Stepwise Holocene aridification in NE Africa deduced from dust-borne radiogenic isotope records

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Abstract

Transfer of tropical heat to higher latitudes is the major driving force of the Earth’s climate. Consequently, sediments in regions to the north and south of the tropics potentially retain an archive of past major climate reconfigurations. The climate of one such region, around the Arabian Sea, sensitively depends on the coupled Asian and African monsoons that also control the dust transport. Here, we use the Sr–Nd isotope ratios of the dust fraction from Core 905 (Arabian Sea off Somalia), as a novel tool to deduce the Holocene weathering history of the Horn of Africa with emphasis on the climate transition that took place from a wet early to a dry late Holocene. The highly variable Sr isotope ratios are interpreted to reflect mainly changes in the evaporation/precipitation balance over NE Africa whilst the Nd isotope measurements record no significant variations and point to a prevailing NE African dust source. The Sr isotope record shows that the first aridification step occurred at 8.5 kyr BP followed by an unstable transitional period up to 6 kyr BP, characterized by decadal-scale high-amplitude variations in the evaporation/precipitation balance. A second aridification step began at 6 kyr BP and ceased at 3.8 kyr BP when modern-day dry climate was established. The combined Sr and Nd isotope records probably reflect north–south shifts of the Intertropical Convergence Zone controlling the evaporation/precipitation balance over NE Africa.

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]

References (40)

  • A. Berger et al.

    Insolation values for the climate of the last 10 million years

    Quat. Sci. Rev.

    (1991)
  • P. deMenocal et al.

    Abrupt onset and termination of the African humid period; rapid climate responses to gradual insolation forcing

    Quat. Sci. Rev.

    (2000)
  • P. Hoelzmann et al.

    Environmental change and archeology: lake evolution and human occupation in the Eastern Sahara during the Holocene

    Palaeogeogr. Palaeoclimatol. Palaeoceol.

    (2001)
  • D. Anderson et al.

    A 300 kyr record of upwelling off Oman during the late Quaternary; evidence of the Asian southwest monsoon

    Paleoceanography

    (1993)
  • S. Clemens et al.

    Eccentricity forcing of Pliocene-early Pleistocene climate revealed in a marine oxygen-isotope record

    Nature

    (1997)
  • R. Tiedemann et al.

    Astronomic timescale for the Pliocene Atlantic δ18O and dust flux records of Ocean Drilling Program site 659

    Paleoceanography

    (1994)
  • R. Gillespie et al.

    Post-glacial arid episodes in Ethiopia have implications for climate prediction

    Nature

    (1983)
  • J.C. Ritchie et al.

    Sediment and pollen evidence for an early to mid-Holocene humid period in the eastern Sahara

    Nature

    (1985)
  • F. Gasse, Hydrological changes in the African tropics since the last glacial maximum, in: K.D. Alverson, F. Oldfield,...
  • F. Gasse et al.

    The arid-humid transition in the Sahara and the Sahel during the last deglaciation

    Nature

    (1990)
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