Magnetic Mineral Diagenesis Research Articles

Possible correlation between miocene global climatic changes and magnetic proxies, using neuro fuzzy logic analysis in a stratigraphic well at the Llanos foreland basin, Colombia

In this work we have assessed the hybrid algorithm of NeuroFuzzy logic (NFL), to establish a correlation between global climatic changes (benthic foraminiferal δ18O data), experimental S-ratio (factor characterizing stability of remanent magnetization) and magnetic susceptibility (κ). Magnetic proxies have been measured in 44 samples of the Colombian stratigraphic well Saltarin 1A (distal Llanos foreland basin). κ and Sratios were linked to global δ18O data assuming a constant accumulation rate for a 305 meters thick stratigraphic interval flanked by the two palynological age constrains available. This interval encompasses, from top to base, the bottom of the Guayabo formation, the Leon, and the upper unit of the Carbonera formations (lower to middle Miocene). The best inference is accomplished applying a Takagi-Sugeno-Kan (TSK) fuzzy model with four fuzzy rules and the δ18O, S-ratios and κ data used in a linear form to train the system. These results are interpreted as the outcome of a significant influence of global climatic changes upon magnetic proxies. A stronger correlation is perhaps prevented by the likely influence of local and regional tectonic events and climatic changes that could have affected the distal segment of the Colombian Llanos foreland basin during Miocene times. We argue that late diagenesis of primary magnetic minerals and the assumption of a constant accumulation rate might have a minor influence on these results.

Early diagenesis of magnetic minerals in marine transitional environments: geochemical signatures of hydrodynamic forcing

The low-field magnetic susceptibility ( χ) of surficial sediments from the Ria de Pontevedra (NW Spain) shows a characteristic, hydrodynamically driven textural control, where diamagnetic sandy bioclastic carbonates occur toward the coastline, leaving the finer muddy sediments to be deposited in the central and deeper areas of the ria. Three gravity cores were collected from the inner (core 1), middle (core 2) and outer (core 3) zones of the clayey-rich central axis of the ria, from which the magnetic properties, geochemical characteristics and hydrodynamic regime of the uppermost 90 cm of the sediment record were reconstructed. χ profiles have a distinctive pattern that is controlled by depth. Isothermal remanent magnetization (IRM) acquisition curves facilitated the identification of three well-defined zones with depth, depending on their degree of magnetic saturation, and thus of their dominant magnetic behaviour: magnetite-like, goethite-like and greigite-like. This zonation is interpreted as the result of mineralogical changes during early diagenesis. The relative depths of each zone are mostly dependent on distance to the open sea, with the magnetite-like zones being thicker in the outermost part of the ria. Toward the inner part of the ria, the three zones become shallower and thinner. In the innermost areas, only greigite-like coercivities are observed. Good correlation between magnetic and geochemical parameters such as χ vs. 2Fe/S, or H cr vs. C/S, demonstrates that magnetic property variations with depth are diagenetically controlled, and that these parameters can be used as magnetic proxies for early diagenesis. Similar significant positive correlations were found between χ of the clay fraction and modelled wave heights, and between other magnetic and hydrodynamic parameters, providing magnetic evidence of cryptic wave climate forcing of the diagenetic pathway. The combined analysis of the magnetic, geochemical and hydrodynamic data ultimately indicates that the sediment distribution and subsequent diagenetic pathway in this type of transitional environment mostly depend on the local distribution of wave energy and water depth, rather than on estuarine-like circulation processes.

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.

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

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

Monitoring Termination II by environmental magnetic records from subtropical South Atlantic sediments

Termination II, the transition from oxygen isotope substages 6.2 to 5.5 at around 130 ka, is the penultimate drastic change from extreme glacial to interglacial climatic conditions, invoking shifts in terrigenous and biogenous influx to the deep ocean. Environmental magnetic methods are applied to reveal these shifts in an ocean‐spanning transect of the subtropical South Atlantic. Magnetic mineralogy, grain size, and coercivity reflect intensified eolian inflow of volcanogenic material from the Andes due to enhanced continental aridity and wind intensity during substage 6.2. An African dust supply is only observable in continental vicinity. On the Santos Plateau, magnetic attributes indicate fluvial influx of eroded flood basalts. Layers of reductive magnetic mineral diagenesis were identified only at the westernmost sites and appear to reflect paleoredox boundaries related to enhanced Corg deposition at the termination and during stage 5.5. In all unaltered sections, stage 5.5 is magnetically similar to the Holocene.

Diagenesis of magnetic minerals in the intertidal sediments of the Yangtze Estuary, China, and its environmental significance

In this study, diagenesis of iron oxides in intertidal sediments of the Yangtze Estuary, China, has been investigated by combined environmental magnetic and geochemical methods. The results indicated that the magnetic properties of the sediments were dominated by ferrimagnetic magnetite. The content of Fe, DCB- and AOD-extractable iron oxides correlated positively with the concentration of fine grained magnetite near the super paramagnetic/stable single domain (SP/SSD, ∼0.03 μm) boundary, and with anti-ferromagnetic minerals (hematite/goethite). The magnetic parameters for core SDK indicated a substantial decrease in magnetite concentration from a depth of approximately 20 cm toward the surface, together with a shift in the grain-size distribution of magnetic minerals toward the coarse end, suggesting selective dissolution of fine grained magnetite under reducing conditions. The reduction of iron oxides inferred from magnetic measurements was supported by the similar decrease in the concentration of Fe and Mn and a lower ratio of Mn/Fe. Magnetic measurements on another core from elsewhere also indicated substantial reductive dissolution of iron oxides. In conjunction with the results of heavy metal analysis, it was suggested that the dissolution of iron oxides had a direct effect on the cycling of heavy metals. Therefore, magnetic measurements may provide useful information as to early diagenesis within intertidal sediments, which greatly influences the behavior of heavy metals in coastal environments.

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.

Rock-magnetic signature of gas hydrates in accretionary prism sediments

Sediments from two Ocean Drilling Program Leg 146 sites from the Cascadia margin of western North America have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates. Two indices combining coercivity, remanence, and susceptibility parameters, D JH ( = {J rs/J s}{H cr/H c”) and D S ( = {J rs/k}H cr), when combined with thermo-magnetic data, can be diagnostic of these changes. At Site 892, D S values are distinctly higher and more scattered above the bottom simulating seismic reflector (BSR), which marks the base of the hydrate stability zone. Within the hydrate stability zone at Site 892, D JH shows two trends: an increase from about 50 mbsf to the BSR at 73 mbsf, corresponding to an expected increase in hydrate concentration near the BSR; and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf, associated with hydrate recovered in cores above 19 mbsf. At Site 889/890 D JH increases downhole to about 285 mbsf, substantially below the BSR at 225 mbsf. High D JH sediments within a low Cl − zone at this site have magnetic mineralogies which are dominated by fine-grained magnetic sulfides, whereas sediments from above and below this zone are characterised by magnetite-magnetic sulfide mixes. This trend at Site 889/890 is consistent with an interpretation based on pore-water geochemistry (low Cl −) and bottom-water temperature that a ‘fossil gas hydrate zone’ extended downwards to about 295 mbsf during the last glacial. The observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through SD greigite to pyrite (or to overgrowth of SD greigite to MD size). Diagenetic growth of magnetic iron sulfides (greigite and/or pyrrhotite) has been reported in other accretionary wedge sediments. Thermal demagnetization of multi-component isothermal remanent magnetization (mIRM) indicates the presence of a low-coercivity magnetic mineral with an unblocking temperature (T ub) between 310° and 350°C. High J rs/k ratios suggest that the low-coercivity, low unblocking temperature mineral is predominantly greigite rather than pyrrhotite. A low- to medium-coercivity mineral with T ub ca. 580°C — magnetite — is also present in varying amounts. Hydrate apparently controls the presence of greigite by incorporating H 2S, shown to be present as a hydrate phase together with methane in hydrate recovered at Site 892. Release of H 2S below the base of the hydrate layer allows overgrowth of greigite grains to MD size or the conversion of some of the greigite to pyrite.

Magnetic mineral diagenesis in suboxic sediments at Bettis Site W‐N, NE Pacific Ocean

The effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from Bettis Site W‐N in the NE Pacific Ocean. As part of the oxidative decomposition of organic matter, ultrafine‐grained authigenic magnetites are produced immediately above the iron reduction zone. This zone is commonly observed in deep‐sea sediments as a prominent color change from brown to greenish grey. The magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties. The magnetites are composed of relatively pure Fe3O4 and fall within a restricted size range of 0.06–0.12 μm, which is nominally single domain. Some of these finest‐grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction, leaving a relatively coarser population. Magnetite dissolution appears to continue with depth according to a first‐order surface area reaction process when at steady state. Iron reduction is independent of sulfide formation in these sediments, because solid Sulfur profiles show no increase at depth and most of the sulfur occurs as barite. Despite a well‐constrained carbonate stratigraphy, comparisons of paleomagnetic directions between nearby cores show little agreement, suggesting that the directions are composed of a complex mixture of remanences acquired at different depths. Subsurface magnetic peaks may represent paleoredox markers preserved by non‐steady state redox conditions caused by climatically induced variations in organic matter flux and sedimentation rates.

Diagenesis of magnetic minerals in the recent sediments of a eutrophic lake

Magnetic mineral assemblages in lake sediments are usually interpreted as reflecting changing catchment erosion. Magnetic and chemical assemblages in a small eutrophic lake (Lough Augher, Northern Ireland) suggest diagenetic dissolution of fine‐grained magnetite, however, probably as a result of changing redox conditions associated with eutrophication. Water‐column results show internal cycling of Mn and Fe from the sediments into the anoxic hypolimnion. Calculations suggest that late‐summer hypolimnetic Fe concentrations are controlled by formation of authigenic vivianite; sedimentary pore‐water concentrations of total soluble Fe and P suggest a similar process in the sediments. The loss of Mn from the lake by diffusion from the sediment and washout at overturn is indicated by Mn budget calculations and by a marked decrease in its sedimentary concentration, reflecting onset of eutrophication as indicated by diatom biostratigraphy. The changes in the sediment Mn profile are coincident with, and in some cases identical to, changes in several magnetic parameters. The changes in magnetic stratigraphy suggest a coarsening of the magnetic grain size and reduction in the total concentration; agreement with chemical profiles indicative of changing redox suggest that this coarsening results from dissolution of fine‐grained magnetite. Catchment stability, together with other stratigraphic profiles of eutrophication and changing redox, supports the conclusion that the magnetic profiles are reflecting in situ diagenetic changes rather than altered catchment processes. With the data available it is not possible to identify the actual dissolution process involved or to determine whether it is chemically or bacterially controlled.

Diagenesis of magnetic minerals in the recent sediments of a eutrophic lake

Diagenesis of magnetic minerals in the recent sediments of a eutrophic lake

Diagenesis of magnetic minerals in Recent haemipelagic sediments

Rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the Earth's magnetic field. The objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments. While lacustrine sections are generally restricted to temperate latitude glacial lakes (≤ 20,000 yr old), rapidly deposited marine sediments along continental margins potentially offer continuous high resolution, yet long-term records of geomagnetic secular variation. However, to interpret the sedimentary magnetic record accurately, geochemical processes that affect the reliability of the magnetic signal must be understood. We now report downcore magnetic profiles from undisturbed Kasten cores taken in rapidly deposited laminated sediments from the Gulf of California and in bioturbated haemipelagic muds on the Oregon continental slope which give apparently reliable directions, but show dramatic decreases in the intensities of natural (NRM) and artificial (ARM, IRM) remanences with depth. Downcore porewater and solid sulphur analyses show concave-down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides. The maximum curvature of the sulphide profile occurs directly below the high magnetization zone. Combined with other compositional and mineralogical analyses, these data suggest that due to oxidative decomposition of organic matter, magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth. Iron reduction seems to occur before sulphide formation. Changes in magnetic stability parameters are consistent with selective dissolution of the finer-sized grains causing downcore coarsening of the magnetic fraction.