Magnetic inclination from Brazilian bricks and application to Archeomagnetic dating
Abstract
The Earth's magnetic field (EMF) is recorded in archeological baked materials carrying a stable remanent magnetization. This magnetic record can be used for indirectly date the archeological material by comparison with "reference curves" of the EMF. In this work we present magnetic inclination data from two different sites in southeastern Brazil, an ancient and a modern one: (a) a sampling at the ancient sugarcane mill Engenho Central de Piracicaba (ECP, 1881-1974 AD), and (b) a controlled survey at the modern brickyard Olaria Schiavolin (OS). Both of them are located in the city of Piracicaba, São Paulo State, Brazil. Magnetic measurements included stepwise thermal and alternating field demagnetization, anisotropy of magnetic susceptibility (AMS) and low-field magnetic susceptibility vs. temperature. In OS, we collected 40 oriented bricks from 5 sampling points inside the brickyard oven in order to test the stability and reliability of their remanent magnetizations by comparing them with the local field (measured directly with a fluxgate magnetometer). We observed differences of +/- 3° between the local field and the IGRF. When magnetic inclinations are reported relative to the flat planes of the brick (the situation we face for real ancient bricks), the differences due to inclination of the burning plane and magnetic anomalies inside the oven never exceed 6°. These differences are averaged out by using a minimum of 6 bricks (~24 specimens) per sampling point to compute the inclination. In the ancient site ECP we collected 140 non-oriented bricks from 14 different walls; three of them were previously dated using historical records. Inclinations were defined using at least 6 bricks per wall, showing consistent average values within the same walls. When compared to the IGRF and GUFM1 models, the inclinations obtained for the dated walls agreed within the experimental error. We have then applied the same procedure to estimate the age of the remaining walls. Using this procedure we were able to reconstruct the architectural history of ECP building, with the following ages: ECP5 (1881-1937.5), ECP11 (1924.5-1933.5), ECP1 (1921.5-1936.5), ECP2 (1926.5-1935.5), ECP10 (1931.5-1942.5), ECP3 (1931.5-1949.5) e ECP9 (1931.5-1972.5).
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2013
- Bibcode:
- 2013AGUSMGP21B..01B
- Keywords:
-
- 1503 GEOMAGNETISM AND PALEOMAGNETISM / Archeomagnetism;
- 1532 GEOMAGNETISM AND PALEOMAGNETISM / Reference fields: regional;
- global;
- 1555 GEOMAGNETISM AND PALEOMAGNETISM / Time variations: diurnal to decadal