AFRICAN SPELEOTHEMS: A PROMISING ARCHIVE OF THE SECULAR VARIATION OF THE EARTH'S MAGNETIC FIELD OVER THE LAST MILLENNIA FOR A CONTINENT LACKING IN EXPERIMENTAL DATA

The study of the Earth's magnetic field (EMF) is one of the most interesting and important topics in geophysical research for two reasons. First, it is the only signal from the liquid outer core that can be detected at the Earth's surface, and second, it is potentially continuously recorded. We know that its secular variation is marked by a strong geographic dependence and can present rapid fluctuations [1]. The rates of these variations are still being debated [2]. Answering this questioning involves acquiring new data obtained from reliable archives, which have to be continuous over a significant time interval and distributed as homogeneously as possible at the Earth surface. We cannot model and therefore cannot understand a phenomenon as complex as the EMF if we do not observe it correctly. It is thus relevant to collect data in regions that are still poorly documented. In the Geomagia database [3], Holocene African records, without any selection based on quality criteria, are of only 138 data and 428 data relating to its direction and to its intensity, respectively. This is explained by a rather unique manufacture in Africa of archeological ceramic material based on a technique of drying in the sun and baking at low temperature in open fires. Very few potter kilns have been excavated as in Europe or in the Middle East. The only remains of archeological kilns correspond to metallurgical furnaces that are ill-suited in archeomagnetism given the probable presence of metallic masses near the workshop. The local EMF may be marred by an unknown error. Thus, it is necessary to target alternative archives such speleothems. With this in mind, we analyzed speleothems collected and oriented manually from the Lihouma cave in the town of Lastourville in Gabon. The stalagmites have been sectioned at the base level. The speleothems were dated with the Ur/Th technique. We will present the acquisition of paleomagnetic results, which on this type of archive are always difficult to obtain especially when looking for a signal with high temporal resolution. The quality of our measurements will be discussed by comparing our data with the 2 global models [4] and [5].

Ref:

[1] Shaar, EPSL, 301(1-2), 297–306, 2011.

[2] Hervé, PEPI, 315, 106712, 2021.

[3] Brown, EPS, 67(1), 1–31, 2015.

[4] Hellio, GJI, 214(3), 1585–1607, 2018.

[5] Arneitz, PEPI, 290, 57–75, 2019.