High-coercivity Magnetic Minerals Research Articles

Environmental magnetic properties of core sediments in the Yellow River subaqueous delta and their chronologicalapplications

The Yellow River has shifted frequently in the downstream region since 1885; it has flowed into the sea through the Qingshuigou channel from 1976 onward, thus forming the latest delta lobe (1976–present). During the past ~40 years, the sediment load of the Yellow River has been greatly reduced because of human and natural activities, such as reservoir construction, soil and water conservation, and river avulsion. Beginning in 2002, the water-sediment regulation scheme (WSRS) has also changed the sediment loading patterns of the Yellow River. Several studies have reported on the erosion and deposition characteristics of the latest lobe of the Yellow River Delta over the past several decades, based on remote sensing methods and water-depth measurements. However, continuous sedimentary records of delta erosion and deposition and their responses to natural and human activities in the watershed are lacking. Here, we obtained three sediments cores, YD01, YD02, and YD03, from the subaqueous delta of the latest lobe of the Yellow River Delta. These cores contain sediments deposited from 1976 to 2018. We analyzed the environmental magnetism and lithological characteristics of these cores, and investigated their responses to natural and human activities in the watershed of the Yellow River, as well as possible applications to sediment dating in the Yellow River Delta. The results show that frequency-dependent susceptibility ( χ fd%) and anhysteretic remanent magnetization susceptibility ( χ ARM) values were high in core sediments deposited from 1976 to 1996, indicating abundant contents of fine single-domain (SD) and superparamagnetic (SP) magnetic particles, and therefore a dominant sediment supply from the Chinese Loess Plateau. Small decreases in χ fd% and χ ARM in sediments deposited after 1986 revealed slight reduction of the sediment load from the Yellow River, which was likely caused by sediment retention related to the construction of the Longyangxia Reservoir. Rapid rises in χ fd% and χ ARM occurred in core YD01 sediments deposited during 2002 and ~2007, likely in response to the implementation of the WSRS, which resulted in an increase in sediment transportation. Increases in hard isothermal remanent magnetization (HIRM) values, an indicator of high-coercivity magnetic minerals such as goethite and hematite, in cores YD02 and YD03 from 2002 to ~2007 may also represent the start of the WSRS. This is because the downstream riverbed of the Xiaolangdi Reservoir underwent intense erosion during the implementation of the WSRS, leading to large-scale transportation of coarse sediments enriched in goethite and hematite, which were deposited on the riverbed before the implementation of the WSRS. The lack of increased contents of fine SD and SP magnetic minerals in cores YD02 and YD03 compared with YD01 in the interval deposited between 2002 and ~2007 was attributed to erosion and redeposition in the areas around cores YD02 and YD03. Large drops in SD and SP contents after ~2007 were detected in all three cores, revealing intense erosion near the river mouth likely caused by river channel shift and reduced capability of the WSRS to remove sediments. Contents of SD and SP particles increased again in sediments deposited after 2014 in cores YD02 and YD03, likely related to erosion of the present (after 1996) Yellow River mouth and transportation of fine particles to the region near cores YD02 and YD03. In summary, the magnetic parameters χ fd% and χ ARM and the lithological characteristics of cores YD01, YD02, and YD03 recorded sediment delivery changes caused by four events in the Yellow River watershed: The main river channel shift in 1976, construction of the Longyangxia Reservoir in 1986, the beginning of the WSRS in 2002, and weakened capability of the WSRS to remove sediments after ~2007. The notable changes of χ fd%, χ ARM, and lithology at the 1976, 1986, 2002, and ~2007 horizons may be used as dating points in future research of sediment cores from the Yellow River Mouth.

High-coercivity magnetic minerals in archaeological baked clay and bricks

SUMMARYThe thorough understanding of magnetic mineralogy is a prerequisite of any successful palaeomagnetic or archaeomagnetic study. Magnetic minerals in archaeological ceramics and baked clay may be inherited from the parent material or, more frequently, formed during the firing process. The resulting magnetic mineralogy may be complex, including ferrimagnetic phases not commonly encountered in rocks. Towards this end, we carried out a detailed rock magnetic study on a representative collection of archaeological ceramics (baked clay from combustion structures and bricks) from Bulgaria and Russia. Experiments included measurement of isothermal remanence acquisition and demagnetization as a function of temperature between 20 and >600 °C. For selected samples, low-temperature measurements of saturation remanence and initial magnetic susceptibility between 1.8 and 300 K have been carried out. All studied samples contain a magnetically soft mineral identified as maghemite probably substituted by Ti, Mn and/or Al. Stoichiometric magnetite has never been observed, as evidenced by the absence of the Verwey phase transition. In addition, one or two magnetically hard mineral phases have been detected, differing sharply in their respective unblocking temperatures. One of these unblocking between 540 and 620 °C is believed to be substituted hematite. Another phase unblocks at much lower temperatures, between 140 and 240 °C, and its magnetic properties correspond to an enigmatic high coercivity, stable, low-unblocking temperature (HCSLT) phase reported earlier. In a few samples, high- and low unblocking temperature, magnetically hard phases appear to coexist; in the others, the HCSLT phase is the only magnetically hard mineral present.

Magnetostratigraphic dating of a drill core from the Northeast Plain of China: Implications for the evolution of Songnenpaleo-lake

Lacustrine sediments from the Northeast Plain of China are an ideal archive for recording the climatic history and variability of the East Asian monsoon during the Quaternary in mid-latitude East Asia. However, the lack of a high-resolution stratigraphy and high-precision geochronology of these sediments has limited our understanding of the mechanisms of Quaternary climatic and environmental changes in the region. In this study, we present a high-resolution magnetic susceptibility stratigraphy and precise paleomagnetic and optically stimulated luminescence (OSL) chronologies for core Qiananlingzi (QAL) from the Northeast Plain. Paleomagnetic data were obtained using alternating-field (AF) demagnetization with a peak AF of 80 mT and increments of 2.5−10 mT. In addition, OSL dating was conducted on coarse-grained quartz (90−150 µm) using the single aliquot regenerative dose (SAR) protocol. Correlation of the recognized polarity intervals of the QAL core to the geomagnetic polarity timescale was achieved by combining the magnetostratigraphic and OSL geochronologic data. The results indicate that the sedimentary sequence spans the interval from the Olduvai normal subchron to the end of the Brunhes normal chron. According to the lithology and the magnetic susceptibility record, the core can be divided into four intervals, from bottom to top: the Qian’an Formation (fluvial yellow-green, brown-red and grey-brown silty sand, interbedded with brown-black silty clay, with gravel layers in the lower part); the Lingzi Formation and Lindian Formation (lacustrine grey-green clay interbedded with grey-white silty sand, with fossil bivalves); and a loess-like soil (predominantly brown-yellow clayey silt, alternating with fluviolacustrine grey-white, brown and black clay). The magnetic susceptibilities (MS) of the sediments of the lower fluvial Qian’an Formation and upper loess-like soil strata are generally higher, indicating the neoformation of strongly magnetic minerals under predominantly oxidizing conditions. By contrast, lower MS values occur in the lacustrine sediments of the Lingzi Formation and Lindian Formation, which indicate a reducing environment. Overall, the MS record of the QAL core exhibits clear orbital-scale variations, with higher MS values in coarse-grained silt layers, and lower values in fine-grained clay layers. After the application of a 300 mT DC field, the isothermal remanence (IRM) is only 70%, and a hysteresis loop remained unclosed at 500 mT. These characteristics indicate the substantial presence of high-coercivity magnetic minerals in the intervals with low MS values. Thermomagnetic measurements ( χ - T ) curves show an abrupt decrease in the heating curve at 580°C, indicating that magnetite is the dominant magnetic mineral, and a further decrease above 580°C indicates the presence of hematite. After the application of an applied DC field of 300 mT, the IRM reached 90%, the hysteresis loop was closed, and the heating limb of the χ - T curve did not decrease significantly after 580°C, indicating that low-coercivity magnetic minerals contributed substantially to the magnetic properties within the intervals of higher MS. The lacustrine sediments of the core were dated to ~1180‒450 ka based on paleomagnetic and QSL dating, and interpolation of the age-depth relationship suggests that Songnen paleo-lake existed for ~730 ka. The grain-size record of the loess in the northeastern monsoon region shows no evidence of drought events at 450 ka, and records of MS and FeD/FeT from the deposits of the loess Plateau indicate a trend of increasing summer monsoon precipitation at this time. Therefore, the disappearance of the paleo-lake may be related to the multiple episodes of subsidence of the Yishu fault, which caused a diversion of the discharge of the Songhua River from the Songnen paleo-lake to the Sanjiang Plain. The high-resolution MS record and the precise chronology obtained in this study indicate that the lacustrine sedimentary sequence of QAL core can potentially resolve orbital-scale paleoclimate changes during the middle Pleistocene in mid-latitude East Asia. The Mid-Pleistocene Transition (MPT) is one of the most important climatic events in Quaternary paleoclimatology, and it is necessary to understand the response of different climatic systems to the MPT. Detailed environmental magnetic, sedimentological and geochemical studies of lacustrine sedimentary strata in the Northeast Plain in China should be carried out to explore the paleoclimatic record of the area, including the regional climatic response to the MPT.

Coercivity and Vector Magnetization Analysis of Obsidian Samples from the Trans-Mexican Volcanic Belt

This note presents initial results of a paleomagnetic study of obsidian from twenty localities in the eastern, central and western sectors of the Trans-Mexican volcanic belt in central Mexico. We focus on the coercivity and vector composition of the remanent magnetization, which are critical for paleodirectional and paleointensity studies. Alternating field demagnetization shows that obsidians carry single and two-component magnetizations residing in low- and high-coercivity magnetic minerals, with discrete and overlapping coercivity spectra. Magnetic minerals are likely iron-titanium oxides with fine-grain sizes characterized by pseudo-single domain states.

Unmixing hysteresis loops of the late Miocene-early Pleistocene loess-red clay sequence.

Magnetic paleoclimatic records often represent mixed environmental signals. Unmixing these signals may improve our understanding of the paleoenvironmental information contained within these records, but such a task is challenging. Here we report an example of numerical unmixing of magnetic hysteresis data obtained from Chinese loess and red clay sequences. We find that the mixed magnetic assemblages of the loess and red clay sediments both contain a component characterized by a narrow hysteresis loop, the abundance of which is positively correlated with magnetic susceptibility. This component has grain sizes close to the superparamagnetic/stable single domain boundary and is attributed to pedogenic activity. Furthermore, a wasp-waisted component is found in both the loess and red clay, however, the wasp-waisted form is more constricted in the red clay. We attribute this component to a mixture of detrital ferrimagnetic grains with pedogenic hematite. The abundance of this component decreases from the base to the top of the red clay, a pattern we attribute to decreased hematite production over the Chinese Loess Plateau (CLP) due to long-term climate cooling. This work demonstrates the potential of hysteresis loop unmixing to recover quantitative paleoclimatic information carried by both low and high coercivity magnetic minerals.

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

Abstract. The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before  ∼  320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments. While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land and intra-lake processes.

Magnetic properties of agricultural soil in the Pearl River Delta, South China — Spatial distribution and influencing factor analysis

Abstract Environmental magnetism has been widely applied to soil science due to its speediness, non-destructiveness and cost-effectiveness. However, the magnetic investigation of agricultural soil, so closely related to human activity, is limited, most probably because of its complexity. Here we present a magnetic investigation of 301 agricultural soil samples collected from the Pearl River Delta (PRD, 112°E–115°E and 22°N–24°N), China. The results showed that both low and high coercivity magnetic minerals coexist in agricultural soil. The values of concentration-dependent parameters, low-field susceptibility (χlf), anhysteretic remanence magnetization susceptibility (χARM), and saturation isothermal remanence magnetization (SIRM) were much higher in the PRD plain than in the surrounding areas. The S-ratio (S− 300) showed a similar spatial pattern to the aforementioned parameters. By contrast, frequency-dependent susceptibility (χfd%) and χARM/SIRM were higher in the surrounding hilly and mountainous areas than in the PRD plain. Natural and anthropogenic factors such as parent material, soil type and cultivation methods play important roles in determining agricultural soil magnetic properties. Magnetic minerals were coarser grained and overall indicated higher concentrations in soils from river alluvium and deposited materials. Soils which had suffered long-term water submergence have the lowest magnetic mineral concentration, a result consistent with previous studies. The magnetic properties of agricultural soils are strongly influenced by cultivation methods. Other human activities, such as industrial development and concomitant emitted pollutants, might have had an additional impact on the magnetic properties of agricultural soil.

Onshore–offshore gradient in reductive early diagenesis in coastal marine sediments of the Ria de Vigo, Northwest Iberian Peninsula

Early diagenetic modification of magnetic properties is an important process in marine sediments, but temporal and spatial variability of diagenetic processes have rarely been reported for recent coastal sediments. The magnetic properties of sediments from the Ria de Vigo (NW Spain) define a marked three-part zonation with depth. The uppermost zone is magnetically dominated by (titano-)magnetite. In the intermediate zone, rapid down-core dissolution of (titano-)magnetite increases the relative influence of high-coercivity magnetic minerals, which react more slowly during reductive dissolution than (titano-)magnetite. This zone is characterized by the ubiquitous occurrence of framboidal iron sulphides. Pyrite is the dominant iron sulphide, but framboidal ferrimagnetic greigite is also frequently observed in association with pyrite. The lowermost zone is characterized by an almost complete depletion of magnetic minerals associated with progressive reduction of detrital iron oxides with depth. This zonation is controlled by organic matter diagenesis, which varies with water depth and wave-induced sediment resuspension and organic matter reoxidation in the water column. This leads to a shallowing and thinning of each zone with more intense reductive diagenesis toward the interior of the ria. Such a zonation seems to be a common feature in shallow water marine environments. If preserved, the described zonation and its spatial variability provide a potential tool for detecting estuarine-like environments in the geological record. Magnetic detection of current or past reductive conditions also has important implications for assessing paleoenvironmental proxies that are sensitive to diagenetic redox state.

A magnetic mineral record of Late Quaternary tropical climate variability from Lake Bosumtwi, Ghana

We report magnetic hysteresis results from sediment cores obtained from Lake Bosumtwi, Ghana. As a hydrologically closed basin, the water budget of Lake Bosumtwi is extremely sensitive to changes in the precipitation/evapotranspiration balance. Lake Bosumtwi lies in the path of the seasonal migration of the intertropical convergence zone (ITCZ); hence, the lake is ideally situated to study monsoon variability in West Africa. Five distinctive magnetic mineral zones (A–E) were identified in the 11-m-long sediment cores that span the last 26,000 calendar years. Prior to 12 calendar (cal) ka, low concentrations of multidomain, high-coercivity magnetic minerals are present. Three prominent shifts towards very high concentrations of high-coercivity iron sulfide (greigite) magnetic minerals are centered at 12,470, 17,290, and 22,600 calendar years during the last glacial period (magnetic zones D 1–3). Between 12 and 3.2 cal ka, there is an abrupt shift to moderately high concentrations of mixed multidomain and single-domain, low-coercivity minerals and an organic-rich sapropel lithology. Since 3.2 cal ka, the magnetic mineral parameters reveal a shift to increased amounts of high-coercivity magnetic minerals. These magnetic mineral zones document tropical climate variability on a variety of temporal scales. Glacial age sediments have a high-coercivity magnetic mineralogy due to increased aeolian dust transport from the Sahel to Lake Bosumtwi as well as postdepositional reductive diagenesis. During the last glacial period, the increased strength of Harmattan and North African continental trade winds, the southward depression of the ITCZ, and weakened summer monsoon strength resulted in increased regional aridity and greater dust flux out of Sahel source regions. The greigite-bearing D magnetic zones correspond to brief lowstands in the level of Lake Bosumtwi and likely represent periods of intensified aridity in West Africa. The D magnetic zones closely resemble the timing and duration of Heinrich events and suggest a hemispheric-scale climatic coupling between the tropics and poles. The well-documented African humid period (AHP) is characterized by abrupt shifts in magnetic parameters between 12 and 3.2 cal ka. Dust flux to Lake Bosumtwi is inferred to be very low during this humid interval due to the strengthening of the summer monsoon. Since 3.2 cal ka, the magnetic mineral parameters suggest increased aridity as compared to the AHP. This work demonstrates that the magnetic properties of Lake Bosumtwi sediment are a sensitive recorder of abrupt climate change of global significance.

A magnetic mineral record of Late Quaternary tropical climate variability from Lake Bosumtwi, Ghana

Abstract We report magnetic hysteresis results from sediment cores obtained from Lake Bosumtwi, Ghana. As a hydrologically closed basin, the water budget of Lake Bosumtwi is extremely sensitive to changes in the precipitation/evapotranspiration balance. Lake Bosumtwi lies in the path of the seasonal migration of the intertropical convergence zone (ITCZ); hence, the lake is ideally situated to study monsoon variability in West Africa. Five distinctive magnetic mineral zones (A–E) were identified in the 11-m-long sediment cores that span the last 26,000 calendar years. Prior to 12 calendar (cal) ka, low concentrations of multidomain, high-coercivity magnetic minerals are present. Three prominent shifts towards very high concentrations of high-coercivity iron sulfide (greigite) magnetic minerals are centered at 12,470, 17,290, and 22,600 calendar years during the last glacial period (magnetic zones D1–3). Between 12 and 3.2 cal ka, there is an abrupt shift to moderately high concentrations of mixed multidomain and single-domain, low-coercivity minerals and an organic-rich sapropel lithology. Since 3.2 cal ka, the magnetic mineral parameters reveal a shift to increased amounts of high-coercivity magnetic minerals. These magnetic mineral zones document tropical climate variability on a variety of temporal scales. Glacial age sediments have a high-coercivity magnetic mineralogy due to increased aeolian dust transport from the Sahel to Lake Bosumtwi as well as postdepositional reductive diagenesis. During the last glacial period, the increased strength of Harmattan and North African continental trade winds, the southward depression of the ITCZ, and weakened summer monsoon strength resulted in increased regional aridity and greater dust flux out of Sahel source regions. The greigite-bearing D magnetic zones correspond to brief lowstands in the level of Lake Bosumtwi and likely represent periods of intensified aridity in West Africa. The D magnetic zones closely resemble the timing and duration of Heinrich events and suggest a hemispheric-scale climatic coupling between the tropics and poles. The well-documented African humid period (AHP) is characterized by abrupt shifts in magnetic parameters between 12 and 3.2 cal ka. Dust flux to Lake Bosumtwi is inferred to be very low during this humid interval due to the strengthening of the summer monsoon. Since 3.2 cal ka, the magnetic mineral parameters suggest increased aridity as compared to the AHP. This work demonstrates that the magnetic properties of Lake Bosumtwi sediment are a sensitive recorder of abrupt climate change of global significance.

Quaternary climatic control of biogenic magnetite production and eolian dust input in cores from the Mediterranean Sea

We report high-resolution magnetic measurements from two Mediterranean piston cores: LC07 (Sicily Strait) and LC10 (Ionian Sea). Magnetostratigraphic results and δ 18O data provide age constraints for core LC07, where we investigate magnetic property variations for two age intervals (0–600 kyr and 660–1020 kyr). For core LC10, rock magnetic parameters appear to be climatically controlled and are used to derive an astronomically tuned age model for the interval between 780 and 1200 kyr. In core LC07, the dominant control on the magnetic properties appears to be glacial–interglacial variations in the concentration of biogenic magnetite. In addition, an increased contribution from high coercivity minerals (e.g. hematite and/or goethite) probably reflects an enhanced eolian input during glacial periods. Climatic control of magnetotactic bacterial populations has been previously suggested in other environments, but this is the first such report from the Mediterranean. In contrast, the rock magnetic response to Quaternary climatic variability in core LC10 seems to be better expressed by variations in the concentration of high coercivity magnetic minerals. The contrast between a dominantly detrital/eolian flux and a dominantly biogenic flux at the same time for the two Mediterranean settings might relate to the presence of an active current regime in the Sicily Strait, which might decrease delivery of an eolian component to the seafloor compared to the deep Ionian Sea.

Mineral-magnetic record of Late Quaternary climatic changes in a high Alpine lake

The magnetic properties of a sediment core from a high altitude lake in the Swiss Alps were compared with palynological and geochemical data to link climatic and mineral magnetic variations. According to pollen data, the sediments extend from the present to the Younger Dryas, i.e., they cover more than 10,000 years of environmental change in the Alps. The major change in magnetic properties corresponds to the climatic warming of the early Holocene. High-coercivity magnetic minerals that characterize the Late Glacial period almost disappeared during the Holocene and the concentration of ferrimagnetic minerals increased sharply. The contribution of superparamagnetic grains also decreased in the Holocene sediments. Similar variations in SP content and coercivity, of smaller magnitude, are found in the Holocene and are interpreted to represent minor climatic variations. Comparison with the historical record of the last 1000 years confirms this interpretation. The magnetic mineralogy, the superparamagnetic contents, and the IRM intensity in the coarse-grained, Late Glacial sediments are similar to those measured in the catchment bedrock. This indicates a detrital origin. The different properties and the higher concentration of magnetic minerals in the Holocene sediments are due to authigenic phases. Magnetic properties provide a high resolution record of climatic change. They are sensitive even to small variations that are not recorded in the pollen or LOI data. Magnetic parameters show fine-scale variation and constitute a valuable supplement to conventional climatic indicators.

Changes of the geomagnetic field vector obtained from lava sequences on the island of Vulcano (Aeolian Islands, Sicily)

A combined geochronologic ( K Ar ) and palaeomagnetic study has been conducted on five volcanic sections (70 flows) on the island of Vulcano. The sections span the interval from 15 ± 2 to 135 ± 4 ka, with large hiatuses. Rock magnetic investigations indicate that the natural remanent magnetisation (NRM) is carried by titanomagnetites in the four oldest sections, with a contribution from high coercivity magnetic minerals in the youngest section. Palaeomagnetic directions obtained by thermal demagnetisation document a large magnetic excursion, at around 110 ka, with virtual geomagnetic poles (VGPs) that lie over eastern Siberia. These directions may represent a volcanic record of the termination of the Blake event. Only about 15% of the samples (from 19 flows) proved suitable for palaeointensity (Thellier and Thellier) determinations. These measurements indicate that the geomagnetic field has varied at Vulcano between 16 and 53 μT in the explored time interval. When combined with previous results obtained at Mt. Etna for the period from 140 to 60 ka (Tric et al., 1994, Phys. Earth Planet. Inter., 85: 113–129), these results document a broad low at around 115 ka. The palaeointensity values from Vulcano and Etna are consistent with global values, suggesting the presence of a dominant dipole-field contribution. These values, on the other hand, are significantly smaller than those recently obtained from La Réunion in the southern hemisphere. This suggests that large, long-lived, non-dipolar components of the geomagnetic field may be present at La Réunion in the interval from 130 to 90 ka. These observations are consistent with the fact that the VGP angular dispersion, calculated from non-excursional data, is significantly smaller at Vulcano than at La Réunion.