Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A detailed rock-magnetic and archeointensity study was carried out on materials baked by a western Mexican artisan following traditional techniques to produce faithful reproductions of archeological pieces of the Michoacan region (Western Mesoamerica). The field strength at the site (41.0 +/- 0.51 mu T) was measured with a fluxgate magnetometer and the temperature of the furnace during the baking process was monitored continually by means of a thermocouple placed in the middle of the baking cavity.;Rock-magnetic experiments performed on the raw material (clay and paste) and on in situ prepared baked ceramics and bricks included measurement of thermomagnetic curves (susceptibility and strong-field magnetization versus temperature), first-order reversal curves (FORC), anisotropy of magnetic susceptibility (AMS) and anisotropy of thermoremanent magnetization (A-TRM). Magnetite and probably hematite are present in the samples as carriers of the remanence. Hysteresis ratios suggest that the samples fall in the pseudo-single-domain grain size region, which may indicate a mixture of multidomain and a significant amount of single-domain grains.;Ceramic pieces and brick fragments were subjected to the Thellier-Coe archeointensity method and to an alternative paleointensity experiment, with a TRIAXE magnetometer, in order to check whether they are faithful recorders of the local geomagnetic field strength.;Mean raw-intensity of sample M1 (pottery) overestimates a 7% the expected site intensity, while those corresponding to the brick samples (LQ1 and LQ2) underestimate it 15%. Brick sample LNQ shows a slightly lower intensity (7%), but agrees with the expected site intensity within the experimental uncertainty. The intensity retrieved from the volcanic fragment also included closely reproduces the expected intensity. After A-TRM and cooling-rate corrections, all mean raw values move closer to the expected intensity.;Measurement of temperatures at different parts inside the kiln (bottom and upper parts of both central and peripheral parts) revealed the existence of significant thermal gradients, similar to those observed in ovens from other localities. Different cooling rates are then expected in a single oven.;The scatter in the intensity determinations observed in this study, retrieved from pieces elaborated together in the same oven, could arise from this differentiated cooling rate within the oven and thus, to an inappropriate cooling rate correction in the archeointensity protocol. As this situation was probably reproduced in the baking of ancient ceramic artifacts, a better knowledge of the temperature distribution inside these types of kiln would be desirable in order to choose the appropriate cooling rate correction.
ISSN: 0031-9201

hal-00644559
https://hal.archives-ouvertes.fr/hal-00644559
DOI : 10.1016/j.pepi.2011.06.007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present new, full geomagnetic field vector results from three Neolithic ovens discovered at the archaeological site of Portonovo (Marche, Italy). The discovered structures are a rare example of very well preserved underground ovens from the Early Neolithic period. Standard thermal demagnetization procedures were used to isolate the direction of the Characteristic Remanent Magnetization acquired by the baked clay during the ovens' last firing. The corresponding archaeointensities were determined by the multi-specimen procedure (MSP-DSC) and show a clear intensity low during the Neolithic period. Both directional and intensity results are of high quality, offering the first contribution of full geomagnetic field vector data for this period in Italy. The new data are compared with other contemporaneous data from Europe and with global geomagnetic field models. Independent archaeomagnetic dating of the three ovens was also performed by means of the SCHA.DIF.14k model. The obtained results are in excellent agreement with available radiocarbon dates and confirm that all ovens belong to the Neolithic. These new data importantly enrich our knowledge of the geomagnetic field during the Neolithic period that is poorly documented by data, not only in Italy but also in the whole of Europe and show that archaeomagnetic dating can provide precise results even for prehistoric periods.
ISSN: 0012-821X

hal-01357276
https://hal.archives-ouvertes.fr/hal-01357276
DOI : 10.1016/j.epsl.2016.05.003

Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
Environmental magnetism of speleothems is still in its early stage of development. Here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems, and what are the factors that control its preservation and reliability. To address these issues, we used a multidisciplinary approach, including rock magnetism, petrography, scanning electron microscopy, stable carbon and oxygen isotope compositions, and major and trace element concentrations. We applied this to a set of samples from different stages of speleothem evolution: present-day dripwater (glass plates), a weathered stalactite, a fresh stalagmite, cave sediments, and terra rossa soils. These samples come from the Penico and Excentricas caves, located in two distinct aquifers of the Algarve region, South Portugal. Our results show that the main magnetic carriers of the speleothems under study are primary (detrital) and consist of maghemite (and magnetite?). Similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater. Hence, they represent a regional environmental signature. Interestingly, a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite, whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities. We propose that these low intensities can be the result from (i) different remanence acquisition mechanisms between stalagmite and stalactite and/or (ii) iron dissolution by fungal activity. We also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock (soil)-atmosphere, while oxygen isotope composition and alkaline-earth element concentrations inform about calcite-water interaction processes. These results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo-environmental signal.
ISSN: 0148-0227

hal-01114953
https://hal.archives-ouvertes.fr/hal-01114953
DOI : 10.1002/2014JB011381

Éditeur(s) : HAL CCSD Integrated Ocean Drilling Program
Résumé : International audience
A detailed study of magnetic properties was carried out on ~60 samples from three holes cored during Integrated Ocean Drilling Program Expedition 324, Shatsky Rise. The measurements include scanning electron microscopy (SEM), hysteresis, and first-order reversal curve (FORC) diagrams, as well as measurements of susceptibility at high and low temperatures and variation of saturation isothermal magnetization at low temperature. SEM measurements show the presence of a complex mixture of Fe-Ti oxides with various Ti content x, which is confirmed by the measurements of susceptibility variation with temperature and low-temperature measurements. According to FORC diagrams, the magnetization is carried by single-domain to pseudosingle-domain grains.
ISSN: 1930-1014

hal-00795484
https://hal.archives-ouvertes.fr/hal-00795484
DOI : 10.2204/iodp.proc.324.201.2013