Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments
Introduction
African climate varied greatly during the late Quaternary with alternating periods of aridity and humidity (Singarayer and Burrough, 2015). The last major humid phase in Northern Africa, i.e., the African Humid Period (AHP, deMenocal et al., 2000; Shanahan et al., 2015; Tierney et al., 2011), occurred during the early and mid-Holocene (∼11–5 ka BP, Gasse, 2000). The development of the so-called “Green Sahara” during the AHP allowed the establishment of large lake systems in Northern Africa and vegetation in areas that are now desert. Human colonization was possible at latitudes 800 km further north than now (Kropelin et al., 2008; Kuper and Kropelin, 2006). This wet African phase has been attributed to the northward migration of the rain belt associated with the Intertropical Convergence Zone (ITCZ) induced by precession-forced insolation changes. These changes started at the end of the Last Glacial Maximum (LGM, Caley et al., 2011; Tuenter et al., 2003). The magnitude of this wet phase was also exemplified by the deposition of the most recent sapropel (S1) in the eastern Mediterranean Sea. Enhanced Nile river runoff led to surface water buoyancy gain, resulting in reduced deep-water ventilation and oxygen deficiency at depth (see Rohling et al., 2015; Rossignol-Strick et al., 1982; Vadsaria et al., 2019).
The termination of the AHP remains highly controversial, albeit frequently studied at many locations in and around the African continent (e.g., Collins et al., 2017; Shanahan et al., 2015; Tierney and deMenocal, 2013). Some studies suggest an abrupt transition that occurred at ∼5 ka BP (Collins et al., 2017; deMenocal et al., 2000; McGee et al., 2013; Tierney and deMenocal, 2013). Other studies propose that a gradual AHP termination (Berke et al., 2012; Foerster et al., 2012) or a termination that involved several phases of decreasing humidity (Liu et al., 2017; Loomis et al., 2015). In contrast, the onset of AHP is less controversial and began on the African continent around 15–14 ka (Shanahan et al., 2015; Trauth et al., 2018). However, within the Nile delta, and within the Eastern Mediterranean Sea, there are relatively few sites with a sufficient time resolution covering the last 20 ka. Thus, the onset of the AHP is less well described than its termination. On the Pleistocene time scale, humid periods are characterized by high sedimentation rates and a large proportion of smectite and Fe-rich silicate minerals within the Nile delta sediment (Caley et al., 2011; Emeis et al., 2000; Kroon et al., 1998; Revel et al., 2010; Zhao et al., 2011). These characteristics reflect increased physical erosion and material transport mainly originating from the Ethiopian highland during higher rainfall regimes related to summer monsoon precessional variability. Thus, the sediment records preserved at the Nile deep-sea fan provide a suitable archive to study basin-wide environmental changes in vegetation cover, lithology, and soil erosion/weathering at a high time resolution mainly during humid periods such as the AHP (Blanchet et al., 2014; Castañeda et al., 2016; Hamann et al., 2009; Hennekam et al., 2015; Weldeab et al., 2014).
Sedimentary provenance of terrigenous material in the Nile deep-sea fan can be characterized through its neodymium isotopic signature (εNd, Bastian et al., 2017; Blanchet et al., 2014; Blanchet, 2019; Revel et al., 2015; Weldeab et al., 2002). No significant fractionation occurs during chemical weathering so εNd reflects the isotopic composition of the parent bedrock and the origin of the material (Bayon et al., 2015). Since the Nile basin encompasses the Precambrian African basement as well as the Ethiopian and Somalian Cenozoic basalts with contrasting Nd isotopic compositions, geochemical studies on its sediment loads can distinguish between the sources of the material (Fig. 1A).
In parallel, glycerol dialkyl glycerol tetraethers (GDGTs), core membrane lipids of Achaea and Bacteria, are ubiquitous in marine to lacustrine waters and soils (e.g., Karner et al., 2001; Keough et al., 2003; Weijers et al., 2006). Specifically, the branched to isoprenoid tetraether ratio (BIT-index, Hopmans et al., 2004) and its first order derivatives are effective as tracers of continental inputs in estuarine settings (Blanchet et al., 2014; Kim et al., 2015; Ménot et al., 2006; Soulet et al., 2013). However, their environmental significance is controversial (French et al., 2015; Kaiser et al., 2015; Zell et al., 2015). The BIT-index depends on the fluctuations of branched GDGTs (brGDGTs) and variation of the GDGT isoprenoid crenarchaeol (Hopmans et al., 2004). The index is therefore dependent on processes that affect these GDGTs. BIT values are not just dependent on the inputs of soil brGDGTs, but also on the levels of marine Archaea. For example, sites with equal inputs of terrestrial brGDGTs but different local marine productivity can display different BIT values (Fietz et al., 2011; Herfort et al., 2006; Smith et al., 2010). Furthermore, differential degradation in the water column may affect the relative preservation of different compounds. Field and laboratory experiments have tested the stability of brGDGTs from soils to river mouths (Peterse et al., 2015; Zhu et al., 2011). Exposure of anoxic sediments to oxygen produced little degradation of brGDGTs. This was likely due to their enhanced protection by the sedimentary matrix inhered in soils. However, this protection might differ for water-column produced isoprenoid GDGTs (Huguet et al., 2008; Kim et al., 2009). To avoid these possible biases, the abundance ratio of the hexa-to penta-methylated brGDGTs ratio (IIIa/IIa) was proposed (Xiao et al., 2016) and its modified form ΣIIIa/ΣIIa (Martin et al., 2019). In this study we used the ΣIIIa/ΣIIa ratio along with the proportions of tetra-, penta- and hexa-methylated brGDGTs, and the #ringstetra index defined as the weighted mean number of pentacycles in tetramethylated brGDGTs (Sinninghe Damsté, 2016).
Using inorganic (Nd isotopes) and organic (GDGTs) proxies, we characterized the erosion products in the Nile delta to assess past hydrological changes in its basin. We validated the reliability of the targeted sedimentary archive by testing our toolbox on the termination of the AHP. We then discuss the timing and pattern of this wet period as recorded in the Nile delta in GDGT-based indices and clastic εNd signatures.
Section snippets
Present day hydrology and lithology of the Nile watershed
The Nile River system has a large drainage basin which extends over more than 30° in latitude from its sources to the delta (Fig. 1A). During its journey through Eastern Africa, the river crosses hydroclimates ranging from subtropical at 15°N, with high precipitation (∼1400 mm/year) to the Sahara Desert which receives ∼5 mm/year (Korecha and Barnston, 2007). The precipitation regime along the Nile catchment is mainly related to the West African monsoon modulated by the Indian monsoon dynamic.
εNd and sedimentation rate
The new εNd measurements (Fig. 2A, red open circles) are consistent with previous εNd results (Revel et al., 2015). The εNd values varied from ∼–4 during the African Humid Period (AHP) to −11 during Last Glacial Maximum (LGM) and late Holocene arid periods (Fig. 2A, Table S1). From 14.5 ka BP and during the AHP, εNd values approached the Ethiopian Traps value (about 0). When combined with evidence of correspondingly high sedimentation rates (Fig. 2C, Table S1), the more positive radiogenic εNd
Evaluation of GDGTs as a proxy of soil and marine inputs
When compared to the available soil and marine sediment global database (Davtian et al., 2016; Xiao et al., 2016), the sedimentary ΣIIIa/ΣIIa values in core MS27PT clearly reflect a terrigenous signature (Fig. 3). Most of the ΣIIIa/ΣIIa values ranged below the threshold of 0.59 as do 90% of the world soil values (Xiao et al., 2016). This is further confirmed by the proximity of most of the ΣIIIa/ΣIIa values in core MS27PT to the mean value of the few soils from the Nile watershed contained in
Conclusions
A comparison of organic and inorganic signatures in a sedimentary sequence from the Nile deep-sea fan over the last 20 ka as well as a centennial time resolution for key intervals such as the onset and termination of the AHP produced the following conclusions:
- 1.
The reliability, with qualifications, of the use of brGDGTs as a terrigenous flux tracer in a deltaic environment,
- 2.
The diversity of sedimentary settings in the Nile delta based on a synthesis of local scale publications,
- 3.
The fluctuations of
Declaration of competing interest
The authors declare that no conflict of interest exists on this manuscript submission: none of us has financial or personal relationships that inappropriately influence his or her actions.
Acknowledgements
We would like to thank Cécile Blanchet, the anonymous reviewers and the editor for their thoughtful reviews and constructive comments on this manuscript. CEREGE research was supported by the Collège de France and studies focusing on low latitudes were supported by the BNP Paribas Foundation (Climate Initiative – project CPATEMP). Internship gratification for SP was financed through a project LABEX OT-Med (Objectif Terre: Bassin Méditerranéen; http://www.otmed.fr/; Project “MEDiterranean surface
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