Reversal records in marine marls and delayed acquisition of remanent magnetization

The upper and lower Thvera sedimentary geomagnetic reversal records from southern Sicily

“…In addition, if the ChRM is the vector sum of normal and reversed directions caused by smoothing, then the intensity is low while the geomagnetic field intensity during the normal and reversed directions may be strong. The VGP paths of the upper and lower Thvera are most probably the result of smoothing of the stable directions before and after the transition [3,5]. In addition, it is unlikely that the complex behaviour of the upper Thvera record is a registration of the transitional geomagnetic field because the directional changes do not completely match directional changes of the Calabrian upper Thvera record only about 250 km away.…”

“…In the case of authigenic (biogenic) formation of magnetic minerals, however, a chemical remanent magnetization (CRM) may be acquired at depths or in depth intervals that depend on redox conditions rather than mainly on compaction. The "lock-in depth" or depth lag may then be considerably larger [5]. Post-depositional diagenetic processes controlled by changing redox conditions, may, in addition, lead to the migration of Fe ions, which can cause a CRM at a depth of more than 1 m below the sediment-water interface [1,16].…”

“…This seems rather unlikely because the rock magnetic parameters do not indicate secondary minerals in this part of the record. Recently, Van Hoof and Langereis [5] showed that the HT (and LT) component can acquire their remanence at a considerable depth below the sediment-water interface and that this depth lag was not constant throughout the lithology. It was suggested that the magnetic minerals carrying the components were authigenically formed under [2].…”

“…Detailed records of geomagnetic transitions have been obtained from different sources, the most important of which are those recorded in lava sequences and those in sedimentary sequences. Although sedimentary sequences concern appropriate time control and continuous registration of the geomagnetic signal, the recording mechanism in sediments is still badly understood and may easily lead to sedimentary arte- facts [3][4][5][6]. However, Laj et al [7] and Tric et al [8] concluded, from studies of the VGP paths of several transitions (showing a wide distribution in both time and place) that the VGP paths are predominantly confined to a longitudinal band over the Americas or its antipode, and that there may have been a strong dipolar field during transitions.…”

A critical re-evaluation of the Miocene/Pliocene boundary as defined in the Mediterranean

“…Moreover, the polarity reversals are located in exactly the same sedimentary cycles as found earlier in other Trubi sections [7,8,50]. The two normal polarity intervals shown represent the Thvera and Sidufjall Subchrons [see 7] and the position of the upper Thvera reversal boundary at Eraclea Minoa has been slightly revised based on new data [51]. The upper Thvera boundary is now pinpointed in the lower part of cycle 16 and the lower Thvera at the transition from cycle 5 to cycle 6.…”