Sunlight-driven nitrate loss records Antarctic surface mass balance

Abstract

Standard proxies for reconstructing surface mass balance (SMB) in Antarctic ice cores are often inaccurate or coarsely resolved when applied to more complicated environments away from dome summits. Here, we propose an alternative SMB proxy based on photolytic fractionation of nitrogen isotopes in nitrate observed at 114 sites throughout East Antarctica. Applying this proxy approach to nitrate in a shallow core drilled at a moderate SMB site (Aurora Basin North), we reconstruct 700 years of SMB changes that agree well with changes estimated from ice core density and upstream surface topography. For the under-sampled transition zones between dome summits and the coast, we show that this proxy can provide past and present SMB values that reflect the immediate local environment and are derived independently from existing techniques.

Fig. 1. Schematic diagram of the NO₃⁻ photolytic process in Antarctica.

After NO₃⁻ containing either ¹⁴N (blue) or ¹⁵N (red) is deposited on the Antarctic snowpack surface (1), sunlight in the photic zone can trigger photolysis of NO₃⁻ that favors NO₃⁻ with a ¹⁴N atom, which leaves the residual NO₃⁻ enriched in ¹⁵N (2). Because sites with lower surface mass balance will accumulate less snow over a given period of time than high surface mass balance sites (3), the NO₃⁻ at lower surface mass balance sites will remain in the photic zone longer, experience more photolytic mass loss before burial in the archived zone, and have higher δ¹⁵N_NO3arc values (4).

Fig. 2. The relationship between Antarctic snow δ¹⁵N_NO3arc and surface mass balance (SMB).

a Map of East Antarctic sites sampled for δ¹⁵N_NO3arc along different scientific and logistic transect routes. Colored circles indicate the locations and δ¹⁵N_NO3arc values of samples included in our field data set, with δ¹⁵N_NO3arc data from the EAIIST (pink) and CHICTABA (yellow) transects newly reported here. The base map SMB data were modeled by MAR and adjusted for dry site bias (see Methods) with elevation contours from REMA overlaid. Preservation of NO₃⁻ is not expected in blue ice zones (gray solid polygons) due to very low or negative SMB and wind scouring. Presently occupied stations in the CONMAP database are shown as labeled triangle icons for spatial reference. b Scatter plot of δ¹⁵N_NO3arc vs. SMB for all sites in the field dataset. The color of the points corresponds to the transects where the samples were collected as shown in a, and the shape of the points corresponds to the sampling method (i.e., snow core, snow pit, or 1-m depth layer). c Scatter plot and linear regression of (1) using all sites in the field dataset. The linear regression (gray solid line) is shown with shaded 95% confidence intervals, and regression parameters are displayed at lower left.

Fig. 3. δ¹⁵N_NO3arc values modeled by (1) across East Antarctica based on surface mass balance (SMB).

The spatial variability of δ¹⁵N_NO3arc values across East Antarctica are modeled by applying the field data regression of ln(δ¹⁵N_NO3arc + 1) vs. SMB⁻¹ to the 1979–2015 mean SMB output (35 km resolution) from the MAR, adjusted for dry site bias (see Methods). Values of δ¹⁵N_NO3arc are undefined (gray) at some locations near the coast with very low or negative SMBs due to high sublimation and wind scouring. Preservation of NO₃⁻ is not expected in these locations, which often correspond to blue ice zones (blue polygons, zones with >100 km² extent shown). Samples of δ¹⁵N_NO3arc from the field database are illustrated by colored circles with the same color gradient as the modeled δ¹⁵N_NO3arc values. Regions with SMB less than or greater than 40–200 kg m⁻² a⁻¹ (i.e., the SMB range targeted by the δ¹⁵N_NO3arc proxy described here) are illustrated with hatching and crosses, respectively. Presently occupied stations in the CONMAP database are shown as triangle icons for spatial reference, and the Aurora Basin North (ABN) site is indicated with a red star.

Fig. 4. Reconstructions of surface mass balance (SMB) for an Antarctic ice core from ABN.

a SMB for Aurora Basin North based on δ¹⁵N_NO3arc data from the ABN1314-103 ice core. Reconstructed SMB_δ15N values are shown by the red stepped lines with the 50-yr running mean±1σ overlaid as a darker thick line and shaded zone. b Comparison of SMB values reconstructed from δ¹⁵N_NO3arc (red) with those from ice density (gray) and upstream GPR isochron depth. The SMB_δ15N and SMB_GPR values were aggregated to match the 1-m resolution of the SMB_density data. For SMB_δ15N and SMB_density, smoothed LOESS curves are overlaid to more clearly show long-term patterns. c SMB_δ15N values after the upstream topographic impact on SMB has been removed, with 50-yr running mean±1σ values overlaid. The resulting residuals may better illustrate SMB variability due to climate change.

Fig. 5. Topography and accumulation patterns upstream of the Aurora Basin North (ABN) drill site.

a Local surface topography of the ice sheet around the ABN ice core drilling site, shown as a hillshade derived from the REMA digital elevation model with 100x vertical exaggeration. Ground-penetrating radar measurements were taken along a 60 km transect upstream of the drill site relative to local ice sheet flow, and the ice contained in the ABN1314-103 core corresponds to the first 11.5 km of the transect. b Local accumulation rate variability with depth along the upstream ABN transect determined from radar identification of isochronal internal reflective horizons, reflecting past changes in surface mass balance. Regions of relatively higher or lower accumulation preserved with depth likely represent the influence of long-lived surface topographic features. Accumulation rates have an original depth resolution of 0.5 m which is smoothed through a moving age-depth average with a cosine weighting window to reduce isochron artifacts.