Secular Variation and Paleointensity in Mexico during the Plio-Quaternary – SVPIntMex

As lava cools and solidifies, it preserves the direction of the ambiant field so looking at rocks that has formed over time gives us an idea of the evolution of the Earth’s magnetic field through time. The same phenomenon occurs during the cooling of an archeological artefact (ovens, potteries, ceramics), even though the magnetic mineralogy can be slightly different. Therefore, the methodology to be applied is the same in both cases, just changing the temporality of the material.

The techniques that will be used for the project are mainly :

•. Rock magnetism, optical microscopy (transmitted and reflected lights) and electron microscopy (SEM) to define the magnetic carriers,

•. Thermal and alternating field demagnetizations to isolate the direction(s) of magnetisation carried by the rocks,

•. Paleointensity analysis with the Thellier-Thellier to compare the thermoremanent magnetisation acquired in a known laboratory field with the natural magnetisation of the rocks and define the intensity of the filed during cooling,

•. Radiometric ages to precisely date the cooling of the rocks and therefore the age of the magnetisation..

in progress

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For the last two decades, experimental and numerical models of the Earth magnetic field have advanced significantly, approaching more and more a realistic geodynamo model. For example, many of the models designed now to reproduce the movement in the Earth core are able to simulate reversals. However, to further constrain these models, the evolution of the Earth magnetic field through time has to be known with much more details, from historical time down to geologic time. This will improve our understanding of the functioning of the core through time, the core-mantle interactions, and provide realistic input parameters for even more precise geodynamo models. New, reliable data are therefore badly needed, especially from central and south America, which represent for the moment only a few per cent of the paleointensity dataset.
On the other hand, when the secular variation curve is well known for a given region, it can be a very powerful tool to date archaeological materials. As Mexico is a very rich archaeological area but with a secular variation curve, and especially its intensity, still poorly defined, here again new data are essential.
Finally, we propose to use paleointensity and rock magnetism as a new method to look for the provenance of obsidian artefacts.
The overall purpose of this project is therefore to acquire new and reliable experimental data of the geomagnetic field behaviour (direction and intensity), using archaeological and volcanic materials from Central America. The methodology to be applied is the same for both archaeomagnetism and paleomagnetism, just changing the temporality of the material, historical to geological, respectively. We will focus more precisely the investigation of this proposal on Mexican material. Mexico is an extremely pertinent area for this type of study, firstly because of the abundance of archaeological and volcanic material, but also because of the small number of studies focused on that matter up to now. In this context, our project is timely and strongly justified.
Up to now, we have contact with many archaeologists that will provide us archaeological artefacts and access to ovens for sampling, and we will also sample well-dated, oriented volcanic flows and obsidians covering a period from Pliocene to Present. Therefore, we are confident that, during the course of our project, we will get significant knowledge about the behaviour of the magnetic field during the past five million years and will also greatly improve the rock magnetic and paleomagnetic tools for archaeology.