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.