N₂O record spanning the penultimate deglaciation from the Vostok ice core

Nitrous oxide (N₂O) is an important atmospheric trace gas whose concentration is determined by numerous sources and sinks. Over the past 200 years, N₂O has increased by 17±3% due primarily to increased anthropogenic emissions. Over the last glacial/interglacial transition, data from ice cores show that N₂O increased by 35%. This increase may be related to increased terrestrial N₂O emissions resulting from a global climate amelioration. Here I describe a new dry extraction technique for measuring the elemental and isotopic composition of N₂O in ice core samples. Results from a shallow GISPII ice core spanning the last 200 years provide an estimate for the preanthropogenic N₂O concentration of 266±4 ppb. In addition, 11 ice samples from the last interglacial period show fairly uniform N₂O values (269±9 ppb). During the penultimate glacial period (131-160 ka), N₂O concentrations of air trapped in the glacier fluctuated between 190 ppb and 320 ppb, while CO₂ and CH₄ concentrations were relatively low and stable. Two local N₂O maxima were observed during the penultimate glacial period that actually exceeded "normal" interglacial N₂O values (∼265 ppb). The maxima were located in portions of the core which exhibit both elevated dust concentrations and elevated bacterial counts. The δ¹⁵N and δ¹⁸O of N₂O in this portion of the Vostok core were15‰ higher and 12‰ lower than during the warm Eemian period, respectively. The isotopic data and coincidence between the elevated N₂O concentrations and the high bacterial counts may be related to insitu N₂O production by bacteria. The lower δ¹⁸O values are consistent with the isotopic composition of N₂O produced by in-situ nitrification.