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100,000 Years of African Monsoon Variability Recorded in Sediments of the Nile Margin

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100,000 Years of African Monsoon Variability Recorded in Sediments of the Nile Margin Quaternary Science Reviews 29 (2010) 1342–1362 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev 100,000 Years of African monsoon variability recorded in sediments of the Nile margin Marie Revel a,*, E. Ducassou b, F.E. Grousset b, S.M. Bernasconi c, S. Migeon a, S. Revillon d, J. Mascle a, A. Murat e, S. Zaragosi b, D. Bosch f a Geosciences Azur, Observatoire Oce´anologique, La Darse, B.P. 48 06235 Villefranche/Mer, France b Universite´ Bordeaux 1, CNRS, UMR 5805-EPOC, avenue des faculte´s, 33405 Talence cedex, France c ETH Zurich, Geologisches Institut, 8092 Zurich, Switzerland d IFREMER, De´partement Ge´osciences Marines, BP70, 29280, Plouzane´, France e Cnam-Intechmer, BP324, 50103 Cherbourg, France f Laboratoire de Tectonophysique, Universite´ de Montpellier II, 34095 Montpellier, France article info abstract Article history: Multiproxy analyses were performed on core MS27PT recovered in hemipelagic sediments deposited on Received 20 April 2009 the Nile margin in order to reconstruct Nile River palaeohydrological fluctuations during the last 100,000 Received in revised form years. The strontium and neodymium isotope composition of the terrigenous fraction and the major 17 December 2009 element distribution reveal large and abrupt changes in source, oscillating between a dominant aeolian Accepted 4 February 2010 Saharan contribution during arid periods and a dominant Nile River contribution during pluvial periods. Iron content shows a strong correlation with strontium and neodymium isotopes. This allows the use of a high-resolution continuous Fe record as a proxy of Blue Nile sediment input over the last 100,000 years. The detailed Fe record, with approximately 10 years resolution during pluvial periods, is consistent with subtropical African records of well-dated lake level fluctuations and thus constitutes a first continuous high resolution record of the East African monsoon regime intensity over Ethiopia. The detailed Fe record shows the two main known pluvial periods attributed to strengthening of the African monsoon over Ethiopia, the Nabtian period from 14 to 8 ka cal BP and the Saharan period from 98 to 72 ka BP. For the first time, the last glacial period (Marine Isotope Stage (MIS) 2, 3 and 4) is documented with a continuous record showing large oscillations between high and low East African palaeo-monsoon regimes. The end of the Nabtian period occurred at 8 ka in core MS27PT, i.e. much earlier than on the East Equatorial African region where it ended around 5.5 ka. We interpret this as evidence that the southward shift of the rain belt occurred 3000 years earlier over the Eastern Ethiopian Highland and propose that the gradual southward migration of the rain belt was associated with highly variable intensity and longer rainy seasons from 8 to 5 ka. During the last glacial period, two wet periods are present around 60–50 ka BP and 38–30 ka BP. These two humid periods are in phase with the rise of atmospheric CH4 concen- trations suggesting that wetland tropical African area was one of the sources of the atmospheric CH4 during the MIS 3. During the Last Glacial Maximum and MIS 4, high Saharan aeolian influxes in phase with records of aeolian dust deposited in East Antarctica are documented. This study highlights the importance of reconstructions of monsoon rainfall fluctuation at high temporal resolution to better understand the link between low- and high-latitude climate variability at millennial timescales. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction conveyed and redistributed by atmospheric and oceanic circulation towards high latitudes. The changing strength of this meridional Recently, palaeoclimatic data and climate models have high- heat transfer has strongly contributed to past global climatic lighted the key role of the tropics in global climate (Alpert et al., changes. 2006). At these low latitudes, heat and water vapour are The South-Eastern Mediterranean region experiences particular climatic conditions because of its mid latitude position and its link with the North Atlantic system (Sanchez-Goni et al., 2008) and with * Tel.: þ33 493763744; fax: þ33 493763766. the African subtropical monsoon system (Rohling et al., 2009; E-mail address: [email protected] (M. Revel). Almogi-Labin et al., 2009). The monsoonal system that originates 0277-3791/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.02.006 M. Revel et al. / Quaternary Science Reviews 29 (2010) 1342–1362 1343 in the tropical Atlantic and the southern Indian ocean, passes over Atbara River accounts for about 56% of the total annual Nile water N–E Africa and is associated with the low-latitude rainfall system discharge (rising to 68% during maximum summer flow) and for that influence the hydrology of the Levantine Basin through Nile more than 95% of the suspended sediment load (Foucault and River outflow. The seasonal migration of the Inter Tropical Stanley, 1989; Williams et al., 2000). Over a year the Nile River Convergence Zone (ITCZ) – a narrow latitudinal zone of wind has a unimodal discharge curve, with summer floods linked to the convergence and precipitation – determines the onset, duration northward migration of the ITCZ from the equator (20N in August) and termination of the monsoon-rainy season in the tropics and causing heavy monsoon rainfall over the headwaters, and espe- subtropics. The intensity of the summer African/Asian monsoon cially over the Ethiopian Highlands (Fig. 1). During winter, the rainfall is mainly controlled by the amount of solar radiation North African landmass cools relative to the adjacent ocean and the received at low latitudes, modulated by the Earth’s astronomical regional atmospheric circulation reverses. The ITCZ is pushed precessional cycle (Rossignol-Strick, 1985; Fontugne and Calvert, southward and dry conditions and northeast trade winds 1992; Rohling, 1994). predominate. One of the most distinctive features of the Eastern Mediterra- Nile basin hydrology, which represents the main discharge in to nean is the Nile River (Fig. 1). It has a large drainage basin extending the Levantine basin, is closely linked to the intensity of the African/ over more than 30 in latitude, and connecting several different Asian monsoon and large fluctuations in discharge and sediment climatic zones. Its main sources are located in the Ethiopian high- transfer during the Quaternary have been driven by changes in lands (Lake Tana) and the equatorial zone (Lakes Albert and global climate (Woodward et al., 2001; Hassan, 1981; Williams and Victoria). Runoff from the Ethiopian Highlands via the Blue Nile and Adamson, 1980). Periods of higher frequency in Nile river floods Fig. 1. Map of North East Africa showing the ITCZ position in summer and location of sites (African lakes and altitude) or regions called in text and figures. Shaded areas represent the most productive source of Libyan and Egyptian aeolian dusts (from Prospero et al., 2002). The yellow arrow represents dust storm plumes emitted from Libyan/Egyptian desert and transported over the Mediterranean Sea to Crete/Cyprus by south-easterly winds (from Ganor et al., 1991 and Prospero et al., 2002). Purple arrows show the counter-clockwise Mediterranean Sea surface circulation. Also marked are the Cenozoic Basalt outcrops (dotted red line) adapted from Stein et al. (2007) and the Precambrian crystalline basement. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 1344 M. Revel et al. / Quaternary Science Reviews 29 (2010) 1342–1362 and related high levels recorded in North-East Africa palaeo-lakes Eastern Libyan Desert into western Egypt. These sources are active have been correlated with the periodic monsoon intensification during much of the year with intensified activity in May–June. called pluvial periods (Said, 1993; Szabo et al., 1995; Gasse, 2000; Quaternary marine sediment records in the Eastern Mediter- Williams et al., 2000; Lamb et al., 2007; Williams, 2009). ranean sea are characterized by the rhythmic deposition of organic Continental records of the Nile fluvial regime have the potential to carbon rich layers, called sapropels which correspond mainly to yield crucial data to reconstruct changes in hydrological regime and pluvial periods over north Africa, and are commonly formed Northeastern African climate (Woodward et al., 2001; Woodward during interglacial periods (Venkatarathnam and Ryan, 1971; et al., 2007; Cohen et al., 2007; Scholz et al., 2007). However, conti- Stanley and Wingerath, 1996; Foucault and Me´lie`res, 2000; nental climate records based on lacustrine sequences, show hiatuses Wehausen and Brumsack, 2000; Calvert and Fontugne, 2001; due to desiccation and subsequent erosion and/or non-deposition of Larrassoano et al., 2003). Many studies have focused on the very sediment and some only provide records with low- temporal reso- late Pleistocene and early Holocene and in particular on sapropel lution (Gasse, 2000; Gasse et al., 2008). In addition, continental S1 (9500–6600 cal ka BP; Emeis et al., 2000). Sapropel S1 is records are often difficult to precisely date by radiocarbon due to the thought to result from increased freshwater supply into the hard water effect and thus make correlation to marine records diffi- Mediterranean Sea by the Nile River. The runoff from the Nile cult (Said, 1981, 1993; Zaki, 2007; Williams, 2009). would have either increased biological production and/or Depositional sequences with high temporal resolution recording increased organic matter preservation in the sediments because of Quaternary climate fluctuations are exceptionally well preserved in inhibited water mass circulation and presence of anoxic deep the Nile margin sediments. The Nile margin is the largest sedi- waters (Rossignol-Strick, 1985; Murat and Got, 2000; Emeis et al., mentary accumulation in the Eastern Mediterranean and was 2000; Ariztegui et al., 2000; Krom et al., 2002; Bard et al., 2002; formed by sediment supplied from the Nile River.
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