No abstract

The geomagnetic field variations on the continent of Africa are still largely undeciphered for the past two millennia. In spite of archaeological artefacts being reliable recorders of the ancient geomagnetic field strength, only few data have been reported for this continent so far. Here we use the Thellier-Coe and calibrated pseudo-Thellier methods to recover archaeointensity data from Burkina Faso and Ivory Coast (West Africa) from well-dated archaeological artefacts. By combining our 18 new data with previously published data from West Africa, we construct a reference curve for West Africa for the past 2000 years. To obtain a reliable curve of the archaeointensity variation, we evaluate a penalized smoothing spline fit and a stochastic modelling method, both combined with a bootstrap approach. Both intensity curves agree well, supporting the confidence in our proposed intensity variation during this time span, and small differences arise from the different methodologies of treating data and uncertainties. Two prominent peaks at around 740 AD and 1050 AD appear to be common in ours and several reference curves from other locations, indicating a general westward movement from China to Hawaii of a rather stable feature of the geomagnetic field. However, independent smaller peaks that do not correlate in different locations may hint to localized expressions of the geomagnetic field as a result of temporarily varying non-dipole sources.Archaeological artefacts have already in the 1930s been proven to be excellent recorders of the geomagnetic field (GMF) for the past 10 millennia 1 . This type of material acquires a thermoremanent magnetization (TRM) at the time of its heating and subsequent cooling that is proportional to the ancient GMF intensity, or archaeointensity. This TRM is dependent on many factors including the ancient heating temperature, the type of magnetic mineral carrying the magnetization, and the reheating process.Since the beginning of archaeomagnetism, retrieving archaeointensities has been considered a demanding process. Several methods have been developed, for example the thermal Thellier-family methods 2-4 , the microwave technique 5,6 , the multispecimen methods 7,8 and non-heating methods 9,10 . One of the non-heating methods, the relative pseudo-Thellier technique 11 , makes use of alternating magnetic fields to demagnetize the samples and has recently been successfully calibrated and applied to volcanic rocks 12-14 . However, the calibrated pseudo-Thellier method has never been applied to archaeological artefacts before. The Thellier-family methods are considered as the most reliable of all techniques, because they have been used since nearly 100 years and include many quality assessments. These methods can comprise up to 50 time-consuming heating steps. On the contrary, the pseudo-Thellier method is much faster and avoids chemical alteration of the samples.Archaeological artefacts have been collected from many locations world wide to construct palaeosecular variation curves, or reference c...

No abstract

We report comprehensive rock-magnetic and archaeointensity investigations from 21 well-constrained pottery fragments from the Catamarca province of northwest Argentina. The absolute ages of the studied sites are ascertained by several high-quality radiometric ages and range between 1940 to 114014C yr BP. Magnetic mineralogy experiments indicates that the remanence is carried by thermally stable Ti-poor titanomagnetites. Forty-seven samples belonging to 11 out of 98 studied potsherds yielded reliable absolute intensity determinations judging from the quality parameters associated with the Thellier double-heating experiments. Moreover, we analyzed the available absolute geomagnetic intensities associated with the radiometric ages to construct the first intensity paleosecular variation curve (PSVC) for South America using thermoremanent magnetization carried by burned archaeological artifacts obtained in the present investigation and 79 other selected archaeointensities (out of 213 published in the literature). The dataset is used to build the PSVC reference curve by combined bootstrap and temporal P-spline methods. The variation curve shows significant differences with the global prediction model SHA.DIF.14k mainly based on the GEOMAGIA database. This intensity PSVC curve shows reasonably good agreement with paleosecular variation curves for Europe between 850 through 1150 BC and for Asia between 1000 and 1500 BC. This regional curve may be used as most reliable archaeomagnetic dating tool for the major part of South America (Peru, Brazil, Argentina, Chile, and Bolivia) for the last two millennia.