GP41C-01
A New Archeomagnetic Master Curve for Mesoamerica
A new archeomagnetic reference curve contains all available and updated results reported since early 70th and 27 new archaeomagnetic directions obtained during last six years. The most of samples are taken from the burned floors and walls. In some case, lime plasters are also used. The age interval covers from 100 BC to present. Alternating field demagnetization were carried out to obtain the directions of primary remanence. Pseudo single grain size titanomagnetite seems to be main magnetic carrier as deduced from rock-magnetic experiments. The active collaboration with the archaeologists proved to be fundamental to better constrain the age of archeological events.
GP41C-02
Systematic Archaeomagnetic Datings at Teopancazco- Teotihuacan Area
Since 2000, archaeomagnetic were carried out at Teopancazco - a residential area near Teotihuacan. We report here some new results of detailed sampling of more than 40 different floors of Teopancazco. The floors constitute stratigraphic sequences with temporary control by 14C and archeological contexts. Most of these floors had been affected by fire and carry thermoremanent magnetization. However, some of them are covered by lime plasters (so-called stuccos) containing volcanic ashes and grinded scoria. The samples were demagnetized by alternating fields to get the characteristic magnetization. Rock magnetic experiments yielded very simple magnetic mineralogy dominated by almost-magnetite phase. Rock magnetic fabric had been determined by anisotropy of magnetic susceptibility experiments. The results obtained are in good agreement with previous studies.
GP41C-03
Archeointensity from 1300-200 BP Brazilian Pottery
The evolution of the Earth's magnetic field in the 10 to 100s of years time-period can provide important information about geodynamo variations. Accurate curves for the last 2000 years have been developed for Europe and Western Asia. In contrast, continents in the southern hemisphere count much less data; South America contributes with less than 1% of the world archeointensity database. In order to derive a South American archeointensity master-curve, we have initiated a systematic paleointensity survey in well-dated (TL and/or 14C methods) ceramic fragments from southeast and northeast Brazil. Here we report the first results on pottery collected in 24 Guarani sites with ages from 1300 BP up to 200 BP, and colonial historical sites with ages from 350 BP to 200 BP. Rock magnetic measurements indicated that the main magnetic carrier in all pottery samples is magnetite in the SD state domain. Paleointensity measurements were performed using the double-heating Thellier-Thellier method and pTRM (partial thermoremanent magnetization) checks with success rates of 60%. Results were consistent within sites and between potteries of similar ages. Archeointensity values vary from 30 μT at around 870 BP up to two peaks of 52 μT at 900 BP and 540 BP, and decrease continuously towards the present-day value of ~30 μT.
GP41C-04
Archeomagnetic Results From Spain
A first secular variation (SV) curve for the Iberian Peninsula was computed by hierarchical Bayesian method using a total of 134 archaeomagnetic directions with ages ranging from 775 BC to 1959 A.D. A general agreement is observed between the Iberian curve and the French and German SV curves, excepted between the 11th and 14th centuries. The analysis of these three reference curves indicates that SV in western Europe is characterized by three major directional changes at 125, 200, and 1350 A.D. It is suggested that these cusps are regional features of the geomagnetic field. Archeointensity studies were also conducted on 24 Spanish archeomagnetic sites (220 AD to 1959 AD). Intensity data were obtained using the original Thellier method with anisotropy and cooling rates corrections. Our new 24 data, together with 62 previously published results, were used to recover, by Bayesian modelling, the geomagnetic field intensity evolution over the past two millennia for western Europe. Our results indicate that the geomagnetic intensity remains more or less constant between the 1st and 4th centuries, and between the 13th and 15th centuries, whereas an important decrease occurs during the last 5 centuries. The evolution of the Earth's magnetic field intensity during High Middle Ages remains uncertain. The available geomagnetic global models predict reasonably well our data.