Variability of nitrogen deposition and preservation over the Greenland Ice Sheet

Abstract

This work represents an analysis of the spatial and temporal variability of nitrate deposition and preservation recorded in multiple ice core and snow pit records from the Greenland ice sheet. The primary objective of this research was to develop improved estimates of variability in the preserved ice core record of nitrate to aid in the interpretation of paleoatmospheric concentrations of reactive nitrogen compounds. Three separate works are presented, each focusing on a unique component of variability. The first is a study related to the direct preservation of nitrate over a single year. The second and third topics are related to analysis of ice core records collected during NASA's Program for Arctic Regional Climate Assessment (PARCA) which was initiated in 1993 and continued through 2002.The first study of preservation demonstrated that nitrate, despite possible post-depositional cycling and alteration, was well preserved throughout the year, such that the total flux measured in a snow pit taken to represent the previous year, was representative of snow surface concentrations during the past year. The small difference in preserved concentrations from observed surface snow concentrations gives evidence of only 7% post-depositional loss at this site (mean annual accumulation ~23 g cm-2 yr-1). Results from these studies indicate that at this site accumulation is the most significant process affecting preservation of nitrate in the firn.In the second study, the temporal variability of preserved nitrate was evaluated through time series analysis and correlation studies with available paleoclimate proxy records. Six Greenland ice cores covering the period 1794-1995 show correlated co-variability of nitrate concentration for periods greater than ten years and a ~60% increase in average concentration during the last 75 years. The changes in concentration yield ~30% higher nitrate flux (2.5 to 3.2 g m-2 a-1) and ~11% greater variability during 1895-1994 period versus the prior 100 years. Nitrate trends in the cores during the last 100 years are also correlated with global nitrate emissions, with an average r-value of 0.93 for the six cores.The last study focused on spatial variability of nitrate, and the relation of deposition to components of the earth system including temperature and accumulation. The objective of the study was to assess the contribution of spatial (latitude, longitude, and elevation) and climate (accumulation and temperature) components to the preserved record. Furthermore, the study evaluated the influence of anthropogenic activities on the spatial distribution of nitrate of the Greenland ice sheet. Large scale spatial variability exists as a result of accumulation gradients, with concentration 5% greater in the northern plateau, yet flux over the northern plateau is 30% lower than the dry snow zone as a whole. While spatially, flux appears to be more dependent on accumulation, preservation of flux shows increasing dependence on concentration with increasing accumulation. The relationship between concentration and accumulation is non-linear, showing less dependence in the low accumulation regions versus high accumulation regions. Accumulation alone is insufficient to account for the observed variability in nitrate flux in the low accumulation regions, and evidence supports an additional component to a transfer function model for nitrate that includes photochemistry, temperature, and possibly sublimation. In high accumulation regions, evidence points to a dilution effect, with concentration decreases resulting from increased accumulation. Flux estimates over the ice sheet are compared with a GEOS-CHEM model estimate of reactive nitrogen vertical fluxes showing the model captures a significant component of the variability in the southern portion of the ice sheet, but under-represents the flux and variability in the northern half of the ice sheet by a factor of 4.