Climate driven changes in nitrate deposition and preservation over the West Antarctic Ice Sheet

Thesis (Master's)--University of Washington, 2018Nitrate preserved in ice core records has the potential to be used as a proxy for historic atmospheric NO x concentrations and oxidant abundances. Nitrate deposited to ice sheets can undergo photolysis to NO x , oxidation, and re-deposition. It is necessary to understand nitrate photolysis and recycling to understand the origin of nitrate preserved in ice cores. We use snow pit observations of nitrate concentration and δ 15 N(NO − 3 ) to assess a snow chemistry model. Using the model we explore the impact of various snow and atmospheric properties on the preservation and recycling of nitrate. We use the model to calculate the amount of nitrate lost from the snow pack, deposition flux of nitrate from NO x source regions, and the fraction recycled nitrate within the snow in the present day. − 15 We report measurements of δ 15 N(NO − 3 ) from the WAIS Divide deep ice core. δ N(NO 3 ) was 6±2 ‰ in the Holocene, 26±8 ‰ during the glacial-Holocene transition period, and 34 ± 5 ‰ during the last glacial period. Comparisons between δ 15 N(NO − 3 ) from WAIS Divide and a Green- land ice core show δ 15 N(NO − 3 ) has an inverse relationship with temperature for both regions though temperatures, snow accumulation rates, and dust concentrations were very different for each loca- tion. The primary deposition flux of nitrate from NO x source regions at WAIS Divide is calculated to be 129±14 μmoles nitrate m −2 yr −1 in the Holocene, 142 ± 14 μmoles nitrate m −2 yr −1 during the glacial-Holocene transition, and 121 ± 35 μmoles nitrate m −2 yr −1 during the last glacial pe- riod. Modeling at glacial conditions shows changes in snow accumulation rate and light absorbing particulate concentrations alone cannot account for the observed changes in δ 15 N(NO − 3 ) betweenthe Holocene and glacial periods. We attempt to calculate the fraction of preserved nitrate that is recycled over the ice sheets but find that we cannot without further constraints on the fraction of snow-sourced NO x that is either exported from or re-deposited to the ice sheet