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Synchronous rise of African C4 ecosystems 10 million years ago in the absence of aridification

Matters Arising to this article was published on 02 July 2020

Abstract

Grasslands expanded globally during the late Cenozoic and the development of these ecosystems shaped the evolution of many faunal groups, including our hominin ancestors. The emergence of these ecosystems has been dated in many regions, but the origins of the iconic African C4 savannah grasslands remain poorly known, as do the causal factors that led to their establishment. Here we document their origins with the distinct carbon isotope signature from the hot-, arid- and low-CO2-adapted C4 grasses that dominate modern savannahs and grasslands. We use the carbon isotope values of leaf-wax molecules in deep-sea drill cores to measure the rise of African C4 ecosystems. We also reconstruct African palaeohydroclimate change from leaf-wax hydrogen isotope values and dust deposition rates in these cores. We find that C4-dominated ecosystems expanded synchronously across Northwestern and East Africa after 10 million years ago. This was not accompanied by substantial changes in palaeohydrology or dust deposition, precluding aridification as a causal factor. The expansion of C4 grasses was coincident, however, with dramatic high-latitude cooling and increased pole–Equator temperature gradients. We suggest that declining atmospheric CO2 levels were a direct cause of the C4 grassland expansion.

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Fig. 1: Latitudinal patterns of modern African rainfall, carbon isotope values of long-chain plant-waxes and vegetation cover.
Fig. 2: Plant-wax records of Neogene vegetation and hydrologic change.
Fig. 3: Evolution of global SST and African dust flux compared with C4 expansion and African hydrologic change.

Data availability

All data presented in this paper are freely available in Supplementary Tables 17.

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Acknowledgements

We thank I. Raffi and J. Backman for assistance with updating nannofossil ages, W. Ryan for discussions and G. Janigian, K. Jackson and N. deRoberts for laboratory assistance. This research used samples and data provided by the IODP. This is Lamont-Doherty Earth Observatory Contribution 8331. This research was supported by the US National Science Foundation through graduate research fellowships to C.R. and S.R.P (grant no. DGE16-44869) and undergraduate student participation through the LDEO intern programme (grant no. OCE13-59194) and by the Center for Climate and Life at Columbia University.

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C.R., P.J.P., K.T.U. and P.deM. conceived the study. C.R., P.J.P., K.T.U. and S.R.P. conducted the investigation. All authors wrote the manuscript. P.J.P. and P.deM. supervised the study.

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Correspondence to Pratigya J. Polissar or Cassaundra Rose.

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Supplementary information

Supplementary Information

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Supplementary Table 1

Site information

Supplementary Table 2

Site 659 composite depth

Supplementary Table 3

Site 659 age model

Supplementary Table 4

Site 959 age model

Supplementary Table 5

Leaf-wax data

Supplementary Table 6

Leaf-wax epsilon

Supplementary Table 7

Site 659 dust flux

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Polissar, P.J., Rose, C., Uno, K.T. et al. Synchronous rise of African C4 ecosystems 10 million years ago in the absence of aridification. Nat. Geosci. 12, 657–660 (2019). https://doi.org/10.1038/s41561-019-0399-2

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