Geomagnetic long-term secular variations in Italian Lower Cretaceous shallow-water carbonates

The remanent magnetic properties of an 88 m bore core are unrelated to eith er the dolomite content or the sedimentological textures and are considered to be carried primarily by biogenetic magnetite that was cemented in durin g very early diagenesis. Individual readings represent time intervals of c. 720 +/- 32 yr and, after 40 mT partial demagnetization, they provide an al most continuous record of averaged geomagnetic secular variations over a pe riod of some 3.17 Myr. The magnitude of directional secular variation is tw ice that of the present day, despite being smoothed, and the secular variat ions appear to grade into polarity transitions, suggesting no difference in their mechanisms. The rates of change in direction between subjacent level s in the core have a log-normal distribution which extends smoothly beyond 90 degrees and has a median value of 13 degrees/700 yr, the same as for uns moothed European secular variation during the last 2000 yr. The intensity o f remanence, after 40 mT partial demagnetization, appears to provide a reas onable approximation to geomagnetic field intensity. This tends to be weake r when the direction is moving faster, reflecting averaging, but is unrelat ed to the distance of the vector from the mean direction; that is, it depen ds on the rate of change and not on the virtual pole latitude. The virtual poles, after correction for tectonic rotations about horizontal and vertica l axes, have latitudes that form a strongly platykurtic Fisherian distribut ion, while their longitudes have a circular distribution on which are super imposed two Gaussian peaks, 180 degrees apart. This bore core thus provides detailed information of smoothed geomagnetic secular variation in the Lowe r Cretaceous (127 +/- 3 Ma) which shows clear regularities in behaviour, so me related to changes in the Earth's orbital parameters.