We have analyzed the soluble portion of impurities trapped in solid precipitation that accumulated at Summit (central Greenland) from 1193 A.D. to the present. Seventy-three ice layers show elevated concentrations of ammonium and formate, caused by high-latitude biomass burning debris reaching Greenland. While a mixture of ammonium and formate close to the molar ratio is generally observed in these ice layers, a large depletion of formate relative to ammonium is found in a few cases. The chemical composition of such layers indicates the presence of a mixture of ammonium, formate, and nitrate with a NH4+/(HCOO+NO3-) molar ratio close to 1. These differences may be related to the fire type (flaming versus smoldering) or to meteorological conditions encountered by plumes during their transport toward Greenland. The high-resolution ammonium and formate profiles are used to reconstruct the frequency and the intensity of high-latitude biomass burning input having reached central Greenland since 1193 A.D. Three periods of enhanced biomass burning input over central Greenland are identified: 1200--1350 A.D., 1830--1930 A.D., and to a lesser extent 1500--1600 A.D. The 1200--1350 A.D. time period coincides with warm and dry conditions which characterized the Medieval Warm Period. After a period of infrequent biomass burning input during the coldest period of the Little Ice Age (1600--1850 A.D.), the frequency was enhanced at the turn of the last century and then decreased throughout this last century. Aside from high-latitude biomass burning, the background levels of formate show a slight and persistent decreasing trend over the last 800 years probably reflecting the deterioration of the boreal vegetation from North America. ¿ 1998 American Geophysical Union