Authigenic Greigite Research Articles

Environmental magnetic records derived from lacustrine sediments in the Western Guangdong Province, China: Implications for Late-Holocene climatic/environmental changes

The western Guangdong Province (WGDP) in China, influenced by both the East Asian and the Indian summer monsoon, is a pivotal region for exploring the Asian summer monsoon (ASM) evolution. So far, there still lacks detail studies on late Holocene climatic/environmental changes here. In this study, multiple magnetic parameters were measured on the lake sediments of Hedong section in this area. Results indicated a dominance of pseudo-single-domain detrital (titano-)magnetite or maghemite mixed with paramagnetic and anti-ferromagnetic minerals in sediments, and the impacts of authigenic greigite, bacterial activity, post-depositional dissolution, and dilution effects were trivial. Changes in rainfall-induced surface erosion intensity were determined as a role in sediment magnetic properties. Intensified surface erosion deduced by increased rainfall favored enhanced concentrations and more coarse-grained magnetic minerals in sediments, and vice versa. Thus, the magnetic parameters can be used to reflect varying ASM strength in the past ~4300 years. They revealed four relatively humid (strengthened ASM) (i.e., 4300–4000 cal. yr BP, 3500–3130 cal. yr BP, 2640–1950 cal. yr BP, and 1250–850 cal. yr BP), and three dry periods (weakened ASM) (i.e., 4000–3500 cal. yr BP, 3130–2640 cal. yr BP, and 1950–1250 cal. yr BP) which coincided with the Bond events 3, 2, and 1 respectively. In combination with the three significant cycles (i.e., 1250-, 75-, and 65-year) revealed by spectral analysis on SIRM record, we inferred both the external forcing (e.g., solar activity) and internal hemispheric-scale atmospheric processes (e.g., ENSO, North Atlantic Oscillation) have played an integral role in late Holocene climate changes in the study region.

Enigmatic mixture of magnetite magnetofossils and diagenetic greigite as the magnetic carriers of the Early Miocene lacustrine sediments from the Most Basin in Central Europe

The Early Miocene lacustrine sediments of the Most Basin in the Czech Republic preserve a European continental paleoenvironmental archive. A number of paleoenvironmental and magnetostratigraphic studies have been carried out on sediment cores from boreholes due to ongoing coal mining in the basin. However, the magnetic carriers of the studied sediments have not been identified clearly. Here, we present a detailed paleo-rock magnetic study from the Burdigalian sediments near the Bilina mining area, Most Basin. The studied clay sediments cover the period of local lakes and a basin-wide lake above the main coal seam. Our results suggest that the magnetic carriers of the studied section in the Most Basin are mixtures of authigenic greigite and magnetite magnetofossils with overlapping magnetic signatures. Greigite is formed by migration of pore water through the sediment column, where iron from siderite grains reacts with these fluids with limited H2S, which then favors greigite precipitation. The co-existence of greigite and magnetite indicates a partial dissolution of magnetofossils due to H2S deficiency. Diagenetic greigite has been problematic in paleomagnetic studies due to an unknown time lag between the depositional remanence and the chemical remanent magnetization (CRM). A ghost polarity interval reveals that greigite acquired at least ∼45 kyr delayed CRM. The revealed timing of remanence acquisition brings a new perspective to the chronostratigraphic structure of the Most Basin.

Mineral magnetic properties of surface soils from the Broknes and Grovnes Peninsula, Larsemann Hills, East Antarctica

This study presents the mineral magnetic, particle size, and organic content data of surface soils from Larsemann Hills, East Antarctica. The analysis of isothermal remanent magnetization and the high specific magnetic susceptibility values –mean (±S.D.) values of 117.7 (±175.0) × 10−8m3kg−1 for Broknes Peninsula and of 330.9 (±217.4) × 10−8m3kg−1 for Grovnes Peninsula– indicate high concentrations of low-coercivity magnetic minerals. The magnetic minerals are coarse-grained in the multidomain and pseudo-single domain range, and the significant correlation between some magnetic parameters suggests the dominant control of multidomain grains. The remanent acquisition coercivity H1/2 shows mean (±S.D.) values of 38.2 (±4.9) for Broknes and 38.3 (±3.7) mT for Grovnes soils suggesting magnetite dominance. The soils lie in the sand and loamy sand textural classes, and the concentration of organic matter is very low. Values of percentage frequency-dependent susceptibility indicate insignificant proportions of superparamagnetic grains, and therefore no significant evidence for pedogenic magnetic minerals was observed in these soils. The magnetic signal of Larsemann Hills soils was primarily terrigenous with no contributions from bacterial magnetite, authigenic greigite and anthropogenic magnetic minerals.

Exploring Spatiotemporal Paleoenvironmental and Paleoceanographic Changes on the Continental Shelf Using Authigenic Greigite: A Case Study From the East China Sea

AbstractThe lack of suitable indicators of changes in such as sea‐level and circulation has been a major limit to paleoenvironmental and paleoceanographic investigations in continental shelf regions. This paper presents an environmental magnetic study by comparing two late‐Quaternary sediment cores (DH02 and DH03) from the outer shelf of the East China Sea (ECS). Late and early Marine Isotope Stage (MIS) 3 sediments were deposited in a prodelta under cold coastal currents and an open‐shelf with the Taiwan Warm Current and upwelling. The dominant iron‐bearing minerals of the late and early MIS 3 sediments are authigenic greigite (Fe3S4) and pyrite (FeS2), respectively, which were assumed to be formed nearly syndepositionally. The overlying sediments, however, are magnetically dominated by detrital magnetite. This pattern corresponds well to the temporal changes in sea‐level over this period. The widespread occurrence of greigite in the late MIS 3 sediments can also be used for future stratigraphic division and correlation in the ECS. Additionally, compared to microfossil assemblages, rock magnetic parameters based on greigite may be more sensitive to environmental changes on continental shelves. Furthermore, the inter‐borehole spatial comparisons imply not only a sedimentary hiatus/erosion of at least 30‐m thickness in core DH02, most probably during the Last Glacial Maximum, but also that core DH02 was in a more reductive environment than core DH03 during late MIS 3. The findings highlight the potential of authigenic greigite as an indicator of spatiotemporal changes in paleoenvironmental and paleoceanographic conditions on the continental shelf at orbital or even suborbital timescales.

Authigenic greigite in late MIS 3 sediments: Implications for the Yellow Sea Cold Water Mass and Yellow Sea Warm Current evolution

Yellow Sea current conditions during the late Marine Isotope Stage 3 (late MIS3, 40–30 ka B.P.) highstand remain unknown. Here, environmental magnetism, geochemistry, particle size, and scanning electron microscope methods are employed to analyze core YS01A with a basal age of 40 kyr from the South Yellow Sea mud deposits. Greigite formed in marine sedimentary layers with thickness >7 m during early diagenesis. Greigite formation is related to the redox state and total organic content (TOC) availability, which are controlled, respectively, by the Yellow Sea Cold Water Mass (YSCWM) and the Yellow Sea Warm Current (YSWC). In MIS1 (7–0 ka B.P.), a strong YSWC brought greater TOC, and a strong YSCWM (thickness of 48 m) resulted in a strongly reducing low-oxygen environment. This led to massive magnetite dissolution and pyrite preservation. In late MIS3 (37–31 ka B.P.), the weak Kuroshio Current and East Asian Winter Monsoon (EAWM) resulted in a low-intensity YSWC that provided less TOC. A weak YSCWM (thickness of 35 m), caused by shallow water depths and a weak EAWM, contributed to a less reducing sedimentary environment. Thus, pyritization was interrupted and greigite was preserved. Greigite content can, thus, be used to trace the development of Yellow Sea currents. During a period of slow sea level rise at 41–37 ka B.P., minor greigite formed with most magnetite being preserved, which indicates that the environment started to become reducing. This can be interpreted as due to initial formation of a thin and weak YSCWM. During the 37–31 ka B.P. highstand, the greigite content increased, which suggests more reducing conditions related to establishment of a thicker and intensified YSCWM. In addition, the greigite content decreased progressively, which may have resulted from YSWC weakening. Our results suggest that greigite can be used as a simple tool to reflect current evolution in the Yellow Sea, which can give a better understanding of continental shelf environment change.

Genesis and preservation of authigenic magnetite and greigite in the cold seep sediments, Bay of Bengal

We report a new mechanism of authigenic greigite and magnetite formation and preservation in an anomalous “sulfide-rich” interval in the cold (methane) seep sediments from the Bay of Bengal. Rock magnetic proxies (SIRM/χlf, ARM/SIRM) coupled with microscopic (TEM, SEM) observations provided compelling evidence for the presence of nanoscale authigenic magnetite and greigite particles in the methanic sediments. Multiple peaks of SIRM/χlf coupled with high Hcr (>53 mT) and DJH values suggest significant contribution from authigenic greigite particles to the bulk sediment magnetism. High ARM/SIRM in the sulfide-rich intervals within methanic zone suggest that nanoscale authigenic magnetite formation is closely linked with the microbial iron reduction process fuelled by anaerobic oxidation of methane (AOM). We proposed that enrichment of reactive secondary iron (oxyhydr) oxides through selective leaching of Fe2+ during diagenetic dissolution and maghemitization of detrital magnetic particles and iron-sulfide mineral oxidation at the paleo-SMTZ fronts, hindered the pyritization processes and favoured microbial iron reduction which led to the formation and preservation of authigenic greigite and magnetite in methanic sediments.

Magnetic properties of gas hydrate-bearing sediments and their association with iron geochemistry in the Sea of Marmara, Turkey

The anaerobic oxidation of methane, a key geochemical process that is involved in the cycling of sulfate and iron (oxyhydr)oxides in marine sediments, results in the formation of iron sulfides. Although ferrimagnetic iron sulfides have been identified in seepage systems, the link between iron migration and sediment magnetic properties remains poorly understood. Here, we investigate two cores from the Sea of Marmara to evaluate biogeochemical iron cycling and iron sulfide mineralogy in gas hydrate-bearing sediments. Magnetic analyses indicate the presence of greigite and pyrrhotite in a core from a hydrate-rich site with a high hydrocarbon flux, which contrasts with a lack of these minerals in a core characterized by only mild seepage. This is supported by the results of rock magnetic and scanning electron microscope analyses of the sediments. The presence of authigenic greigite is critical for assessing local redox records and together with the occurrence of monoclinic pyrrhotite may suggest specific diagenetic processes in gas hydrate environments. Our analysis demonstrates the usefulness of these ferrimagnetic minerals, with a high saturation isothermal remanent magnetization to magnetic susceptibility ratio (SIRM/χ > 15 kAm−1) and a high index of hysteresis parameters (DJH > 0.2) indicative of magnetic mineralogy changes, for evaluating variability in the intensity of seepage fluxes and for estimating gas hydrate distributions.

Diagenetic analysis of shallow and deep-seated gas hydrate systems from the Bay of Bengal

This is the first systematic high-resolution rock magnetic study complemented by mineralogical and petrological observations conducted on sediment cores representing shallow (active cold seep; SSD-45/Stn-4/GC-01) and deep-seated (NGHP-01–15A) gas hydrate systems in the Bay of Bengal which addresses two key questions: (i) how magnetic minerals respond to the geochemical environment at two different diagenetic settings experiencing variable fluid sequence, and (ii) elucidates the control on the magnetite and greigite authigenesis in sulfidic, methanic, and gas hydrate bearing sediments. Titanomagnetite is the dominant detrital magnetic mineral identified in the studied cores from the Krishna-Godavari (K-G) basin along with diagenetic (pyrite) and authigenic (magnetite, greigite) minerals. Large detrital titanomagnetite and silicate-hosted (fine-grained) magnetic inclusions survived diagenetic dissolution during high sedimentation events. Magnetic proxies (ARM/SIRM and SIRM/χlf) provided useful insights on the diagenetic and authigenic formation and preservation of magnetic particles in the studied cores. Preferential diagenetic dissolution of finer (detrital) magnetic particles in sulfidic and authigenic formation of magnetite in methanic environment is clearly evident in the ARM/SIRM record of cores from both sites. Elevated values of SIRM/χlf in cores SSD-45/Stn-4/GC-01 (sulfidic and hydrate bearing) and NGHP-01–15A (methanic) indicate formation and preservation of ferrimagnetic authigenic (SP) greigite particles. Based on the magnetic proxies, we demonstrated that the formation of authigenic magnetite in the methanic environment of both sites is tightly linked with the microbial iron-reduction process. Multiple occurrences of authigenic carbonate provided evidence on the episodic intensification of anaerobic oxidation of methane (AOM) at active seep and silicate weathering coupled to microbial methanogenesis at deep-seated gas hydrate site respectively.

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.

Particle-size dependent magnetic property variations in the Yangtze delta sediments of late Holocene: Effects of pedogenesis and diagenesis

Late Holocene is a key period for the formation of modern deltas, and environmental magnetism is commonly used to infer paleoenvironmental information. Magnetic properties of bulk sediments are influenced by both hydrodynamic sorting and post-depositional pedogenesis and diagenesis, and the relationship between them is unclear. Detailed magnetic study on particle size fractions is a tool for understanding this issue and has been used in surface sediments, while the applications in delta core sediments are rare. Here we investigated the bulk and particle size dependent magnetic properties of the Yangtze River Delta core sediments over the past 3 ka, to determine the main mechanism for bulk magnetic property changes under the effects of pedogenesis and diagenesis. Our results showed that, in all particle size fractions, pedogenesis occurred in the top ∼2 m (within 0.3 ka) and the dissolution of fine magnetic minerals driven by early diagenesis appeared below this depth and completed at ∼20 m in depth (∼1.5 ka). These effects, including the dissolution of fine-grained ferrimagnetic minerals and the precipitation of authigenic greigite, were most obvious in the <16 μm fraction and decreased with coarsening particle sizes. The grain sizes of magnetic minerals were the finest in the <16 μm sediments and the coarsest in the 32–63 μm sediments, with intermediate similar features observed in the 16–32 and 63–125 μm sediments. Bulk magnetic properties were dominated by the <16 μm sediments. Our findings distinguished the differences between the effects of pedogenesis and early diagenesis among size fractions, which may provide a better approach to recognize redox states and provenance in delta sediments since the late Holocene.

Decadal Geomagnetic Secular Variations From Greigite Bearing Dead Sea Sediments

AbstractArcheomagnetic data from the Levant revealed periods within the Holocene with fast and extreme changes in the geomagnetic field. Yet, the availability of the archeomagnetic data is sporadic and the correlation with the available sedimentary records from the region is rather poor. To further explore decadal variations in the directions of the field, we investigate three outcrops of the late Holocene Dead Sea that are exposed along the western retreating shores of the modern lake. The sediments were deposited under spatially varying limnological‐environmental conditions, influencing their magnetic properties. The southern section, located near Ein‐Gedi Spa (EG section) is dominated by detrital titanomagnetite whereas the northern sections, Nahal Og (Og section) and Ein‐Feshkha (EF section), are dominated by authigenic greigite. The chronology of the sections was established by radiocarbon dating of short lived organic debris. The magnetic data were obtained in a 2 cm resolution. The EF section, spanning the time interval from ca. 2,500 cal yr BP to ca. 1,000 cal yr BP, is dominated by greigite and thus providing the most robust geomagnetic record with precise paleomagnetic directions. Greigite forms very early in the sediment and the effects of smoothing and the inclination shallowing are negligible. The new data reveal a maximal deviation of 20° from the geocentric axial dipole field between 2,400 to 2,200 cal yr BP accompanied with a fast swing in inclination from 60° to 35° over about a century. This suggests high geomagnetic field activity associated with the Levantine Iron Age geomagnetic anomaly.

Authigenic Greigite as an Indicator of Methane Diffusion in Gas Hydrate-Bearing Sediments of the Hikurangi Margin, New Zealand

Authigenic ferrimagnetic iron sulfides, essentially greigite (Fe3S4), are commonly found in gas hydrate-bearing marine sediments of active accretionary prisms. Greigite is a by-product, either intracellular or extracellular, of microbial activity, and therefore provides good indication of microbial processes which are closely related to the occurrence of gas hydrate. A high-resolution rock magnetic study was conducted at Site U1518 of International Ocean Discovery Program Expedition 375, located in the frontal accretionary wedge of the Hikurangi Margin, offshore New Zealand. Samples were collected throughout the entire recovered stratigraphic sequence, from the surface to ∼492 m below seafloor (mbsf) which includes the Pāpaku fault zone. This study aims to document the rock magnetic properties and the composition of the magnetic mineral assemblage at Site U1518. Based on downhole magnetic coercivity variations, the studied interval is divided into five consecutive zones. Most of the samples have high remanent coercivity (above 50 mT) and first-order reversal curves (FORC) diagrams typical of single-domain greigite. The top of the hanging wall has intervals that display a lower remanent coercivity, similar to lower coercivities measured on samples from the fault zone and footwall. The widespread distribution of greigite at Site U1518 is linked to methane diffusion and methane hydrate which is mainly disseminated within sediments. In three footwall gas hydrate-bearing intervals, investigated at higher resolution, an improved magnetic signal, especially a stronger FORC signature, is likely related to enhanced microbial activity which favors the formation and preservation of greigite. Our findings at the Hikurangi Margin show a close linkage between greigite, methane hydrate and microbial activity.

Authigenic Iron Sulfides Indicate Sea‐Level Change on the Continental Shelf: An Illustration From the East China Sea

AbstractAlthough sea‐level is the primary control over sedimentary architectures on continental shelves, deciphering such changes is still challenging, except for the easily recognized transgression‐regression cycles. This problem becomes more prominent in sedimentary units with relatively homogenous and fine‐grained lithologies that have been deposited over short periods. Here, we focus on a dominantly clayey silt section of a sediment core from the outer shelf of the East China Sea. Detailed rock magnetic and electron microscopy analyses indicate that authigenic greigite (Fe3S4) is the major magnetic component in an almost 4‐m‐thick interval that was deposited during the late Marine Isotope Stage (MIS) 3, while pyrite (FeS2) is enriched in the underlying middle MIS 3 sediments. According to microfossil analyses, the identified greigite formed in a prodelta setting that was affected by cold coastal currents after the middle MIS 3, when the Taiwan Warm Current and its associated upwelling were prevailing within a middle‐outer shelf setting favorable for pyrite formation. Large‐amplitude regression occurred subsequently during the MIS 2, limiting sulfate supply and favoring the greigite preservation. Our results, along with the previous studies, demonstrate that both the material and environmental conditions that are crucial to iron sulfide survival have been largely controlled by sea‐level change. Moreover, we make a preliminary proposition that a water depth range of 30–60 m is more likely to lead to the enrichment of greigite on continental shelves. We, therefore, provide a promising avenue for the assessment of sea‐level change on continental shelves over orbital and even suborbital timescales.

Magnetic properties of surface sediments in Schirmacher Oasis, East Antarctica: spatial distribution and controlling factors

PurposeWe investigated the magnetic properties (abundance, grain size, and mineralogy) of iron oxides present in Lake L-55 sediments, Schirmacher Oasis, East Antarctica, with an aim to understand their spatial distribution and the underlying mechanisms that control their formation and distribution.MethodsTwenty-five surficial sediments retrieved from different parts of Lake L-55 were subjected to the entire range of environmental magnetic (magnetic susceptibility, anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM)) measurements (at different field strengths). Inter-parametric ratios (χARM/SIRM, χARM/χlf, χARM/χfd, IRM20 mT/SIRM, IRM20 mT/ARM, S-ratio, L-ratio) provided insights into the magnetic properties (abundance, grain size, and mineralogy of iron oxides). Scanning electron microscopic-energy dispersive X-ray spectroscopic (SEM-EDS) analysis was performed on magnetic extracts from a few sediments. Besides, organic matter (%) was also calculated for the sediment samples. Principal component analysis was performed to gain information on the presence of different components and their relative dominance.ResultsThe iron oxides are strongly magnetic (high values of concentration-dependent parameters). The principal iron oxide is magnetite (S-ratio > 0.90) which is coarse-grained (multi-domain (MD) and stable single-domain (SSD) grains), and there is no influence of authigenic greigite, bacterial magnetite, and anthropogenic magnetite. The mineralogy is confirmed by SEM-EDS data. The iron oxides are of different grain sizes, and their contribution is in the order of MD > SSD > SP as shown by the principal component analysis. Pedogenic iron oxide minerals seem to be present in the samples whose formation is due to the oxidation of magnetite into hematite. However, they are of SSD size and not SP, suggesting that the intensity of pedogenesis is not sufficient to form SP grains.ConclusionThe iron oxide minerals are mainly terrigenous, and the biogenic activity within the lake is not sufficient to modify the ferrimagnetic minerals. Spatial distribution patterns suggest the non-uniform distribution of magnetite/titanomagnetite of varying sizes in the lake basin which is transported by both melt water streams and winds.

Magnetic Properties of Late Holocene Dead Sea Sediments as a Monitor of Regional Hydroclimate

AbstractDiagenetic processes in anoxic sedimentary environments influence sediment magnetic properties mainly through dissolution of detrital magnetite and precipitation of authigenic greigite. Recently exposed late Holocene Dead Sea sediments provide an opportunity to study the processes governing greigite formation and preservation, and their relation to different hydrological settings. Magnetic data and pore‐fluid compositions were obtained from three Holocene sections along a N‐S transect on the western Dead Sea shore: Og, Ein‐Feshkha (EF), and Ein‐Gedi. The northern sections are closer to the major freshwater source to the Dead Sea‐the Jordan River. Detrital titanomagnetite is present at all sections, but greigite is the dominant magnetic phase at Og and EF. Bulk rock magnetic data vary between and within the sections by over 3 orders of magnitude, where higher values indicate higher greigite concentrations. At the three sites, pore fluids have similar or lower salinity than the modern and Holocene Dead Sea brine, with variable and dissolved iron (Fe2+) and sulfate (SO42−). Magnetic property changes are reflected by iron and/or sulfate microbial reduction that controlled sedimentary greigite formation. We propose that the N‐S greigite decrease suggests that anoxic microbial activity was controlled by labile organic matter and/or reactive iron brought by, or formed as a result of, freshwater influx from the Jordan River. Hence, greigite concentration changes depended on past freshwater input to the hypersaline lake and proximity to the freshwater source. The apparent relationship between hydrological conditions and magnetic properties provides a new method to trace past hydrological changes in the Dead Sea.

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.

Formation of greigite (Fe3S4) in the sediments of saline lake Lop Nur, northwest China, and its implications for paleo-environmental change during the last 8400 years

We present the results of rock magnetic and SEM/EDX analyses of the sediments of Lop Nur, a former saline lake in the Tarim Basin, China. The aim of the study was to assess the impacts of authigenic iron sulfide minerals (e.g., greigite) on the sedimentary magnetic properties and their possible relationship with environmental and climate change. Five layers with relatively high values of χ, χARM, SIRM, S-ratio, χARM/χ and SIRM/χ are identified within organic-rich sediments; the five layers are dated to ca. 8150–7000, 5300–4900, 3600–3100, 2750–2550 and 1820–1680 cal. yr BP. Authigenic greigite is the dominant magnetic mineral within the five layers and it is present in the stable single domain state. The magnetic properties of the intervening layers are dominated by detrital multidomain magnetite. Combined with results of a previous study, we infer that the greigite-bearing sediments were deposited during cold and wet periods which were probably related to the North Atlantic cold events during the past 8400 yr. During these periods, greigite formation was likely driven by an enhanced organic matter supply, but pyritization was constrained by the decreased sulfate content and high sedimentation rate. The greigite and its diagnostic magnetic parameters (e.g., SIRM/χ) can be regarded as an indicator of the hydrological and climatic evolution of the Tarim Basin.

Paleomagnetism in Lake Pannon: Problems, Pitfalls, and Progress in Using Iron Sulfides for Magnetostratigraphy

AbstractDating of upper Miocene sediments of the Pannonian Basin (Hungary) has proven difficult due to the endemic nature of biota, scarcity of reliable radio isotopic data, and generally inconsistent magnetostratigraphic results. The natural remanent magnetization (NRM) is mostly residing in greigite (Fe3S4), which complicates NRM interpretation. We reinvestigate the viability of these sediments for magnetostratigraphy using samples from recently drilled well cores (PAET‐30 and PAET‐34) from the Paks region. Significant intervals of the cores contain composite NRM behavior. Thermal demagnetization results include multipolarity (M‐type) samples consisting of a low‐temperature (LT, above ~120 °C), a medium‐temperature (MT), and a high‐temperature (HT) component, within distinct temperature ranges and all exhibiting dual polarities. The LT and HT components have the same polarity and are antiparallel to the MT component. Rock magnetic and scanning electron microscopy results indicate that all magnetic components reside in authigenic greigite. The LT and HT components represent the characteristic remanent magnetization and are of early diagenetic origin. The MT component records a late diagenetic overprint. Alternating field demagnetization cannot resolve the individual components: it yields polarities corresponding to the dominant component resulting in erratic polarity patterns. Interpretation of LT and HT components allows a reasonably robust magnetostratigraphic correlation to the geomagnetic polarity time scale with the base of PAET‐30 at ~8.4 Ma and its top at ~6.8 Ma (average sedimentation rate of ~30 cm/kyr). The base of PAET‐34 is correlated to ~9 Ma and its top to ~6.8 Ma (average sedimentation rate of 27 cm/kyr).

Mineral magnetic properties of Holocene sediments in the subaqueous Yangtze delta and the implications for human activity and early diagenesis

This study examines the temporal and spatial distribution of magnetic properties in Holocene sediment cores taken from the subaqueous Yangtze delta, China, to evaluate depositional environmental changes including the sediment with an anthropogenic provenance and changes in the bottom-water chemistry. Our approach is to compare the magnetic properties in Holocene cores with those from a suite of surficial sediment samples taken at various locations from the Yangtze delta and the adjacent continental shelf. The results indicate that the magnetic properties in sediment cores change generally with sedimentary facies, mainly due to their in-phase changes with sediment grain size and redox conditions, but that they have also been significantly altered by effects of human activity and early diagenesis. Magnetic parameters that exhibit soil erosion associations show remarkable increases over the past ∼800 years, reflecting an increase in the terrestrial supply of fine-grained magnetic minerals induced by the intensification of human activity in the Yangtze River catchment. Early diagenesis was strong in core HZK8, located at the depocenter of the subaqueous Yangtze delta, including dissolution of fine-grained ferrimagnetic minerals and the precipitation of authigenic greigite and pyrite, as evidenced by both room-temperature and thermo-magnetic analyses. The dissolution of ferrimagnetic minerals coincides with changes in sedimentation rate at different sites and time periods, suggesting that this factor is important for controlling the rate of early diagenesis. Authigenic iron sulfides suggest the function of sulfate-reducing bacteria, which implies a hypoxic environment at this site from ∼6000 cal. yr BP.

Magnetic properties of tidal flat sediments on the Yangtze coast, China: Early diagenetic alteration and implications

Measurements of magnetic properties, total organic carbon (TOC) and total sulphur (TS) were performed on recent tidal flat sediments from the Feng-Xian (FX) and Bei-Bu-Gang (BBG) areas of the Yangtze coast and on Holocene tidal flat sediments from core SL67 from the southern Yangtze delta plain, China. The results indicate that greigite has likely formed in the recent upper and middle tidal flat sediments of cores FX and BBG, which are enriched in TOC and TS. Greigite is also present in association with pyrite in the early to middle Holocene saltmarsh and tidal flat sediments of SL67. The abundance of greigite in the early Holocene basal saltmarsh sediments of core SL67, along with the extremely low values of TS/TOC, suggest the presence of either a limited sulphate supply from seawater or a major terrestrial source of organic matter that decomposed slowly. The presence of greigite in mid-Holocene lower tidal flat sediments that contained low amounts of TOC indicates an upward diffusion of CH4 and H2S from the underlying upper tidal flat sediments. A comparison between the sediments of core SL67 and those of the modern tidal flat suggests that early diagenesis (including selective dissolution and pyritisation) continued after the sediments were buried during the Holocene, resulting in the depletion of soil-derived superparamagnetic (SP) particles and the enhanced magnetic properties because of authigenic greigite. However, antiferromagnetic magnetic minerals have not been altered by early diagenesis. Thus, magnetic parameter hard isothermal remanent magnetisation (HIRM) which preserved the primary magnetic signals reflects fluctuations in the detrital mineral input to the core site and records a two-stage 8.2 ka cooling event.