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The time-transgressive termination of the African Humid Period

Abstract

During the African Humid Period about 14,800 to 5,500 years ago, changes in incoming solar radiation during Northern Hemisphere summers led to the large-scale expansion and subsequent collapse of the African monsoon. Hydrologic reconstructions from arid North Africa show an abrupt onset and termination of the African Humid Period. These abrupt transitions have been invoked in arguments that the African monsoon responds rapidly to gradual forcing as a result of nonlinear land surface feedbacks. Here we present a reconstruction of precipitation in humid tropical West Africa for the past 20,000 years using the hydrogen isotope composition of leaf waxes preserved in sediments from Lake Bosumtwi, Ghana. We show that over much of tropical and subtropical Africa the monsoon responded synchronously and predictably to glacial reorganizations of overturning circulation in the Atlantic Ocean, but the response to the relatively weaker radiative forcing during the African Humid Period was more spatially and temporally complex. A synthesis of hydrologic reconstructions from across Africa shows that the termination of the African Humid Period was locally abrupt, but occurred progressively later at lower latitudes. We propose that this time-transgressive termination of the African Humid Period reflects declining rainfall intensity induced directly by decreasing summer insolation as well as the gradual southward migration of the tropical rainbelt that occurred during this interval.

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Figure 1: Comparison of proxy and model estimates of hydrologic variations at Lake Bosumtwi.
Figure 2: Proxy records of West African monsoon variability over the past 20,000 years.
Figure 3: Hydrologic changes across North Africa over the past 20,000 years.

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Acknowledgements

This work was supported by NSF grants EAR0601998, EAR0602355, AGS0402010, ATM0401908, ATM0214525, ATM0096232 and AGS1243125.

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Contributions

Palaeolake-level reconstructions were carried out by T.M.S. and J.T.O. Biomarker analysis was performed by T.M.S. and K.A.H. Analysis of TraCE-21 simulations was conducted by N.P.M. and B.O-B. Field work was conducted by T.M.S., J.T.O., J.P., C.W.H., J.K. and C.A.S. Interpretation was carried out by T.M.S., N.P.M., J.T.O. and K.A.H.

Corresponding author

Correspondence to Timothy M. Shanahan.

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The authors declare no competing financial interests.

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Shanahan, T., McKay, N., Hughen, K. et al. The time-transgressive termination of the African Humid Period. Nature Geosci 8, 140–144 (2015). https://doi.org/10.1038/ngeo2329

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