Reconstructing palaeoclimate and hydrological fluctuations in the Fezzan Basin (southern Libya) since 130 ka: A catchment-based approach

Abstract

We propose a novel method to evaluate regional palaeoclimate that can be used to alleviate the problems caused by the discontinuous nature of palaeoenvironmental data found in deserts. The technique involves processing satellite imagery and DEM's to map past rivers, catchments and evaluate the areas and volumes of palaeolakes. This information is used to determine the new Lake Evaluation Index (LEI) that allows a qualitative estimate of the amount of sediment received by lakes and how long-lived those lakes are. Lakes with considerable longevity and large sediment stores are selected for study. Validation is performed using image interpretation of remote sensing data, UltraGPR surveys and fieldwork. These techniques are also used to identify and study spring deposits and fluvial landforms that provide valuable palaeoclimate information. The method is applied to the Fezzan Basin in southern Libya focusing on the Wadi ash Shati and Wadi el-Agial catchments. Results indicate that the palaeohydrology is accurately mapped except within dune fields. We analysed the sedimentology of the key deposits identified by this methodology, developing a chronology using optically stimulated luminescence (OSL) and radiocarbon dating. We find evidence for relatively humid conditions during MIS 5c/d and e, as well as during the early to middle Holocene. Larger lakes and more extensive river systems were present during MIS 5 than are found during the Holocene, suggestive of greater humidity. The Holocene humid period started at ~11 ka and continued until ~5 ka being interrupted by abrupt periods of aridity at ~8.2 ka and ~6 ka that coincide with North Atlantic cooling. After each of these arid events the climate was less humid than previously, suggesting that they were superimposed upon 2 an overall drying trend. The termination of the Holocene humid period in the Sahara has received much scrutiny in recent years, and sediments of Palaeolake Shati provide a continuous record of this. We do not find evidence to support the hypothesis of either sudden or gradual aridification of the Sahara at ~5 ka, instead we find that that aridity started to develop at ~6.5 ka, whereupon the lake levels oscillated until finally drying-up by 5.3 ka. Most of the other lakes in the Fezzan also dried up at ~ 5ka. We suggest that thousands of years of aridification prior to 5 ka shrunk these lakes so that additional aridity at this time led to their final desiccation. Because lakes are prodigious dust sources this mechanism potentially explains the rapid rise in dust flux to the Atlantic at 5 ka, with this final drying being the culmination of longer term aridity, albeit overprinted with considerable climate variability.

Holocene resource exploitation along the Nile: diet and subsistence strategies of Mesolithic and Neolithic societies at Khor Shambat 1, Sudan

“…The Mesolithic and Neolithic occupation at KSH1 corresponds to the African Humid Period (Gasse 2000; Kuper & Kröpelin 2006; Drake et al . 2018), when the climate in Sudan was characterised by high rainfall, frequent Nile floods, and seasonal lakes in deserts to the east and west of the Nile, allowing the expansion of savannah vegetation and Sahelian fauna. KSH1 was one of several locations along the Nile that provided extensive opportunities for hunting, gathering and fishing and, eventually, for cattle herding.…”

Sedimentary and geomorphic evidence of Saharan megalakes: A synthesis

“…Magnetostratigraphy of these sediments demonstrates that numerous high-stands occurred during the Miocene (Hounslow et al, 2017). Younger lake sediments in the basin yield MIS5 and Holocene ages (Drake et al, 2018), though in both cases these lakes were much smaller features (maximum surface area of ~1600km 2 ) than the megalake phases recorded in the Miocene limestone deposits. The evidence for these Late Pleistocene lake stands consists of birdsfoot deltas, coquinas and shell rich sands that contain sedimentary structures consistent with wave action and delta progradation at a lake margin (Drake et al, 2018).…”

“…Ghienne et al, 2002;Schuster et al, 2005;Drake and Bristow 2006;Leblanc et al, 2006;Bouchette et al, 2010). These include evidence for palaeo-shorelines eroded into the landscape, littoral sediment accumulations (deltas, spits and berms, Drake and Bristow, 2006;Schuster et al, 2005;Drake et al, 2018) and fine-grained deposits within the deeper parts of the basin that contain diatoms (Gasse, 2002), ostracoda and molluscs (Bristow et al, 2018) indicative of freshwater conditions. Dating of numerous shorelines suggest megalake Chad existed between 5-11 ka (Armitage et al, 2015) and during MIS5, with two beach ridges dated to 114.2±14 and 125.4±11.6 ka (Drake et al, 2011).…”

A Window into Africa’s Past Hydroclimates: The SISAL_v1 Database Contribution

“…However, it is clear that during the early Holocene, parts of MIS 5 and earlier times in the Quaternary Lake Chad has been very large (up to 350,000 km 2 ) and there has been increased surface water in the Fezzan basin. Wet conditions in the Fezzan basin during the early Holocene are suggested by surface water in Lake Shati, [125], evidence of vegetation in the region and multiple carbonate-forming springs [126]. There are also OSL-based dates of similar deposits of various ages in Fezzan within MIS 5 [75].…”