N₂O record spanning the penultimate deglaciation from the Vostok ice core

Nitrous oxide (N 2 O) is an important atmospheric trace gas whose concentration is determined by numerous sources and sinks. Over the past 200 years, N 2 O has increased by 17±3% due primarily to increased anthropogenic emissions. Over the last glacial/interglacial transition, data from ice cores show that N 2 O increased by 35%. This increase may be related to increased terrestrial N 2 O emissions resulting from a global climate amelioration. Here I describe a new dry extraction technique for measuring the elemental and isotopic composition of N 2 O in ice core samples. Results from a shallow GISP II ice core spanning the last 200 years provide an estimate for the preanthropogenic N 2 O concentration of 266±4 ppb. In addition, 11 ice samples from the last interglacial period show fairly uniform N 2 O values (269±9 ppb). During the penultimate glacial period (131–160 ka), N 2 O concentrations of air trapped in the glacier fluctuated between 190 ppb and 320 ppb, while CO 2 and CH 4 concentrations were relatively low and stable. Two local N 2 O maxima were observed during the penultimate glacial period that actually exceeded “normal” interglacial N 2 O values (∼265 ppb). The maxima were located in portions of the core which exhibit both elevated dust concentrations and elevated bacterial counts. The δ 15 N and δ 18 O of N 2 O in this portion of the Vostok core were 15‰ higher and 12‰ lower than during the warm Eemian period, respectively. The isotopic data and coincidence between the elevated N 2 O concentrations and the high bacterial counts may be related to in situ N 2 O production by bacteria. The lower δ 18 O values are consistent with the isotopic composition of N 2 O produced by in‐situ nitrification.