Full vector archaeomagnetic field in Central Europe

The main goal of the project is to establish a full vector archaeomagnetic data set for Central Europe for the past 4000 years. This data set is aimed to investigate the hypotheses of a correlation between sudden changes in the movement of the archaeomagnetic direction, called `archaeomagnetic jerks` and high archaeointensity. This phenomenon may control centennial climate change. The Earth`s magnetic field vector undergoes changes on time scales within years to 100 000 years which are called geomagnetic secular variation. Human artefacts like pottery, hearths or kilns record the magnetic field vector and preserve it until today. This feature is called archaeomagnetism and it allows us to reconstruct the history of the Earth`s magnetic field by investigation of well dated archaeological fired structures. If the temporal variation of the magnetic field is already known, such a secular variation curve can serve as a dating tool for archaeology by comparing the obtained field vector from a heated archaeological structure of unknown age with the curve. As secular variation is a regional phenomenon such curves exist for several countries of Europe (Austria, Bulgaria, France, Germany, Great Britain, Hungary) and they all show for example a change in the movement of archaeomagnetic direction during mediaeval times and in the Roman epoch. For France these cusps in direction (`archaeomagnetic jerks`) are associated with maxima in intensity and in these two time intervals there is a sufficient data base. Two further `archaeomagnetic jerks` are suspected in the `Dark Ages` as well as at the end of the Bronze age. For these time intervals the archaeodirectional as well as the archaeointensity data sets are poor and require further investigation. Up to now the data set supporting the hypothesis of the correlation between `archaeomagnetic jerks` and intensity maxima comes from different archaeological sources found in different sites. Accordingly, the temporal correlation of direction and intensity is based mainly on dating by archaeological evidence which may not be precise enough. Archaeomagnetic full vector data avoid this problem and can help to support such a correlation directly. During the project it is intended to investigate especially archaeomagnetic directions from sites dated in times BC or dated in the `Dark Ages` in order to fill the gaps in the directional data base of Central Europe. About 30 structures from these periods have already been sampled in France and Germany. Together with the European archaeodirectional data base the new results will allow to study, if `archaeomagnetic jerks` occur contemporaneous in Central Europe. For investigation of the correlation of these jerks with high archaeomagnetic intensities a full vector data set for Central Europe will be established. This means that from sites for which archaeodirection is already obtained, the archaeointensity will be determined. If this is not possible for the material of the in situ structure, displaced materials, like potsherds or bricks, which are temporally closely associated with the heated structure and have a provenance close to site (i. e. potsheds and pottery kiln) will be taken for archaeointensity determination. Such a full vector data set allows to search directly for the possible coincidence of intensity rise and change in movement of the direction. A full vector archaeomagnetic data set can also refine archaeomagnetic dating as a third parameter will reduce the error margins obtained for the age.

The project`s main aim was to test the hypothesis of a possible relationship between the behaviour of the Earth`s magnetic field and changes of climate. It was claimed that during the past 3000 years the secular variation (change of direction and intensity of the field on decadal and centennial timescales) curve shows relatively rapid changes in the movement of direction (cusps). Using single archaeointensity data from France and Syria four cusps seem to be correlated with maxima of the Earth`s magnetic field intensity and this phenomenon was named `archaeomagnetic jerk`. A relation between the `archaeomagnetic jerks` and advance of Alpine glaciers was suggested. Mechanisms for such a link between the geomagnetic field and climate are discussed very controversially. By investigating the complete European archaeomagnetic directional data set in the frame of the project, it can be shown that five cusps of secular variation coincide all over Europe on centennial time scale. Then, a full vector archaeomagnetic data set using archaeological sites from Austria, Germany and Eastern France has been measured. This data set covers the past 3500 years with at least one full vector per century obtained also from archaeological structures, which recorded the secular variation cusp signals. Although significant variations of archaeointensity occur during the past 3500 years, a correlation of high intensity and cusps of directional curves is not seen, except in the 8th century AD. On the contrary during the cusp found in the 8th century BC very low intensities were measured. Nevertheless, the only two very pronounced intensity maxima occur in the 5th century BC and the 8th century AD. The strong increase of archaeointensity lasting from 800 to 500 BC is correlated with increasing rainfall and a change to colder climate all over Europe. The same is true for the archaeointensity maximum found around 800 AD. Both time intervals are also correlated with lower rates of radiocarbon production which lead to long plateaus in the radiocarbon calibration curve. Accordingly, it seems that the existing link between the Earth`s magnetic field and radionuclide production may also affect processes in the high atmosphere governing cloud production. However, an increasing archaeomagnetic data base for better constraining global magnetic field models is still needed. Furthermore, the project provides a secular variation curve which extends the range of archaeomagnetic dating in Central Europe to the past 3500 years. This new dating tool for proto-history may overcome the problems of radiocarbon plateaus in Late Bronze and First Iron age.