Age and origin of closed depressions in the Paris Basin: A majority of marl pits dug since the Neolithic period?

Marling (limestone alkaline amendment) agrarian practices have a plurimillennial influence on soil pH and on soil-associated ecosystems. Although the earliest written records in Europe date back to antiquity, the origin of this agrarian practice is not well known. In order to trace the evolution of this practice in the early agrarian societies of Western Europe, we searched for topographic anomalies of ancient limestone quarries that may have been used for marling. Four hundred circular closed topographic depressions (CDs) were identified under two areas of ancient forest in northern France with Lidar digital elevation models (DEMs). Soils, morphology and archaeological context were studied and filling sediments were dated in several dozen CDs.The gaps in the carbonate rocks, soils with calcaric properties (scattered limestone fragments), unstratified fills, the absence of buried paleosols, and dates (OSL, C-14) later than the first local agricultural societies made it possible to recognise the CDs that may have served as limestone quarries. This was the case for most of the 30 CDs studied, leaving a minority of natural origin (suffosion, depressions of thermokarstic origins). Amongst the quarries identified, the preferential excavation of soft (friable) limestone and the location of these quarries in fossil agrarian plots led us to hypothesize that they were used for alkaline amendments of the cultivated fields. The quarries were filled from the Middle Neolithic to Late Antiquity, shortly before the period of afforestation of these former agrarian territories.