Koji Fukuma

Magnetic discrimination of pyrrhotite‐ and greigite‐bearing sediment samples

By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite- and greigite-bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite-bearing samples showed unambiguous TB temperature spectra, greigite-bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero-field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.

Variable shape of magnetic hysteresis loops in the Chinese loess-paleosol sequence

Shape of magnetic hysteresis loops of the Chinese loess-paleosol sequence is variable with low-field susceptibility and is weakly constricted in samples with intermediate susceptibility (~1.0 × 10−6 m3/kg). The analyses of the hysteresis loops show that both low- (ferrimagnetic) and high-coercivity (antiferromagnetic) components are present and the ferrimagnetic component dominate the magnetic characteristics. The ratio of ferrimagnetic over antiferromagnetic minerals (S ratio) and the superparamagnetic fraction increase with increasing susceptibility. Neither simple two-component mixtures of ferrimagnetic and antiferromagnetic minerals nor of single-domain and superparamagnetic grains fully explain the constricted hysteresis loops. We interpret the variation of the loop shape with susceptibility in the following way. When the ratio of the ferrimagnetic to antiferromagnetic contribution is relatively low (low susceptibility), the broad loop is controlled by lithogenic ferrimagnetic and antiferromagnetic minerals. For samples with intermediate susceptibility values, constricted shape originates from an addition of a broad loop from the lithogenic fraction and a narrow loop from a pedogenic fraction with high superparamagnetic content. Then with further susceptibility increase, the constricted shape almost disappears and the loop is dominated by the pedogenic fraction. The variation of hysteresis loop shape with susceptibility can be a useful indicator of the degree of pedogenesis for loess-paleosol samples.

Monte Carlo simulation of two-dimensional domain structures in magnetite

A Monte Carlo method was applied for micromagnetic studies of two-dimensional domain structures of a 1-μm magnetite cube in zero field. By using this method we could incorporate the effect of thermal agitation into micromagnetic modeling. Starting from an initial single-domain structure (a saturated state), we obtained a closure domain structure with three body and four closure domains. This structure is quite different from the checkerboard-like structure obtained by a conjugate gradient method, and it gives a much lower energy and saturation remanence ratio. An initial lamellar two-domain structure evolved into a vortex structure, which was also reached from an initial quasi-vortex structure. Although such a vortex structure has a lower energy than the closure domain structure, it was not attainable from an initial single-domain structure at room temperature. The Monte Carlo method is effective in finding a path to escape from unstable local energy minima and reach a stable local energy minimum, although not necessarily a global minimum, at a given temperature. The structure corresponding to such a stable local minimum should represent a realistic domain structure, comparable to what would be attained in nature with the aid of thermal fluctuations of spins.

Origin of the absence of magnetic lineations in the Yamato Basin of the Japan Sea: Magnetic properties of mafic rocks from Ocean Drilling Program Hole 794 D

We investigated magnetic properties of mafic rocks recovered from Ocean Drilling Program Hole 794 D in the Yamato Basin of the Japan Sea. In contrast to other back-arc basins in the western Pacific, lineated magnetic anomaly patterns have not been recognized in the Yamato Basin. Our objective is to clarify the reason for the absence of lineated magnetic anomalies by direct magnetic measurements of crustal rocks. The dominant magnetic mineral of Hole 794 D is titanomaghemite as commonly observed for typical oceanic basalts, but the grain size is much larger (greater than several tens of microns). Remanence properties, i.e., low natural remanent magnetization intensities (≈1.5 A/m), low Koenigsberger ratios (≈1.0), high viscous remanent magnetization, and mixed magnetic polarities, indicate that the mafic rocks cannot be a source of lineated magnetic anomalies. While Curie temperature, saturation magnetization, and susceptibility values are not significantly different from those of typical oceanic basalts, magnetic properties related to stability are greatly reduced. The large grain size of the mafic rocks from Hole 794 D plays a primary role in differentiating the magnetic properties from those of typical oceanic basalts, and is caused by the formation of sill-sediment complexes beneath the Yamato Basin. Sill-sediment complexes, rather than extrusive basalts in typical oceanic crust, are responsible for the absence of lineated magnetic anomalies in the Yamato Basin.

Is amino acid chronology applicable to the estimation of the geological age of siliceous sediments

Abstract There are few conventional dating methods that can be used to estimate the geological age of siliceous sediments on the order of 104–105 yr. In contrast, methods such as δ18O are available for dating carbonate-containing sediments in this geological age range. We focused on amino acid chronology as an alternative dating method for siliceous sediments. We analyzed the enantiomeric ratio ( D -isomer/ L -isomer) of aspartic acid (Asp) in bulk diatom assemblages in two siliceous sediment cores collected at Station (St.) 3 (approx. lat. 50°N, long. 165°E) and St. 5 (approx. lat. 40°N, long. 165°E) in the northwestern North Pacific. Radiocarbon and paleomagnetic ages were also obtained from both cores to use as reference ages. Two models, a reversible first-order kinetic model and a parabolic model, were used to determine the relationship between the D / L ratios of Asp and reference ages from the core at St. 5. By using these models, Asp ages were then estimated for the core at St. 3, and these ages were compared to paleomagnetic ages from that core. There was a large difference between Asp ages estimated by the first-order kinetic model and the reference ages. On the other hand, Asp ages estimated by the parabolic model were consistent with the reference ages. Therefore, an Asp dating method using the parabolic model is suitable for dating siliceous sediments. However, although generally the D / L ratio of Asp increased with increasing depth in the core at St. 5, the ratio did not continue to increase below about 10 m depth. The D / L ratio of Asp and the paleomagnetic age at that depth were 0.37 and 350 kyr BP, respectively. Therefore, the Asp racemization reaction apparently does not continue to progress in diatom frustules older than this age. This finding implies that Asp chronology can be used to determine ages up to about 350 kyr BP in sediments composed of diatom ooze. Although the Asp dating method using the parabolic model has a limitation of 350 kyr BP for siliceous sediments, it is available for the estimation of ages on the order of 104–105 yr BP, which is beyond the time range (up to 50 kyr BP) datable by the 14C method.