Impacts of the photo-driven post-depositional processing on snow nitrate and its isotopes at Summit, Greenland: a model-based study

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 (delta N-15 and Delta O-17) 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 snow-pack 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 delta N-15 seasonality, while seasonal variations in delta N-15 of primary nitrate are needed to reconcile the timing of the lowest seasonal delta N-15. In contrast, post-depositional processing can only induce less than 2.1% seasonal Delta O-17 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 delta N-15 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 delta N-15 variability in ice-core nitrate.

Automated summary

The study suggests that post-depositional processing of snow nitrate can cause significant redistribution of nitrate in the upper snow-pack, with up to 21% loss and/or redistribution after deposition. However, at the annual scale, the net effect of post-depositional processing on atmospheric signals is relatively small under current Summit conditions. Nevertheless, at longer timescales where snow accumulation rates change significantly, post-depositional processing could become a major driver of delta N-15 variability in ice-core nitrate.