Mid- and Late Holocene fluvial chronology of Tunisia

Abstract

Regarding fluvial response to recent climate change, it is important to identify and understand previous changes of fluvial dynamics related to past climatic shifts. In this paper, periods of increased flood activity in Central Tunisian river systems are presented. Changes in Holocene flood dynamics are reconstructed through a comprehensive analysis of 195 radiocarbon dates from alluvial archives. Holocene floodplain deposits in Central Tunisia normally consists of cohesive overbank fines, which exhibit excellent preservation conditions against lateral erosion and, therefore, permit chronostratigraphic records of the entire Holocene at most. Based on published radiocarbon dates, two 14C cumulative probability density plots are created for Central Tunisia: the former represents ‘activity dates’ indicating phases of increased fluvial dynamics, the latter documents ‘stability dates’ showing periods of soil formation and enhanced landscape stability within the floodplain and the catchment. Due to geomorphological considerations, ‘activity dates’ and ‘stability dates’ should be mutually exclusive. There is a good match for the Mid-Holocene with increased periods of flooding from 6.2 to 6.0, 4.8 to 4.5, 4.1 to 3.7, and 3.3 to 3.0 ka. These flood periods coincide with low soil formation signals. On the other hand, periods of increased alluvial soil formation from 6.8 to 6.6, 5.9 to 4.8, 4.5 to 4.1, 3.7 to 3.3, 3.0 to 2.8, and 1.8 to 1.6 ka coincide with periods of low flood plain aggradation. The data reveal a coupling with North Atlantic cooling. Short-term periods of enhanced fluvial dynamics in Central Tunisia match well with North Atlantic cooling. The findings indicate a strong climatic link from 4.7 ka until today. Compared to the Holocene floodplain dynamics in Tunisian drylands, the temperate mid-latitudes show Mid-Holocene switches in fluvial dynamics with opposite signs. Especially after 5 ka North Atlantic driven flooding in the temperate Central European river systems contrast sharply to flood dynamics in arid to semiarid Mediterranean Tunisia. While under temperate climate conditions floodplain sedimentation decreases, it increases under more arid conditions. This may indicate that fluvial archives of dissimilar climatic zones reveal different threshold values and sedimentation patterns, and, therefore, different responses to climatic shifts.