Archeointensity study on baked clay samples taken from the reconstructed ancient kiln: implication for validity of the Tsunakawa-Shaw paleointensity method

Abstract

In 1972, a reconstruction experiment of a kiln had been done to reproduce an excavated kiln of the seventh century in Japan. Baked clay samples were taken from the floor surface and −20 cm level, and they have been stored after determinations of the paleomagnetic directions by partial alternating field demagnetizations. We recently applied the Tsunakawa-Shaw method to the samples to assess how reliable archeointensity results are obtained from the samples. A suite of the rock magnetic experiments and the scanning electron microscope observations elucidate that dominant magnetic carriers of the floor surface samples are Ti-poor titanomagnetite grains in approximately 10 nm size with single-domain and/or super-paramagnetic states, whereas contributions of multi-domain grains seem to be relatively large for the −20-cm level samples. From the floor surface samples, six out of the eight successful results were obtained and they give an average of 47.3 μT with a standard deviation of 2.2 μT. This is fairly consistent with the in situ geomagnetic field of 46.4 μT at the time of the reconstruction. They are obtained with a built-in anisotropy correction using anhysteretic remanent magnetization and without any cooling rate corrections. In contrast, only one out of four was successful from the −20-cm level samples. It yields an archeointensity of 31.6 μT, which is inconsistent with the in situ geomagnetic field. Considering from the in situ temperature record during the firing of the kiln and the unblocking temperature spectra of the samples, the floor surface samples acquired full thermoremanent magnetizations (TRMs) as their natural remanent magnetizations whereas the −20-cm level samples only acquired partial TRMs, and these differences probably cause the difference in the archeointensity results between the two sample groups. For archeointensity researches, baked clay samples from a kiln floor are considered to be ideal materials.

Reconstructing the Geomagnetic Field in West Africa: First Absolute Intensity Results from Burkina Faso

“…The obtained values of anisotropy correction agree with values found by other authors or are lower; e.g., Tema et al 22. obtained changes of <8% of archaeointensity due to ATRM correction in brick furnaces; Yamamoto et al 23. observed typical values between (5.9–12.0)% for baked clay from experimental furnaces; and Kovacheva et al 24.…”

Paleointensity Study on the Holocene Surface Lavas on the Island of Hawaii Using the Tsunakawa–Shaw Method

“…Paleointensity was estimated from the linear segment of the NRM-TRM1 * diagram when the ARM correction was judged to be valid based on the linear segment of the TRM1-TRM2 * diagram. We adopted the following selection criteria, which are similar to those used with the Tsunakawa-Shaw method in recent paleointensity studies (e.g., Yamamoto et al, 2010Yamamoto et al, , 2015Yamazaki and Yamamoto, 2014): Rubin et al (1987);…”

“…Cromwell et al (2017) argued that total TRM-based paleointensity methods, including the Shaw method, are likely to underestimate the expected geomagnetic field strength in general because they do not test for SD-like behavior. The TsunakawaShaw method, on the other hand, places much emphasis on the selective removal of MD-like components by lowtemperature and AF demagnetizations and also on correction to compensate possible magnetostatic interactions among the magnetic assemblages (e.g., Yamamoto and Tsunakawa, 2005;Yamamoto and Hoshi, 2008) and anisotropy of remanences (e.g., Yamamoto et al, 2015) using ARM. These improvements have not been incorporated into most of the Thellier-type experiments.…”

A LabVIEW software for Thellier paleointensity measurements with an automated three-component spinner magnetometer TSpin

“…Published paleointensity data have been compiled in databases (Biggin et al 2009;Brown et al 2015) that allow users to find lack of reproducibility in the existing paleointensity results. Field tests for materials cooled in the known geomagnetic field revealed significant variations in obtained paleointensities within a single sampling site (e.g., Herrero-Bervera and Valet 2009;Yamamoto et al 2015). Paleointensity experiments usually need to repeat double heatings at many temperature steps as in the Thellier method (Thellier and Thellier 1959).…”