A 110,000-YEAR HISTORY OF CHANGE IN CONTINENTAL BIOGENIC EMISSIONS AND RELATED ATMOSPHERIC CIRCULATION INFERRED FROM THE GREENLAND ICE-SHEET PROJECT ICE CORE

The 110,000-year record of ammonium concentrations from the Greenland Ice Sheet Project 2 (GISP2) ice core provides the basis for an analysi s of terrestrial biological production and atmospheric circulation pat terns involved in the transport of biologically produced ammonium to t he Greenland atmosphere. The directly measured concentration series wa s selected for analysis, rather than that of estimated ammonium flux, after a detailed analysis of the relationship among ice core glacioche mical concentrations and a high-resolution simultaneous record of snow accumulation from the GISP2 core. Analysis of the ammonium concentrat ion series shows that maxima in background levels of ammonium in the G reenland atmosphere are strongly related to and synchronous with summe r forcing associated with the precessional cycle of insolation. Minima in background levels, on the other hand, are delayed relative to mini ma in summer insolation at those times when ice volume is significant. The duration of these delays are similar in magnitude (approximate to 6000 years) to other paleoclimatic responses to changes in ice volume . Decadal and centennial scale variation about background levels of am monium concentration exhibit two modes of behavior when compared to a record of polar atmospheric circulation intensity. During warmer perio ds ammonium transport to Greenland is similar to present patterns. Und er coldest conditions the low levels of ammonium transported to Greenl and are the result of extreme southerly excursions of the predominantl y zonal polar circulation. The rapid transitions (approximate to 200 y ears) between these two climatic conditions appear to be associated wi th a critical volume or extent of the continental ice sheets.