Dating of thaw depths in permafrost terrain by the palaeomagnetic method: experimental acquisition of a freezing remanent magnetization

The acquisition of a freezing remanent magnetization (FRM) has been studied in controlled magnetic and thermal environments by successive freezing and thawing (−18 to +20°C) of samples of natural sediments from a frost polygon near Ny Ålesund, Spitsbergen. Successive freeze-thaw cycles cause a significant decrease in the intensity of the initially induced shock remanent magnetization (SRM), associated with directional trends towards the ambient magnetic field direction during the freezing phase. A slow increase in intensity commences after seven to 10 freeze-thaw cycles. The acquisition of a FRM in samples carrying an isothermal remanent magnetization shows a significantly smaller reduction in intensity and only minor directional variations. This result indicates that only a fraction of the magnetic grains in a natural sediment contributes to the natural remanent magnetization. Insignificant changes in lengths and directions of the principal susceptibility ellipsoid axes also indicate that magnetic fabric and remanent magnetization are carried by partly different populations of magnetic grains.
The acquisition of a FRM in nature has yet to be explored. If such a process is confirmed, however, it has the potential for obtaining age estimates of ancient thaw depths and for providing insights into material transport processes in frost polygons.     

A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals

Summary. Susceptibility, thermo-remanent magnetization (TRM) and isothermal remanent magnetization (IRM) anisotropy ellipsoids have been determined for several rock samples. The results indicate that the ellipsoid of initial susceptibility is less anisotropic than the TRM and low field IRM ellipsoids which are found experimentally to be of identical shape. This suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non-destructive low field IRM measurements. Such IRM measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured. The results for a series of artificial anisotropic samples containing magnetite particles of different sizes (in the range 0.2–90 μm) were very similar to those for the rocks. In contrast, a comparison of the susceptibility and IRM ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory. Such results imply that susceptibility and IRM ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single-domain particles (magnetization confined to the easy axis) or whether the particles are essentially multidomain.     

The need to correct for the Suess effect in the application of δ<Superscript>13</Superscript>C in sediment of autotrophic Lake Tanganyika,as a productivity proxy in the Anthropocene

The change in dissolved inorganic δ¹³C in the ocean resulting from the change in δ¹³C in atmospheric CO₂ owing to anthropogenic activities (the Suess effect) is well known. The need to correct for the Suess effect when applying δ¹³C in organic matter in lacustrine sediment deposited during the anthropocene as a productivity proxy, is widely although not universally acknowledged. This paper reviews conceptions about the Suess effect in lacustrine δ¹³C_org and methods to adjust for the Suess effect when δ¹³C_org is used to infer recent changes in aquatic productivity. Lake Tanganyika is used as an example to illustrate the necessity of the correction. When the Suess effect is not considered, interpretations of sediment core data can result that are opposite to those achieved with the correction applied, as is here shown in Lake Tanganyika and in other lakes. A new method to correct for the Suess effect is provided which has the advantage of being applicable to data for a larger period (1700–2000) than methods currently available. In addition, Lake Tanganyika is shown to be a net sink for CO₂.