Paleomagnetism and archeology: the perfect combination to reveal the mysteries of Africa
- Written by Sara Stopponi
A boundless continent, the cradle of human civilization. A territory rich in history and scientific potential, which is still little studied by researchers from all over the world.
Africa and its paleomagnetic field have been the privileged object of a recently published research which has allowed us to add a new piece in the history of knowledge of the earth's magnetic field and its anomalies.
We interviewed Anita Di Chiara, INGV researcher and author of the study that led to the creation of the first geomagnetic model of Africa in the last 4.000 years, to let us tell her more about this work and the potential applications of this method for the study of areas of our planet that have not yet been sufficiently investigated by the scientific literature.
What is the Earth's magnetic field and why is it important to study it?
The Earth's magnetic field, or geomagnetic field, is a field that is thought to be generated mainly by electric currents which are in turn the result of mass movements in the outer core of the Earth. The earth's magnetic field acts as a protective shield from solar radiation, modulating the radiation flux that reaches the earth's surface.
Since its origins, the magnetic field has shown variations both in terms of direction (i.e. orientation with respect to the north magnetic pole) and in intensity (its strength). The study of these variations is possible thanks to paleomagnetic analyses, i.e. studies of rocks and materials which, during their formation, "recorded" the earth's magnetic field. The palaeomagnetist, therefore, deals both with studying materials of known age to reconstruct the variations of the earth's magnetic field in the geological past, and with using this information as tools for dating and correlating finds.
Studying the magnetic field, therefore, is important for many disciplines of Earth Sciences but also, for example, as a dating tool for archaeological and historical objects.
What have you achieved with your latest magnetic field study?
The study was the result of a collaboration with the University of Madrid. It contains a first part of data analysis and a second part of actual modeling, created by Doctor Javier Pavon Carrasco of the Spanish University. The work presents the first paleomagnetic model of the Earth's magnetic field relating to Africa in the last 4.000 years. In fact, the initial idea was to cover the entire Holocene (i.e. the last 12.000 years), but the data available were very limited, so in this first phase we focused on a smaller time frame.
What prompted you to focus on studying the geomagnetic field of Africa?
Essentially the lack of other studies that investigated the topic. Our interest, in fact, arose when we wondered about how many and which studies, relating to Africa over the last 12.000 years, were available to date: only 48, just think. The hope that motivated us to carry on with this study, therefore, was to encourage further work on this huge continent, which presents elements of interest both from an anthropological point of view (with the whole line of investigation on the origin of the first hominids , for example), and - and it is on this aspect that we have focused on - from the point of view of paleomagnetism. The study of the magnetic field, in fact, is a very valid and powerful tool for dating numerous finds: from archaeological objects to volcanic deposits. Africa, being rich both in archaeological sites and in volcanoes (particularly in the Rift zone), is an exceptional resource in this sense.
And then I must say that the pandemic has certainly made its contribution!
In what sense?
Well I would actually be a country geologist, who goes into the field and materially collects data and information. The lockdown forced us indoors, so the idea for this study was born behind a desk while I was wondering: "What can we do?". Modeling was the answer. At the time I was working at Lancaster University on an ICDP project (International Continental Drilling Program) on the study of 250.000-year-old sediments from an African lake: the "choice" of Africa as a 'case study' arose a little from here, from the curiosity for the absence of a volume of data congruous to the enormous potential of that continent.
What do you think causes this lack of data?
Definitely from a geographical component: desert areas and dense forests hinder many campaigns ,. But also from socio-political factors, let's think of the war zones, widespread in Africa, which naturally discourage visits by foreign researchers. Furthermore, the collaboration between paleomagnetists and archaeologists is actually quite recent, therefore still not widespread even where it would potentially be possible, such as in the universities of the continent that have more resources available.
Returning to the earth's magnetic field, what kind of information does your study provide?
It provides various types of information: by studying the magnetic (and, specifically, paleomagnetic) field, we are able to define its direction (in terms of declination and inclination) and intensity. It is thanks to this kind of study that it was possible to highlight the existence of the so-called "South Atlantic Anomaly", which also affected the African region.
What is this "Anomaly"?
This is a magnetic field structure that is observed in satellite data. It is an "anomaly" which, over time, "migrated" from east to west and today affects all of South America and part of the South Atlantic, where the earth's magnetic field is about 35% weaker than what we know one would expect at those latitudes. This has consequences on satellite systems in daily use for both civilian and military purposes, which are directly affected by the strength of the magnetic field. Studying its variations is also fundamental because all of this also regulates the production of the so-called 'cosmogenic isotopes' which, indirectly, influence our climate.
What we still haven't been able to understand, however, is the profound meaning of this anomaly in terms of the evolution of our magnetic field: being an anomaly, it is in fact not observed anywhere else in the world.
When did it first appear?
It is still an open question, in the sense that over the years various studies have followed one another which have gradually identified it in different eras: to date, the first certified "observation" dates back to 5 million years ago.
How does this anomaly relate to the paleomagnetic model you have created?
We had the objective of building a model of a regional nature that would cover the African continent precisely for the purpose of trying to understand and reconstruct, on the basis of archaeomagnetic data (i.e. palaeomagnetic data reconstructed starting from archaeological objects) the evolution over the centuries of the Anomaly of the South Atlantic. So it is closely related to 'Why' of our research.
What kind of data did you use?
We analyzed data of various kinds: just over 50% comes from archaeological finds, the rest is divided between volcanic and sedimentary finds (coming from lakes on the continent). The latter, however, due to too marked discrepancies that would have invalidated the effectiveness of the model, were analyzed but not used for the actual modeling in this first study.
Could this model also have applications in other areas of study?
Absolutely yes. It is a model of a regional nature, which differs from the global one since the latter tends to mediate too rapid variations or anomalies. A regional type model is a model that has already been mathematically and physically applied in Europe in numerous studies (also thanks to the enormous amount of data available for this continent).
The hope is that this model can be applied, somewhat following our 'footprints', also in other areas of the world that are still little studied, such as South America or China for example.
Do you have any further developments of your work in the pipeline?
The model, as we have seen, is based on data; to collect data a whole chain of work is required which is always a little longer than one would like. For Africa, specifically, we often talk about helicopter research o “neo-colonial research”: Wealthier countries and universities literally 'land' in more remote and isolated places, with fewer resources, take samples and go home without involving the local community or university in the study. As far as we are concerned, however, the archaeologist - i.e. the first fundamental player in this type of study - should be local, know the site very well, its age and the archaeological objects present there: the fact that in reality there is no this link leads to a considerable delay in the entire work chain.
My hope, therefore, is that this model will inspire new collaborations as well as new studies: I myself do not yet have a well-defined program on the next steps we will be able to take, but I sincerely hope to be able to take new ones soon.