Abstract
Abstract. Atmospheric information embedded in ice-core nitrate is disturbed by post-depositional processing. Here we used a layered snow photochemical column model to explicitly investigate the effects of post-depositional processing on snow nitrate and its isotopes (δ15N and Δ17O) at Summit, Greenland, where post-depositional processing was thought to be minimal due to the high snow accumulation rate. We found significant redistribution of nitrate in the upper snowpack through photolysis, and up to 21 % of nitrate was lost and/or redistributed after deposition. The model indicates post-depositional processing can reproduce much of the observed δ15N seasonality, while seasonal variations in δ15N of primary nitrate are needed to reconcile the timing of the lowest seasonal δ15N. In contrast, post-depositional processing can only induce less than 2.1 ‰ seasonal Δ17O change, much smaller than the observation (9 ‰) that is ultimately determined by seasonal differences in nitrate formation pathway. Despite significant redistribution of snow nitrate in the photic zone and the associated effects on δ15N seasonality, the net annual effect of post-depositional processing is relatively small, suggesting preservation of atmospheric signals at the annual scale under the present Summit conditions. But at longer timescales when large changes in snow accumulation rate occur this post-depositional processing could become a major driver of the δ15N variability in ice-core nitrate.
Highlights
Nitrate (NO−3 ) is one of the most abundant and commonly measured species in ice cores
We denote the nitrate fluxes as “FY” following Erbland et al (2015), with Flux of primary nitrate (Fpri), FP, FD and FA representing the primary nitrate flux from long-range transport, the nitrate flux that originates from photolysis of snow nitrate, the deposition flux (FD) of atmospheric nitrate and the archived snow nitrate flux that is buried under the photic zone, respectively
The results suggest that the photodriven post-depositional processing is active at Summit, causing strong redistribution of snow nitrate accompanied by isotope effects in the photic zone
Summary
Nitrate (NO−3 ) is one of the most abundant and commonly measured species in ice cores. This is close to the estimate of 16 %–23 % loss based on ice-core δ15N(NO−3 ) (Geng et al, 2015) These results are qualitatively consistent with the observations of NO2 and HONO fluxes from snowpack at Summit which were attributed to snow nitrate photolysis (Dibb et al, 2002; Honrath et al, 2002). The spatial variations in the photodriven nitrate recycling at the air–snow interface and its impact on snow δ15N(NO−3 ) in Greenland have been studied by Zatko et al (2016) using a global 3-D chemical transport model (GEOS-Chem) Their model captures the increasing trend in snow δ15N(NO−3 ) from the coast to inland as snow accumulation rate decreases, which enhances the degree of post-depositional processing. A comparison of the model results with observations should add insight into the preservation of nitrate at high snow accumulation sites and shed light on the interpretation of ice-core nitrate and its isotopes
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