A Late Holocene geomagnetic secular variation record from Erhai Lake, southwest China

A secular variation record of the geomagnetic field direction for the last 6.5  kyr has been obtained from the magnetization of sediment cores from Erhai Lake, southwest China. In order to make a comparison with this record, secular variation in east-central China was investigated by combining available magnetic field data from historical records and archaeomagnetic measurements since about 350 bc . The secular variation in Erhai Lake shows features consistent with the combined record, except for the oldest three observed declination swings in Sian from 720 to 900 ad . Many features of declination and inclination in China also occur in Japan. From 500 to 1000 ad , declination was westerly ranging from about −20° to −5° in Erhai Lake, east-central China, and Japan.     

Palaeosecular variation in Central Mexico over the last 30 000 years: the record from lavas

13 lava flows of known age (ages from ¹⁴C dating), which have been erupted in the last 30 000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 × 10²² A m². Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field.     

New palaeomagnetic result from Vendian red sediments in Cisbaikalia and the problem of the relationship of Siberia and Laurentia in the Vendian

A palaeomagnetic study of Vendian red sediments from the Lena River section on the western margin of Lake Baikal in the region of Cisbaikalia (54°N, 108°E) has isolated a stable remanence with direction D = 296.3°, I = −27.7° (high-temperature component) and a corresponding pole of 2.7°S, 168.2°E. The primary nature of this remanence is confirmed from a positive fold test, dual polarities and the presence of detrital haematite. This result, together with all late Precambrian–Early Cambrian palaeomagnetic data from Siberia, indicates that Siberia occupied low latitudes during that time. It has been proposed on the basis of palaeomagnetic data that Laurentia occupied high latitudes during the Vendian, so it would appear that there cannot have been any Laurentia–Siberia connection at that time. A review of Vendian to Cambrian Laurentian palaeomagnetic data shows that such an interpretation is ambiguous. An alternative interpretation places Laurentia in low latitudes and confirms the Laurentia–Siberia fit of Hoffman (1991 ) and Pelechaty (1996 ). However, the lack of Late Vendian palaeomagnetic data for Siberia still allows the possibility that it could have occupied high latitudes during that time.     

Environmental magnetic record of Antarctic palaeoclimate from Eocene/Oligocene glaciomarine sediments, Victoria Land Basin

The onset of continent-wide glaciation in Antarctica is still poorly understood, despite being one of the most important palaeoclimatic events in the Cenozoic. The Eocene/Oligocene boundary interval has recently been recognized as a critical time for Antarctic climatic evolution, and it may mark the preglacial–glacial transition. Magnetic susceptibility, intensity of natural and artificial remanences, hysteresis parameters and magnetic anisotropy of the lower half (late Eocene/early Oligocene) of the CIROS-1 core (from McMurdo Sound, Antarctica) reveal alternating intervals of high and low magnetic mineral concentrations that do not correspond to lithostratigraphic units in the core. Pseudo-single-domain magnetite is the main magnetic mineral throughout the sequence, and sharp changes in magnetite concentration match changes in clay mineralogy beneath and at the Eocene/Oligocene boundary. The detrital magnetite originated from weathering of the Ferrar Group (which comprises basic extrusive and intrusive igneous rocks). Weathering processes and input of magnetite to the Victoria Land Basin were intense during periods when the Antarctic climate was warmer than today, but during intervals when the climate was relatively cool, chemical weathering of the Ferrar Group was suppressed and input of detrital magnetite to the Victoria Land Basin decreased. Our results also indicate that a cold and dry climate was not established in Antarctica until the Eocene/Oligocene boundary, with major ice sheet growth occurring at the early/late Oligocene boundary. Some earlier cold intervals are identified, which indicate that climate had begun to deteriorate by the middle/late Eocene boundary.