Magnetic Grain-size Variations Research Articles

Quantitative relationships between magnetic enhancement of modern soils and climatic variables over the Chinese Loess Plateau

Environmental magnetism has been widely employed to reconstruct past climate changes on the Chinese Loess Plateau (CLP), and several climofunctions based on the magnetic properties of loess have been developed. However, systematic investigation of the quantitative relationship between topsoil magnetic enhancement and modern climate remains uncommon. In this study, we obtained surface soil samples from 257 sites over the CLP and adjacent regions. From this set, we used 180 samples from sites unaffected by potential contamination to investigate the relationship between the commonly measured magnetic properties of magnetic susceptibility and magnetic remanence and modern climatic variables. The spatial distribution of the results demonstrates a strong NW–SE gradient of the magnetic enhancement of surface soils. The results of more detailed magnetic parameters indicate that pedogenic viscous superparamagnetic and stable single-domain particles are mainly responsible for the magnetic enhancement of the soils in the studied region; and that the magnetic grain-size distribution of the ferrimagnetic components of pedogenic origin remains almost constant, independent of pedogenic intensity. The uniform mechanism of magnetic enhancement, mainly linked with the concentration of the pedogenic components rather than with variations in magnetic grain-size, decreases the level of ambiguity in climate reconstructions based on magnetic measurements of Chinese loess. Statistical analyses, including correlation, Principal Component Analysis (PCA) and multiple regression analysis, suggest that annual rainfall rather than temperature exerts the dominant effect on soil magnetic enhancement. Finally, we used the results to develop several transfer functions to reconstruct mean annual precipitation (MAP). Transfer functions based on frequency-dependent susceptibility (χfd) and anhysteretic remanent magnetization (χARM) provide the most reliable estimates of MAP. This study significantly improves the understanding of the relationship between soil magnetic properties and climate.

Six million years of magnetic grain-size records reveal that temperature and precipitation were decoupled on the Chinese Loess Plateau during ~ 4.5–2.6 Ma

Magnetic grain-size variations have been used as sensitive paleoclimate proxies to investigate the evolution of the East Asian summer monsoon, but their relationship with temperature and precipitation is not entirely clear. Here we find that two magnetic grain-size proxy records (χARM/χLF and χARM/SIRM, where χARM, χLF and SIRM are anhysteretic remanent magnetization susceptibility, magnetic susceptibility measured at 470Hz and saturation isothermal remanent magnetization, respectively) of Chinese loess and red-clay sediments co-vary during the last 6Ma, except between ~4.5 and 2.6Ma, when these two records had opposite trends. We attribute this disparate behavior to the different responses of χARM/χLF and χARM/SIRM to temperature and precipitation during ~4.5–2.6Ma, when temperature and precipitation on the Chinese Loess Plateau were decoupled. A comparison of the loess and red-clay χARM/χLF and χARM/SIRM records with the global ice-volume proxy records reveals that χARM/χLF is more sensitive to temperature variations than χARM/SIRM. The results suggest that temperature on the Chinese Loess Plateau had a cooling trend from ~4.5 to ~2.6Ma, whereas rainfall tended to increase. Our studies demonstrate that joint analysis of loess χARM/χLF and χARM/SIRM records can reveal paleoclimatic information that cannot be revealed by a single parameter.

Magnetic studies and elemental analysis of river sediments: a case study from the Ponnaiyar River (Southeastern India)

Fil: Chaparro, Marcos Adrian Eduardo. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Exactas. Instituto de Fisica Arroyo Seco; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

Magnetite grain-size analysis and sourcing of Mediterranean obsidians

The potential of magnetic grain-size variations as an obsidian source characteristic is investigated using geological and archaeological obsidians from five islands of the Mediterranean Sea: Lipari, Sardinia, Palmarola, Pantelleria, Melos. Four parameters are used: magnetic (χ) and anhysteretic (χa) susceptibilities, saturation isothermal remanent magnetizations at room (SIRM293) and liquid nitrogen (SIRM77) temperature. The ratio ST = SIRM77/SIRM293, which depends on the superparamagnetic grains relative abundance, varies little in each individual site, with the exception of Lipari which is characterized by large variations and the highest content of superparamagnetic grains. The χa vs. χ plot (King et al., 1982) shows some within-site dispersion of the samples; but the ratio Qa = χa/χ, which is strongly influenced by the single domain grains content, is characteristic for each site. The combined use of the King and Qa vs. ST plots discriminates the samples from most of the sites and suggests that the grain-size analysis is a promising approach in sourcing obsidian archaeological artefacts. Moreover, the measurements of the four parameters used are simple, quick and feasible with no or little damage to archaeological finds.

Long-term secular variation in geomagnetic field inclination during Brunhes Chron recorded in sediment cores from Ontong-Java Plateau

A paleomagnetic study has been conducted on three gravity cores obtained from the Ontong-Java Plateau (OJP) in the western equatorial Pacific in order to investigate long-term secular variations during the Brunhes Chron. The cores consist of nanofossil foraminiferal ooze, and covers the last 500,000–800,000 years with average sedimentation rates of 6–9 m/m.y. Downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less, and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters, ratio of anhysteretic remanent magnetization (ARM) to saturation isothermal remanent magnetization (SIRM), and S ratio. These results indicate that the sediments are rock-magnetically homogeneous in general, and are hence suitable for relative paleointensity estimation. I chose SIRM as a normalizer for relative paleointensity. The three cores could be precisely correlated with each other using relative paleointensity variations. Depth-to-age conversion was carried out by tying to the Sint-800 paleointensity stack (Guyodo and Valet, 1999). The ages are consistent with the oxygen-isotope stratigraphy available for one of the three cores. Inclination variations were obtained after alternating-field demagnetization. Large inclination anomaly, about −7° in average, was observed as has been suggested to occur in the western equatorial Pacific by some studies. The inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity. Long-term inclination variations of the order of 10,000–100,000 years were recognized. The inclinations tend to deflect toward negative when relative paleointensity is low. The variations are similar in general to the record from the West Caroline Basin (Yamazaki and Oda, 2002), which supports their model that long-term inclination variation is controlled by relative strength of the geocentric axial dipole (GAD) and persistent non-dipole components.

The rock magnetic signal of climate change in the maar lake sequence of Lac St Front (France)

SUMMARY Rock magnetic properties of the maar lake sediments of Lac St Front (Massif Central, France) reflect environmental changes during the last climatic cycle. High magnetic concentrations are measured in the sediments deposited under glacial climatic conditions, while lower concentrations correspond with more temperate climatic periods. Low- and high-temperature measurements indicate that the remanence is carried by (titanium-poor) magnetite. However, some maghemite and haematite is present in sediments deposited under temperate conditions. Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clearly higher for the sediments deposited during the temperate climatic periods of the Eemian, St Germain I, II and Mid-glacial than for glacial sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possible effects of grain interactions, it is not straightforward to relate this different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with an optical laser granulometer. Dissolution of smaller grains is the most likely explanation for this larger grain size. Changes in magnetic composition and grain size are extremely limited for the glacial sediments, but magnetic concentration varies considerably. Magnetic concentration maxima in the glacial sediments of Lac St Front correlate with those of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the δ18O record of the GRIP ice cores (Thouveny et al. 1994) suggests that magnetic concentration maxima may correspond with short cold climatic episodes, associated with Heinrich events.

Partly pedogenic origin of magnetic variations in Chinese loess

QUATERNARY sequences of interbedded aeolian silts (loess) and buried soils (palaeosols) potentially provide one of the best terrestrial records of past climates1–5. Magnetic susceptibility variations in loess and palaeosol sequences from China are strongly correlated with climate-induced fluctuations of oxygen isotope ratios in deep-sea sediments2–4,6–8. As an explanation of this correlation, it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non-magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6–8. It has also been suggested that such a model, involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles, provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7. Here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size. Our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units. This suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non-magnetic loess is inadequate. Our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols.

Controls on magnetic grain-size variations and concentration in the Argentine Basin, South Atlantic Ocean

Measurement of two magnetic parameters, magnetic susceptibility (ξ) and anhysteretic remanent moment (ARM), was performed on 192 core-top samples from the Argentine Basin of the South Atlantic Ocean. Susceptibility indicates the concentration of magnetic grains in the sediment sample, while ARM estimates changes in magnetic mineral grain size: large ARM values indicate finer grain sizes. ARM normalized for magnetic concentration variation (ARM/ξ) increases precision in magnetic grain-size estimation. In the Argentine Basin, the magnetic characteristics reflect broad changes in magnetic mineral concentration rather than a substantial change in magnetic grain size across the basin. Data variations are consistent with deep-water circulation and sediment sources in the basin. The center of the basin acts as a sediment sink and is characterized by uniform magnetic grain concentrations and small grain sizes. Localized sediment input to abyssal depths from the South American continental margin dramatically modifies the magnetic pattern along the western margin of the basin. The data also show a marked downstream decrease in bulk magnetic grain content of the sediment as Antarctic Bottom Water enters the basin to the southeast and moves first westward and then northward along the margin of the basin.