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Abstract Ice core measurements of the concentration and stable isotopic composition of atmospheric nitrous oxide (N₂O) 74,000–59,000 years ago constrain marine and terrestrial emissions. The data include two major Dansgaard‐Oeschger (D‐O) events and the N₂O decrease during global cooling at the Marine Isotope Stage (MIS) 5a‐4 transition. The N₂O increase associated with D‐O 19 (~73–71.5 ka) was driven by equal contributions from marine and terrestrial emissions. The N₂O decrease during the transition into MIS 4 (~71.5–67.5 ka) was caused by gradual reductions of similar magnitude in both marine and terrestrial sources. A 50 ppb increase in N₂O concentration at the end of MIS 4 was caused by gradual increases in marine and terrestrial emissions between ~64 and 61 ka, followed by an abrupt increase in marine emissions at the onset of D‐O 16/17 (59.5 ka). This suggests that the importance of marine versus terrestrial emissions in controlling millennial‐scale N₂O fluctuations varied in time. 

Abstract Carbonyl sulfide (COS) was measured in firn air collected during seven different field campaigns carried out at four different sites in Greenland and Antarctica between 2001 and 2015. A Bayesian probabilistic statistical model is used to conduct multisite inversions and to reconstruct separate atmospheric histories for Greenland and Antarctica. The firn air inversions cover most of the 20th century over Greenland and extend back to the 19th century over Antarctica. The derived atmospheric histories are consistent with independent surface air time series data from the corresponding sites and the Antarctic ice core COS records during periods of overlap. Atmospheric COS levels began to increase over preindustrial levels starting in the 19th century, and the increase continued for much of the 20th century. Atmospheric COS peaked at higher than present‐day levels around 1975 CE over Greenland and around 1987 CE over Antarctica. An atmosphere/surface ocean box model is used to investigate the possible causes of observed variability. The results suggest that changes in the magnitude and location of anthropogenic sources have had a strong influence on the observed atmospheric COS variability.