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Indo-Pacific Walker circulation drove Pleistocene African aridification

Abstract

Today, the eastern African hydroclimate is tightly linked to fluctuations in the zonal atmospheric Walker circulation1,2. A growing body of evidence indicates that this circulation shaped hydroclimatic conditions in the Indian Ocean region also on much longer, glacial–interglacial timescales3,4,5, following the development of Pacific Walker circulation around 2.2–2.0 million years ago (Ma)6,7. However, continuous long-term records to determine the timing and mechanisms of Pacific-influenced climate transitions in the Indian Ocean have been unavailable. Here we present a seven-million-year-long record of wind-driven circulation of the tropical Indian Ocean, as recorded in Mozambique Channel Throughflow (MCT) flow-speed variations. We show that the MCT flow speed was relatively weak and steady until 2.1 ± 0.1 Ma, when it began to increase, coincident with the intensification of the Pacific Walker circulation6,7. Strong increases during glacial periods, which reached maxima after the Mid-Pleistocene Transition (0.9–0.64 Ma; ref. 8), were punctuated by weak flow speeds during interglacial periods. We provide a mechanism explaining that increasing MCT flow speeds reflect synchronous development of the Indo-Pacific Walker cells that promote aridification in Africa. Our results suggest that after about 2.1 Ma, the increasing aridification is punctuated by pronounced humid interglacial periods. This record will facilitate testing of hypotheses of climate–environmental drivers for hominin evolution and dispersal.

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Fig. 1: Regional oceanographic settings of IODP Site U1476 (2,166 m water depth).
Fig. 2: Climatic and oceanographic records over the past seven million years.
Fig. 3: MCT and eastern African aridity records with distinct glacial and interglacial intervals over the past 3.3 Myr.

Data availability

All new benthic oxygen isotopic and lithogenic grain-size data of the spliced record of IODP Site U1476 are available via www.pangaea.de at https://doi.org/10.1594/PANGAEA.933831 and https://doi.org/10.1594/PANGAEA.933833, respectively.

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Acknowledgements

We are grateful to the government of Mozambique for their permission for deep-sea drilling operations in the context of palaeoclimate and palaeoceanography research. This research used samples and data provided by the Integrated Ocean Drilling Program (IODP). We appreciate the entire shipboard party of IODP Expedition 361 ‘South African Climates’ for their tireless efforts at sea. We thank S. Conn, M. Hagen, S. Lordsmith, A. Nederbraght, L. Owen, M. P. Prins, S. Rumping, S. Slater, C. W. Nooitgedacht, F. van Bakel, S. de Bie and C. van Eijbergen for laboratory assistance. Funding for this work was provided in part by the Dutch Research Council (NWO) (Open Programme grant number 824.01.005 to H.J.L.v.d.L., and Talent Programme grant number 016.Vidi.171.049 to J.C.A.J.) and the UK Natural Environment Research Council (grant number NE/P000037/1 to I.R.H. and grant number NE/P000878/1 to S.B.). We acknowledge R. D. Norris and N. Mantke for X-ray fluorescence scanning the spliced record of Site U1476.

Author information

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Authors

Contributions

IODP Expedition 361 was led by I.R.H. and S.R.H. S.B. produced the benthic oxygen isotope record and J.J. produced the X-ray fluorescence bulk chemistry records. H.J.L.v.d.L. and I.R.H. designed the research, H.J.L.v.d.L. performed the grain-size analysis with input from T.F.B. and performed further data analyses with input from I.R.H. and A.S. H.J.L.v.d.L. conducted oceanographic and climatic analyses with input from B.C.B. H.J.L.v.d.L. wrote the manuscript with contributions from I.R.H., J.C.A.J. and A.S. All authors contributed to the data interpretation and commented on the final manuscript. T.F.B. edited the figure layout for publication.

Corresponding authors

Correspondence to H. J. L. van der Lubbe or I. R. Hall.

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The authors declare no competing interests.

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Peer review information Nature thanks John Andrews, Clara Bolton, Matt O’Regan, Benjamin Petrick and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Coupled oceanographic and atmospheric circulation of the Indian Ocean region.

a, Surface circulation indicated by mean geostrophic velocities and directions. IODP Site U1476 is situated in the Mozambique Channel, which experiences a net southward flow of the Mozambique Channel Throughflow (MCT). The tropical Indian gyre receives and redistributes inflow from the Indonesian Throughflow (ITF). The main components of the tropical Indian gyre: East African Coastal Current (EACC), Southern Equatorial Current (SEC), Southern Equatorial Counter Current (SECC) and Northeast Madagascar Current (NEMC) are highlighted with a dark grey outline marking the mean extension of the tropical gyre63,64. At ~60°E, the SEC bifurcates into two main branches feeding into the NEMC and the Southeastern Madagascar Current (SEMC) as it crosses bathymetric highs including the Mascarene Plateau, highlighted by the solid grey bathymetric contour at 2,200 m water depth. The SEMC is outside the outline of the tropical gyre as it is part of the anti-cyclonic subtropical gyre. b, Mean wind stress (gray arrows) and wind stress curl of the Indian Ocean indicate the basin-wide negative (positive) wind stress curl forcing the tropical (subtropical) gyre. A black line indicates the zero wind stress curl. c, Mean dynamic sea surface topography indicating a sea-level low between 10-5°S at the centre of the tropical Indian gyre; the Seychelles–Chagos thermocline ridge (SCTR)9. The thicker contour of 1.05 m denotes the northern extent of the subtropical gyre, whereby the blues (reds) highlight lower (higher) sea surfaces, which are associated with the cyclonic (anti-cyclonic) circulation of the tropical (subtropical) gyres. The bathymetric, oceanic (1993-2012) and surface wind data (1979-2019) are derived from: General Bathymetric Chart of the Oceans (GEBCO)65, CNES-CLS18 MDT66, ERA5 monthly-averaged data on single levels (DOI: 10.24381/cds.6860a573), respectively. These maps are generated with MATLAB and Mapping Toolbox, version 9.8.0.1323502 (R2020a, Natick, Massachusetts: The MathWorks Inc., United States).

Extended Data Fig. 2 Dynamic sea surface topography in meters superimposed by the mean of monthly Indian Ocean sea surface height (SSH) maps.

a, Composite of all monthly SSH between 1999-2019, superimposed on dynamic sea surface topography, marks sea-level low of the SCTR (5°S–12°S, 45°E–90°E)9. Solid red line refers to cross section along longitude 52°E between latitudes 25 to 5°S, across the SEC, see b. b, Mean dynamic sea surface gradient at 52°E (red thick line) as marked by solid black line in a, c, d, e and f. Meridional surface height gradient and associated near-surface pressure gradient south of the SCTR drives (influenced by Coriolis force) the westward deep-reaching South Equatorial Current (SEC). Positive (negative) SSH anomalies formed during ENSO and IOD propagate westward as downwelling (upwelling) Rossby waves in ca. 6 months10,19,24,67. The dynamic sea surface topography averaged over 6 months following positive Indian Ocean Dipole (+IOD), negative Indian Dipole (-IOD), positive El Niño Southern Oscillation (ENSO) - El Niño, and negative ENSO - La Niña are shown in c, d, e and f, respectively. g, IOD and ENSO time series with solid vertical lines indicating long-term mean with ±1SD and ±2SD (dashed lines). Red dots mark the months that are selected after a positive IOD (c) and ENSO (e) events, while blue dots mark the months after negative IOD (d) and ENSO (f) events. SSH anomalies induced by ENSO and IOD often reinforce each other10, since they are linked at interannual1 to decadal time-scales63, at least over the last millennium68. Sea surface topography, SSH maps, IOD SST index1, El Niño 3.4 SST Index69 are from Ssalto/Duacs-Cnes; https://www.aviso.altimetry.fr, https://psl.noaa.gov/gcos_wgsp/Timeseries/DMI/, https://psl.noaa.gov/gcos_wgsp/Timeseries/Nino34. These maps are generated with MATLAB® and Mapping Toolbox, version 9.8.0.1323502 (R2020a, Natick, Massachusetts: The MathWorks Inc., United States).

Extended Data Fig. 3

3d representations of the mean sea surface topography. a, b, Averaged over 6 months after (a) warm (positive) and (b) cold (negative) ENSO-IOD phases, respectively induced by regional anti-cyclonic (AC) positive and cyclonic (C) negative wind stress curls along the equatorial Indian Ocean west of 100°E, which is coupled to the atmospheric Pacific Walker Cell circulation10,24. In contrast, sea surface height (SSH) variability in the eastern Indian Ocean is derived from the western Pacific Ocean via the Indonesian Throughflow (ITF; Extended Data Figs. 1a, 2c-f). Black arrows indicate schematic representation of the zonal Walker circulation. During positive ENSO phases, the center of atmospheric deep convection shifts eastward, resulting in anomalous descending air masses over the western Pacific Ocean and Maritime Continent63. The corresponding anomalous easterlies induce down-welling Rossby waves in the central Indian Ocean that propagate westward as positive SSH anomalies, increasing the thermocline depth at the SCTR while decreasing the meridional SSH gradient and corresponding SEC in the western Indian Ocean10,24 (Extended Data Fig. 2b). Conversely, the SSH gradient and in turn the SEC flow increases during negative ENSO phases. The associated westerlies/easterlies induce upwelling and thermocline shoaling (dark blue) in the western/eastern Indian Ocean, and in turn promote deep atmospheric convection and excess rainfall over the Maritime Continent/eastern Africa via sea atmospheric interactions10,17. The Mozambique Channel Throughflow (MCT) that is related to the westward flow of the SEC north of Madagascar therefore increases following a negative-cold ENSO-IOD phase (b) and vice versa (a).

Extended Data Fig. 4 Precipitation difference (mm/day) of mean precipitation between 2000-2012 minus that of 1979-1999.

The precipitation difference indicates systematically drier conditions in eastern Africa during the last decade17, which coincides with intensified Southern Equatorial Current (SEC)17 and Mozambique Channel Throughflow (MCT)70. Green triangles indicate the eastern African hominin sites (1. Omo-Turkana Basin, 2. Baringo and Tugen Hills, 3. Kanjera 4. Olorgesailie, 5. Laetoli and Olduvai), from which long-term carbon isotope records of soil carbonates are available serving as long-term proxy of eastern African aridity. The rainfall data is from Global Precipitation Climatology Project (GPCP) monthly precipitation analysis (1979–present; DOI 10.7289/V56971M6) and visualized with MATLAB and Mapping Toolbox, version 9.8.0.1323502 (R2020a, Natick, Massachusetts: The MathWorks Inc., United States).

Extended Data Fig. 5 Flow speed reconstructions using sortable silt mean grain size and abundance in the Mozambique Channel.

a, 3-part diagram of sortable silt properties of flow speed in the Mozambique Channel (Clock-wise). The relationship between sortable silt abundance (% lithogenic fraction between 10-63 μm; SS%) and mean grain size (\(\overline{{\rm{SS}}}\))54,71,72 for the spliced record from IODP Site U147614 and modern surface sediments (64PE304-47, −51, −56, −63, −66 and −68)53. The strong correlation (Pearsons correlation coefficient = 0.79) indicates that sortable silt deposition was subject to current sorting and selective transport71. The \(\overline{{\rm{SS}}}\) of U1476 and nearby surface sediments are well within the calibration range for universal near-bottom current flow speed reconstructions71. The inferred flow speeds for the surface sediment samples using the universal \(\overline{{\rm{SS}}}\) flow speed calibration correspond to the +2 s.d. from the mean near-bottom currents obtained from nearby mooring stations. The mean flow directions are southwards except for those of the Mozambique undercurrent (MUC) that is confined to the eastern African margin below 1.5 km water depth15. The \(\overline{{\rm{SS}}}\) is likely somewhat biased towards to higher near bottom flow speeds, as the finest fractions that may have selectively deposited during slow near-bottom current conditions were preferentially removed under intra- and interannually increased near-bottom currents. Additionally, slight deviations from the mean flow speed might be further attributed to the regional nature of the lithogenic sediments, as most \(\overline{{\rm{SS}}}\) flow speed calibrations are defined for the northern Atlantic Ocean71. However, the sensitivity is comparable amongst the local calibrations and therefore the inferred relative changes in flow speed are also accurate for the Mozambique Channel, albeit the absolute values may slightly differ. In this study, the coarsest \(\overline{{\rm{SS}}}\) and in turn highest flow speeds are obtained from near and within the topographic depression, where the continuation of the MUC passes through the Davie Ridge73. b, W-E transect across the Mozambique Channel with long-term flow contours (cm s−1)15 as in Fig. 1c, but superimposed by the mean flow speeds of individual moorings of mean near-bottom currents across the Mozambique Channel (cm s−1)2173, whereby negative (positive) values reflect southward (northward) flow. c, Mozambique Channel bathymetric map with transect across Site IODP U1476 (S2) and the mooring transect (S1) with station (small solid black squares). The mean near-bottom flow speeds vectors recorded at the mooring transect are indicated by dashed73 and solid21 lines. The locations of the surface sediment samples are marked by white dots with black outlines (64PE304-47, −51, −56, −63, −66 and −68)53.

Extended Data Fig. 6 Age model of the spliced record of IODP Site U1476.

a, From top to bottom: The chronology of spliced core of IODP Site U1476 is based on the correlation of the 𝛿18O benthic record with the global 𝛿18O benthic probabilistic stack (Prob-stack)40 using 38 tie points for the last ~2 Ma. Benthic 𝛿18O record of the spliced record of IODP Site U1476 over the last ~2 Ma (blue) and Prob-stack (black)40. Minimal number of tie-points that are used to tune the 𝛿18O benthic record of U1476 to Prob-stack are indicated by vertical lines in a following a similar approach as57. The Prob-stack overlain by the tuned benthic 𝛿18O record of the spliced record of IODP Site U1476 demonstrates the similarity between both records. b, The age-depth relationship agrees with the biochronology of calcareous nannofossils and planktonic foraminifera with only some minor deviations from the 𝛿18O bayesian age model. Bayesian age-modeling60 of U1476 with ±1SD (dark grey) and ±2SD (light grey) based on the benthic 𝛿18O tuning points and biostratigraphy of calcareous nannofossils over the last 3 Ma with the accumulation rates. The shipboard biostratigraphy of calcareous nannofossils14, which has been further refined by Tangunan et al.(2018)59 displaying internal agreement. c, Idem as b for the last ~7 Ma, Deviations of the shipboard biochronology of planktonic foraminifera might be partly attributable to the use of general low-latitude calibrations rather than specific calibrations for the tropical western Indian Ocean14,51.

Extended Data Fig. 7 7-Ma long records of sortable silt mean (\(\overline{{\rm{SS}}}\)) and derived flow speed changes together with lithogenic properties.

The accompanying elemental compositions are obtained through X-ray fluorescence (XRF) analyses of sediments at Site U1476 (Methods). The glacial periods are indicated for the last 5.3 Ma74 by vertical light blue bars. The 10-point running means of the lithogenic ln (Zr/Rb) record reflect the relative deposition of dense Zr grains that are sorted, via selective deposition, together with silt-sand sized terrigenous particles, and Rb that is mainly present in clay minerals as substitution for K. Additionally, the 10-point running means of XRF bulk record (ln) Ca/(Ca/Ti) record that represents the relative deposition of carbonates (including foraminiferal shells) versus the terrigenous fraction53. Reconstructed enhancements in flow speed after 2.1 ± 0.1 Ma correspond to increases in coarse-grained lithogenic sediments together with increases in marine carbonates, which suggests selective deposition and removal of the fine-grained lithogenic sediment fractions.

Extended Data Fig. 8 Long-term Sea Surface Temperature (SST) records and Indo-Pacific Walker Cell circulation.

a, Long-term SST records from the Indian and Pacific Ocean. The Mg/Ca-based SST records of DSDP 21435, ODP 709C36, 806 and 84743 that are calcuated and corrected for dissolution at depth75 are mainly derived from Globigerinoides sacculifer, therefore recording temperatures about 20-30 m below the surface. Divergence of the SST records at ~2.1 Ma reflects the onset of the modern Indo-Pacfiic Walker cell circulation. b, Representation of present-day coupled Indo-Pacific Walker cell circulation, which is characterized by climatological low-level westerlies11 and easterlies over the equatorial Indian and Pacific Oceans, respectively. The corresponding moisture-laden air masses of both oceans ascend over the Maritime Continent in southeastern Asia and the associated atmospheric deep convection induces excess rainfall. In contrast, the subsiding dry air masses over the cool western Indian Ocean cause arid conditions in eastern Africa. c, Ocean map color gradients show climatological mean Sea Surface Temperature (SST) and black arrows represent the atmospheric surface circulation of the Indian and Pacific Ocean basins (ERA5 monthly-averaged data on single levels; https://doi.org/10.24381/cds.6860a573). The low-level Pacific easterlies and Indian westerlies are driven by the temperature contrast between the center of the Indo-Pacific Warm Pool (IPWP; SST >28 °C) and upwelling areas in the eastern Pacific Ocean (Cold tongue) and western Indian Ocean (Seychelles-Chagos Thermocline Ridge (SCTR), western Indian Ocean and Arabian Sea). The yellow dots indicate IODP Site U1476 (this study), and U133728, DSDP Site 21435, ODP Sites 709C36, 721-72212,46,62, 806 and 84743 providing long-term SST records, as well as sites GeoB12610-239 and GeoB10038-449 that date back to the Last Glacial Maximum (LGM). The SST map has been derived from NOAA Extended Reconstructed Sea Surface Temperature (ERSST) (Version 5, NOAA National Centers for Environmental Information, DOI:10.7289/V5T72FNM) is plotted with MATLAB and Mapping Toolbox, version 9.8.0.1323502 (R2020a, Natick, Massachusetts: The MathWorks Inc., United States).

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van der Lubbe, H.J.L., Hall, I.R., Barker, S. et al. Indo-Pacific Walker circulation drove Pleistocene African aridification. Nature 598, 618–623 (2021). https://doi.org/10.1038/s41586-021-03896-3

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