Paleomagnetic directions and thermoluminescence dating from a bread oven‐floor sequence in Lübeck (Germany): A record of 450 years of geomagnetic secular variation

A record of about 450 years of geomagnetic secular variation is presented from a single archaeological site in Lübeck (Germany) where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today. The age dating of the oven‐floor sequence is based on historical documents, 14 C‐dating and thermoluminescence dating. It confines the time interval from about 1300 to 1800 A.D. Paleomagnetic directions have been determined from each oven floor by means of 198 oriented hand samples. After alternating field as well as thermal demagnetization experiments, the characteristic remanent magnetization direction was obtained using principal component analysis. The mean directions of 24 oven floors are characterized by high Fisherian precision parameters (>146) and small α 95 confidence limits (1.2°–4.6°). For obtaining a smooth curve of geomagnetic secular variation for Lübeck, a spherical spline function was fitted to the data using a Bayesian approach, which considers not only the obtained ages, but also stratigraphic order. Correlation with historical magnetic records suggests that the age estimation for the upper 10 layers was too young and must date from the end of the sixteenth to the mid of the eighteenth century. For the lowermost 14 layers, dating is reliable and provides a secular variation curve for Germany. The inclination shows a minimum in the fourteenth century and then increases by more than 10°. Declination shows a local minimum around 1400 A.D. followed by a maximum in the seventeenth century. This is followed by the movement of declination about 30° to western directions.