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North Pacific seasonality and the glaciation of North America 2.7 million years ago


In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitch's orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the role of moisture supply in the initiation of large Northern Hemisphere ice sheets has remained unclear. The subarctic Pacific Ocean represents a significant source of water vapour to boreal North America, but it has been largely overlooked in efforts to explain Northern Hemisphere glaciation. Here we present alkenone unsaturation ratios and diatom oxygen isotope ratios from a sediment core in the western subarctic Pacific Ocean, indicating that 2.7 million years ago late-summer sea surface temperatures in this ocean region rose in response to an increase in stratification. At the same time, winter sea surface temperatures cooled, winter floating ice became more abundant and global climate descended into glacial conditions. We suggest that the observed summer warming extended into the autumn, providing water vapour to northern North America, where it precipitated and accumulated as snow, and thus allowed the initiation of Northern Hemisphere glaciation.

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Figure 1: Palaeoceanographic data and model time series through the time interval marking the onset of major Northern Hemisphere glaciation.
Figure 2: Output for two equilibrium states of the CLIMBER-2 Earth system model.
Figure 3: Simulated area of permanent snow cover (shaded) for the ‘cold orbit’ configuration in the destratified (a) and stratified (b) equilibria.

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We thank M. Sarnthein, H. Thierstein, M. Zhao and S. Honjo for discussions. J. Barron, J. Onodera and K. Takahashi provided insight on diatoms C. marginatus and C. radiatus and their seasonal fluxes in the North Pacific, and H. Sloane helped with the diatom δ18O analyses. We thank the Ocean Drilling Program (ODP) and the scientific party and crew of ODP Leg 145 for their efforts in the drilling of Site 882. This work was supported by the Deutsche Forschungsgemeinschaft (DFG), Schweizer Nationalfonds (SNF), the US National Science Foundation (NSF) and BP and the Ford Motor Company through the Princeton University Carbon Mitigation Initiative.

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Correspondence to Gerald H. Haug.

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Haug, G., Ganopolski, A., Sigman, D. et al. North Pacific seasonality and the glaciation of North America 2.7 million years ago. Nature 433, 821–825 (2005).

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