The Sahara Megalakes Project
Figure 1. SRTM 1 km resolution digital elevation model (DEM) showing Saharan palaeolakes over 500 km2. See Research methods for the techniques used to map these lakes and rivers (Image by Nick Drake)
The Sahara Desert is presently the most extensive desert on Earth, but during the Pleistocene and Holocene it was also home to some of the world’s largest freshwater lakes. Four lakes were very large, each with estimated surface areas greater than 30,000 km2 (approximately the size of Belgium).
Three of these large palaeolakes were found in adjacent closed basins, which traverse the Sahara from northwest to southeast. In the south of the Sahara is the Lake Chad catchment, a large closed basin that contained a truly giant lake called Lake Megachad during the early Holocene (Ghienne et al, 2002; Schuster et al, 2006; Drake and Bristow 2006). Drake and Bristow (2006) recognise twelve lake shorelines and thus a number of different lake phases and lake area estimates. The most prominent shoreline yields a lake area of 361,000km2, while higher shorelines produce larger lake area estimates ranging from 413,000 to 837,000 km2.
To the north of Chad lies the Fezzan basin in south-west Libya. At times during the Pleistocene, the Fezzan Basin contained a very large lake that we have called Lake Megafezzan, which attained an area of about 130,000 km2 (Thiedig et al, 2000; Brooks et al, 2003; Drake et al, 2006; Armitage et al, 2007; Drake et al, 2008). To the west, the Fezzan catchment adjoins a large catchment that drains the Tassili n-Ajjer and Hoggar Mountains and terminates in the basin of the Chotts, where the Chotts Megalake developed to a maximum size of 30,000km2 at times during the late Pleistocene and early Holocene (Causse et al, 2003).
Lake Megafezzan sediment outcrops in the Ubari Sand Sea (Photograph by Toby Savage)
By chance these megalake catchments form a corridor across the Sahara. Synchronous humidity is recorded in the three basins in the early Holocene, at about 100 ka, and perhaps between (Drake et al, 2011) suggesting that aridity was completely ameliorated in the central Sahara desert at these times.
The aim of the Sahara Megalakes Project is to:
- Further our understanding of the chronology of climate change recorded in Lake Megachad, Lake Megafezzan and the Chotts Megalake basins.
- Combine these chronologies to determine synchronous megalake high stands and thus pan-Saharan humidity.
- Develop ‘time-slice’ maps of the Saharan lakes that can be used to parameterise and validate models of climate change.
Significance of Saharan megalakes
Each megalake provides a detailed palaeoclimate record. Such records are rare in the Sahara, but relatively abundant in the Megalake basins, for reasons which are poorly understood. The information provided by studying all three lakes together is greater the sum of the individual parts.
- They can provide useful information for furthering our understanding of African biogeography and palaeoanthropology. The Sahara Desert currently provides a formidable barrier to animal and hominid migration from central/southern Africa to Arabia and the Levant. However, there is abundant evidence that on several occasions in the past, creatures which evolved in central/southern Africa were able to populate adjacent landmasses, indicating that this barrier did not always operate in the past. Understanding the long-term climatic evolution of the Sahara region is therefore a particularly important question for biogeography and palaeoanthropology. By chance, three of the megalake basins link to form a corridor across the Sahara. Thus the palaeolake sediments they preserve can be used to determine whether there was previously a corridor of humidity across the Sahara by looking for evidence of synchronous lacustrine activity in all three basins.
- The Saharan palaeolakes are also significant because of the sheer volume of standing water that they are able to accommodate. If the megalakes ever reached their maximum possible size at the same time, they would have attained a combined area of about one million km2 and covered c.10 per cent of the Sahara with standing water. Because lake-generated precipitation forms a significant portion of the hydrological budget of large lakes (Coe and Bonan, 1997), these large expanses of open water could well have affected the regional climate and perpetuated the humidity in the region. Palaeoclimate data from the Saharan Megalakes can be used both to parameterise starting conditions for climate models, and to validate their output by determining whether they can predict lakes with the correct area at the right time.
For further details click on the links below:
1 Research methods
2 Lake Megachad
3 Lake Megafezzan
4 The Chotts Megalake
5 Megalakes and the Saharan Palaeoclimate record
6 The corridor across the Sahara and the ‘out of Africa hypothesis’