Greenland ice core reconstruction of millennial changes in North American wildfire and soil activity over the last glacial cycle

Abstract:

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 Dansgaard-Oeschger (DO) events. However prior to the last deglaciation, little is known about the response of North American vegetation to such rapid climate changes and especially about the response of biomass burning, an important factor for regional changes in radiative forcing. Here we present continuous, high-resolution ammonium (NH₄⁺) records derived from the NGRIP and GRIP ice cores, where background NH₄⁺ concentrations in the ice document North American ammonia emissions from soils while wildfire derived NH₄⁺ peaks superimposed on the background concentrations allow to reconstruct changes in North American wildfire frequency over the last 110,000 yr.

Soil emissions increased on orbital timescales with warmer climate, related to the northward expansion of vegetation due to reduced ice-covered areas. During Marine Isotope Stage 3 DO warm events, a significantly higher fire recurrence rate is recorded, while ammonia soil emissions rose only slowly during longer interstadial warm periods, in line with slow ice sheet shrinkage and delayed ecosystem changes. Our results indicate that sudden warming events had little impact on NH₄⁺ aerosol production from soil emissions and on NH₄⁺ aerosol transport to Greenland during the glacial but triggered a significant increase in the frequency of fire occurrence.