The Isotopic Composition of Nitrate in West Antarctica at Present and Since the Last Glacial Stage

Abstract:

Nitrate is one of the major ions found in polar and alpine snow. The oxygen isotopic composition of nitrate offers unique potential for examining the oxidation chemistry of past atmospheres. Additionally, nitrogen isotope ratios may contain information abut the contribution of the nitrogen oxide precursors (NO_x = NO + NO₂) to atmospheric nitrate from different sources (e.g., fossil fuel combustion, biomass burning, soil microbial emissions, lightning and stratospheric injection). Nitrate in snow, however, is sensitive to post-depositional processing and isotopic alteration, thereby obscuring the atmospheric record ultimately archived in an ice core. At sites with very low snow accumulation rates (such as East Antarctica), nitrate is particularly vulnerable to photolytic loss due to long residence times near the surface. However, under higher accumulation regimes (such as Summit, Greenland), previous work has shown that loss can be more limited and nitrate isotopic composition preserved. Here we present results from a two-part study assessing the modern and paleo isotopic composition of nitrate in West Antarctica. We present seasonally-resolved snowpit and shallow core records from 7 West Antarctic sites which span a range of accumulation rates in order to evaluate the spatial heterogeneity of deposited nitrate and how preservation varies with snowfall. This work is requisite to an accurate interpretation of a new nitrate isotopic record from the West Antarctic Ice Sheet Divide deep ice core, from which we show decadal- to centennial-scale measurements since the last glacial stage.