Dune ages in the sand deserts of the southern Sahara and Sahel

Abstract

In this paper we aim to document the history of aeolian processes within the southern Sahara as part of the INQUA Dune Atlas. We review available luminescence ages for sand dunes across the southern Sahara to develop an improved understanding of the dune chronology on a regional basis and attempt to correlate periods of sand accumulation. This was achieved by analysing dune age by country, as well as by latitude and longitude. The results show a very patchy spatial distribution of dune ages with large gaps that encompass some of the largest sand seas. Despite these gaps, some related patterns in dune morphology and stratigraphy appear to be consistent between northern Nigeria and southern Mali where older linear dunes are distinct from younger Late Holocene transverse and barchanoid dunes. Elsewhere in Mauretania linear dunes with different orientations appear to have accumulated at different times, most likely in response to changes in atmospheric circulation. Regional climatic changes are identified where dunes are transgressed by lake deposits within endorehic basins. We identify four locations where dune accumulation is terminated by lacustrine transgressions, two of which, in Lake Chad and the Bodélé Depression, occur shortly after the last glacial maximum (LGM). The third example at Gobiero in Niger occurred later, in the early Holocene, around 8.4 ka and a fourth marks a later transgression of Palaeolake MegaChad after 4.7 ka. Larger-scale latitudinal and longitudinal distributions in dune ages across the southern Sahara do not show any consistent patterns, though this may be due to the small sample size relative to the study area. In addition, local variations in external controls such as wind regime, rainfall, vegetation and sand supply need to be considered, sometimes on a site by site basis. Limiting the analysis to dune ages determined using the single-aliquot regenerative-dose (SAR) protocol indicates a lack of dune preservation during the LGM and the Younger Dryas, times associated with increased dust input to the oceans which is assumed to indicate increased aeolian activity. The SAR dune dates suggest that preservation of dunes at the onset of succeeding humid intervals is an important component of the dune record. The most striking examples of this phenomenon occur where dunes are preserved within endorehic basins by lacustrine transgressions.

A late Holocene palaeoenvironmental ‘snapshot’ of the Angamma Delta, Lake Megachad at the end of the African Humid Period

“…The dune age of 4700 BP overlaps the calibrated radiocarbon age for the bioclastic silty sands and this can be reconciled if the dune ages are interpreted as dune stabilisation ages, caused by a lake level rise and transgression at 4700 cal BP. This scenario is common in endorheic basins in the Sahara (Bristow and Armitage, 2016) but contrasts with groundwater-fed lakes, such as the Ounianga, that persists to this day (Kroepelin et al, 2008).…”

“…However, the channels do not cut through the beach ridge which defines the delta front, which is known as the cordon littoral (Servant et al, 1969). The fact that the beach ridge is not cut by the fluvial distributary channels indicates that the fluvial drainage from the northern catchments ceased before the lake-level fell (Armitage et al 2016). Had the rivers continued to flow after the lake-level had fallen, then the rivers would have incised through the beach ridge to create a falling stage delta.…”

Analysis of the deposit conditions of sedimentary series of fluvio-lacustrine and aeolian origin in the Djourab desert, in Northern Chad

“…All the fossil sites in northern Chad form a strip of outcrop, oriented east-west, covering an area of approximately 2,500km 2 . It constitutes a key zone which has played a role of oscillating buffer according to hydroclimatic fluctuations (Armitage et al, 2015;Bristow and Armitage 2016;Sewell, 2019).…”

A fluid past: Socio‐hydrological systems of the West African Sahel across the long durée

“…Of course, many of these techniques have been used in other regions, such as tracking pollen data to reconstruct vegetation communities (Hopcroft, Valdes, Harper, & Beerling, ) and precipitation levels (Peyron et al, ). However, as archeology is generally concerned with change over time, scholars observing and quantifying such changes in charcoal taxa (Eichhorn & Neumann, ), zoogeography (Drake, Blench, Armitage, Bristow, & White, ), dune formation events (Bristow & Armitage, ; Stokes, Bailey, Fedoroff, & O'Marah, ) and even sedimentary DNA ( sed DNA) (Bremond et al, ) in any one locale or across an entire region, have contributed significantly to refining the spatial and temporal resolution of our climate reconstructions.…”

“…Fine sands in the water column of Lake Yoa in northeastern Chad indicate higher wind speed and/or dune formation across the region (Francus et al, ). The remobilization of dunes (Bristow & Armitage, ), and a “slight degree” of local block elevation, constricted the meandering of the Senegal River, driving its generally fixed and incised position (i.e., Maley & Vernet, ). To the east, the discharge of the Niger reduced significantly and the Interior Delta shrank by 50% of the size it had reached during the previous humid periods (Hassan, ).…”