Archeointensity of the Four Corners Region of the American Southwest

Abstract

New paleointensity experiments were conducted using the IZZI protocol (a variation on the Thellier‐Thellier method) on 289 specimens from 54 baked pottery fragments collected from four archeological sites in the American Southwest. Anisotropy experiments were conducted to correct for anisotropy of remanence, a common problem in archeological material. Additionally, the effect of cooling rate was evaluated and the calculated paleointensity values were adjusted accordingly. Using the Thellier graphical user interface program, the specimen results were analyzed, averaged by sample (i.e., pottery fragment), filtered for the highest quality, and converted to Virtual Axial Dipole Moments (VADMs). Stylistic evidence, historical documentation, and dendrochronology analyses provide age constraints with up to decade resolution for the VADM results. We compared these new results with the highest quality previously published paleointensity values from the Four Corners region of the American Southwest—defined here as the four states of New Mexico, Arizona, Utah, and Colorado. None of the previously published data were corrected for cooling rate and only some were corrected for anisotropy, resulting in a systematic bias between data sets. To accommodate this difference, an estimated cooling rate correction was applied to all the previously published data. No correction can be made for anisotropy as this is specimen specific. Our estimated cooling rate correction and the empirical correction applied to the new data both require an estimation of the historical cooling rate of the pottery. Experimental pottery firings using one ancient technique and outfitted with thermocouples were conducted to determine the historical cooling rate.

First Full Vector Archeomagnetic Data From Northern Mexico

“…Archeological baked clays constitute 80% of the data set. Consistently with Jones et al (2020), we corrected the Southwestern USA mean values by a cooling rate effect of 6.73%, experimentally determined in their study. 32 volcanic data are available for Mexico and one for Utah.…”

“…Data were retrieved from GEOMAGIA50 (Brown et al, 2021) Open (full) circles on Zijderveld plots correspond to a vertical (horizontal) plane projection, respectively. and from regional compilations (Hervé, Perrin, Alva-Valdivia, Tchibinda Madingou, et al, 2019;Jones et al, 2020Jones et al, , 2021Mahgoub, Juárez-Arriaga, Böhnel, Manzanilla, & Cyphers, 2019;Soler-Arechalde et al, 2019). Data were relocated to Casas de Fuego via the Virtual Geomagnetic Pole (VGP) and the Virtual Axial Dipole Moment (VADM) assumptions for directions and intensities respectively.…”

“…For the last 2 kyrs, 192 intensity averages were found within 2,000 km from Casas de Fuego: 141 from Mexico (see Alva-Valdivia et al, 2021;Garcia et al, 2021;Hervé, Perrin, Alva-Valdivia, Tchibinda Madingou, et al, 2019;Mahgoub, Juárez-Arriaga, Böhnel, Manzanilla, & Cyphers, 2019) and 51 from Western North America (e.g., Bowles et al, 2002;Champion, 1980;Jones et al, 2020;Sternberg, 1982). Archeological baked clays constitute 80% of the data set.…”

“…The whole data set is plotted in Figure 7a, together with the new intensities that are within the wide range of VADM values (70-150 ZAm 2 ) observed in the first half of the second millennium CE. As several studies (Hervé, Perrin, Alva-Valdivia, Tchibinda Madingou, et al, 2019;Jones et al, 2020;Mahgoub, Juárez-Arriaga, Böhnel, Manzanilla, & Cyphers, 2019) showed the large role of the uneven data quality in this dispersion, we applied the following selection criteria: (a) only data acquired with the original or derived Thellier-Thellier and microwave methods, (b) at least three specimens, (c) a standard deviation (sd) lower than 10%, and (d) an age uncertainty less than ±200 years. Furthermore, paleointensity values on ceramics must have been corrected for anisotropy effects using the TRM tensor when the laboratory field was not applied parallel to the NRM.…”

“…There has been a long tradition of archeomagnetic studies in Northern and Central America, with the first studies in the mid 60s (e.g., Bucha et al, 1970;Lee, 1975 for Mexico and see Jones et al, 2021 for USA) and continuous improvements since (e.g., Alva-Valdivia et al, 2020Dubois, 1989;Hagstrum & Champion, 2002;Hervé, Perrin, Alva-Valdivia, Tchibinda Madingou, et al, 2019;Jones et al, 2020Jones et al, , 2021Lengyel & Eighmy, 2002;Mahgoub, Juárez-Arriaga, Böhnel, Manzanilla, & Cyphers, 2019;Morales et al, 2009;Pétronille et al, 2012;Rodríguez-Trejo et al, 2019;Schaafsma et al, 2002;Soler-Arechalde et al, 2019;Sternberg, 1989;Wolfman, 1990). For the past 2 kyrs, around 325 intensities and 690 directions, respectively 11% and 12% of the worldwide archeomagnetic data set (GEOMAGIA50.v3.4; Brown et al, 2021), have been published.…”