Dynamics of Green Sahara Periods and Their Role in Hominin Evolution

Abstract

Astronomically forced insolation changes have driven monsoon dynamics and recurrent humid episodes in North Africa, resulting in green Sahara Periods (GSPs) with savannah expansion throughout most of the desert. Despite their potential for expanding the area of prime hominin habitats and favouring out-of-Africa dispersals, GSPs have not been incorporated into the narrative of hominin evolution due to poor knowledge of their timing, dynamics and landscape composition at evolutionary timescales. We present a compilation of continental and marine paleoenvironmental records from within and around North Africa, which enables identification of over 230 GSPs within the last 8 million years. By combining the main climatological determinants of woody cover in tropical Africa with paleoenvironmental and paleoclimatic data for representative (Holocene and Eemian) GSPs, we estimate precipitation regimes and habitat distributions during GSPs. Their chronology is consistent with the ages of Saharan archeological and fossil hominin sites. Each GSP took 2–3 kyr to develop, peaked over 4–8 kyr, biogeographically connected the African tropics to African and Eurasian mid latitudes, and ended within 2–3 kyr, which resulted in rapid habitat fragmentation. We argue that the well-dated succession of GSPs presented here may have played an important role in migration and evolution of hominins.

Middle Palaeolithic find spots with Nubian cores from the Southern Negev and the Arava, Israel

“…The mean annual precipitation is 50 mm, usually falling in a few rainfall events per year. Past climatic records indicate that during glacial and interglacial periods between MIS 6 and MIS 4 the central Negev was mostly dry (Amit et al 2006;Vaks et al 2007), whereas short humid episodes (Negev Humid Periods, (2017) NHP), possibly resulting from a northern shift of the African Monsson system, occurred during interglacial times (Blome et al 2012;Kutzbach & Liu 1997;Larrasoaña et al 2013;Torfstein et al 2015;Vaks et al 2010). The last of the NHP (NHP-1) is dated to 142-109 ka and was synchronous with pluvial periods in the Sahara, the Egyptian Desert and the Arabian Peninsula (Osborne et al 2008;Osmond & Dabous 2004;Petit-Maire et al 2010;Rosenberg et al 2011;Yehudai et al 2017;Vaks et al 2006;.…”

Source-to-sink magnetic properties of NE Saharan dust in Eastern Mediterranean marine sediments: review and paleoenvironmental implications

“…For example, short-lived (e.g., 4-8 kyr) past periods of increased monsoonal precipitation in the Sahara, called "green Sahara" periods (GSPs), led to development of lakes and permanent river systems and to the spread of savannah vegetation through the NES (Kuper and Kröpelin, 2006;Drake et al, 2011;Lézine et al, 2011;Larrasoaña et al, 2013). Paleoprecipitation estimates for the Holocene (∼6-11 ka) and Eemian (∼121-128 ka) GSPs, which can be taken as representative of late Pleistocene-Holocene GSPs, indicate mean annual precipitations of around 100-300 mm/year (Kuper and Kröpelin, 2006;Larrasoaña et al, 2013) and 400-600 mm/year (Kowalski et al, 1989;Kieniewicz and Smith, 2009;Larrasoaña et al, 2013) throughout the core of the NES, respectively. Wetter conditions may have prevailed during earlier GSPs (Drake et al, 2008;Geyh and Thiedig, 2008;Larrasoaña et al, 2013).…”

“…Paleoprecipitation estimates for the Holocene (∼6-11 ka) and Eemian (∼121-128 ka) GSPs, which can be taken as representative of late Pleistocene-Holocene GSPs, indicate mean annual precipitations of around 100-300 mm/year (Kuper and Kröpelin, 2006;Larrasoaña et al, 2013) and 400-600 mm/year (Kowalski et al, 1989;Kieniewicz and Smith, 2009;Larrasoaña et al, 2013) throughout the core of the NES, respectively. Wetter conditions may have prevailed during earlier GSPs (Drake et al, 2008;Geyh and Thiedig, 2008;Larrasoaña et al, 2013). It is the fine-grained material transported by rivers from the highlands to the lowlands during GSPs, which accumulated in distal alluvial and lacustrine settings, that fuels the bulk of dust produced during subsequent hyper-arid desert phases (Prospero et al, 2002;Maher et al, 2010).…”

“…Interbedded within these background sediments are thin (typically <20 cm) sapropel layers, which have dark colors associated with their elevated organic carbon contents (typically 2-10% and up to 30%) Brumsack, 1999, 2000;Emeis et al, 2000). Sapropels mark peak humid conditions associated with GSPs; their formation is, therefore, linked to periods of enhanced monsoonal precipitation driven by boreal summer insolation maxima associated with precession minima (Hilgen et al, 1995;Emeis et al, 2000;Wehausen and Brumsack, 2000;Larrasoaña et al, 2013;Rohling et al, 2015) (Figure 3). Due to the impact of eccentricity modulation of insolation on monsoon variability, sapropels (and hence GSPs) tend to cluster around 400-and 100-kyr eccentricity maxima (Hilgen et al, 1995;Emeis et al, 2000;Wehausen and Brumsack, 2000; Larrasoaña et al (2003a,b), and Ti/Al data are from Wehausen and Brumsack (2000).…”

“…Given the often discontinuous nature of the continental sedimentary record, these marine magnetic records of eolian dust deposition provide valuable sources of information on past climate variability in continental regions (Liu et al, 2012a). In the case of the Eastern Mediterranean Sea, magnetic records of eolian dust deposition have been used to provide novel information on paleo-environmental conditions in the neighboring Sahara Desert (Larrasoaña et al, 2003a;Köhler et al, 2008), which has important implications for better understanding paleo-monsoon dynamics (Trauth et al, 2009) and of any climatic influence on human evolution (Larrasoaña et al, 2013).…”