Millennial changes in North American wildfire and soil activity over the last glacial cycle


Climate changes in the North Atlantic region during the last glacial cycle were dominated by the slow waxing and waning of the North American ice sheet as well as by intermittent, millennial-scale Dansgaard–Oeschger climate oscillations. However, prior to the last deglaciation, the responses of North American vegetation and biomass burning to these climate variations are uncertain. Ammonium in Greenland ice cores, a product from North American soil emissions and biomass burning events, can help to fill this gap. Here we use continuous, high-resolution measurements of ammonium concentrations between 110,000 to 10,000 years ago from the Greenland NGRIP and GRIP ice cores to reconstruct North American wildfire activity and soil ammonium emissions. We find that on orbital timescales soil emissions increased under warmer climate conditions when vegetation expanded northwards into previously ice-covered areas. For millennial-scale interstadial warm periods during Marine Isotope Stage 3, the fire recurrence rate increased in parallel to the rapid warmings, whereas soil emissions rose more slowly, reflecting slow ice shrinkage and delayed ecosystem changes. We conclude that sudden warming events had little impact on soil ammonium emissions and ammonium transport to Greenland, but did result in a substantial increase in the frequency of North American wildfires.

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Figure 1: Climate and environmental changes recorded in Greenland ice cores.
Figure 2: High-resolution records of NH4+ soil emissions and wildfire activity in NA during MIS2 and MIS3.
Figure 3: High-resolution records of NH4+ soil emissions and wildfire activity in NA during MIS4 to MIS5b.
Figure 4: High-resolution records of NH4+ soil emissions and wildfire activity in North America during the last glacial/interglacial transition.


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The authors of this paper are indebted to the late D. Wagenbach, who contributed to and inspired this research in numerous discussions. This paper has also greatly benefited from the Sir Nicholas Shackleton fellowship, Clare Hall, University of Cambridge, UK, awarded to H.F. in 2014. The Division for Climate and Environmental Physics, Physics Institute, University of Bern acknowledges the long-term financial support of ice core research by the Swiss National Science Foundation (SNSF) and the Oeschger Centre for Climate Change Research. E.W.W. is supported by a Royal Society professorship. NGRIP is directed and organized by the Department of Geophysics at the Niels Bohr Institute for Astronomy, Physics and Geophysics, University of Copenhagen. It is supported by funding agencies in Denmark (SNF), Belgium (FNRS-CFB), France (IPEV and INSU/CNRS), Germany (AWI), Iceland (RannIs), Japan (MEXT), Sweden (SPRS), Switzerland (SNSF) and the USA (NSF, Office of Polar Programs).

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M.B. and R.R. performed the CFA measurements in the field at NGRIP, and together with S.S. carried out raw data analysis. H.F. developed the time series analysis approach, and together with R.M. and E.W.W. developed the concept for reconstruction of atmospheric concentrations. G.G. provided the back-trajectory analysis used in the transport model, T.E. contributed to the deposition model. All authors discussed the results and contributed to the interpretation and to the manuscript, which was written by H.F.

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Correspondence to Hubertus Fischer.

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Fischer, H., Schüpbach, S., Gfeller, G. et al. Millennial changes in North American wildfire and soil activity over the last glacial cycle. Nature Geosci 8, 723–727 (2015).

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