The Earth's magnetic field is generated by the motion of liquid iron-rich material in the outer core. One of the most drastic manifestations of the dynamics in the outer core are polarity reversals of the magnetic field. The processes controlling geomagnetic reversals, however, are still poorly understood. The mathematical formulation of the dynamics of the liquid outer core show such a degree of complexity that a universal numerical model still remains elusive. Given that the last reversal occurred about 780,000 years ago, direct observations of a reversal have never been possible. Thus we are left with records of ancient reversals recorded in sequences of sedimentary and igneous rocks. Documenting any systematics in reversal processes will provide substantial information about the outer core and core mantle boundary conditions. However, despite the advances in deciphering the behaviour of the field during polarity transitions, reversal records yield controversial results and thus answers to several key questions are still enigmatic.

Detailed studies of palaeodirectional and absolute palaeointensity patterns of geomagnetic reversals are scarce and are restricted to the Cenozoic so far. In order to verify or reject concepts developed on the basis of this dataset, reversal records which occurred in the more distant geological past of the Earth are needed. This work presents the results obtained from the Siberian Trap Basalts (Russia) which are coeval with the Permo-Triassic boundary (250 Ma). The sequence yields the by far oldest hitherto studied detailed record of a geomagnetic transition from reversed to normal polarity and provides new insights in transitional field behaviour.

Three sections (Talnakh, Listvjanka and Abagalakh) comprising a total of 86 lava flows have been sampled in the Noril'sk region, located at the northwestern rim of the Siberian Trap Basalt province. They provide a complete coverage of the lava pile outcropping in the area. The samples have been subjected to palaeomagnetic direction analysis and to Thellier-type palaeointensity experiments. Extensive rockmagnetic investigations and microscopical studies have been carried out to asses the reliability of the palaeomagnetic information recorded by the lava flows.

Magnetite and Ti-poor titanomagnetites were identified to be the carriers of the characteristic remanent magnetisation. The reversibility of the thermomagnetic curves and the observation of exsolution lamellae by ore microscopy give clear evidence for a primary high-temperature oxidation of the titanomagnetite. It can thus be inferred that the measured palaeodirectional and intensity information obtained from these flows was acquired shortly after extrusion of each flow.

The demagnetisation of the natural remanence reveals only one direction of magnetisation for most samples. Thermal and alternating field demagnetisation methods are equally effective in isolating the characteristic remanent magnetisation. Occasional overprints have maximum unblocking temperatures of 350°C or remanence coercivities less than 20 mT. Reliable palaeointensity estimates were obtained for approx. 50% of the samples. The relatively high success rate can be attributed to the enhanced magnetic and thermal stability of high-temperature oxidised titanomagnetites.

In the lower part of the sequence reversed polarity of the Earth's magnetic field is identified. The associated palaeointensities yield values around 10 µT. The subsequent flows recorded transitional configurations. A tight cluster of virtual geomagnetic poles (VGPs) in mid northerly latitudes, comprising the results of 15 flows, is observed during the transition. Within the cluster the record shows a pronounced and well defined increase in intensity from around 6 to 13 µT. A doubling of local field intensity infers that large scale dynamic processes in the outer core are responsible for this feature, making a strong case for a reasonable temporal stability (se