Fine-grained Magnetic Minerals Research Articles

Rock magnetic constraints for the Mid-Miocene Climatic Optimum from a high-resolution sedimentary sequence of the northwestern Qaidam Basin, NE Tibetan Plateau

The thick sequence of Cenozoic deposits in the Qaidam Basin of the NE Tibetan Plateau has great potential for understanding the tectonic and climatic evolution of the region. In this study, we performed detailed rock magnetic analyses of a well-dated (~20.7–11.2 Ma) Upper Cenozoic sedimentary sequence from the Huatugou (HTG) section in the NW Qaidam Basin. The magnetic susceptibility of the sequence is relatively low before 17.4 Ma, high during 17.4–14.5 Ma, and lower after 14.5 Ma again. This pattern is consistent with the occurrence of the Mid-Miocene Climatic Optimum (MMCO) during 17.4–14.5 Ma, with global climatic cooling thereafter. Measurements of multiple rock magnetic parameters indicate a peak in the concentration of fine-grained magnetic minerals during 17.4–14.5 Ma. This feature demonstrates that the enhancement of the magnetic susceptibility is due to the pedogenic production of superparamagnetic (SP) and single domain (SD) magnetite, suggesting the occurrence of relatively high effective precipitation and high temperature in the Qaidam Basin at this time. Comparing with regional and global climatic records, the warm and humid climatic conditions coincide with the MMCO, implying that global climate changes were responsible for the climatic evolution in the Qaidam Basin throughout the MMCO. However, after 14.5 Ma, the rapid uplift of the northern Tibetan Plateau caused it to reach a critical elevation which blocked the moisture supply to the Qaidam Basin and may also have contributed to the observed climatic changes.

Environmental magnetic record of a ~ 3000-years subalpine peat core from the western Nanling Mountains, South China

A 95-cm subalpine peat core from the Daping Swamp, located in the transition zone between the middle and southern subtropical regions of China, provides an environmental mineral magnetism record over the past ~ 3000 years. Magnetic concentration-dependent parameters [i.e. magnetic susceptibility (χlf), anhysteretic susceptibility (χARM), and saturation isothermal remanent magnetization (SIRM)] and grain size-dependent parameters [i.e. percentage frequency-dependent susceptibility (χfd%), SIRM/χlf, and χARM/χlf] of core and catchment sediments were measured to characterize provenance and catchment processes. Multi-magnetic parameters revealed that the primary magnetic minerals in the peat samples were mainly detrital magnetic minerals, which originated from catchment surface soil erosion. Changes in vegetation coverage as well as pedogenesis induced by rainfall variation play an important role in controlling the input of magnetic materials. Therefore, we infer that variations of magnetic features in the core bear potential to elucidate the history of environmental changes in the past ~ 3000 years, which are closely related to Asian summer monsoon evolution. Multiple parameters revealed that in three periods (i.e. 2700–2500, 1600–1000, and 500–300 cal year BP), decreased pedogenesis intensity and increased erosion induced by decline in vegetation coverage under dry conditions led to increased input of magnetic minerals, especially the coarser minerals in peat samples. However, in the two periods (i.e. 2500–1600, and 1000–500 cal year BP), multi-magnetic proxies indicate increased pedogenesis and decreased input of magnetic minerals dominated by fine-grained magnetic minerals caused by increased vegetation coverage under relatively wet and warm conditions. These climatic periods are of broad regional consistence across the world. The general trend of the late Holocene climate revealed by magnetic parameters confirms the sensitivity of environmental magnetism in this peat succession from the subalpine western Nanling Mountains to the climatic/environmental variability in the Asian summer monsoon region.

Changing sediment sources in the Bay of Bengal: Evidence of summer monsoon intensification and ice-melt over Himalaya during the Late Quaternary

A gravity core retrieved from the southern Bay of Bengal (BoB) was studied to determine the sources of sediments and factors controlling their distributions during the Late Quaternary. The parameters used for this study was sediment grain size, organic carbon, clay mineralogy and environmental magnetic properties. Radiocarbon ages of sediments indicate that the 3.06 m long sediment core covers a period since ~45 cal kyr B.P. The sediments deposited before ~14 cal kyr B.P. were clayey silt with higher magnetic susceptibility (MS) values (av: 26 × 10−8 m3 kg−1) and those deposited after ~14 cal kyr B.P. were sandy silt with lower MS values (avg: 13 × 10−8 m3 kg−1). The latter period (after ~14 cal kyr B.P.) indicated high energy turbidites from the Himalayan Rivers into the BoB due to the strengthening of summer monsoon and ice-melt in the Himalayas. Higher smectite and lower illite contents during the last glacial maximum (LGM) reflected low sediment supply from the Ganges-Brahmaputra River System (GBRS) and higher illite and lower smectite during ~14 cal kyr B.P. indicated increased sediment supply from GBRS during this interglacial period. The magnetic grain size of sediments deposited from ~15 cal kyr B.P. was more fine-grained at the start of the Bølling-Ållerød (B-A) event and coarse-grained during ~13 cal kyr B.P. (Younger Dryas event). Fine-grained magnetic minerals also showed a peak at ~11 cal kyr B.P. along with higher summer solar insolation. The down-core variations in sediment grain size, magnetic properties and clay minerals were interpreted in relation to the significant changes in sources of sediments in the BoB during different climatic events. The changing sediment supply in the BoB was due to the combined influence of ice-melt by increased solar insolation and intensification of summer monsoon over Himalaya. Increase of fine-grained magnetic minerals at ~15 cal kyr B.P. and coarser sediment fraction at ~14 cal kyr B.P. indicated time gap between intensification of summer monsoon and ice-melt over Himalaya, respectively. The trend of the magnetic grain size record seems to be modulated by orbital-scale variations in June insolation, which coincides with the summer monsoon variations also. These observed climatic variations are correlating well with the climate changes of the Northern Hemisphere during the Late Quaternary.

Response of pedogenic magnetite to changing vegetation in soils developed under uniform climate, topography, and parent material

Pedogenesis produces fine-grained magnetic minerals that record important information about the ambient climatic conditions present during soil formation. Yet, differentiating the compounding effects of non-climate soil forming factors is a nontrivial challenge that must be overcome to establish soil magnetism as a trusted paleoenvironmental tool. Here, we isolate the influence of vegetation by investigating magnetic properties of soils developing under uniform climate, topography, and parent material but changing vegetation along the forest-prairie ecotone in NW Minnesota. Greater absolute magnetic enhancement in prairie soils is related to some combination of increased production of pedogenic magnetite in prairie soils, increased deposition of detrital magnetite in prairies from eolian processes, or increased dissolution of fine-grained magnetite in forest soils due to increased soil moisture and lower pH. Yet, grain-size specific magnetic properties associated with pedogenesis, for example relative frequency dependence of susceptibility and the ratio of anhysteretic to isothermal remanent magnetization, are insensitive to changing vegetation. Further, quantitative unmixing methods support a fraction of fine-grained pedogenic magnetite that is highly consistent. Together, our findings support climate as a primary control on magnetite production in soils, while demonstrating how careful decomposition of bulk magnetic properties is necessary for proper interpretation of environmental magnetic data.

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.

Frequency-dependent AMS of rocks: A tool for the investigation of the fabric of ultrafine magnetic particles

In some geological processes, new very fine-grained magnetic minerals may originate. The variation in content of these minerals is routinely investigated by frequency-dependent magnetic susceptibility, which is traditionally interpreted in terms of the presence of viscous superparamagnetic (SP) particles in addition to stable single domain (SSD) and multidomain (MD) magnetic particles. In addition, the fabric of these grains can be investigated through the frequency-dependent AMS. Through standard AMS measurement at different frequencies, one can evaluate the contribution of SP particles to the whole-rock AMS; appropriate methods were developed. Various rocks, soils and ceramics, showing frequency-dependent magnetic susceptibility, were investigated. Measurable changes of AMS with operating frequency were revealed and attempts are made of their fabric interpretation.

Out-of-phase evolution between summer and winter East Asian monsoons during the Holocene as recorded by Chinese loess deposits

We analyzed climate proxies from loessic-soil sections of the southern Chinese Loess Plateau. The early Holocene paleosol, S0, is 3.2m thick and contains six sub-soil units. Co-eval soils from the central Loess Plateau are thinner (~1m). Consequently higher-resolution stratigraphic analyses can be made on our new sections and provide more insight into Holocene temporal variation of the East Asian monsoon. Both summer and winter monsoon evolution signals are recorded in the same sections, enabling the study of phase relationships between the signals. Our analyses consist of (i) measurements of magnetic properties sensitive to the production of fine-grained magnetic minerals which reflect precipitation intensity and summer monsoon strength; and (ii) grain-size analyses which reflect winter monsoon strength. Our results indicate that the Holocene precipitation maximum occurred in the mid-Holocene, ~7.8–3.5cal ka BP, with an arid interval at 6.3–5.3calka BP. The winter monsoon intensity declined to a minimum during 5.0–3.4cal ka BP. These results suggest that the East Asian summer and winter monsoons were out of phase during the Holocene, possibly due to their different sensitivities to ice and snow coverage at high latitudes and to sea-surface temperature at low latitudes.

Tracking the migration of our Chinese ancestors using molecular science

The Bible tells the story of Noah, who, under the direction of God, constructed a giant boat (Noah’s Ark) to save his family and all the world’s animals from the Great flood. In China, the hero Dayu (∼4000 years ago) spent 13 years managing and fighting floods to save human lives. Yet, the wisest thing our ancestors did when facing floods was moving to higher grounds.The pattern of migration has thus been of great interest to archaeologists. However, the link between human migration and hydrological changes in ancienthistoryhasbeendifficult tofind because of the lack of valid proxies. Xie et al. [1] demonstrated that the combination of two independent proxies (the hopanoid biomarkers from aerobic bacteria living in peat bogs and the fine magnetic particles deposited in cave stalagmite) reasonably explained the events of floods associated with human migration in central China between 1000 and 13 000 years ago. Organic biomarkers are commonly used to reconstruct paleotemperatures. They appear, however, also to be sensitive to hydrological changes in the terrestrial environment. Hopanoids can be produced by a variety of microorganisms (primarily bacteria) including both aerobic and anaerobic species [2]. Linking hopanoids to ancient hydrology is based on the idea that hopanoid-producing aerobes in peatlands will be more abundant when the water table is low and the airsaturated surface zone is thicker. Thus, periods of high hopanoids abundance should reflect less rainfall and a dry climate, whereas periods of low hopanoids abundance should reflect greater rainfall and flooding. Indeed, Xie et al. observed patterns of high and low hopanoids at time scales consistent with the dry and wet periods’ characteristics of the Asian monsoon system in the past 13 000 years in the Dajiuhu peatland, 600 km northwest of Wuhan. Furthermore, Xie et al. surveyed extensively the archaeological sites in central China and found fewer archaeological sites in the lowlands in periods of low hopanoids, indicating human migration to higher ground during wet periods. Biomarkers are effective proxies for paleoenvironmental and paleoclimatic studies [3]. Xie et al. further showed that hopanoids correlated poorly with temperature inferred by othermethods.They also prudently validated the hopanoid abundance—oxic water table hypothesis using molecular DNA containing the specific gene sqhC that is responsible for the biosynthesis of hopanoids. The results indicate that 90%–94% of the sqhC sequences were attributed to aerobic organisms. One assumption of this claim is that the DNA of hopanoid producing organisms has been well preserved and the organisms have not continued to grow after burial. The predominance of aerobic organisms in the tested sqhC sequences indicates that this might be the case. However, caution needs to be taken when interpreting DNA recovered from ancient deposits. To validate the hopanoid proxy, Xie et al. employed an inorganic proxy. This proxy is based on the relative abundance of fine-grained magnetic minerals in total ferromagnetic particles collected from a stalagmite in the Heshang Cave in central China, ∼200 km south of the Dajiuhu peatland. The term anhysteretic remanent magnetization (ARM) is commonly used to characterize the magnetic property of fine-grained particles having a single domain; the term saturation isothermal remanentmagnetization (SIRM) characterizes the total magnetic property in a sample containing not only single-domain particles but also pseudo-single-domain and multidomain grains that are commonly coarser than the single-domain particles. In soil environments, fine-grained magnetic particles (e.g. magnetite and/or greigite) can be produced by microorganisms, such as magnetotactic bacteria and ironreducing bacteria [4] or by pedogenic processes.Coarser-grainedmagnetic particles (magnetite,maghemite, titanomagnetite or hematite), in contrast, are commonly derived from weathering of igneous or metamorphic rocks, so-called detrital sources. Xie et al. were unable to identify any of these minerals from the stalagmite because of the difficulty of isolating them without chemical damage. However, the authors cleverly linked the ratio of ARM/SIRM of the stalagmite in the sub-ground to the hydrological dynamics of the soil above the ground, with low ARM/SIRM ratios indicating a high water level in the soil resulting from greater rainfall. One caveat of the ARM/SIRM proxy is that the fine magnetic particles formed in the soil can go through transformation or degradation [5] during

Magnetic analyses of soils from the Wind River Range, Wyoming, constrain rates and pathways of magnetic enhancement for soils from semiarid climates

In order to constrain the rate of magnetic enhancement in soils, we investigated modern soils from five fluvial terraces in the eastern Wind River Range, Wyoming. Profiles up to 1.2 m deep were sampled in 5-cm intervals from hand-dug pits or natural riverbank exposures. Soils formed in fluvial terraces correlated to the Sacajawea Ridge (730–610 ka BP), Bull Lake (130–100 ka BP) and Pinedale-age (∼20 ka BP) glacial advances. One soil profile formed in Holocene-age sediment. Abundance, mineralogy, and grain size of magnetic minerals were estimated through magnetic measurements. Magnetic enhancement of the A-horizon as well as an increase in fine-grained magnetic minerals occurred mostly in Bull Lake profiles but was absent from the older profile. Such low rates of magnetic enhancement may limit the temporal resolution of paleosol-based paleoclimate reconstructions in semiarid regions even where high sedimentation rates result in multiple paleosols. A loss of ferrimagnetic and an increase in antiferromagnetic minerals occurred with age. Our findings suggest either the conversion of ferrimagnetic minerals to weakly magnetic hematite with progressing soil age, or the presence of ferrimagnetic minerals as an intermediate product of pedogenesis. Absolute and relative hematite abundance increase with age, making both useful proxies for soil age and the dating of regional glacial deposits. All coercivity proxies are consistent with each other, which suggests that observed changes in HIRM and S-ratio are representative of real changes in hematite abundance rather than shifts in coercivity distributions, even though the modified L-ratio varies widely.

Sources of fine-grained magnetic minerals in sediments: a problem revisited

Progress in the discrimination of fine-grained magnetic minerals in sediments and sediment sources using measurements of magnetic susceptibility at two frequencies and of magnetic remanence generated by a combination of direct and alternating currents is summarized. The former measurements yield values for low field and frequency-dependent susceptibility (χlf and χfd, respectively). The latter generate an Anhysteretic Remanent Magnetisation (ARM) that, when normalized to the DC bias field, can be represented as the susceptibility of ARM (χARM). Using quotients derived from χlf, χfd and χ ARM to generate bi-logarithmic plots, it is possible to discriminate between assemblages dominated by magnetic minerals formed through pedogenesis and those arising from the presence of magnetotactic bacteria that use chains of magnetic particles to seek optimum environments in surface sediments. Sediments to which both types of magnetic mineral contribute occupy positions between the envelopes of values typical of soil-derived and magnetosome-derived assemblages. Fire also appears to give rise to a distinctive envelope of values indicative of smaller magnetic grain sizes, whilst the signature of extracellular magnetite is indicative of the finest grain sizes here recorded.

Roles of sorting and chemical weathering in the geochemistry and magnetic susceptibility of Xiashu loess, East China

Uncertainties in paleoenvironmental interpretations for traditional chemical analysis of bulk samples result from different grain-size sub-populations of sediments containing variable distributions of elements and minerals. Therefore, it is important to understand the elemental and mineral distribution in different grain sizes in determining the quantitative relationship between chemical weathering and climatic change. We sieved a series of Xiashu loess samples into three sub-populations of different grain sizes (<2, 2–45 and >45 μm, respectively), and then analyzed each population for rubidium (Rb), strontium (Sr), rare earth elements and magnetic susceptibility. In comparison with elemental concentrations of bulk samples, clay mineralogy and illitic crystallinity, our results show that distinct elemental distributions and magnetic susceptibilities for different grain-size sub-populations are controlled by sorting and/or chemical weathering, although we also suggest that the Xiashu loess may have the same provenance as the Central Chinese Loess. Maximum concentrations of Rb and fine-grained magnetic minerals in the less than 2 μm sub-population, coupled with our finding of maximum Sr in the larger than 45 μm fraction, indicate that Sr was lost during chemical weathering. Grain-size sub-population analysis is, therefore, an effective method for extracting paleoenvironmental information, because individual sub-populations show minimal variations in initial Rb/Sr ratios compared to bulk analysis of all sizes together. Furthermore, a negative correlation between Rb/Sr ratios and Sr concentrations for the <2 μm fraction ( R 2 = 0.97) may indicate that clay is a sensitive indicator of intensity of chemical weathering and is an ideal sub-population for determining Rb/Sr ratios, but not for magnetic susceptibility.

Signature of magnetic enhancement in a loessic soil in Nebraska, United States of America

Our study of a loessic soil profile from east-central Nebraska shows that the A horizons of the modern soil are characterized by higher concentrations of fine-grained (<0.1 μm) magnetic minerals. This pedogenic magnetic component leads to higher values of concentration-dependent parameters, such as magnetic susceptibility ( χ), isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM), combined with increases in frequency-dependent susceptibility ( χ fd) and ARM/IRM ratios. Hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component. The magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite. Analyses of “soft” IRM (sIRM) and “hard” IRM (hIRM), however, do show that approximately 80–90% of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri- and antiferromagnetic minerals. The pedogenic magnetic component is most likely caused by the conversion of paramagnetic, iron-bearing minerals to ferri- and antiferromagnetic minerals. Soil compaction, lessivage or decalcification cannot explain the observed magnetic soil properties. Magnetic analyses of loess-paleosol sequences from the midwestern United States may yield valuable information about regional variability of paleoclimate. Based on the fine-grained nature of the pedogenic magnetic component, we expect grain-size-dependent proxies (ARM, ARM/IRM, χ fd) to yield better paleoclimatic information than low-field magnetic susceptibility used in previous analyses.

Acquisition of Natural Remanent Magnetization for Dirt-Snow Containing Rock Dust.

Magnetization process of dirt-snow containing fine-grained magnetic minerals was investigated in order to understand the acquisition mechanism of natural remanent magnetization (NRM) by dirt-ice layers in Antarctica. Dirt-snow acquired NRM from the direction of geomagnetic field (GMF) direction at rooms -10°C and -20°C, and inclination becomes shallower than the GMF after 50 days.The following two types of NRM acquisition mechanisms in the GMF are proposed; (1) magnetic grains align along the GMF when they drop onto the surfaces of underlying snow due to vaporization of snow; (2) the magnetic grains are rotated to align in the GMF direction on the snow surfaces by the torque with the GMF and the NRM. The compaction of the natural snow at formation of depth hoar works to flatten the NRM direction.Probably the Antarctic dirt-ice has acquired NRM during the periods of accumulation of snow and magnetic grains and formation of the depth hoar. The NRM of dirt-snow and -ice are usuful for understanding the glacial dynamics and paleomagnetism of the Antarctic ice sheet, although many problems still remain.

The nature of magnetic single domains in volcanic glasses of the KBS Tuff

Magnetic and optical variations in volcanic glasses of the East African Rift have been investigated by means of experimental and analytical study. The results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature, origin, and significance of fine‐grained magnetic minerals and on the magnetic petrology of glassy rocks. The samples that we have studied in detail, glass shards from the KBS Tuff of northern Kenya, contain a superparamagnetic precipitate of microcrystals, ∼2 to ∼10 nm in size. The precipitate is likely to have formed at elevated temperature, during or subsequent to eruption, about 1.9 m.y. ago. We have studied these microcrystals by means of transmission electron microscopy (TEM), ac and dc magnetic methods, electron spin resonance (ESR), Mössbauer spectroscopy, and optical spectroscopy. Our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism(s) responsible for the optical and magnetic behavior of these glasses. The particle size determination for the larger of these microcrystals has been achieved using TEM imaging and Mössbauer spectroscopy. The mineralogy is nonstoichiometric magnetite, based on Mössbauer, X ray diffraction, and electron diffraction results. The ratio of saturation rémanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single‐domain theory for a uniaxial anisotropy, the origin of which may lie with stress or surface effects. We interpret magnetic and ESR results to infer the possible existence of microcrystals or amorphous clusters of Fe atoms, ∼1 nm or smaller, in KBS glasses that appear homogeneous in high‐resolution TEM images. Results from optical absorption experiments show that the color variations in KBS shards arise due to optical absorption by the precipitate, a result consistent with classical electromagnetic theory.