A progressively wetter climate in southern East Africa over the past 1.3 million years

Abstract

African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial–interglacial change intensified worldwide1,2,3. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year (precession) beat, driven by orbital forcing of summer insolation, global ice volume and long-lived atmospheric greenhouse gases4. Here we present a 1.3-million-year-long climate history from the Lake Malawi basin (10°–14° S in eastern Africa), which displays strong 100,000-year (eccentricity) cycles of temperature and rainfall following the Mid-Pleistocene Transition around 900,000 years ago. Interglacial periods were relatively warm and moist, while ice ages were cool and dry. The Malawi record shows limited evidence for precessional variability, which we attribute to the opposing effects of austral summer insolation and the temporal/spatial pattern of sea surface temperature in the Indian Ocean. The temperature history of the Malawi basin, at least for the past 500,000 years, strongly resembles past changes in atmospheric carbon dioxide and terrigenous dust flux in the tropical Pacific Ocean, but not in global ice volume. Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals.

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Figure 1: The location and bathymetry of Lake Malawi, African rainfall response to the Indian Ocean dipole, and the drill site age model.
Figure 2: Vegetation (hydroclimate) and temperature history of the Lake Malawi basin.
Figure 3: Correlation of the Malawi temperature record with atmospheric carbon dioxide and dust.
Figure 4: Northern Hemisphere summer insolation and the Indian Ocean SST gradient.

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Acknowledgements

We thank the engineering and design team of the Lake Malawi Scientific Drilling Project for overcoming substantial technical challenges associated with drilling on Lake Malawi, especially the efforts of D. Schnurrenberger, M. Pardy and Lengeek Vessel Engineering. B. Otto-Bliesner and S. Albani provided advice on climate model results relevant to this study. J. King provided the palaeomagnetic reversal data that contributed substantially to the age model of the Malawi sediment record. We thank the scientists and technicians of LacCore, University of Minnesota, for their assistance in the splitting, initial analyses, sampling and archiving of the sediment cores obtained by the Lake Malawi Drilling Project. Financial support was provided by the US National Science Foundation EAR and P2C2 programmes and by the International Continental Scientific Drilling Program. S.S. and J.S.S.D. were supported by the Netherlands Earth System Science Centre (NESSC), which is financially supported by the Dutch Ministry of Education, Culture and Science (OCW).

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Authors

Contributions

T.C.J., J.P.W. and E.T.B. conceptualized the project. C.A.S. and T.C.J. were two of the Principal Investigators on the Lake Malawi Drilling Project. J.P.W., J.S.S.D. and S.S. supervised and interpreted the biomarker analyses conducted by A.A., M.B., J.H., S.C. and S.G. E.T.B. supervised the X-ray fluorescence analyses for calcium. A.D. provided Ar–Ar dates on tephra. R.P.L. provided the lake level history. B.A.S. conducted the statistical analyses. T.C.J. and E.T.B. wrote the manuscript with substantial contributions from J.P.W., A.A., M.B., B.A.S., S.C., S.S. and J.S.S.D. All authors reviewed the paper prior to submission.

Corresponding author

Correspondence to T. C. Johnson.

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The authors declare no competing financial interests.

Additional information

The data used in this study are available as Supplementary Data.

Reviewer Information

Nature thanks K. Freeman, P. Polissar and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Correcting temperature data for lapse rate effect.

a, b, Uncorrected TEX86 temperature (a) and temperature corrected for lapse rate effect (b) plotted against burial depth at drilling site MAL05-1. The light solid and dashed lines represent the 1σ and 2σ ranges of uncertainty in both graphs. c, Lake-level history, from LPC114, which is the basis for the lapse-rate correction to temperature (Methods).

Extended Data Figure 2 A predominant 100-kyr (eccentricity) cycle in hydroclimate since the Mid-Pleistocene Transition.

Blackman–Tukey spectral power versus per year (‘1/Y’) of 500-kyr intervals from the present back to one million years ago, of the lake-level record portrayed in Extended Data Fig. 1, when dated with the tuned age model depicted in Extended Data Fig. 6. Bandwidth is 0.0001 and the 80% error estimate on the power spectra is 0.626. We note the strong eccentricity cycle back to 900 kyr ago, and its diminishing influence before then. Whereas the temperature record would display this cycle simply because it was tuned to LR04, the lake level record was derived independently of the temperature data14.

Extended Data Figure 3 A contrast in hydroclimate history on the African continent.

The leaf wax δ13C31 record indicates that the Malawi basin became progressively wetter since the Mid-Pleistocene Transition around 900 kyr ago (a), while much of the continent to the north of Lake Malawi maintained a trend towards drier conditions over the past three million years or more, as indicated by soil carbonate δ13C values in northern Tanzania, Kenya and Ethiopia (summarized in ref. 47) (b) and in marine sediment records of terrigenous dust input from northern Africa48, as shown in ODP Sites 721 and 722 from the Gulf of Aden (c).

Extended Data Figure 4 Accepted (blue) and rejected (orange) temperatures and BIT data.

Acceptance criteria are explained in the Methods.

Extended Data Figure 5 δ13C of the C29, C31, C33 n-alkanes.

δ13C of the C29, C31 and C33 n-alkanes (a, b, c), and the weighted mean average (WMA) of these values (d).

Extended Data Figure 6 Aligning the temperature record to LR04 to refine the age model.

a, Age versus depth for drill site MAL05-1, depicting ages based on 14C, tephra, magnetic reversals and alignment (tuning) of corrected temperature with LR04. The dashed pink line is a linear fit through the dates derived from radiocarbon, tephra and magnetic reversals only, described by the equation: Age (in kyr before present) = −12.44 + 3.602z (r2 = 0.9984). b, Blackman–Tukey analysis (spectral power versus per metre) of the corrected temperature data in the upper 200 m of drill site MAL05-1, showing a 33.6-m cycle, which corresponds to about 121 kyr. Bandwidth is 0.0001 and 80% error estimate on the power spectrum is 0.626. c, Temperature plotted against age based solely on radiocarbon, tephra and magnetic reversal dates aligned to the LR04 age scale (green dashed lines), in order to assign ages in MAL05-1 between 75 kyr ago (Toba ash horizon) and 590 kyr ago (Ar–Ar). The red lines depict tephra and magnetic reversal ages, which constrain the temperature alignment. B/M, Brunhes–Matuyama; UJ, Upper Jaramillo; LJ, Lower Jaramillo; UCM, Upper Cobb Mountain; LCM, Lower Cobb Mountain. Data are from ref. 14.

Extended Data Figure 7 The Indian Ocean west-minus-east gradient in SST since 130 kyr ago.

Alkenone records of SST in the western Indian Ocean (core MD85668: 0° 01′ N, 46° 02′ E) (a)30 and the eastern Indian Ocean (core GeoB 10038-4: 5° 56.25′ S, 103° 14.76′ E) (b)29. The west-minus-east temperature gradient (IOD) derived from these two records is displayed in c.

Extended Data Table 1 Locations of the cores analysed in this study
Extended Data Table 2 Sediment dates that underlie the age model of this study

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Johnson, T., Werne, J., Brown, E. et al. A progressively wetter climate in southern East Africa over the past 1.3 million years. Nature 537, 220–224 (2016). https://doi.org/10.1038/nature19065

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