Barbara A. Maher

Magnetic properties of modern soils and Quaternary loessic paleosols: paleoclimatic implications

The magnetic properties of paleosols developed in Quaternary sequences of loess have been used for: stratigraphic definition; correlation with other terrestrial and deep-sea sequences; and paleoclimatic (paleorainfall) reconstruction. In some loess/paleosol sequences, including those of the Chinese Loess Plateau, Tajikistan and the Czech Republic, maxima in magnetic susceptibility values correspond with the paleosol horizons, and minima with the least-weathered loess layers. In other loess/soil sequences, including those of Siberia, Alaska and Argentina, the relationship is completely opposite, with susceptibility minima associated with the most developed paleosols. To account for these opposite relationships, the respective roles of: (1) magnetic enhancement and (2) magnetic depletion and/or dilution in determining soil magnetic properties are investigated for a range of modern soil types. Most magnetic enhancement is seen in the upper horizons of well drained cambisols. Absence or loss of magnetic iron oxides is apparent in acid, podsol profiles and waterlogged soils. For the cambisol profiles, significant correlation is found between susceptibility and organic carbon, cation exchange capacity and clay content. The mineralogy, morphology and grain size of soil magnetic carriers, extracted from three modern enhanced soils and a paleosol from the Chinese Loess Plateau, are also identified by independent petrographic means (microscopy and X-ray diffraction of magnetic extracts). Magnetite and maghemite of ultrafine grain size (from 0.4 to < 0.001 I¼m) are the major contributors to the magnetically enhanced soils. Weathering can concentrate detrital magnetic grains, especially in the fine silt size fractions of soils. The magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site-specific basis, taking into account the possibilities of pedogenic enhancement, pedogenic dilution or depletion, and allochthonous inputs of magnetic minerals. Excessively arid, wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets. Well drained, intermittently wet/dry soils, with reasonable buffering capacity and a substrate source of Fe, show most magnetic enhancement. For those soils which favour magnetic enhancement processes, correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall. The almost unique pedogenic system in the Chinese loess plateau, where variation in soil-forming factors other than climate is reduced to a global minimum, allows use of the paleo-susceptibility values as proxy paleorainfall values. To identify the mechanism of any link between magnetic properties, for example, susceptibility, and climate change, the mineralogy and grain size of soil magnetic carriers, requires detailed investigation. Given the prospect of: (1) quantitative paleorainfall records from continental loess records; (2) predictions of present and future climate change; and (3) the value of paleoclimate data for testing of numerical climate models; such magnetic investigations can contribute significantly to our understanding of past and future environmental change.

Quaternary climates, environments, and magnetism

Preface Raymond S. Bradley and Friedrich Heller 1. Introduction B. A. Maher, M. W. Hounslow and R. Thompson 2. The North Atlantic as a Quaternary magnetic archive J. S. Stoner and J. T. Andrews 3. Palaeomonsoons I: the magnetic record of palaeoclimate in the terrestrial loess and palaeosol sequences B. A. Maher and R. Thompson 4. Palaeomonsoons II: magnetic records of aeolian dust in Quaternary sediments of the Indian Ocean B. A. Maher and M. W. Hounslow 5. Bacterial magnetite and the Quarternary climate record P. Hesse and J. F. Stolz 6. Incidence and significance of magnetic iron sulphides in Quaternary sediments and soils I. Snowball and M. Torii 7. Holocene environmental change from magnetic proxies in lake sediments J. Dearing 8. Magnetic monitoring of air, land and water pollution E. Petrovsky and B. B. Ellwood 9. Environmental factors affecting geomagnetic field palaeointensity estimates from sediments S. P. Lund and M. Schwartz 10. Magnetic cyclostratigraphy: high resolution dating in and beyond the Quaternary and analysis of periodic changes in diagenesis and sedimentary magnetism C. G. Langereis and M. J. Dekkers Index.

Characterisation of soils by mineral magnetic measurements

Processes of iron mineral authigenesis, diagenesis, and dissolution, in interaction with primary inputs of iron minerals, act to produce vertical differentiation of soil magnetic properties. Thus, magnetic iron oxides, in common with other iron forms, may both respond to and reflect soil forming processes. Investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes. Evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance, in several areas of environmental study: for example, the observed magnetic variation can be used to discriminate between individual soils and soil horizons, for the purposes of soil surveying, sediment ‘tagging’ and tracing, and empirical modelling of source-sediment linkages. This paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance. The effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters (including magnetic susceptibility, frequency dependent susceptibility, and anhysteretic and saturation remanences). Contrary to current pedological thought, but in accord with other, magnetically-based, studies (e.g., Mullins ), the presence of magnetite/maghemite (typically of superparamagnetic-single domain grain size) within soils has been found to be an extremely widespread phenomenon. Its contemporary neoformation within the soil environment is inferred. However, the input of artificially-generated ferrimagnetic material is indicated for one of the magnetically ‘enhanced’ soils examined here, and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic, processes of magnetic enhancement are inferred. From the data presented, discrimination of industrially-derived magnetic particles appears possible, on the basis of their very low levels of frequency dependent susceptibility (χfd), low anhysteretic remanence (ARM), and dissociation from the finer, clay-sized ( Not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation. Eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content; variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons. Within gleyed (waterlogged) soils, processes of magnetite dissolution appear to be grain size-specific; preferential reduction of those grains of ultrafine (superparamagnetic) and fine (single domain) magnetic grain size is indicated. Where soil forms a major contributor to catchment sediment loads, this has significance in the area of source-sediment modelling. Soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments; further, only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record.

Mineral magnetic record of the Chinese loess and paleosols

Variations in the magnetic susceptiblity of the Chinese loess and its interbedded paleosols correlate strikingly well with the deep-sea oxygen isotope record. Low susceptibility values are found for the loess layers, and high values are found for the soil horizons. Two interpretative models have been advanced to account for these magnetic variations. Both of these models discount any significant in situ formation of magnetite during soil-forming periods. Instead they infer relative concentration of detrital or atmospheric magnetite at these times. We critically examine the assumptions made in these models and, on the basis of some new mineral magnetic data and fromalized magnetic flux calculations, offer an alternative explanation of the loess magnetic record. Our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols. The formation and preservation of this pedogenic magnetite is dependent on soil-froming conditions and hence reflects the regional climate.

Spatial and temporal reconstructions of changes in the Asian palaeomonsoon: A new mineral magnetic approach

Abstract A new quantitative approach is proposed for estimating palaeoprecipitation across the Chinese Loess Plateau. At present, there is a strong rainfall gradient across the plateau from ∼ 300 mm/yr in the near-desert conditions in the northwest to over ∼ 750 mm/yr in the southeast, just 700 km distant. We find that the concentration of ferrimagnetic iron oxide minerals in nine modern soil types (represented by 37 individual soil profiles) is strongly correlated with this contemporary rainfall gradient. The ferrimagnetic concentration rises along this gradient, from 0.01% in the northwest to over 0.2% in the southeast. The nine modern soil types have been used in the construction of a rainfall vs. magnetic susceptibility (least squares regression) climofunction. Past variations of loess-soil iron oxide content are easily established through magnetic susceptibility measurements and so can be used to reconstruct the rainfall of former interglacial and glacial periods. The physical and pedological basis of the rainfall vs. susceptibility relationship is discussed and potential limitations of our rainfall reconstruction method are explored. Our palaeoclimate reconstructions indicate dramatic changes in rainfall due to variations in the structure of the Asian monsoon. The rainfall variations are about four times greater than has been suggested for this region by atmospheric general circulation modelling. Our data indicate increased rainfall throughout central China both in interglacial periods and in the early Holocene. The increases in monsoonal rain were particularly pronounced at our westernmost sites, adjacent to the northeastern edge of the Tibetan plateau. In contrast, for glacial periods a reduction in rainfall is found across the whole loess area, with the greatest decreases in the southeast.

Paleoclimatic significance of the mineral magnetic record of the Chinese loess and paleosols.

The origins of the magnetic susceptibility variations of the Chinese loess and paleosols are explored by scanning and transmission electron microscopy of magnetic extracts, and by magnetic modeling of magnetic hysteresis data, to provide quantified estimates of the major magnetic components. Microscopy identifies several distinct size and shape characteristics in the magnetic carriers. Lithogenic magnetites, intact and abraded, dominate the coarse-grained magnetic fraction. The smallest of the coarse grains is 2 I¼m. The remaining magnetic materal is ultrafine in size, with two types of magnetite particles present. Type A particles strongly resemble soil magnetites produced by inorganic precipitation. Type B particles, which occur rarely, are probably bacterial in origin. Quantitative modeling of these magnetic assemblages shows that over 90% of the susceptibility variations is accounted for by the superparamagnetic magnetite component. Compared to the loess units, the paleosols are richer in magnetite, particularly of superparamagnetic size, and have a threefold higher ratio of magnetite to hematite. We identify pedogenic formation of magnetite as the major contributor to the loess magnetic record. Matching this record against other paleoclimatic records, we find an extremely high correlation with the standard 18O record. The Chinese loess sequences record a very high resolution magnetic stratigraphy directly related to changing climate.

Early Pleistocene human occupation at the edge of the boreal zone in northwest Europe

The dispersal of early humans from Africa by 1.75 Myr ago led to a marked expansion of their range, from the island of Flores in the east to the Iberian peninsula in the west. This range encompassed tropical forest, savannah and Mediterranean habitats, but has hitherto not been demonstrated beyond 45° N. Until recently, early colonization in Europe was thought to be confined to the area south of the Pyrenees and Alps. However, evidence from Pakefield (Suffolk, UK) at ∼0.7 Myr indicated that humans occupied northern European latitudes when a Mediterranean-type climate prevailed. This provided the basis for an ‘ebb and flow’ model, where human populations were thought to survive in southern refugia during cold stages, only expanding northwards during fully temperate climates. Here we present new evidence from Happisburgh (Norfolk, UK) demonstrating that Early Pleistocene hominins were present in northern Europe >0.78 Myr ago when they were able to survive at the southern edge of the boreal zone. This has significant implications for our understanding of early human behaviour, adaptation and survival, as well as the tempo and mode of colonization after their first dispersal out of Africa.

Magnetic biomonitoring of roadside tree leaves: identification of spatial and temporal variations in vehicle-derived particulates.

We report here the novel use of rapid and non-destructive magnetic measurements to investigate the spatial and temporal pattern of urban dust loadings on leaves of roadside trees. More than 600 leaves were collected from birch trees and their remanent magnetization (IRM300 mT) determined and normalized for the leaf area. The results show that this normalised 2-D magnetization is dominantly controlled by the trees distance to the road. The magnetic analyses enabled detailed mapping of the spatial and temporal variations of vehicle-derived particulates. Higher 2D-magnetizations, indicating higher magnetic dust loadings, were measured for leaves collected adjacent to uphill road sections than for those next to downhill sections. This suggests that vehicle emissions, rather than friction wear or resuspended road dust, are the major source of the roadside magnetic particles. Additional magnetic analyses suggest that the particle size of the magnetic grains dominantly falls in the range classified for airborne particulate matter as PM2.5 (<2.5 µm), a particle size hazardous to health due to its capacity to be respired deeply into the lungs. Thus, the leaf magnetizations relate directly to release into the atmosphere of harmful vehicle combustion products. For leaves from individual trees, magnetization values fall significantly from high values proximal to the roadside to lower values at the distal side, confirming the ability of trees to reduce aerosol concentrations in the atmosphere. Magnetic analysis of leaves over days and weeks shows that rainfall produces a net decrease in the leaf magnetic loadings. Key words: Magnetic measurements, vehicle pollution, biomonitoring.

Magnetic mineralogy of soils across the Russian Steppe: climatic dependence of pedogenic magnetite formation.

Formation of ferrimagnets in well-drained, buffered, unpolluted soils appears to be related to climate, and especially rainfall. If robust, this magnetism/rainfall couple can be used to estimate past rainfall from buried soils, particularly the multiple soils of the Quaternary loess/soil sequences of Central Asia. However, dispute exists regarding the role of climate vs. dust flux for the magnetic properties of modern loessic soils. Here, we examine the mineralogical basis of the magnetism/rainfall link for a climate transect across the loess-mantled Russian steppe, where, critically, dust accumulation is minimal at the present day. Magnetic and independent mineralogical analyses identify in situ formation of ferrimagnets in these grassland soils; increased ferrimagnetic concentrations are associated with higher annual rainfall. XRD and electron microscopy show the soil-formed ferrimagnets are ultrafine-grained (<50 nm) and pure. Ferrimagnetic contributions to MA¶ssbauer spectra range from 17% in the parent loess to 42% for a subsoil sample from the highest rainfall area. Total iron content varies little but the systematic magnetic increases are accompanied by decreased Fe2+ content, reflecting increased silicate weathering. For this region, parent materials are loessial deposits, topography is rolling to flat and duration of soil formation effectively constant. The variations in soil magnetic properties thus predominantly reflect climate (and its co-variant, organic activity) â�� statistical analysis identifies strongest relationships between rainfall and magnetic susceptibility and anhysteretic remanence. This magnetic response correlates with that of the modern soils across the Chinese Loess Plateau. Such correlation suggests that the rainfall component of the climate system, not dust flux, is a key influence on soil magnetic properties in both these regions.

Quantifying grain size distribution of pedogenic magnetic particles in Chinese loess and its significance for pedogenesis

Quaternary glacial/interglacial cycles have been imprinted on the Chinese loess/paleosol sequences through pedogenesis. In order to accurately decode the paleoclimatic signals carried by these pedogenic particles it is essential to quantify the pedogenically produced magnetic particles in terms of mineralogy as well as grain size distribution (GSD). To date, the GSD has not been accurately determined because of the dearth of available means for analyzing extremely fine grained (nanometer-scale) pedogenic magnetic particles. Using low-temperature techniques, we systematically investigated the temperature dependency of χ fd (defined as χ1Hz − χ10Hz, where χ1Hz and χ10Hz are AC magnetic susceptibility measured at 1 and 10 Hz, respectively) from two characteristic loess profiles, one located at the western Chinese Loess Plateau and the other in the central plateau. On the basis of Neel theory for a shape anisotropy dominant grain and experimental analysis at low temperatures, a quantitative GSD for pedogenic particles in Chinese loess/paleosols was constructed. We found that the dominant magnetic grain size lies just above the superparamagnetic/single-domain threshold (∼20–25 nm) and that the GSD is almost independent of the degree of pedogenesis. This observation agrees well with other constraints from previous studies. This new GSD model improves our understanding of the pedogenic processes in Chinese loess, enabling further explicit linkage of environmental magnetism to paleoclimate changes.