http://www.earth-prints.org The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material. Search the Channel search http://www.earth-prints.org/simple-search http://hdl.handle.net/2122/3997 Title: Relative geomagnetic paleointensity from the Jaramillo Subchron to the Matuyama/Brunhes boundary as recorded in a Mediterranean piston core <br/> <br/>Authors: Dinarès-Turell, J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Roberts, A. P.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK <br/> <br/>Abstract: Piston core LC07, located west of the Sicily Strait in the Mediterranean Sea, unambiguously records the Matuyama/ Brunhes (M/B) and the upper Jaramillo polarity reversals, with similar average sediment accumulation rates (SARs) for the Brunhes Chron (2.29 cm/kyr) and late Matuyama Chron C1r.1r (2.19 cm/kyr). We report a relative paleointensity record for the interval spanning the M/B boundary down into the Jaramillo Subchron, which is unique in the Mediterranean because existing records from this basin cover only the last 80 kyr. The average SAR in core LC07 is used to translate the depth-related paleointensity record to the time domain; the ratio of anhysteretic remanent magnetization to low-field magnetic susceptibility is climatically sensitive and is used to tune the age model. This correlation produces a good fit to the global ice volume model derived for summer insolation at 65°N. With this age model, a paleointensity minimum in association with the M/B boundary has a duration of about 4-5 kyr, while the directional change has a duration of <3 kyr. A second paleointensity minimum of similar duration is found about 16 kyr below the M/B boundary. This feature (precursor or 'dip' in the literature) has previously been recognized at the same time interval in many marine records, which reinforces the validity of our age model. Other relative paleointensity minima are found within chron C1r.1r, and, within the uncertainties of the respective age models, these minima coincide with those observed from the few published coeval paleointensity records. In particular, there is good correspondence between the ages of minima at about 0.92 and 0.89 Ma, which probably correlate with two geomagnetic excursions (Santa Rosa and Kamikatsura, respectively) that have been recorded in lava flows and dated using the 40Ar/39Ar technique. In contrast, a recently dated excursion at 0.83 Ma from La Palma seems to correspond to a paleointensity maximum. This observation is opposite to that expected and this excursion needs to be confirmed. In contrast to some recently published paleointensity records, spectral analysis of the LC07 record does not reveal identification of significant power at the orbital obliquity frequency. http://hdl.handle.net/2122/3996 Title: Comment on "New radiometric dating of volcanic ash layers in Periadriatic foredeep basin system, Italy" <br/> <br/>Authors: Florindo, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Marra, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia <br/> <br/>Abstract: Bigazzi et al. (2000) report new geochronological data (fission-track dating) from two volcanic ash marker layers interbedded in Upper Miocene (Messinian) -Early Pleistocene sedimentary sequences in three sites from the Adriatic side of the Italian peninsula. As Bigazzi and co-authors correctly state, these data have the potential for the establishment of sound regional chronostratigraphic markers. Unfortunately, the entire stratigraphic framework in which the new fission-track analyses are considered by Bigazzi et al. (2000) was superseded by several studies conducted in the past decade and this limits the actual potential use of the new results. We stress in this note some necessary clarifications. http://hdl.handle.net/2122/3995 Title: Quaternary climatic control of biogenic magnetite production and eolian dust input in cores from the Mediterranean Sea <br/> <br/>Authors: Dinarès-Turell, J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Hoogakker, B. A. A.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK; Roberts, A. P.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK; Rohling, E. J.; School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK; Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia <br/> <br/>Abstract: We report high-resolution magnetic measurements from two Mediterranean piston cores: LC07 (Sicily Strait) and LC10 (Ionian Sea). Magnetostratigraphic results and δ18O data provide age constraints for core LC07, where we investigate magnetic property variations for two age intervals (0-600 kyr and 660-1020 kyr). For core LC10, rock magnetic parameters appear to be climatically controlled and are used to derive an astronomically tuned age model for the interval between 780 and 1200 kyr. In core LC07, the dominant control on the magnetic properties appears to be glacial-interglacial variations in the concentration of biogenic magnetite. In addition, an increased contribution from high coercivity minerals (e.g. hematite and/or goethite) probably reflects an enhanced eolian input during glacial periods. Climatic control of magnetotactic bacterial populations has been previously suggested in other environments, but this is the first such report from the Mediterranean. In contrast, the rock magnetic response to Quaternary climatic variability in core LC10 seems to be better expressed by variations in the concentration of high coercivity magnetic minerals. The contrast between a dominantly detrital/eolian flux and a dominantly biogenic flux at the same time for the two Mediterranean settings might relate to the presence of an active current regime in the Sicily Strait, which might decrease delivery of an eolian component to the seafloor compared to the deep Ionian Sea. http://hdl.handle.net/2122/3994 Title: Inter-laboratory calibration of low-field magnetic and anhysteretic susceptibility measurements <br/> <br/>Authors: Sagnotti, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Rochette, P.; CEREGE, University of Aix-Marseille 3, BP80 13545, Aix en Provence, Cedex 4, France; Jackson, M.; Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, USA; Vadeboin, F.; CEREGE, University of Aix-Marseille 3, BP80 13545, Aix en Provence, Cedex 4, France; Dinarès-Turell, J.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Winkler, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; "Mag-Net" Science Team; various European Universities and Institutes <br/> <br/>Abstract: Inter-laboratory and absolute calibrations of rock magnetic parameters are fundamental for grounding a rock magnetic database and for semi-quantitative estimates about the magnetic mineral assemblage of a natural sample. Even a dimensionless ratio, such as anhysteretic susceptibility normalized by magnetic susceptibility (Ka/K) may be biased by improper calibration of one or both of the two instruments used to measure Ka and K. In addition, the intensity of the anhysteretic remanent magnetization (ARM) of a given sample depends on the experimental process by which the remanence is imparted. We report an inter-laboratory calibration of these two key parameters, using two sets of artificial reference samples: a paramagnetic rare earth salt, Gd2O3 and a commercial "pozzolanico" cement containing oxidized magnetite with grain size of less than 0.1 m according to hysteresis properties. Using Gd2O3 the 10 Kappabridges magnetic susceptibility meters (AGICO KLY-2 or KLY-3 models) tested prove to be cross-calibrated to within 1%. On the other hand, Kappabridges provide a low-field susceptibility value that is ca. 6% lower than the tabulated value for Gd2O3, while average high-field susceptibility values measured on a range of instruments are indistinguishable from the tabulated value. Therefore, we suggest that Kappabridge values should be multiplied by 1.06 to achieve absolute calibration. Bartington Instruments magnetic susceptibility meters with MS2B sensors produce values that are 2–13% lower than Kappabridge values, with a strong dependence on sample centering within the sensor. The Ka/K ratio of ca. 11, originally obtained on discrete cement samples with a 2G Enterprises superconducting rock magnetometer and a KLY-2, is consistent with reference parameters for magnetites of grain size <0.1 m. On the other hand, Ka values from a 2G Enterprises magnetometer and K values from a Bartington Instruments MS2C loop sensor for u-channel and discrete cement samples, will produce average Ka/K values that are unrealistically high if not properly corrected for the nominal volume detected by the sensors for these instruments. Inter-laboratory measurements of K and Ka for standard paleomagnetic plastic cubes filled with cement indicate remarkable differences in the intensity of the newly produced ARMs (with a standard deviation of ca. 21%), that are significantly larger than the differences observed from the calibration of the different magnetometers employed in each laboratory. Differences in the alternating field decay rate are likely the major source of these variations, but cannot account for all the observed variability. With such large variations in experimental conditions, classical interpretation of a "King plot" of Ka versus K would imply significant differences in the determination of grain size of magnetite particles on the same material.