[en] Complete text of publication follows. Requirements for the Thellier and Thellier (1959) double heating palaeointensity determination technique are equality of blocking temperatures of partial natural remanent magnetisation (NRM) and thermoremanent magnetisation and linear field dependence of TRM. While the equality of blocking temperatures is tested during the experimental procedure by partial TRM checks and tail checks, the field dependence is in general not checked. However, two palaeointensity determination experiments can be derived from both aforementioned requirements: 1. the temperature is changed and the field is kept constant, i.e. the classical Thellier-Thellier technique and 2. the temperature is kept constant and the magnetising field is varied. We applied both techniques to promising baked limestone samples from a Roman limestone kiln at Tournai (Belgium) and from a pottery kiln of the same period from Bruyelle (Belgium) in order to test if both independent methods yield the same results. The studied limestone samples were taken inside the combustion chamber wall at two distances from the fire exposed surface, i.e. at 0 to 2.2 cm and at 4.8 to 7 cm. The alteration checks made during palaeointensity experiments fall generally inside the error envelope of the fitted slope. This indicates the absence of thermo-chemical alteration during the experiment and is apparently independent from the distance of the sample to the fire, although a colour gradient from white to black is observed at increasing distance from the fire. Somewhat different palaeointensities are systematically obtained with both palaeointensity determination methods. At 0 to 2.2 cm distance, the value is about 55 μT while at a 4.8 to 7 cm distance the value is about 72 μT. As the latter value is closer to known values for the same time period, the values obtained close to the fire are probably untrustworthy. Possibly, a low-temperature thermo-chemical disequilibrium, due to post-baking weathering, causes alteration below the first heating step (i.e. below 150 deg C) and being equalised by heating at 150 deg C. In this case, newly-formed magnetic grains would contribute only to the laboratory remanence. Consequently, TRM values are increased, implicating a flatter slope and eventually lower palaeointensity. The interior of the limestone slab, i.e. at 4.8 to 7 cm distance would not have been affected by weathering, and yields thus correct values. Palaeointensity determinations from the baked clay sample taken from the combustion chamber of the Roman pottery kiln at Bruyelle were not successful at distances close to the fire due to thermo-chemical magnetic mineral alteration. However, in samples taken between 6.5 and 8 cm from the fire exposed surface, the alteration is negligible and both palaeointensity determination methods yield very similar results of 73 and 76 μT. The good consistency of the results applying both techniques encourages us to use the variable field technique for further investigations, because the thermal stress during the experimental procedure is lower compared to the Thellier-Thellier technique, as one heats only to a constant temperature. However, preliminary alteration tests should be carried out in order to detect low temperature alteration and to choose the optimal heating temperature.

[en] Complete text of publication follows. Acquisition of detrital and/or post-detrital magnetisation (DRM and pDRM) of sediments depends on many factors linked to sediment properties and to the depositional process. Despite the large differences between the timing of natural and artificial deposition in laboratory it is possible to constrain some critical parameters by simple experiments. The present study, which involves laboratory redeposition of carbonate-rich and carbonate poor natural marine sediments preserved in piston cores, is aimed at: constraining the variability in the response curves of the magnetisation to the applied field; determining the importance and consequences of post-detrital reorientation in different environments. In order to meet these goals we used samples with gelatine, which mechanically locks all grains shortly after deposition by cooling down to 3 deg C, and samples without gelatine. Mechanical orientation of magnetic grains was preserved by the gelatine while in samples without gelatine a large part remained free to rotate within the sediment or at the interface with water, because remanence was measured in zero field. A direct consequence is that the DRM of the gelled samples was 2 to 4 times stronger than for the samples without gelatine. The magnetisation intensities derived from the laboratory experiments have been compared to those that were initially measured in natural sediments after normalising the magnetisation by the concentration of magnetic material (using either ARM or susceptibility). The values obtained without gelatine match those of the natural sediment. Therefore, either the part of magnetisation that was not locked-in is associated with unstable magnetic carriers or the same grains were also mechanically free to rotate during the initial measurements of the natural remanent magnetisation (NRM) of the sediment in zero field. Alternating field demagnetisation curves of the DRMs obtained for samples with and without gelatine are quite different. Without gelatine, the median destructive field (MDF) is below 5 mT, but when using gel the MDF is about 30 mT. Grains with stronger remanent coercive forces are still in suspension and contribute only to the total remanence if they are completely mechanically fixed. In comparison to nature, a gelled sample would correspond to a certain depth where the sediment is almost dewatered and consolidated and where the sediment matrix does not allow anymore particle rotations. Concerning our studied sediments, this means that 25 to 50 % of the total remanence would be blocked immediately during sediment deposition in case of no bioturbation) and that the remainder would be locked-in between sediment deposition and sediment consolidation. In dependence of magnetic grain sizes, or the size of sediment particles, carrying remanence carriers, delayed remanence acquisition may be not an unimportant factor in certain deep-sea sediments.

[en] Complete text of publication follows. Magnetic susceptibility (MS) records from Holocene, Pleistocene and Tertiary sedimentary successions have been widely used as palaeoclimatic proxies. The use of MS in Palaeozoic sediments is becoming more common but still suffers from some controversy. One of the problems is the origin of the magnetic minerals. Most sedimentologists accept the hypothesis that magnetic minerals reflect lithogenic inputs. However, it remains unclear if these inputs are of fluvial or of aeolian nature and if they are influenced by climatic, sea-level or tectonic changes, which act probably at different time-scales. In order to differentiate between different impacts, a strong interdisciplinary characterisation of facies, cyclostratigraphy and the MS signal and MS carriers is required. Furthermore, concerning of Palaeozoic rocks, the influence of diagenesis on the magnetic mineralogy has to be assessed. We would like to promote the interdisciplinary IGCP-project 850, recently funded by the UNESCO, in order to invite collaborators to possibly join our research. The project aims at compiling published magnetic susceptibility records from Palaeozoic sedimentary rocks and at acquiring new data for testing the correlation between susceptibility and sedimentological parameters. Another goal is to investigate the cause of the magnetic susceptibility signal in different sedimentary environments and its relationship to climate controlled environmental parameters. Finally, magnetic susceptibility records from different globally distributed sites will be correlated for reconstructions of past climatic variations.