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Mid-Pliocene glaciation preceded by a 0.5-million-year North African humid period

Nature Geoscience volume 17, pages 660–666 (2024) Cite this article

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Abstract

Past North African humid periods caused expanded vegetation over the Sahara, due to northward tropical African rainbelt displacement, opening migration pathways for hominins. Commonly, these precession-timed humid periods ended within 15,000 years due to rainbelt retreat. During North African humid periods, eastern Mediterranean organic-rich layers called sapropels were deposited at least since 8 Myr. Here we combine climate modelling with palaeoclimate proxy data to show that weakened sapropel preservation during the 5.3–3.3 Myr period resulted from nutrient runoff limitation associated with enhanced North African vegetation cover due to a persistently more northward-located African monsoon front, relative to the mid-Pliocene (3.3–3.0 Myr, when glacial intensity increased). Moreover, sapropel absence within the 3.8–3.3 Myr period coincided with maximum monsoon runoff and extensively humid, vegetated conditions throughout North Africa. Our model results indicate that this 0.5-Myr-long pan-North African humid period ended at ~3.3 Myr because of southward monsoon front displacement with Northern Hemisphere glacial intensification. The 3.8–3.3 Myr humid period coincided with the earliest known evidence for hominin coexistence over eastern and central North Africa. We posit that persistent green corridors during this humid phase facilitated early hominin connectivity and migration, expanding their habitat range over the wider North African territory.

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Fig. 1: Eastern Mediterranean sapropels across the past 5 Myr.
Fig. 2: North Africa drying during the M2 glaciation.
Fig. 3: Evidence for enhanced humid conditions across North Africa during the 3.3–3.8 Myr interval.
Fig. 4: The Pan-NAHP and hominin evolution.

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Data availability

ODP 967 Nd isotope data from this study have been submitted to Pangaea (www.pangaea.de) under ‘Plio-Pleistocene Neodymium (Nd) isotope data for Ocean Drilling Program (ODP) Site 967, Eastern Mediterranean’ and are also available as online Supplementary Data accompanying this article. The model data presented in this study are available to download via Zenodo at https://doi.org/10.5281/zenodo.11124493 (ref. 69). Source data are provided with this paper.

Code availability

The model code used in this study is available to download via Github at https://github.com/mom-ocean/MOM5 using the CM2M configuration under version 5.1.0. Input files and restart files for each experiment are available via Zenodo at https://doi.org/10.5281/zenodo.11124493 (ref. 69).

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Acknowledgements

This work contributes to Australian Research Council projects FL120100050 and DP200101157 (E.J.R.), DE190100042 (K.M.G.), DP190100874 (A.P.R., D.H.), DE220100279 (D.K.H.) and the Australia–New Zealand IODP Consortium (ANZIC) Legacy/Special Analytical Funding grant LE160100067 (K.M.G.). We thank Mawson Analytical Spectrometry Services, University of Adelaide for contributing to Nd isotope measurements.

Author information

Authors and Affiliations

  1. Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory, Australia

    Udara Amarathunga, Katharine M. Grant, David Heslop & Andrew P. Roberts

  2. Department of Geosciences, Princeton University, Princeton, NJ, USA

    Udara Amarathunga

  3. Department of Earth Sciences, University of Utrecht, Utrecht, The Netherlands

    Eelco J. Rohling

  4. Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, UK

    Eelco J. Rohling

  5. School of Physics, Chemistry and Earth Sciences, Faculty of Sciences, Engineering and Technology, The University of Adelaide, Adelaide, South Australia, Australia

    Alexander Francke & Robert M. Klaebe

  6. Fenner School of Environment and Society, Australian National University, Canberra, Australia Capital Territory, Australia

    James Latimer

  7. Climate Change Research Centre and Australian Centre for Excellence in Antarctic Science, University of New South Wales, Sydney, New South Wales, Australia

    David K. Hutchinson

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  1. Udara Amarathunga
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Contributions

U.A. designed and led the study, prepared samples for Nd isotope analyses, analysed the data and climate model outputs, developed the hypothesis, generated the figures and wrote the paper; A.F. advised on Nd isotope analysis and data interpretation; R.M.K. carried out Nd isotope analyses; D.K.H. generated the climate model ice sheet configurations and performed modelling experiments; D.H. advised on statistical analyses and performed the box model experiments; K.M.G. and J.L. helped with sample selection and preparation; E.J.R. advised on data interpretation; all authors contributed to paper development.

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Correspondence to Udara Amarathunga.

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Extended data

Extended Data Fig. 1 Preserved sapropel record of the eastern Mediterranean.

a, Conceptual diagram with modern locations of eastern Mediterranean ODP sites. The present water depth at each site is indicated. b, Preserved sapropels at each site. The ODP Site 964 sediment record only goes back to ~3.8 Myr. At Site 966, sediments beyond 4.5 Myr are tectonically deformed16,58. Prolonged sapropel absence during a period of high-amplitude precession minima (centred at ~3.5 Myr) over the entire Mediterranean is marked by vertical blue shading. The ODP Site 967 Ba record indicates enhanced organic carbon burial after 3.2 Myr, which is observed at all sites. The presence of Ba/Al peaks (with a low magnitude) in the Early Pliocene suggests the presence of organic carbon burial, that were oxidized later. Cross section in a drawn using Ocean Data View version 5.6.3 (ref. 80).

Extended Data Fig. 2 Evidence for enhanced freshwater flux to the eastern Mediterranean during the Pan-NAHP.

a, Mediterranean δ18OCalcite sensitivity to sea surface temperature (SST), monsoon runoff and surface-water temperature concentration (Tc) changes (see Methods). For unchanging Gibraltar sill relative sea-level (RSLGib), 1 °C SST increase can cause a ~ 0.2 ‰ δ18O drop. A freshwater runoff increase equivalent to pre-Aswan Nile runoff can cause a ~ 0.28 ‰ δ18O drop. A 1 °C surface freshwater layer temperature increase can cause a ~ 0.21 ‰ δ18O drop. For example, with unchanging SST and temperature concentration across transition 1 (T1), the observed 0.45 ‰ shift to negative δ18O can be generated with a runoff increase of 1.6 times pre-Aswan Nile runoff equivalents. Shaded intervals indicate the standard error associated with sensitivity estimates. b, SST reconstructions available for the Mediterranean41,73,74. The mean SST variability does not indicate an SST rise that can explain the δ18O drop at T1 (See Methods). c, ODP 967 δ18O and global ice volume change18 across the 2.5-4.5 Myr period. The blue dotted line (upper diagram) comprises all δ18O data points across the interval. The thick grey line (lower diagram) indicates global ice volume evolution. Thick blue and black lines represent the 60-kyr moving average for the δ18O and ice volume curves, respectively. The shading for each curve represents 90% confidence intervals. The shifts to more negative δ18O values at T1 and more positive values at transition 2 (T2) are highlighted in grey vertical bars. While T2 was caused mainly by the global ice volume increase (a minimum of 0.3 ‰, see Methods), T1 was largely caused by a mean freshwater runoff increase to the eastern Mediterranean (no relative global ice volume change across T1, see Methods). Green boxes indicate data gaps.

Extended Data Fig. 3 Topography and bathymetry used for model simulations.

a, c, e, Topography used for the early Pliocene, M2 glacial and mid-Pliocene glacial simulations, respectively. The ice sheets were adjusted for each simulation, following the calculations based on ice volume reconstructions. b, d, f, Bathymetry for the early Pliocene, M2 glacial and mid-Pliocene glacial simulations, respectively. g, h, Difference in topography for 2 scenarios, M2 simulation-early Pliocene simulation and mid-Pliocene glacial simulation-early Pliocene simulation.

Extended Data Fig. 4 Climate model result comparison between the Early Pliocene and the M2 glacial.

a, Boreal summer 600 hPa vertical velocity change (M2 glacial–Early Pliocene), with decreased ascending motion over North Africa at ~20°-25°N during the M2 glacial. Contours represent Early Pliocene climatology; continuous red lines indicate descending motion (positive) and dashed blue lines indicate ascending motion (negative). Decreased northward ascending motion indicates a more southward monsoon front during the M2 glaciation, resulting in the observed precipitation decrease centred at ~20°N. b, The change in 850 hPa zonal wind field (M2 glacial–Early Pliocene), which indicates southward displacement of the easterly (negative/dashed blue contours) -westerly (positive/ solid red contours) convergence zone (solid grey contour) centred at ~18°N over North Africa. Contours indicate Early Pliocene climatology, whereas shading indicates the change from Early Pliocene to the M2 glacial.

Extended Data Fig. 5 Climate model result comparison between the Early Pliocene and mid-Pliocene glacials.

a, Boreal summer 600 hPa vertical velocity change (Mid Pliocene glacial mean-Early Pliocene mean), with decreased ascending motion over North Africa centred at ~20°N compared to the Early Pliocene. Contours represent Early Pliocene climatology; continuous red lines indicate descending motion (positive) and dashed blue lines indicate ascending motion (negative). Decreased northward ascending motion indicates a more southward monsoon front during mid-Pliocene glacials, resulting in the observed precipitation decrease centred at ~20°N. b, The change in 850 hPa zonal wind field (Mid Pliocene glacial mean-Early Pliocene mean), indicating a southward displacement of the easterly -westerly convergence zone centred at ~18°N over North Africa. Contour arrangement (type and colour) is similar to Extended Data Fig. 4. Vertical and horizontal wind vigour is smaller during the Mid Pliocene glacials compared to the M2 glaciation (see Extended Data Fig. 4).

Extended Data Fig. 6 Data for ODP sites 967 and 659.

a, Northern Hemispheric ice volume evolution through the past 10 Myr18. b, ODP 967 Ti/Al data, plotted separately for periods of sapropel (blue) and marl (orange) deposition10. Thick blue and orange lines represent the 60-kyr moving average for sapropel and marl intervals. Both sapropelic and marl Ti/Al values indicate an abrupt slope change to higher relative fluvial sediment inputs at 3.8 Myr. At 3.4 Myr, only the marl Ti/Al values indicate the return of increased aeolian input to the eastern Mediterranean. c, ODP 967 δ18O record back to the Miocene–Pliocene boundary. The thick blue line represents the 60-kyr moving average. d, ODP 659 [Al+Fe]/[Si+K+Ti] record back to 10 Myrref. 1. The thick brown line represents the 60-kyr moving average. MSC, Messinian salinity crisis11.

Extended Data Fig. 7 Nd isotope data for ODP Sites 967 and 659, compared with North African sediment source regions.

a, Map of the main aeolian and fluvial sediment source areas for the eastern Mediterranean (ODP Site 967) and west Africa (ODP Site 659). Land areas highlighted in yellow, purple and grey indicate preferential dust source areas (PSAs) for western, central and eastern North African regions (PSAWest, PSACen. and PSAEast, respectively) (data from ref. 30). Main modern fluvial sediment contributors to the eastern Mediterranean (Main, Blue and White Nile) are shown. b, Comparison of data for sites 967 and 659, with modern εNd values from each sediment source region shown in a. Shaded horizontal bars indicate the range of values observed from each source region. Dots within the bars represent εNd values for individual samples, retrieved from published databases. ODP Site 659 values mostly correspond to εNd values from PSAWest and PSACen, as interpreted in ref. 1. ODP Site 967 εNd data from the present study indicate a largely constant sediment origin through the past 5 Myr. Samples representing periods of enhanced organic carbon burial (black diamonds) prior to 3.2 Myr were chosen based on geochemical data (Ba/Al)10.

Extended Data Fig. 8 North Atlantic meridional temperature gradient evolution.

a, Northern Hemisphere ice volume evolution through the past 5 Myr18. Thick red line represents the 60-kyr moving average. Upper and lower shadings represent 95th percentile confidence intervals. b, Global sea level relative to present18. Red circles with error bars represent global mean sea-level (GMSL) benchmarks from Mallorca18. Error bars indicate age uncertainties (horizontal) and 16th and 84th percentile sea-level ranges (vertical). c, Meridional temperature gradient (ΔT) evolution between North Atlantic ODP Site 982 and available tropical Atlantic temperature records40,41. Thick lines represent the 60-kyr moving average for each record. Relatively stable Early Pliocene ΔT drops to the lowest values at 3.6-3.4 Myr. TS, Tropical stack. d, ODP 967 δ18O record back to the Miocene–Pliocene boundary. Thick black line represents the 60-kyr moving average. Upper and lower shadings represent 95th percentile confidence intervals. Blue arrows in c and d demarcate the stepwise ΔT drop and simultaneous δ18O decrease. The eastern Mediterranean sapropel stack is shown above the time axis. e, Map of locations of ODP sites used for the North Atlantic meridional temperature gradient evaluation. Orange labels indicate sites considered for the tropical stack (ODP sites 662, 722, 846, 1146 = alkenone-based temperature reconstructions; ODP Site 806 = Mg/Ca-based temperature reconstruction)40,41.

Extended Data Fig. 9 Pan-NAHP hominin sites and North African landscape.

a, Map of major hominin fossil sites for early hominin coexisting during the Pliocene Pan-NAHP. White dots indicate discovery sites43,44,45,46,47,48 of A. bahrelgazali (Koro Toro, Chad Basin), A. deyiremeda (Woranso–Mille, central Afar) and K. platyops (Lake Turkana, Kenya). Numbered black dots indicate A. afarensis discovery sites (1, Woranso–Mille; 2, Hadar; 3, Omo basin; 4, Koobi Fora; 5, Lomekwi; 6, Kantis; 7, Laetoli). East and Central African rift systems81,82,83 are highlighted in red and purple, respectively. Data from refs. 81,83. b, Cross-sections drawn to scale, along A-B-C and D-E lines on the map in a. The two sections have contrasting geographies along the East African rift valley and the Central African rift system. Lake names are in blue. Vertical grey shading indicates the intersection of two rift systems, where the Turkana depression is located32. The rift intersection was proposed as a bottleneck, where a possible hominin migration path toward the Chad Basin was proposed34 across the CASZ. CASZ, Central African shear zone.

Extended Data Table 1 Ice sheet boundary conditions used for the early Pliocene, M2 glacial and Mid Pliocene glacial simulations
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ODP 967 Nd isotope data.

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Amarathunga, U., Rohling, E.J., Grant, K.M. et al. Mid-Pliocene glaciation preceded by a 0.5-million-year North African humid period. Nat. Geosci. 17, 660–666 (2024). https://doi.org/10.1038/s41561-024-01472-8

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