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The results of a detailed magnetostratigraphic and biostratigraphic study of late Pliocene to early Pleistocene marine marl sequences from the Monte Singa and Crotone areas in Calabria, Italy are presented.The magnetostratigraphy from the Monte Singa sequence ranges from below the Gauss/Matuyama boundary up to and including the lower Olduvai boundary. Normal polarities at a level corresponding to isotope stage 81 most probably represent the R~union subchron. From the lower Olduvai boundary upward, a reliable magnetostratigraphy could not be established due to increased weathering of the marls, resulting in mainly secondary magnetizations.The magnetostratigraphy from the composite sequence of the Crotone area belongs to a large part of the Matuyama Chron and includes the Olduvai subchron. The position of the lower and upper boundaries of the Olduvai subzone could be established more precisely than from earlier results. Moreover, the upper boundary of the Olduvai subzone poses an ambiguity: a relatively long normal polarity interval representing the main Olduvai subchron and corresponding to a duration of 115 ka is followed by a short (30 ka) reversed subchron and the short (15 ka) normal Vrica subchron. Another option, and more in accordance with the duration of the Olduvai subchron in literature, would be to consider the complete N-R-N polarity succession with a total duration of 160 ka as representing the Olduvai subchron, implying that this Olduvai subchron has a short reversed interval in its upper part.Linear interpolation and extrapolation yield ages for the most important late Pliocene-early Pleistocene biostratigraphic datum levels. An age of 1.69 Ma is found for the Pliocene-Pleistocene boundary, using the conventional polarity time scale dated with radiometric results. However Hilgen [1], in correlating the sapropel groups and patterns to the precession curve of the Earth's orbit, obtained significantly different ages for the polarity transitions of the present study. According to this astronomically calibrated polarity time scale, the age of the Pliocene-Pleistocene boundary is 1.81 Ma.

Rock magnetic parameters are often used to recognize variations in the original magnetic mineralogy and for normalizing purposes in palaeointensity studies. Incipient weathering, however, is shown to have a profound but partly reversible influence o n the rock magnetic properties of the mark of the Early Pliocene Trubi formation in southern Sicily (Italy). The remanencc in the mark resides in single-domain (SD) magnetite grains, but the remanent coercive force ( H , , ) shows a strong variation and most values observed are anomalously high ( H ( , range 36-188 mT).The enhanced coercivities arc attributed to stress in the magnetite grains induced by surface oxidation at low temperature. Upon heating to 150°C a reduction of coercivities occurs that can be explained by a stress reduction as a result of a reduction of Fe" gradient due to a higher diffusion rate at elevated temperature. After heating to 150 "C. coercivities are quite uniform throughout the outcrop and the values are characteristic of SD magnetite ( H < , range 30-38mT). The bulk susceptibility increases by 4-24 per cent, and the isothermal remanent magnetization (IRM) decreases by 5-1 1 per cent. The increase in anhysteretic remanent magnetization (ARM) is large: 20-242 per cent. The magnitude of the changes is related to the degree of weathering.Another effect of heating the marl samples to 150°C is a substantial reduction of the coercivities of the secondary overprint in the natural remanent magnetization.After heating, separation of the secondary and primary components by alternatingfield demagnetization is more efficient. The usual difficulties of thermal demagnetization above 300 "C may thus be avoided by a combination of moderate heating to 150 "C and subsequent alternating-field dcmagnctization.

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