Stepwise Holocene aridification in NE Africa deduced from dust-borne radiogenic isotope records
Introduction
In the African and West Asian monsoon systems seasonally varying solar insolation induces north–south shifts in the location of the Intertropical Convergence Zone (ITCZ). In the Arabian Sea these shifts are manifested as the summer SW monsoon (Fig. 1) and winter NE monsoon winds. The seasonally varying solar insolation also induces a north–south oscillation of the major rainfall belts controlling the precipitation over continental North Africa and the runoff into the Mediterranean via the River Nile. The long-term history of the African and West Asian monsoon systems is a reflection of variations in the Earth’s orbital parameters [1], [2], [3]. Also the continental climate records from North Africa, in line with a reduction in solar insolation, indicate that the early Holocene was generally more humid than the late Holocene (African Humid Period (AHP) [4], [5], [6], [7]). In addition, a number of millennial–centennial-scale arid–humid climate alternations [4], [8], [9], [10] have been reported superimposed on the long-term Holocene climate trend from wet to dry. We report a detailed study of the dust-borne sediment fraction in Core 905 off Somalia by using the combined Sr and Nd isotope records as a novel tool to establish a continuous weathering record that reflects the regional NE African climate history during the transitional period from wet to dry conditions. The data record a stepwise aridification in NE Africa. The first step was abrupt and occurred at 8.5 kyr BP and the second step started at 6 kyr BP and ended at 3.8 kyr BP. The transitional period between 8.5 kyr BP and 6 kyr BP records a number of centennial and decadal arid–humid alternations. These variations appear to reflect a transitional, climatically unstable period associated with the change from a wet early to a dry late Holocene over Africa.
Section snippets
Materials and methods
Piston Core 905 was retrieved from a water depth of 1580 m offshore Somalia during the Netherlands Indian Ocean Program 1992/93 (Fig. 1). The age model for the last 31 kyr BP of the core is based on 33 AMS14C dates [11], [12], [13], [14]. Fig. 2 shows the Holocene stable oxygen isotope records of the planktonic foraminifer Globigerina bulloides from Core 905 and the AMS14C dates for that section (note that the AMS14C dates for the early Holocene section between 6 and 10 kyr BP per sample
Dust in Core 905
Dust transport to the western Arabian Sea occurs almost exclusively during summer [16], [17], [18]. Core 905 potentially receives dust brought in by winds from NE Africa (SW monsoon) and/or the Arabian Peninsula/eastern Egypt (Northwesterlies, Fig. 1a). Today, dust advection from the Arabian Peninsula and the Middle East mainly controls the lithic sediment input to the eastern and central Arabian Sea [17], [18], [19]. Local dust sources from NE Africa dominate the dust advection towards the
Results and discussion
To identify changes in dust provenance the Sr and Nd isotopes were measured and the results are displayed in Fig. 3. Conspicuous in our isotope records is the nearly flat Nd isotope profile. The Nd isotope values in Core 905 should primarily reflect variations in dust provenance because Sm/Nd ratios are not greatly fractionated by most magmatic processes associated with crustal genesis so that sedimentary rocks derived from them have very similar Sm/Nd ratios [22]. In addition, most common
Acknowledgments
This study is part of the EU-TMR network ERBFMRXCT960046 ‘The marine record of continental tectonics and erosion’ and is NSG Publication 2004.03.01. We thank Viviane Bout-Roumazeilles for discussions. We also thank Peter de Menocal and five anonymous reviewers for their constructive criticisms.[BARD]
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2022, Global and Planetary ChangeCitation Excerpt :Moreover, it is uncertain where and if the aeolian detritus contained in the marine core records originated from the rift valley of eastern Africa. Radiogenic study of Holocene dust flux suggests that, rather than interior eastern Africa, coastal areas are the more likely sources for the aeolian detritus transported to the marine core sites (Jung et al., 2004). It has been assumed that the presence of the same volcanic ashes within the marine core records and the rift valley lake basins suggest that the windblown detritus of the cores derived from hominin sites of the rift (Feakins et al., 2005; Uno et al., 2016).