Archeomagnetism in Brazil: New archeointensity data from the Bahia region over the past five centuries

There is a clear need of new archeomagnetic data from the southern hemisphere to better constrain the global geomagnetic field models over the past few millennia. In particular, less than 5% of all available absolute archeointensity data have been obtained from this hemisphere. We will present the first archeointensity data obtained for Brazil from the analysis of several groups of architectural brick fragments dated mainly from the so-called colonial period (with ages ranging between ~1550 AD and ~1850 AD). The fragments collected were produced in or close to the city of Salvador, Bahia State (13°S, 38.5°W) and their dating is ascertained with archives and archeological evidences, yielding precise age controls with, in most cases, uncertainties of less than 10 years. Intensity experiments were performed using the Thellier and Thellier (1959) method as modified by Coe (1967). The stability of the magnetic mineralogy was monitored using partial thermoremanent magnetization (pTRM) checks and pTRM tail tests (Riisager and Riisager, 2001) were used to detect possible biases due to the presence of multidomain grains. High-temperature intensity measurements were also carried out using the Triaxe magnetometer (Le Goff and Gallet, 2004). For both experimental procedures, effects related to the anisotropy of the TRM and to the dependence of TRM acquisition on the cooling rate were taken into account. A good agreement is observed when comparing the mean intensity values obtained using the two methods. We will further compare our new archeointensity results with the intensities expected in Brazil from global geomagnetic field models over the past 500 years.