Yongjae Yu

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.

Paleointensity determination on the Late Precambrian Tudor Gabbro, Ontario

Paleointensity determination by the Coe-modified Thellier method was carried out on the ≈1100 Ma Tudor Gabbro of southern Ontario. The Tudor has a bivectorial natural remanent magnetization (NRM). The C NRM is a Grenvillian uplift thermoviscous remagnetization which overprints the A NRM, the surviving primary thermoremanent magnetization (TRM) of the Tudor intrusion. A sharp junction at ≈530°C between the A and C vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480–500°C in nature when the difference in time scales is taken into account. The unblocking temperature distributions of the A NRM and of laboratory TRM also match. A is carried by single-domain or small pseudo-single-domain grains, probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580°C. Arai plots of NRM versus partial TRM are linear from 520°C or 530°C to 580°C for 45 specimens of 19 samples from 9 sites. The 45 reliable paleointensity values from these specimens give a virtual axial dipole moment (VADM) of 4.6±0.8 × 1022 A m2 around 1100 Ma. This is about one half of the modern field intensity but is similar to the mean Cretaceous-Cenozoic intensity and to most Archean and Proterozoic VADMs. Keweenawan rocks, whose ages are within ≈20 Myr of the Tudor Gabbro, have a VADM of 11.4×1022 A m2. The difference between the Tudor and Keweenawan VADMs is similar to differences recorded over similar time intervals in the Cretaceous and seems to represent normal variation in geomagnetic field strength throughout Earths history.

Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

Abstract We report geomagnetic field intensities for the A (∼1000 Ma) and B (∼850 Ma) natural remanent magnetizations of the Cordova Gabbro of southern Ontario. We interpret A and B to be secondary thermal overprints produced during regional uplift and cooling following the Grenvillian Orogeny. Multivectorial A+B paleointensity data were analyzed using pairs of Arai diagrams for the separated A and B vectors. From 18 reliable A results and 15 reliable B results, we find paleofield values of 18.1±2.1 μT and 7.1±1.5 μT, respectively. The corresponding virtual axial dipole moments (VADMs) are (3.12±0.36) and (1.82±0.38)×10 22 A m 2 , about 70% and 40% of the average Phanerozoic dipole moment based on data from 0.3–300 Ma. The Cordova data and other Precambrian paleointensities are generally within a range of approximately 0.5–1.5 times the Phanerozoic mean VADM. Three sets of paleofield values as a function of paleolatitude, including the Cordova data, support a dipole configuration for the Earth’s field in 25–150 Ma time windows around 2700 Ma, 1100 Ma, and 850 Ma.

Characteristic low-temperature magnetic properties of aluminous goethite [α-(Fe, Al) OOH] explained

Received 12 June 2006; revised 8 October 2006; accepted 26 October 2006; published 16 December 2006. (1) Goethite (a-FeOOH) is an antiferromagnetic iron oxyhydroxide that forms as a weathering product of iron-bearing minerals. We systematically investigated the low- and room temperature properties of well-defined aluminous goethites (a-(Fe, Al)OOH) with varying grain size and Al content. A marked decrease in the Neel temperature with increasing Al content for goethite lowers the blocking temperature distribution, which produces an increase in the remanent magnetization on cooling. The zero-field-cooled (ZFC) and field-cooled (FC) curves are irreversible. This is due to an additional partial thermal remanent magnetization (pTRM), which is acquired at low temperatures during the FC process because the initial 300 K remanence is far from being saturated due to the extremely high saturation field of goethite. This pTRM can be thermally demagnetized when reheating a sample back to 300 K. Finally, a sharp decrease in the bulk coercivity at 11-13 mol % Al is mostly caused by a broad coercivity distribution due to nonuniformity of Al substitution, which becomes more significant with increasing Al content. The positive correlation between pTRM acquisition during the FC process and the bulk coercivity strongly indicates that the ZFC/FC behavior is controlled by the bulk coercivity, which is, in turn, determined by Al substitution. This explanation of the characteristic low-temperature magnetic properties of Al goethite provides important constraints for identifying natural Al goethite bearing samples and quantitatively estimating the contributions of this mineral to the bulk magnetic properties of such samples.

Intensity and Polarity of the Geomagnetic Field During Precambrian Time

There are only 24 Thellier-type paleointensity estimates for Precambrian rocks. Because orogenesis is episodic, these cluster in a few time intervals: 7 between 820 and 1240 Ma, 7 between 1850 and 2215 Ma, 8 between 2450 and 2765 Ma, and 2 earlier Archean results (3470 Ma). Most late Archean-early Proterozoic results are from dikes. Two results are from large intrusions with multiple phases and slow cooling. Late Precambrian results from the slowly uplifted Grenville Province have an even longer cooling history but partial thermoremanent magnetizations (TRMs) can be dated fairly accurately by 40 Ar/ 39 Ar geochronology. The geographic distribution is uneven. Most results are from Canada, Greenland and Baltica, with only one study each from Africa and Australia. The African and Australian results are thermal overprints rather than primary TRMs. Numbers of acceptable results are often small and standard deviations large. Despite these limitations, the virtual dipole moments for almost all studies lie within a range 0.5-1.5 times the published 0.3-300 Ma average, and only two results are conspicuously high (>10 23 Am 2 ). There is no obvious record of onset and growth of a dynamo field in the Archean or early Proterozoic. A detailed record of a Middle Proterozoic polarity transition has all the expected features: 180° reversal along a great circle path, precursor excursion on the same path, a major drop in intensity during the reversal, and reduced intensity before and after. Reversal frequency may have been lower than in recent times, with several chrons of length 20-50 myr around 1460-1400 Ma, 1150-1050 Ma, and 1050-820 Ma, but there are also records of 3 myr and shorter chrons in continuous stratigraphic sections. The dipolar nature of the field is debatable. Roughly contemporaneous results confirm the expected dipole dependence of field strength and paleosecular variation on paleolatitude. On the other hand, there is an apparent excess of directional results with low inclinations and paleolatitudes compared to the dipole prediction but this may be due to bias built into the method.

Are ARM and TRM analogs? Thellier analysis of ARM and pseudo-Thellier analysis of TRM

Abstract We test the possibility of using the pseudo-Thellier method as a means of determining absolute paleointensity. Thellier analysis of anhysteretic remanent magnetization (ARM) and pseudo-Thellier analysis of thermoremanent magnetization (TRM) have been carried out on a large collection of sized synthetic magnetites and natural rocks. In all samples, the intensity of TRM is larger than that of ARM and the ratio R (=TRM/ARM) is strongly grain size dependent. The best-fit slope ( b TA ) from pseudo-Thellier analysis of TRM shows a linear correlation with R . The ratio b TA / R yielded approximately correct paleointensities, although uncertainties are larger than in typical Thellier-type determinations. For single-domain and multidomain magnetites, alternating field and thermal stabilities of ARM and TRM are fairly similar. However, for ∼0.24 μm magnetite, ARM is both much less intense and less resistant to thermal demagnetization than TRM, reflecting different domain states for the two remanences and resulting in severely non-linear Arai plots for Thellier analysis of ARM.

Magnetic discrimination between Al-substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance

Received 22 June 2011; revised 20 December 2011; accepted 24 December 2011; published 23 February 2012. [1] Hematite, a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas, is weakly magnetic. However, it carries a stable natural remanent magnetization, and thus can reflect paleoenvironment changes. To quantify the influence of Al content in hematite on its magnetic properties, two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension (HFh* series) and by thermal dehydration of goethite (HG* series). Crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively. More specifically, the HFh* series samples display oblate (plate-like) morphologies, while the HG* series samples are prolate (highly acicular). HFh* series samples display higher saturation magnetization but lower magnetic coercivity than that of the HG* series. It is tenable that a better lattice ordering of Al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite, resulting in weaker saturation magnetization for HG* series samples. The origin of single domain (SD) hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity: a positive correlation indicates the presence of pure (Al-free) SD hematite, while a negative correlation indicates a chemical origin of SD Al-substituted hematite. These results bear new information on decoding the complex magnetic properties of SD Al-hematite in nature environments, and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by Al-hematite.

Archeomagnetism of Ontario potsherds from the last 2000 years

An archeomagnetic study was carried out on potsherds samples from sites in Ontario with ages ranging from A.D. 90 to A.D. 1640 as determined by 14C dating. Thellier double-heating paleointensity experiments were performed in air on 65 specimens of 52 samples from seven sample sets. Reliable paleointensity estimates were obtained for 49 specimens. Alternating field and thermal demagnetization, temperature dependence of weak-field susceptibility, and hysteresis measurements indicate that magnetite of pseudo-single-domain grain size is the carrier of natural remanent magnetization. The paleointensity results follow a half-cycle sine curve, with a steady decrease from 54.0±5.9 μT to 37.6±5.7 μT between A.D. 90 and A.D. 885 and a monotonic increase from 52.0±6.1 μT to 59.4±1.7 μT between A.D. 1200 and A.D. 1900. The paleointensities determined yield virtual axial dipole moments (VADMs) of the Earths magnetic field that agree well with those from other parts of North America, except between A.D. 900 and A.D. 1400, when they are systematically lower. This discrepancy is probably caused by a substantial non-dipole field in southwestern North America from the tenth to the fifteenth century, since secular variation studies using potsherds from Arizona and lake sediments from Minnesota show different inclination variations during that period.

Magnetic properties of Kurokami pumices from Mt. Sakurajima, Japan

Both chromite and low-Ti titanomagnetite are carriers of natural remanent magnetization (NRM) in Kurokami andesitic pumices of Mt. Sakurajima, Japan. Thermal demagnetization of the NRM and of a laboratory isothermal remanence indicate unblocking of chromite remanence between 200 and 260‡C and of titanomagnetite remanence between 400 and 520‡C. Electron microprobe analyses support the compositions Fe1:9Cr1:11O4 and Fe2:9Ti0:1O4 indicated by the respective Curie temperatures of 260‡C and 520^530‡C. Chromite is thermally unstable and converts to magnetite during thermal demagnetization, leading to a self-reversal of NRM between 500 and 580‡C. The chromite contributes about 25% of the NRM intensity of the sample. fl 2001 Elsevier Science B.V. All rights reserved.

Effects of the core‐shell structure on the magnetic properties of partially oxidized magnetite grains: Experimental and micromagnetic investigations

The relationship between magnetic hysteresis parameters and the degree of oxidation of ultrafine magnetite particles is examined by both experimental measurements (distributed particle assemblage with median grain size of ∼80 nm and standard deviation 0.43) and micromagnetic simulations (single particles from 40 nm to 140 nm). Experimental results show that both coercivity (Bc) and the ratio of saturation remanence to saturation magnetization (Mrs/Ms) increase slowly, as the oxidation parameter z increases from 0 to ∼0.9. Thereafter, both parameters decrease sharply as magnetite becomes completely oxidized to maghemite. Numerical simulations of hysteresis loop and microstructure using a micromagnetic model with a core-shell geometry (a stoichiometric core surrounded by an oxidized shell) show three categories of behavior for magnetic grains during oxidation. First, the coercivity of SD particles decreases as oxidation proceeds, but their remanence magnetization remains in a uniform state. Second, for PSD sized particles near the critical SD boundary (80 nm to 100 nm), the initial vortex domain structure changes to a SD as oxidation occurs and returns to a vortex state upon complete maghemitization, resulting in an initial rise and then fall of Bc and Mrs. Finally, larger PSD grains remain a vortex state throughout the maghemitization, with less variations of Bc and Mrs. The predicted magnetic properties exhibit good agreement with experimental observations and suggest that the domain arrangement is likely to be dominated by a core-shell structure with strong exchange coupling at their interface. Overall, the partially oxidized magnetite in SD-PSD range can reliably record palaeomagnetic signals.