The Field of Sherds: Reconstructing Geomagnetic Field Variations from Peruvian Potsherds

Abstract

Well dated potsherds of Peruvian ceramics, comprising nine different cultural phases from 1000 BC to 1400 AD were studied providing the unique opportunity to establish a geomagnetic field intensity curve for Peru. Rock magnetic experiments revealed magnetite and maghemite as main carrier of the thermoremanent magnetization (TRM). Archaeointensities are determined using a Thellier-type technique (MT4), including checks for magnetomineralogical changes during laboratory treatment and multidomain (MD) bias. Additionally, TRM anisotropy tensors and cooling rate dependencies are measured and corrected for. Both experiments, carried out for the first time on Peruvian ceramics, emphasise that these corrections are critical factors in archaeointensity determinations. Our new high quality data set shows that the average intensity of the investigated cultural phases is about 35% higher than the present day local geomagnetic field. Besides three intensity maxima at 350 ± 50 BC, 210 ± 120 AD and 720 ± 100 AD a significant decline around 250 ± BC, complying with today’s magnetic field strength is found. Comparing the new established intensity curve with French and Syrian data, almost no concordance is found. Beneath the intensity maximum in the year 200 AD two further outliers of the epoch of the Middle Horizon (620–820 AD) and the epoch of the Late Intermediate Period (1000–1400 AD) give hints of three archaeomagnetic jerks, which fit very well to the French jerks.