M. E. Evans

Baoji-type pedostratigraphic section, Loess Plateau, north-central China

Abstract The Baoji section is judged to be the most complete pedostratigraphic section on the Loess Plateau of north-central China. It is located about 5 km north of the city of Baoji and consists of 37 paleosols formed during more than the last 2.5 Ma. It is designated here as the type pedostratigraphic section with formally named geosols following the rules of the North American Commission on Stratigraphic Nomenclature. Geochronological control is based largely on magnetostratigraphy with the Brunhes, Matuyama and Gauss epochs, and Jaramillo and Olduvai events clearly recognized. The Baoji geosols can be correlated with equivalent pedostratigraphic units throughout the Loess Plateau by a combination of magnetostratigraphy and character, position and the association of the pedostratigraphic units within the Quaternary succession. The variation of the geosols suggests climatic cycles of varying intensity and duration; but generally warmer and moister conditions when compared with the loess units that suggest colder and drier periods.

Magnetostatic interactions in a natural magnetite‐ulvöspinel system

[ 1] Magnetostatic interactions have been investigated in anintergrown material consisting of similar to 200-nm magnetite blocksseparated by similar to 30-nm-wide ulvospinel lamellae. First-orderreversal curve (FORC) measurements provide a direct measure of theinteraction fields, giving a value for the full width at half maximum

Preliminary magnetostratigraphy of the red clay underlying the loess sequence at Baoji, China

Magnetic remanence and bulk susceptibility measurements are reported from 93 samples spanning the uppermost 27m of the Red Clay underlying the loess sequence at Baoji, Shaanxi province, China. If a new, previously undetected, short (about 20ka) excursion or reversed interval near 3.3Ma be accepted, then a reasonable fit to the standard time scale emerges. A linear accumulation rate of 1.5cm/ka is implied and the correlation coefficient is 0.992. No significant hiatus occurs between the Red Clay and the overlying loess, so the 27m investigated reach back to about 4.3Ma. Fourier analysis of the bulk susceptibility data yields a strong peak near 400ka in agreement with astronomical calculations of the orbital parameters responsible for climatic forcing.

Magnetoclimatology: Teleconnection between the Siberian loess record and North Atlantic Heinrich events

New environmental magnetic data from loess and paleosol successions in outcrops in the upper reaches of the Ob River drainage, southern Siberia, track the major climatic variations over the last glacial-interglacial cycle. Profiles of magnetic susceptibility and alternating deposition of loess and soil-formation events correspond to oxygen isotope stages 1–5. The magnetic-susceptibility data, in association with the stratigraphic succession, confirm that the wind-vigor magnetoclimatological model is a viable alternative to the classic pedogenic model. Interpretation of magnetic-susceptibility data from loess- paleosol successions must therefore consider eolian dynamics, available source materials, and transport directions, in addition to pedogenic processes. Rapid magnetic fluctuations are also observed. These are identified—for the first time in Siberian records—as the signature of the abrupt cold pulses responsible for the Heinrich layers in North Atlantic marine sediments. The data thus form a component of climatic teleconnections across the Northern Hemisphere, allowing correlations to be made among (1) Siberian magnetic susceptibility stratigraphy, (2) data recorded from other loess-paleosol successions in China, European Russia, Europe, and North America, (3) North Atlantic ice-rafted detritus, and (4) sea-surface temperatures derived from molecular stratigraphy of marine sediments off the northwest coast of Africa.

Loess in the Czech Republic: Magnetic properties and paleoclimate

SummaryWe have studied 454 oriented samples from seven loess outcrops in the Czech Republic for comparison and correlation of the magnetic properties with those of the loess profiles in China and Central Asia. Three sections at Sedlec (Prague), Zeměchy and Dolní Věstonice cover the time span of the last glacial-interglacial cycle. Loess from the middle Pleistocene period including pedocomplexes V and VI was studied at Sedlešovice (Znojmo) and Karlštejn. Loess of uncertain — possibly lower to middle Pleistocene age — was sampled at Sedlec near Mikulov. At Červený Kopec (Red Hill, Brno) we investigated the oldest loess near the Matuyama/Brunhes (M/B) boundary. The characteristic remanence (ChRM) determined after magnetic clearning is of normal polarity throughout all sections except at Červený Kopec. There, two polarity changes from reversed to normal were found within two sections initially thought to represent two successive stratigraphic intervals. However, the marked similarities between lithological, susceptibility, declination and inclination profiles lead us to suggest that the two polarity changes are in fact one and the same. The two sections are therefore laterally equivalent and both have recorded the M/B field reversal. The susceptibility variations were used to mutually correlate the Czech sections and to construct a composite profile across the present day loess outcrops in the Czech Republic. They also serve as a paleoclimate proxy which can be compared and correlated with the paleoclimatic records observed in the Chinese and Central Asian loess. The origin of the low field susceptibility variations was studied by measuring the frequency dependence of susceptibility which revealed that the enhancement within the pedocomplexes is controlled by the amount of fine-grained ferromagnetic minerals present.

Correlation and interpretation of paleosols and loess across European Russia and Asia over the last interglacial–glacial cycle

Abstract Loess-paleosol sequences of the last interglacial-glacial cycle are correlated from European Russia to central Siberia and the Chinese Loess Plateau. During cold periods represented by marine oxygen isotope stages (OIS) 2 and 4, loess deposition dominated in the Russian Plain and the Loess Plateau. In central Siberia, loess deposition took place also, but five to seven thin, weakly developed paleosols are identified in both stages. OIS 3, in the Chinese Loess Plateau near Yangchang, consists of a loess bed that is flanked by two weakly developed paleosols. At Kurtak, Siberia, OIS 3 is represented by two distinct, stacked paleosols with no loess bed separating the paleosols. In the Russian Plain, OIS 3 consists of a single, possibly welded paleosol, representing upper and lower stage-3 climates. Brunisols and Chernozems dominate the profiles in China and Siberia, whereas Regosols, Luvisols, and Chernozems are evident in the northern and southern Russian Plain, respectively. OIS 5 is represented in China and the Russian Plain by pedo complexes in a series of welded soils, whereas in contrast, the Kurtak site consists of six paleosols with interbedded loess. The paleosols consist largely of Brunisols and Chernozems. Although the three areas examined have different climates, geographical settings, and loess source areas, they all had similar climate changes during the last interglacial-glacial cycle.

Magnetic record of Lake Baikal sediments: chronological and paleoclimatic implication for the last 6.7 Myr

Magnetic remanence vectors for 1472 samples taken from a 601 m core through Lake Baikal sediments are reported along with a complete magnetic susceptibility profile obtained from a pass-through system. Matching the stable remanence directions to the standard geomagnetic polarity time scale (GPTS) provides a robust chronology from the present back to V6.7 Ma and yields a remarkably constant sediment accumulation rate of 3.9 cm/kyr. For earlier times ^ represented by depths s 270 m ^ correlation to the GPTS is more problematic. Susceptibility fluctuations reflect climatic changes that can be matched to the marine oxygen isotope pattern for the last 6.7 Myr. Spectral analysis of the resulting susceptibility time series then indicates that, for the most part, the Milankovitch obliquity signal dominates. However, when the temporal evolution of the frequency content is investigated by analyzing sequences of time windows, a complex picture emerges in which eccentricity and precession power appear during some intervals. Furthermore, there is persistent evidence for significant power in a ‘non-Milankovitch’ band between 28 and 35 kyr. A 2003 Elsevier Science B.V. All rights reserved.

Palaeomagnetic evidence for stationary sources of geomagnetic secular variation

Abstract Asymmetrical curvilinear patterns of secular variation with approximately superposed “outward” and “return” trajectories can be attributed to stationary magnetic sources in the core. Some appropriate palaeomagnetic examples are presented here.

A micromagnetic investigation of magnetite grains in the form of Platonic polyhedra with surface roughness

Micromagnetic calculations have been carried out for spherical magnetite particles with surface roughness consisting of patterns of conical bumps based on regular (Platonic) convex polyhedra. The purpose was to examine the effect of surface irregularities while avoiding overall shape anisotropy, which generally plays a dominant role in determining hysteresis properties. We considered three morphologies based on the tetrahedron (4 apices), the icosahedron (12 apices), and the dodecahedron (20 apices). Grains of three sizes were considered: 30 nm (single-domain, SD), 90 nm (on the single-domain/pseudo-single-domain boundary, SD/PSD), and 120 nm (stable pseudo-single-domain, PSD). We find that the morphologies investigated have very little effect on the hysteresis parameters of SD and marginal SD/PSD grains. However, in the PSD grains, coercivity increases significantly as bump amplitude increases from 0.1 to 0.9. This lends support to the long-standing notion that surface protuberances on larger grains are a possible source of paleomagnetically significant stable remanence, although the very high coercivities (on the order of 100 mT) observed in some rocks cannot be achieved. Classical Stoner-Wohlfarth shape anisotropy remains the only explanation for such ultra-stable remanence in magnetite-bearing rocks. This is confirmed by a specific example of a model “skeletal” grain consisting of three orthogonal parallelepipeds.

Magnetostratigraphy of the Lodève Basin, France: Implications for the Permo-Carboniferous reversed superchron and the geocentric axial dipole

New results from the La Lieude Formation now complete the magneto-stratigraphic coverage of the Permian redbeds preserved in the Lodève Basin of southern France. The majority of the samples yield reversed polarity, with a mean of declination D = 187.4°, inclination I = −0.5° (Fisherian statistics k = 44.2, α95 = 5.6°, n = 16). Together with previously published results, these data indicate that the entire basin lies within the Permo-Carboniferous Reversed Superchron (PCRS). But the stratigraphically highest sample exhibits normal polarity, suggesting that the transition marking the end of the PCRS may be close. The unusual behaviour of the geodynamo that generates superchrons prompts one to ask if there are concomitant influences on the morphology of the field. The intersecting palaeomeridian method offers a means of pursuing this question. An updated analysis suggests that there are currently no compelling reasons for adding significant higher-order terms to the geocentric axial dipole (GAD) model for the Late Permian.