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Three piston cores (C1067, C1201, C1202) from the continental shelf and slope in the Salerno Gulf and Cilento offshore preserve small-scale, long-term change of Earth's past magnetic field (Palaeo Secular Variation) between about 115 ka B.P and the present. The cores overlap as follows: C1201 spans the most recent 25 ka and overlaps C1067 and C1202, which respectively cover much of the last approximately 115 ka B.P. Cores C1067 and C1202 lack the record from about 20 ka to about 11 ka B.P., an absence that is due to slope erosional processes. The palaeomagnetic field behaviour, when correlated to curves of global relative palaeomagnetic field intensity and dated records of PSV for western Europe and Great Britain, coupled with discreet tephrochronology and geochronology dating (C14), allow us to compile a chronology for the combined cores. The resulting chronology for the first most recent 44 ka has application for placing time constraints on the marine geology and stratigraphy of the margin of the eastern Tyrrhenian Sea. So far, catastrophic events such as large-scale submarine slumps, volcanic eruptions, turbidite deposition, and abrupt changes in sedimentation rate have been documented. In particular, the changes in sedimentation rate seem to be linked to global rapid sea-level pulses and climatic events that induced concurrent reduction and/or abundance in the sediment supply from the adjacent coastal margin.

El trasllat de dades geomagnètiques al llarg de distàncies de centenars de quilòmetres, suposant que el camp terrestre és dipolar, és una pràctica comuna en arqueomagnetisme. Els errors associats a procedir d'aquesta manera gairebé mai es tenen en compte. Aquest treball analitza la distribució d'aquests errors en l'espai i en el temps i n'extreu conclusions útils per als arqueomagnetistes. El traslado de datos geomagnéticos a lo largo de distancias de centenares de kilómetros suponiendo que el campo terrestre es dipolar es una práctica común en arqueomagnetismo. Los errores asociados a proceder de esta manera casi nunca se tienen en cuenta. Este trabajo analiza la distribución de estos errores en el espacio y el tiempo y extrae conclusiones útiles para los arqueomagnetistas. The relocation of geomagnetic data across distances of hundreds of kilometers and assuming a dipolar geomagnetic field is a common practice in archaeomagnetism. The errors due to this practice are rarely taken into account. This work analyses the time and space distribution of these errors and some useful conclusions for archaeomagnetists have been highlighted.

Accademia Nazionale dei Lincei, Atti Convegni Lincei

Rend. Acc. Sc. Fis. Nat., Napoli, Special Issue

SUMMARY Secular variation ‘master’ curves are built up using geomagnetic historical observations or archaeomagnetic data from a limited area and their use is usually restricted to regions of around 1000 km radius. Relocation of data within this distance is a common practice to enable comparison of data, although the errors due to such process are rarely taken into account. A detailed analysis of the distribution of relocating geomagnetic data has been done using three popular sets of geomagnetic models (IGRF-9, GUFM and CALS7K-2). This study improves the error analysis of relocating geomagnetic directions made up to date and expands it to geomagnetic intensities. Maximum errors correlate with the non-dipole to dipole field ratio. Archaeomagnetists could use this analysis to valuate the error introduced by reducing data.

Results are presented for a paleodirectional and paleointensity study of 16 Popocatepetl lava flows. Popocatepetl is a tall, active, dacitic-andesitic stratovolcano; it forms the southern end of a N-S-trending Quaternary volcanic range at the eastern edge of the Basin of Mexico. Studied units are mainly andesites and one trachyandestite, and all possess similar trace- and rare-earth-element compositions. Rock-magnetic experiments show that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution of original titanomagnetite during flow cooling. Unblocking temperature spectra and high coercivities point to "small" pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. A strong, lightning-produced magnetization overprint was detected for one site. Combining the new paleomagnetic data provides a mean paleodirection of I = 35.4degrees, D = 345.7degrees, k = 21, alpha(95) = 8.5degrees, N = 15 for the Popocatepetl volcano region. All studied flows yield normal polarity magnetization, which supports a maximum age within the Brunhes chron.

Long-term change of Earth's magnetic field (palaeomagnetic secular variation, PSV) during much of the last approximately 120 ka was recorded in cored sediment from the slope margin of the eastern Tyrrhenian Sea. The PSV record has been correlated to master curves of global palaeomagnetic field intensity and PSV records for western Europe. Tephrochronology and radiometric dating ( 14 C and 40 Ar/ 39 Ar) have also been used to constrain the age of the PSV record. The combination of the new data with prior PSV and petrophysical data from the area provides a chronological framework for geological events such as large-scale submarine slumps, stratigraphic gaps and short-term changes in deposition rate on the continental margin. These latter changes are linked to the combined action of relative sea-level oscillations, climate events, and consequent variations in land exposure through time. Moreover, new data concerning the thickness and dispersal of Campanian Plain pyroclastic deposits in the marine setting enable volcanic-hazard evaluation. Finally, a pyroclastic deposit (tephra X-6) found offshore in the Southern Campanian marine environment was 40 Ar/ 39 Ar dated for the first time at 108.9 ± 1.8 ka BP.

The stable component of the natural remanent magnetization (NRM) of the Upper Triassic “Calcari con Selce” limestone formation at Marsico Nuovo (Lagonegro Basin, Southern Italy) has an inclination which is about 45° too steep for the expected paleogeographic position. In order to explain this abnormal magnetization direction, several rock magnetic properties such as the dependence of susceptibility on temperature and alternating field frequency, anisotropy of low field susceptibility, anisotropy of anhysteretic remanent magnetization (ARM) and characteristics of acquisition and thermal decay of isothermal remanent magnetization (IRM) have been investigated. Strong inverse fabrics of both susceptibility and ARM have been recognized, which are due to magnetite in the superparamagnetic state on the one hand and in the stable single domain grain size on the other hand. Although internal deformation in the investigated rocks has not been reported in the geological literature, this may be the cause of the observed anisotropies. The anisotropy is probably responsible for the anomalous NRM directions of these limestones.

Detailed paleomagnetic and rock magnetic studies of 14 previously undated lava flows from the Somma-Vesuvius volcanic complex were undertaken. The magnetic carriers consist of Ti-poor titanomagnetites typified by pseudo-single domains. Characteristic magnetization directions determined from detailed stepwise alternating field and thermal demagnetizations gave well-defined mean directions for each flow unit. Paleodirectioris for 10 of the 14 lavas appear to be related to the early 17(th) century segment of the Italian paleosecular variation reference curve; thus their ages are easily retrieved. The remaining four sites yielded distinct mean directions. which cannot be simply cor-related to any given sector of the reference curve. within the little frame expected for the activity. The paleomagnetic directions for these lavas may correspond to little intervals outside the period covered by the reference secular variation curve, or they may reflect local structural disturbance and have no geomagnetic significance.

SUMMARY A palaeomagnetic study on Vesuvius lava flows, emplaced after the 79 AD event, has been carried out in order to correlate and/or discriminate deposits and to verify their age of emplacement using the new SISVC (Southern Italy secular variation curve). Results from 18 sites located on the W, S and E slopes of the volcano, are presented. IRM (isothermal remanent magnetization), along three orthogonal directions, ARM (anhysteretic remanent magnetization), low field susceptibilities measurements and coercivity analyses have been carried out on three pilot specimens per each site in order to both identify the magnetic carriers and determine the magnetic grain sizes. The study of TRM (thermal remanent magnetization) of each investigated lava flow was carried out by subjecting each specimen to complete PTD (progressive thermal demagnetization). The primary directions were defined by linearity analysis following very stringent criteria. It is shown that three of the investigated lava flows, outcropping on the W–S slopes of the volcano, must be associated to the 1631 AD big eruption. This result widens the number of sites to be associated to this eruption, in accordance with previous palaeomagnetic works. The site mean direction of 1906 lava flow is anomalous with reference to the SISVC and further investigations are needed to clarify this point.

Different procedures in terms of sampling techniques and data analyses for the magnetic stratigraphy studies carried out at Etna over the past twenty three years, are presented and discussed. Mean directions of magnetization of lava flows have been derived from Fisher statistics on the data obtained by three different methods: NRM measurements and remanence measurements of partial PAFD of uncut samples; remanence measurements of cores subjected to complete PAFD; primary components defined by very strict PCA from cores subjected to complete PTD or PAFD. Comparison of the directions of magnetizations from Vesuvius and Arso in Ischia Island west of Vesuvius lava flows (about 350 km from Etna), geomagnetic measurements in Paris and Rome (about 1500 and 500 km from Etna respectively) and archaeomagnetic directions from France, all relocated to Etna via the inclined geocentric dipole, indicates that the third procedure produces a coherent time sequence of geomagnetic/remanence direction variations. Guidelines in terms of complete demagnetization, information on the magnetic carriers, very constricted data analysis procedure and relocation of directions of magnetizations are suggested.