First archaeointensity results from Ecuador with rock magnetic analyses and 14C dates to constrain the geomagnetic field evolution in South America: Enhancing the knowledge of geomagnetic field intensity
Introduction
It has been known for several decades that archaeological artifacts can be reliable recorders of the Earth's magnetic field behavior in the past (e.g. Eighmy and Sternberg, 1990; Shibuya, 1980; Aidona et al., 2010, Aguilar-Reyes et al., 2013). The archaeological baked clays can generally offer a better temporal and spatial resolution than volcanic rocks, mainly due to their precise 14C radiometric dating, and due to the fact that they can be found all around the world without being limited to volcanic areas. The radiocarbon dating of the pieces of pottery of northern Ecuador highlands (Fig. 1) was carried out using charcoal remains, (e.g. maize, corn, organic food residues, Appendix A, Fig. 2), as well as pollen and volcanic ash, for Atuntaqui and Otavalo mounds (e.g. Athens and Osborn, 1974; Athens, 1999, 2003). The radiocarbon dating of La Chimba was based on Athens (1990) and Stahl and Athens (2001) where almost 40,000 specimens were recovered during excavations that started in 1972 at La Chimba site, Pichincha Province, Ecuador. A listing (including weights and counts) of all remains recovered from the 1989 investigations is provided in Athens (1990), including diagnostic ceramics. Identified macrobotanical remains at La Chimba include charred specimens of small maize cobs and kernels, two types of tubers (potatoes and oca), and seeds of quinoa, beans, and other unidentified types Potatoes, oca, and quinoa had not been previously identified in archaeological sites in the northern highlands (Athens, 1990). Fifteen radiocarbon determinations were obtained from TP-7 and appear to encompass the complete occupational history of La Chimba. Using absolute dates assigned to all levels based on a non-quantitatively determined best fit radiocarbon depth-age curve, it appears that initial occupation occurred sometime around 700 B.C. (2640 B.P. calibrated). Occupation was apparently continuous until the site was abandoned around A.C. 250 (1700 B.P. calibrated). The ages of the different test units are summarized in Table 1 and Table 2 of Athens (1990) which presents a generalized chronological scheme for the site (see Athens, 1978). La Chimba is at present the earliest known Ecuadorian ceramic period site north of Quito. Archaeomagnetic data, however, still remain sparse and very unevenly distributed in both time and space to well-describe and understand the pattern of the geomagnetic field's secular variation (SV) over millennial timescales on a global scale (Valet et al., 2008; Pavón-Carrasco et al., 2014a; Panovska et al., 2015). For instance, very few data come from the southern hemisphere and less than 3% of the archaeointensity entries in the GEOMAGIA database (Brown et al., 2015) come from sites with latitudes between −10 and 10°. In particular, archaeomagnetic data from South America are scarce, and in many cases, come from old studies that are characterized by scattering (Goguitchaichvili et al., 2019). Thus, in order to enhance, improve and contribute to the development of reliable global geomagnetic field models, it is necessary to improve the spatial global coverage with well-dated and high-quality reference data (e.g. Pavón-Carrasco et al., 2014b). In this paper, we present a new set of absolute archaeointensity determinations from pottery collections recovered from three archaeological sites located at the highlands of north central Ecuador. The new results were obtained with the modified Thellier-Coe protocol that implies that instead of measuring the NRM first, we preferred to apply a TRM before heating the sample in zero field (e.g. that Aitken et al., 1988, Valet et al., 1998, Herrero-Bervera and Valet, 2005, 2009; Herrero-Bervera et al., 2016), including partial thermoremanent magnetization (pTRM) checks that helps out to monitor possible magnetomineralogical alterations that can take place during experimental heating. The ages of the samples studied were determined through calibrated radiometric data and archaeological evidence based on diagnostic pottery. Only, a few previous available studies from Ecuador, data from neighboring Peru and Colombia covering the same time periods of our study are avaliable and were used for comparison with our new archaeointensity determinations. It is very important to point out that the archaeointensity results of this study will only be very useful and meaningful for the study of the evolution of the generation of the geomagnetic field at equatorial latitudes of ~110 North and for a time windows of our analysis (i.e. 700 BC-1 AD, 1250–1505/1525 AD, 700–440BC, 440–40BC and 44BC-250AD) and within a 600–700 km circle (e.g. Tema and Knodopoulou, 2011) and centered at the northen ecuador sites correlatable to the Earth's core generated signals. Beyond this circle (i.e. radius of 600–700 km) any attempt to correlate to distant locations such as central America, Brazil, Argentina, Uruguay, Paraguay, Chile and Bolivia is meaningless due to the way the geomagnetic field is generated.
Section snippets
Archaeological sites and studied material
The potsherds studied come from three prehistoric archaeological sites situated in northern Ecuador (Fig. 1): Atuntaqui (Mound 30, Lat. 0.34° S, Long. 78.2° W), Otavalo (Mound 3, Lat. 0.24° S, Long. 78.3° W) and La Chimba (Test Pits 5 and 7, Lat. 0.14° S, Long. 78.0° W).
The prehistoric Ecuadorian cultures excelled in the production of pottery, with each cultural phase being characterized by its own style. Therefore, the pottery can also be used as an important cultural marker (Athens, 1990, 1995
Magnetic mineralogy
The magnetic mineralogy of representative samples were investigated at the Paleomagnetics and Petrofabrics Laboratory of the Hawaii Institute of Geophysics and Planetology (HIGP)-SOEST University of Hawaii at Manoa (Honolulu, Hawaii), at the CIMaN-ALP Paleomagnetic laboratory (Peveragno, Italy), and at the Geophysics Institute of the University of Mexico (UNAM, Mexico).
Isothermal remanent magnetization (IRM) curves were obtained after imparting a step-wise increasing direct field with an ASC
Archaeointensity determination
Paleointensity (PI) experiments were performed at the SOEST-HIGP Magnetic Materials, Paleomagnetics and Petrofabrics Laboratory at the University of Hawaii at Manoa (Hawaii, USA). A total of 58 pottery specimens were analyzed using the Coe version (Coe, 1967) of the classical Thellier and Thellier method. However, instead of measuring the NRM first, we preferred to apply a TRM before heating the sample in zero field (Aitken et al., 1988; Valet et al., 1998; Herrero-Bervera and Valet., 2005, 2009
Discussion
We present here new, well-determined absolute paleointensity results from three archaeological sites from Ecuador. The new data come from precisely dated pottery, and therefore offer a very valuable set of information about the past intensity variation in Ecuador. At the same time, they enrich the global reference data from low-latitude ites that are still very scarce woldwide
The new estimated average archaeomagnetic absolute paleointensities obtained in this study of 43.6 ± 1.3 μT for
Conclusions
The studied Ecuadorian ceramics proved to be excellent recorders of the past geomagnetic field, rendering successful archaeointensity determinations with a high success rate. The samples collected from the three different archaeological sites were well dated by means of radiocarbon techniques (Appendix A, Fig. 3) and can therefore be used as reference points for enriching our knowledge of the past Earth's magnetic field intensity secular variation at low latitudes (i.e. equatorial latitudes).
Declaration of competing interest
The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent licensing arrangements),or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed
Acknowledgements
We are pleased to thank Mr James Lau for his assistance during the laboratory experiments. Professor Manuel Calvo Rathert read and critically reviewed the manuscript suggesting corrections, upgrades and improvements that made the manuscript much better than the original version of the article, we thank him for his help. We also thank in advance two anonymous referees for their very constructive, “spot on”, sharp criticisms and overall help. Data reduction was mainly processed using the Paleomac
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New <sup>40</sup>Ar/<sup>39</sup>Ar age of the full vector Upper Mammoth geomagnetic polarity transition recorded in the Pu'u Kualakauila volcanic sequence, Hawaii
2022, Physics of the Earth and Planetary InteriorsCitation Excerpt :Absolute paleointensity determinations (P.I.) experiments were performed using the Coe version (Coe, 1967) of the Thellier and Thellier-experiments, but with a different protocol. Such protocol is applied by instead of measuring the NRM first, we applied a TRM before heating the sample in zero field (Aitken et al., 1988; Aitken, 1990; Valet et al., 2010; Herrero-Bervera and Valet, 2005, 2009; Herrero-Bervera and Acton, 2011, Herrero-Bervera et al., 2020). In this case, magnetomineralogical transformations occur in the presence of a field, resulting in a CRM (chemical remanent magnetization) component which is detected by a deviation of the NRM toward the oven field direction.
Global archaeomagnetic data: The state of the art and future challenges
2021, Physics of the Earth and Planetary InteriorsCitation Excerpt :As we move closer to the equator the amount of available data shrinks with <2% of database entries from between ±10∘ latitude. Six studies have produced new data in this latitude band over the past 10 years, with the first archaeomagnetic data from Kenya (Tchibinda Madingou et al., 2020) and the Ivory Coast (Kapper et al., 2020), and others building on small data sets from Ecuador (Herrero-Bervera et al., 2020) and Colombia (Cejudo et al., 2019). The spatial distribution of directional and intensity data are distinctly different (Fig. 2, Fig. 3 and Fig. 4).