ-2) are probably the paleo-gas hydrate (distributed-type vein filling) intervals. A strong linkage among clay content, formation of veined hydrate deposits, precipitation of authigenic carbonates and greigite preservation is evident. Hydrate crystallizes within faults/fractures formed as the methane gas migrates through the gas hydrate stability zone (GHSZ). Formation of authigenic carbonate layers coupled with clay deposits restricted the upward migrating methane, which led to the formation of distributed-type vein filling hydrate deposits. A closed system created by veined hydrates trapped the sulfide and limited its availability thereby, causing arrestation of pyritization and favored the formation and preservation of greigite in G1, G2, G3.

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.

A rock magnetic perspective of gas hydrate occurrences in a high-energy depositional system in the Krishna-Godavari basin, Bay of Bengal

We conducted a detailed rock magnetic study complemented by sedimentological and mineralogical methods on a 177.2-m-long sediment core of Hole NGHP-01-14A to constrain the influence of high-energy depositional environment on the magnetic mineral diagenesis and formation of gas hydrates in the Krishna-Godavari (K-G) basin, Bay of Bengal. Five sediment magnetic zones were identified based on the downcore variation in rock magnetic parameters. The magnetic mineralogies comprised of detrital titanomagnetite, diagenetically formed magnetic iron sulfides and their mixture. A distinct magnetically enhanced zone (Z-3) close to a bottom simulating reflector (BSR) is dominated by higher magnetite content, sediment bulk density, and grain size and probably represents several sand-rich sediment layers formed as a result of short-duration intense sedimentation events and got rapidly buried and provided conducive environment for the formation of gas hydrate deposits at the studied site. The concurrence of gas hydrates and high gas saturations within the sand-rich layers (Z-3) suggests that the high-energy depositional environment had a larger control over hydrate formation at Hole NGHP-01-14A. Two magnetic iron sulfide bearing sediment intervals in Z-1b and Z-1c below sulfate-methane transition zone (SMTZ) suggest that their formation is controlled by microbially mediated diagenetic reactions fuelled by presence of gas hydrates and probably represents the fossil gas hydrate zones. We propose that the rock magnetic variations in the studied sediment core at Hole NGHP-01-14A is controlled by both differential loading of detrital magnetic minerals as well as hydrate-induced late diagenetic magnetic iron sulfide formation.

Diagenesis of magnetic minerals at the Southwest Pacific DSDP Site 277

ABSTRACTWe present a paleomagnetic and rock magnetic record from a 465.2 m long drill core recovered at DSDP Site 277 in the Southwest Pacific. Our study sought to construct a magnetostratigraphy and thus enable further paleoenvironmental work on this sedimentary succession. Magnetic remanence measurements revealed a stable magnetisation in the upper 34.8 m and lower 58.7 m of the core. However, between 34.8 and 397.5 m sediments are very weakly magnetised, do not carry a stable magnetisation, and rock magnetic analyses revealed a lack of remanence carrying minerals in this interval. We suggest that during the period spanning Paleocene to Oligocene sufficiently high organic matter (reagents) accumulation resulted in complete reduction and dissolution of ferri/ferromagnetic grains.

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

AbstractInternational Ocean Discovery Program (IODP) Site U1437 recovered an 1,800‐m‐long sediment sequence in a volcano‐bounded basin on the Izu rear arc. Pore fluid studies revealed a pattern of repeated fluid inputs, fluid diffusion, and methane and ethane accumulations, which represent “fluid anomalies” that disturb the fluid profiles. First‐order reversal curve analysis, magnetic hysteresis, saturation isothermal remanent magnetization, and low‐temperature remanence cycling reveal a detrital input dominated by vortex‐state and multidomain magnetite, which passes through an initial stage of partial magnetite dissolution and greigite authigenesis in the upper few tens of meters. Progressive magnetic mineral diagenesis comprises the continued loss of fine‐grained magnetite and gradual pyritization of greigite and produces a background logarithmic decrease in saturation isothermal remanent magnetization normalized by magnetic susceptibility. This process implies a continuing slow supply of S2− to depths >1,500 m. Thermally driven diagenesis, which would cause extensive loss of greigite at these depths, does not appear to be significant here. Multidomain magnetite grains dominate the magnetic mineralogy in the deepest part of the sequence, but some single‐domain magnetite survives as inclusions in silicates. Fluid anomalies representing sulfate influx drive locally renewed greigite authigenesis, as do methane and ethane accumulations. In some cases, where methane is accompanied by H2S (“sour gas”), fine‐grained greigite is converted to pyrite. We term these multiple episodes of enhanced magnetic mineral alteration “punctuated magnetic mineral diagenesis.” Despite both progressive and punctuated magnetic mineral diagenesis, enough depositional remanence survives to allow recognition of the magnetostratigraphy to 1,320 m below seafloor.

Magnetic mineral diagenesis in sediments of saline lake Lop Nur

Saline lakes are useful repositories for paleo-climatic records. In recent years, magnetic properties of saline lake sediments have been widely applied to establish paleo-climatic change. However, the influence factors of magnetic properties in saline lakes have not been fully understood, which complicates the paleoenvironmental interpretation. Here, we present a rock magnetic analyses result of LOP1 profile (40°26′09″′N, 90°21′23″E) from Lop Nur, a well-known saline lake, located in the eastern Tarim Basin in northwestern China. We combined the particle size, total organic content, and mineral characteristics analysis to assess the influence factors of magnetic properties in Lop Nur and its environmental significance. The results indicate that early diagenesis is the major influence factor on magnetic properties of Lop Nur saline sediments. Authigenic greigite and pyrite are identified within organic-rich sediments, which produce zones with high and low magnetic susceptibilities, respectively. The different authigenic iron sulfide contents in different layers are related to sedimentary environment changes. Sufficient supplies of organic matter and sulphate and low sedimentation rates favour the pyritization process. Moreover, if pyritization was constrained, intermediate greigite formed and was preserved. In oxidizing environments, sediment magnetic properties are consistent with those of source materials from Tarim Basin and are mainly controlled by particle size and hydrodynamic sorting of mainly detrital magnetite is largely unaffected by early diagenesis. Our study demonstrates that magnetic properties can provide a robust approach for studying depositional environment change in saline lake. In addition, the information obtained in this study would also provide insights into the geochemical processes of iron element in saline lakes.

Magnetic Mineral Diagenesis in a High Temperature and Deep Methanic Zone in Izu Rear Arc Marine Sediments, Northwest Pacific Ocean

AbstractMagnetic mineral diagenesis is an important process in sediments that is responsible for the partial or total destruction of records of Earth's magnetic field variations and also plays an important part in iron and sulfur cycling. A rock magnetic study has been carried out at International Ocean Discovery Program Expedition 350 Site U1437 in the Izu Bonin rear arc to investigate magnetic mineral diagenesis in deeply buried sediments from ~775 to ~1,002 m below sea floor (mbsf) with burial temperatures ranging from ~67 to 85 °C. Nonsteady state geochemical environments occur within an unusual deep methanic zone (below 850 mbsf) because of a release of sulfate below the shallow sulfate reduction zone. A drastic decline in magnetic susceptibility and remanence is observed at ~850 mbsf due to a decrease in ferrimagnetic iron oxide contents. Reduction of (titano)‐magnetite and pyritization occur at this inferred deep sulfate‐methane transition zone. Below ~850 mbsf, lower magnetic mineral contents coincide with further methane release. Geochemical analyses support a change in redox conditions and secondary precipitation of iron sulfides and carbonates in the methane‐rich zone. Magnetic mineral alteration is enhanced in zones where methane accumulates underneath low porosity intervals that act as seals. Although geochemical processes due to methane occurrence are likely dominant, more than one mechanism, possibly involving microbial activity, is probably responsible for the observed magnetic mineral assemblage changes.

Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First‐Order Reversal Curves

AbstractDiagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first‐order reversal curve unmixing method on well‐studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate‐methane transition. Below the sulfate‐methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.

New insights on sediment magnetic remanence acquisition point out complexity of magnetic mineral diagenesis

New insights on sediment magnetic remanence acquisition point out complexity of magnetic mineral diagenesis

Magnetic mineral diagenesis in anoxic laminated sediments from the Southern Gulf of California

Diagenetic effects on the magnetic mineralogy in marine sediments have long been investigated, including oxidation/reduction reactions, magnetic dilution, formation of iron sulfides and oxides, magnetization acquisition mechanisms and reliability of the paleomagnetic record. This study investigates diagenetic effects in low-oxygen depositional environments characterized by recent and past magnetic mineral dissolution zones. We analyze a marine sequence from the Alfonso Basin in the southern Gulf of California in which the topmost sediments show diagenetic effects marked by high magnetic enhancement factors. The susceptibility logs show high values at the top sediments with well-defined small amplitude low frequency fluctuations down core. Magnetic hysteresis loops indicate low coercivity saturation, characteristic of magnetites and low-tititanomagnetites with varying paramagnetic contributions. Intensity of natural remanent, isothermal and anhysteretic magnetizations and coercivity parameters show similar variation patterns with depth. The anhysteretic remanence intensity-susceptibility ratio shows an inverse correlation to magnetic susceptibility, indicating varying concentration of fine grained single domain and superparamagnetic particles. The magnetic logs record diagenetic changes and magnetite authigenesis, with preserved recent and old dissolution zones marked by enriched single-domain/pseudo-singledomain/multi-domain magnetite in between the dissolution fronts. The oxidation/reduction processes relate to climatic and water/sediment interface factors controlling the dissolution processes, which occur in the Alfonso Basin anoxic conditions.

Asian monsoon modulation of nonsteady state diagenesis in hemipelagic marine sediments offshore of Japan

AbstractWe have identified millennial‐scale variations in magnetic mineral diagenesis from Pacific Ocean sediments offshore of Japan that we correlate with changes in organic carbon burial that were likely driven by Asian monsoon fluctuations. The correlation was determined by identifying offsets between the positions of fossil diagenetic fronts and climatically induced variations in organic carbon burial inferred from magnetic and geochemical analyses. Episodes of intense monsoon activity and attendant sediment magnetic mineral diagenesis also appear to correlate with Heinrich events, which supports the existence of climatic telecommunications between Asia and the North Atlantic region. Several lines of evidence support our conclusions: (1) fluctuations in down‐core magnetic properties and diagenetic pyrite precipitation are approximately coeval; (2) localized stratigraphic intervals with relatively stronger magnetic mineral dissolution are linked to enhanced sedimentary organic carbon contents that gave rise to nonsteady state diagenesis; (3) down‐core variations in elemental S content provide a proxy for nonsteady state diagenesis that correlate with key records of Asian monsoon variations; and (4) relict titanomagnetite that is preserved as inclusions within silicate particles, rather than secondary authigenic phases (e.g., greigite), dominates the strongly diagenetically altered sediment intervals and are protected against sulfidic dissolution. We suggest that such millennial‐scale environmental modulation of nonsteady state diagenesis (that creates a temporal diagenetic filter and relict magnetic mineral signatures) is likely to be common in organic‐rich hemipelagic sedimentary settings with rapidly varying depositional conditions. Our work also demonstrates the usefulness of magnetic mineral inclusions for recording important environmental magnetic signals.

Magnetic mineral diagenesis in the river‐dominated inner shelf of the East China Sea, China

AbstractThe inner shelf of the East China Sea is a river‐dominated margin characterized by fine‐grained mud deposits and a rapid sedimentation rate. Three short sediment cores (~2.7 m in length) were examined to characterize spatial variations in magnetic mineral diagenesis. The sediment cores were analyzed for sedimentation rates, magnetic properties, particle size distribution, organic carbon, and total sulfur content. The two more proximal cores with higher sedimentation rates (~2.2 cm/yr and ~0.96 cm/yr) do not exhibit obvious effects of reductive dissolution of magnetite with increasing depth, which is consistent with their lower total sulfur content. The offshore core, A12‐4, which has a lower sedimentation rate, contains clear evidence of magnetite dissolution and increasing total sulfur content with depth. The three cores have a similar sediment source and organic matter input; therefore, we suggest that a higher sedimentation rate will lead to less reductive diagenesis of magnetite, assuming that other factors are constant. The iron‐ to sulfate‐reduction boundary, i.e., revealed by the onset of a rapid decline of magnetic susceptibility, is located 1.0 m below seafloor in core A12‐4. This is much deeper than is reported in many other coastal marine environments and can be explained by the higher sedimentation rate, the presence of refractory terrestrial organic matter, and an abundant input of detrital iron oxides. This study demonstrates that analyses of the magnetic mineral zonation provide a straightforward approach to assess diagenetic organic carbon decomposition pathways in marine environments.

Burial Diagenesis of Magnetic Minerals: New Insights from the Grès d’Annot Transect (SE France)

The diagenetic evolution of the magnetic minerals during burial in sedimentary basins has been recently proposed. In this study, we provide new data from the Grès d’Annot basin, SE France. We analyze fine-grained clastic rocks that suffered a burial temperature from ~60 to >250 °C, i.e., covering oil and gas windows. Low temperature magnetic measurements (10–300 K), coupled with vitrinite reflectance data, aim at defining the magnetic mineral evolution through the burial history. Magnetite is documented throughout the entire studied transect. Goethite, probably occurring as nanoparticles, is found for a burial temperature <80 °C. Micron-sized pyrrhotite is highlighted for a burial temperature >200 °C below the Alpine nappes and the Penninic Front. A model of the evolution of the magnetic assemblage from 60 to >250 °C is proposed for clastic rocks, containing iron sulfides (pyrite) and organic matter. This work provides the grounds for a better understanding of the magnetic properties of petroleum plays.

Diagenesis of magnetic minerals in a gas hydrate/cold seep environment off the Krishna–Godavari basin, Bay of Bengal

We carried out detailed magnetic measurements of the core (MD161/8) located in the vicinity of Site NGHP-01-10, where ~128 m of hydrate is confirmed by drilling/coring, to understand the diagenesis of magnetic minerals in a gas hydrates/cold seep environment. The rock magnetic measurements along with SEM-EDS and XRD analyses show a zone of reduced magnetic susceptibility (zone 2) where most of the magnetic minerals are dissolved. The enhanced concentration of chromium reducible sulfur (CRS) in this zone suggests an intense pyritization process while isotopically depleted authigenic carbonates indicate sulfate reduction via anaerobic oxidation of methane (AOM). Therefore, the dissolution of magnetic minerals is attributed to the HS - released during AOM that has resulted in the reduction in the magnitude of magnetic parameters. Within zone 2, a zone of enhanced susceptibility (zone 2a) is observed between 17.68 and 23.6 mbsf, and is located beneath the present day sulfate-methane transition zone (SMTZ). The frequencydependent magnetic susceptibility and low temperature magnetic measurements suggest the abundance of fine grained superparamagnetic (SP) sized ferrimagnetic particles. The SEM-EDS and XRD analyses show the presence of greigite which occurs in interstices between the pyrite crystals. Such occurrence of greigite in sediments has important implications in the interpretation of paleomagnetic records. We evaluated the likely mechanism for the greigite formation in KG offshore basin and our data suggest that the formation of greigite may be related to either paleo-SMTZ or anaerobic oxidation of pyrite. It is unlikely that the formation of greigite can be explained by the downward diffusion of sulfide below the current depth of SMTZ. However, further investigations are required to ascertain the mechanism for the

Sedimentation rate control on diagenesis, East China Sea sediments

Diagenesis of ferrimagnetic minerals can alter the magnetic properties, erasing partially or entirely the original paleomagnetic and paleoclimatic signals, especially on continental margins where sedimentation rates are usually high. Understanding the mechanisms by which magnetic particles are affected by diagenesis is therefore critical for retrieving paleomagnetic and paleoclimatic information from sediments. High-resolution magnetic analysis was carried out on rapidly deposited (0.4–20 mm/year) Holocene sediments from the East China Sea (ECS) inner continental shelf. The primary magnetic mineral assemblage in the sediment core contains ferrimagnetic minerals, such as magnetite, with minor contributions from hematite. The magnetic properties vary down-core in two steps, due to post-depositional reductive diagenesis. The first occurs at depths of 0.15–1.1 m and is characterized by reduction of both magnetite and hematite in suboxic sediments. The suboxic–sulfate boundary (SSB) is therefore located at 1.1 m. We demonstrate that the depth of the SSB has negative relation to the average sedimentation rate in Chinese marginal seas. The second step change in magnetic properties occurs at 3.2–5.8 m, and contains two intervals with extremely low magnetic mineral content, each corresponding to dissolution fronts associated with the former and present position of the sulfate–methanic transition (SMT). These two intervals correlate well with abrupt changes in sedimentation rate and separate the anoxic zone into two parts: the sulfidic zone and the methanic zone. Our study suggests that sedimentation rate provides a dominant control on magnetic mineral diagenesis, at least in the Chinese marginal seas, which controls not only the type of redox zonation, but also the thicknesses of diagenetic zones.