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Drivers of river reactivation in North Africa during the last glacial cycle

Nature Geoscience volume 14, pages 97–103 (2021)Cite this article

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Abstract

North African greening phases, during which large rivers ran through the Sahara Desert, occurred repeatedly during the Quaternary and are regarded as key periods for the development of past human populations. However, the timing and mechanisms responsible for the reactivation of the presently dormant fluvial systems remain highly uncertain. Here we present hydroclimate changes over the past 160,000 years, reconstructed from analyses of the provenance of terrestrial sediments in a marine sediment record from the Gulf of Sirte (offshore Libya). By combining high-resolution proxy data with transient Earth system model simulations, we are able to identify the various drivers that led to the observed shifts in hydroclimate and landscapes. We show that river runoff occurred during warm interglacial phases of Marine Isotope Stages 1 and 5 due to precession-forced enhancements in the summer and autumn rainfall over the entire watershed, which fed presently dry river systems and intermittent coastal streams. In contrast, shorter-lasting and less-intense humid events during glacial Marine Isotope Stages 3 and 4 were related to autumn and winter precipitation over the Libyan coastal regions driven by Mediterranean storms. Our results reveal large shifts in hydroclimate environments during the last glacial cycle, which probably exerted a strong evolutionary and structural control on past human populations, potentially pacing their dispersal across northern Africa.

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Fig. 1: Map of northern Africa with records relevant for our study.
Fig. 2: Tracers of past fluvial input into the GS.
Fig. 3: Hydroclimate records of core 64PE349-8 compared to transient simulations of precipitation.
Fig. 4: Summer and autumn precipitation as modelled by LOVECLIM during IPM and GPM.

Data availability

All datasets presented here will be made freely available at the data repository PANGAEA (https://pangaea.de/) and are provided as Supplementary tables. Transient LOVECLIM simulations are available at https://climatedata.ibs.re.kr/grav/data/loveclim-784k.

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Acknowledgements

We thank the captain, crew and scientific team of RV Pelagia cruise 64PE349 as well as P. van Gaever and R. Gieles (in memoriam) at NIOZ for analytical assistance. C.L.B. acknowledges the support of GFZ Potsdam through a reintegration grant. R.T. acknowledges financial support by NWO grants INTAX (839.08.434) and SCAN2. W.E. was supported by a grant from the Deutsche Forschungsgemeinschaft (EH89/19-1). A.T. was supported by the Institute for Basic Science (IBS) (IBS-R028-D1).

Author information

Authors and Affiliations

  1. Climate Dynamics and Landscape Evolution, Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany

    Cécile L. Blanchet & Rik Tjallingii

  2. GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany

    Anne H. Osborne, Warner Brückmann & Martin Frank

  3. Department Ocean Systems, Royal Netherlands Institute for Sea Research NIOZ, ‘t Horntje (Texel), The Netherlands

    Rik Tjallingii

  4. Institute for Geophysics and Geology, University Leipzig, Leipzig, Germany

    Werner Ehrmann

  5. Department of Oceanography, School of Ocean and Earth Sciences and Technology, University of Hawaii at Mānoa, Honolulu, HI, USA

    Tobias Friedrich

  6. Center for Climate Physics, Institute for Basic Science (IBS), Busan, South Korea

    Axel Timmermann

  7. Pusan National University, Busan, South Korea

    Axel Timmermann

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  1. Cécile L. Blanchet
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Contributions

C.L.B., A.H.O. and R.T. conceived the study and organized cruise 64PE349, led by W.B. W.B. and A.H.O. retrieved the sediment cores. A.H.O. and M.F. facilitated and measured the radiogenic isotopes; R.T. measured the grain-size distributions, oxygen isotopes and carried out X-ray fluorescence core scanning; W.E. measured the clay mineral assemblages; T.F. and A.T. developed and ran the LOVECLIM simulations. C.L.B. analysed the data and wrote the manuscript with input from all co-authors. A.T., W.B. and M.F. provided logistical and financial support.

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Correspondence to Cécile L. Blanchet.

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Extended data

Extended Data Fig. 1 Extended map of North Africa showing present-day climatology and preferential dust sources (PDS).

a. Topography, climatology and marine surface currents. Location of core 64PE349-8 (red diamond) and relevant records mentioned in the text (MD04-279717, CP10BC18, SL719, MS27PT21 and GeoB77026) or used for the age model (KC0155,81, LC21 and Soreq Cave54) (green diamonds). Map background represents the topography and bathymetry (Gebco 2014 30-arcsecond grid, version 20150318, www.gebco.net), plotted with the open source software R77 and ‘marmap’ package78. Location of current Nile, Niger and Senegal rivers (blue lines) and paleo-river systems (green lines) redrawn from ref. 30 (Kufrah, Sahabi and Irharhar rivers) and ref. 15 (Tamanrasett river). The extent of the paleo-lake Chad was redrawn from ref. 82. Present-day precipitation patterns are indicated as contour lines of monthly precipitation rates in winter (February, in blue) and in summer (August, in red) (UEA CRU TS3p21 climatology c1901-2012 precipitation data47). Present-day surface marine circulation (grey lines) shows the anti-cyclonic circulation in the Gulf of Sirte83. b. Preferential dust sources (PDS) redrawn from remote sensing24 numbered 1 to 7. εNd signature of surface soils and river sediments, rocks, bivalves and deposited aerosols and contour plot of interpolated εNd signature of seafloor sediments (data from ref. 23).

Extended Data Fig. 2 Age model and identification of sapropel layers in core 64PE349-8.

Left: uncorrected core depth, section number (in roman number), line-scan image of the split core surface. a. tie-points for age-depth model shown as diamonds (red: radiocarbon age, orange: upper and lower sapropel limits, purple: tephra layer Y-5), δ18O of planktonic foraminifera G. ruber and correlation tie-points (green diamonds), intervals used to compare sediment populations (sapropel layers S1, S3-S5 and MIS3/4) (orange bars). b. marine isotope stages (MIS, black and white bars), age-depth relationship with identified tie-points and uncertainty envelop, sedimentation rates (black step curve). c. Magnetic susceptibility; d. Log-ratio of barium/aluminium (Ba/Al). The latter two records (c,d) were used to determine the boundaries of the sapropel layers (horizontal grey bars) and tephra layers (marked as ‘T’). Line-scan image, magnetic susceptibility and elemental ratios were measured using an Aavatech XRF scanner at Nioz (Texel, Netherlands).

Extended Data Fig. 3 Grain-size distribution and end-member modeling.

Left panel: Grain-size distribution of individual samples (grey) with overlaid: coefficient of determination (R2) for each grain-size fraction (dashed orange line), end-members EM1 (main mode 3 µm, blue line), EM2 (main mode 15 µm, green line) and EM3 (main modes 3 µm and 45 µm, red line). Right panel: Proportion of variance explained per number of additional end-member. Methodology for end-member modelling analysis follows ref. 6,25.

Extended Data Fig. 4 Definition of interglacial precession minima (IPM) and glacial precession minima (GPM).

a. Reconstructed global-mean surface atmospheric temperature (SAT) for the past 160 kyr76 (black) and precession parameter (green). IPM (red bars) and GPM (blue bars) are defined by time intervals when the precession index is < 0.01 and the simulated global-mean SAT-anomaly is < 1 K (glacial) or > 1 K (interglacial). These IPM and GPM intervals are reported on the simulated MAP for the Tibesti region (b) and the Libyan coastal region (c).

Extended Data Fig. 5 Comparison Model-Data and seasonality of precipitation as modelled by LOVECLIM.

a. Comparison of observed (left) and modelled (right, 0-1 ka averages) present-day seasonality of precipitation (mm/month) over Western North Africa (zonal averaging: 2.8°E-25.3°E). Observed precipitation from reanalysis CMAP Precipitation data provided by the NOAA/OAR/ESRL PSD, Boulder (USA) (https://www.esrl.noaa.gov/psd/). b. Simulated seasonality of precipitation at the LGM (25 ka BP, black), during a GPM (50 ka BP, blue) and during an IPM (125 ka BP, red) over the Libyan coast (left) and the Tibesti (right). c. Precipitation anomalies with respect to the 0-1 ka monthly averages for the same time intervals.

Extended Data Fig. 6 Simulated precipitation over the Tibesti (A–D) and coastal Libya (E–H) for full forcing run and sensitivity simulations.

a and e: Precipitation for full-forcing simulation (black) and simulation using only orbital forcing under a warm background climate (CO2 at 280 ppm, red). b and f: Black line as in a, e and simulation using only orbital forcing under a cold background climate (CO2 at 200 ppm, blue). c,g: Black line as in A, E and simulation using full-forcing except for a constant preindustrial Northern Hemisphere ice-sheet (NHIS). d and h: Black line as in a, e and simulation using full-forcing except for constant preindustrial green-house gas concentrations.

Supplementary information

Supplementary Table 1

Data for plotting regional radiogenic Nd and Sr isotopes.

Supplementary Table 2

Stable O isotopes, radiogenic Nd and Sr isotopes, clay mineral ratio and proportion of grain-size end-members.

Supplementary Table 3

Age model and sedimentary signals in core 64PE349-8.

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Blanchet, C.L., Osborne, A.H., Tjallingii, R. et al. Drivers of river reactivation in North Africa during the last glacial cycle. Nat. Geosci. 14, 97–103 (2021). https://doi.org/10.1038/s41561-020-00671-3

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