High‐resolution ammonium ice core record covering a complete glacial‐interglacial cycle

Abstract

High‐resolution ammonium measurements were performed along the Greenland Ice Core Program (GRIP) deep ice core, covering a complete climatic cycle. No overall anthropogenic increase is observed over the last 300 years; however, springtime concentrations have roughly doubled since 1950. Biomass burning is estimated to be a major source for ammonia emissions for preindustrial times. It contributes between 10% to 40% to the total ammonium deposited on the central Greenland ice sheet during the Holocene. No correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern North America, which is considered to be the main source area for ammonium deposited on the central Greenland ice sheet. This suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core. If unchanged meteorological conditions are assumed for the Holocene, as indicated by the δ18O ice record, a decreasing biomass burning activity toward present time can be derived from the ammonium ice record. Soil and vegetation emissions are responsible for the ammonium background concentrations in the ice. The record therefore may be used to trace back the biomass history of the North American continent. A pronounced decreasing trend in background ammonium is found during the Holocene, reflecting decreasing temperature and therefore lower NH3 emissions in the source region. Variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength, which can be related to the extent of the Laurentide ice sheet. The data suggest that the Laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age.

Ice sheets and nitrogen

“…The lack of an increase has to be treated a little cautiously, not only because of the noisy signal, but also because the conversion of ammonia to aerosol, and therefore its deposition efficiency, both en route and in Greenland, will have altered as sulfate and nitrate have increased. Nonetheless, at face value, this suggests that Greenland in summer (which dominates the annual budget) is still seeing ammonium from undisturbed (northern) North American ecosystems [34] or from marine sources. Unsurprisingly, ammonium concentrations in Antarctic snow show no anthropogenic trend.…”

“…For ammonium, there is no clear recent trend in Greenland [34], although this would have to be discerned from a noisy background, that is largely owing to the dominance of biomass burning spikes in the record [35]. By splitting the annual snowfall into approximately monthly slices, it appears as if an upward trend since 1950 can be observed in the winter/ spring concentrations [34], when Greenland sees a greater proportion of airmasses from Eurasia [36], but not in the summer, when airmasses are most frequently from North America.…”

“…By splitting the annual snowfall into approximately monthly slices, it appears as if an upward trend since 1950 can be observed in the winter/ spring concentrations [34], when Greenland sees a greater proportion of airmasses from Eurasia [36], but not in the summer, when airmasses are most frequently from North America. The lack of an increase has to be treated a little cautiously, not only because of the noisy signal, but also because the conversion of ammonia to aerosol, and therefore its deposition efficiency, both en route and in Greenland, will have altered as sulfate and nitrate have increased.…”

“…In the recent past (during the Holocene period), the dominant features of Greenland records are sharp spikes, typically just one snowfall wide, of high ammonium concentration that have been firmly associated with biomass burning [34,35]. The identification rests on the co-deposition of a number of chemicals that are all connected to biomass burning: organic acids such as formic [52], and in more recent analyses the burning marker, black carbon, and the biomass burning marker, vanillic acid [51].…”

Ammonium in coastal Antarctic aerosol and snow: Role of polar ocean and penguin emissions

“…First, it was shown that ammonium profiles, extracted from Greenland ice cores, are useful for reconstructing the history of forest fires from the boreal zone in the past [Legrand et al, 1992]. Furthermore, Fuhrer et al [ 1996] demonstrated that the ammonium level of central Greenland snow, free of sporadic biomass burning inputs, is mainly related to continental biogenic emissions from soils, and no evidence for the existence of a significant marine NH.…”