Il était une fois des sociétés qui stockaient du carbone en ville; processus de formation et implications sociétales des terres noires urbaines de l'Europe médiévale (Ve-XIe s.)

The underground of European towns is produced by millennia of interactions between societies and environment. These interactions have created urban strata and soils that can be 10 metres thick. After 30 years, the development of geoarchaeology in an artificialised environment and the application of geosciences allow to characterise the pedo-sedimentary processes and human activity systems which lead to the production of those anthropogenic sediments. Thus, it is possible to notice that the strata corresponding to the 4th to 12th century are represented by dark earth layers of several tens of centimetres or meters thick, which seem to be homogenous. This paper proposes a synthesis of the results obtained by the systematic study of Dark Earth at a European scale. These results have been acquired by combining archaeological excavations and geoarchaeological approaches, the application of archaeo-pedology, sedimentology, micromorphology, and geochemistry to the archaeological stratifications. At more than one hundred sites in Great Britain, Belgium, France, and Italy it is possible to notice that the processes leading to Dark Earth formation vary greatly, but are always linked with human occupations. Bioturbation plays a major role in the structuration of those sediments, leading to the disappearance of stratigraphic limits. Dark Earths are very rich in organic matter, in refuse from human domestic or artisanal activities, in excrements, and in degraded architectural materials. The organic carbon content of Dark Earths is very high compared to other ancient urban stratifications, between 9 and 35.9 mg/kg. They are polluted by phosphorus--up to 20.34 g/kg--and by heavy metals. The concentrations of lead can reach 1830 mg/kg. These data allow to better understand the urban behaviours of early medieval societies in their management of waste, their relation with the materiality of the surfaces on which they live, and how the activities were organised within the towns.