Abstract Snow nitrate is vulnerable to photolytic loss that causes isotopic alteration, and thus its isotopes can potentially track the extent of snow nitrate photolysis and its impacts in environments where loss is significant. Large increases in δ15N‐NO3− below the snow surface have been attributed to photolysis and this behavior is generally consistent amongst theoretical as well as lab and field studies. Oxygen isotope ratios are thought to be influenced by photolysis as well as secondary condensed‐phase chemistry, but the competing effects have yet to be reconciled. Here we use a model that simulates nitrate burial, photolytic fractionation, and re‐oxidation in snow to quantitatively assess these processes with the aim of developing a consistent framework for interpreting the photolytic effects of the complete nitrate isotopic composition (δ15N, δ18O, and Δ17O). This study reveals that isotopic effects of nitrate photolysis and aqueous‐phase re‐oxidation chemistry are important sources of uncertainties in modeling δ18O‐NO3−.

Here we use high-precision carbon isotope data (δ 13 C-CO 2 ) to show atmospheric CO 2 during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ 13 C-CO 2 during peak glaciation suggests increased ocean carbon storage. Variations in δ 13 C-CO 2 in early MIS 4 suggest multiple processes were active during CO 2 drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO 2 remained low during MIS 4 while δ 13 C-CO 2 fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO 2 at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO 2 during late MIS 4 (Heinrich Stadial 6, ~63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage.

Surface snow and atmospheric samples collected along a traverse from the coast to the ice sheet summit (Dome A) are used to investigate summertime atmospheric production of nitrate (NO3-) across East Antarctica. The strong relationship observed between delta N-15 and delta O-18 of nitrate in the surface snow suggests a large (lesser) extent of nitrate photolysis in the interior (coastal) region. A linear correlation between the oxygen isotopes of nitrate (delta O-18 and Delta O-17) indicates mixing of various oxidants that react with NOx (NOx = NO + NO2) to produce atmospheric nitrate. On the plateau, the isotopes of snow nitrate are best explained by local reoxidation chemistry of NOx, possibly occurring in both condensed and gas phases. Nitrate photolysis results in redistribution of snow nitrate, and the plateau snow is a net exporter of nitrate and its precursors. Our results suggest that while snow-sourced NOx from the plateau due to photolysis is a significant input to the nitrate budget in coastal snow (up to similar to 35%), tropospheric transport from mid-low latitudes dominates (similar to 65%) coastal snow nitrate. The linear relationship of delta O-18 vs. Delta O-17 of the snow nitrate suggests a predominant role of hydroxyl radical (OH) and ozone (O-3) in nitrate production, although a high Delta O-17(O-3) is required to explain the observations. Across Antarctica the oxygen isotope composition of OH appears to be dominated by exchange with water vapor, despite the very dry environment. One of the largest uncertainties in quantifying nitrate production pathways is the limited knowledge of atmospheric oxidant isotopic compositions. (C) 2018 Elsevier Ltd. All rights reserved.

Snowpits along a traverse from coastal East Antarctica to the summit of the ice sheet (Dome Argus) are used to investigate the post-depositional processing of nitrate (NO3−) in snow. Seven snowpits from sites with accumulation rates between 24 and 172 kg m−2 a−1 were sampled to depths of 150 to 300 cm. At sites from the continental interior (low accumulation, < 55 kg m−2 a−1), nitrate mass fraction is generally > 200 ng g−1 in surface snow and decreases quickly with depth to < 50 ng g−1. Considerably increasing values of δ15N of nitrate are also observed (16–461 ‰ vs. air N2), particularly in the top 20 cm, which is consistent with predicted fractionation constants for the photolysis of nitrate. The δ18O of nitrate (17–84 ‰ vs. VSMOW (Vienna Standard Mean Ocean Water)), on the other hand, decreases with increasing δ15N, suggestive of secondary formation of nitrate in situ (following photolysis) with a low δ18O source. Previous studies have suggested that δ15N and δ18O of nitrate at deeper snow depths should be predictable based upon an exponential change derived near the surface. At deeper depths sampled in this study, however, the relationship between nitrate mass fraction and δ18O changes, with increasing δ18O of nitrate observed between 100 and 200 cm. Predicting the impact of post-depositional loss, and therefore changes in the isotopes with depth, is highly sensitive to the depth interval over which an exponential change is assumed. In the snowpits collected closer to the coast (accumulation > 91 kg m−2 a−1), there are no obvious trends detected with depth and instead seasonality in nitrate mass fraction and isotopic composition is found. In comparison to the interior sites, the coastal pits are lower in δ15N (−15–71 ‰ vs. air N2) and higher in δ18O of nitrate (53–111 ‰ vs. VSMOW). The relationships found amongst mass fraction, δ15N, δ18O and Δ17O (Δ17O = δ17O–0.52 × δ18O) of nitrate cannot be explained by local post-depositional processes alone, and are instead interpreted in the context of a primary atmospheric signal. Consistent with other Antarctic observational and modeling studies, the isotopic results are suggestive of an important influence of stratospheric ozone chemistry on nitrate formation during the cold season and a mix of tropospheric sources and chemistry during the warm season. Overall, the findings in this study speak to the sensitivity of nitrate isotopic composition to post-depositional processing and highlight the strength of combined use of the nitrogen and oxygen isotopes for a mechanistic understanding of this processing.

The nitrogen and oxygen isotopic composition of nitrate in ice cores offers unique potential for examining reactive nitrogen oxide (NOx) budgets and oxidation chemistry of past atmospheres. A low‐latitude record is of particular interest given that the dominant natural sources of NOx and production of hydroxyl radical are most prevalent in the tropics. Any interpretation of nitrate in ice cores, however, must first consider that nitrate in snow is vulnerable to postdepositional loss and isotopic alteration. We report and assess the integrity of nitrate–δ15N, –δ18O, and –Δ17O in a 30 m ice core from a high‐elevation site in the central Andes. Clear seasonality in δ15N, δ18O, and nitrate concentration exists throughout most of the record and cannot be explained by photolysis or evaporation based on our current understanding of these processes. In contrast, nitrate in the upper ~12 m of the core and in a snowpit shows very different behavior. This may reflect alteration facilitated by recent melting at the surface. The relationships between δ15N, δ18O, Δ17O, and concentration in the unaltered sections can be interpreted in terms of mixing of nitrate from discrete sources. Transport effects and an englacial contribution from nitrification cannot be ruled out at this time, but the observed isotopic compositions are consistent with expected signatures of known NOx sources and atmospheric oxidation pathways. Specifically, nitrate deposited during the wet season reflects biogenic soil emissions and hydroxyl/peroxy radical chemistry in the Amazon, while dry season deposition reflects a lightning source and ozone chemistry at higher levels in the troposphere.

UNLABELLED: PREMISE OF THE STUDY:Around the world, tropical glaciers and ice caps are retreating at unprecedented rates because of climate change. In at least one location, along the margin of the Quelccaya Ice Cap in southeastern Peru, ancient plant remains have been continually uncovered since 2002. We used genetic analysis to identify plants that existed at these sites during the mid-Holocene. • METHODS:We examined remains between 4576 and 5222 yr old, using PCR amplification, cloning, and sequencing of a fragment of the chloroplast trnL intron. We then matched these sequences to sequences in GenBank. • KEY RESULTS:We found evidence of at least five taxa characteristic of wetlands, which occur primarily at lower elevations in the region today. • CONCLUSIONS:A diverse community most likely existed at these locations the last time they were ice-free and thus has the potential to reestablish with time. This is the first genetic analysis of vegetation uncovered by receding glacial ice, and it may become one of many as ancient plant materials are newly uncovered in a changing climate.

Radiocarbon dating of well-preserved, in-place vegetation exposed by the retreating Quelccaya Ice Cap of southeastern Peru constrains the last time the ice cap's extent was smaller than at present. Seventeen plant samples from two sites along the central western margin collectively date to 4700 and 5100 cal yr BP and strongly indicate that current ice cap retreat is unprecedented over the past similar to 5 millennia. Seventeen vegetation samples interbedded in a nearby clastic sedimentary sequence suggest ice-free conditions at this site from similar to 5200 to at least similar to 7000 cal yr BP, and place minimum constraint on early- to mid-Holocene ice cap extent. (C) 2009 University of Washington. Published by Elsevier Inc. All rights reserved.

A study of 42 surface sediment samples from the northwestern Weddell Sea documents the relationship between the modern distribution of diatom assemblages and present oceanographic conditions in the region. Three diatom assemblages were identified using R-mode principal components analysis and cluster analysis, while Q-mode cluster analysis was used to distinguish two geographically distinct biofacies. Based on the relative abundance of diatom species, the biofacies of the former Larsen A Ice Shelf (LAIS) is more closely associated with a sea ice diatom assemblage, while the biofacies of the Prince Gustav Channel/Northern Peninsula (PGC/NP) is associated with two open coastal diatom assemblages. This division is consistent with variance observed in the current oceanographic setting, whereby the LAIS is generally characterized by a longer seasonal duration of sea ice coverage and lower primary productivity relative to the PGC/NP which is marked by a shorter seasonal duration of sea ice coverage and greater surface water productivity. This relationship is further reflected in the distribution of the paleoenvironmentally significant diatom species Fragilariopsis curta, Thalassiosira antarctica, and Chaetoceros subg. Hyalochaete spp. as well as in absolute abundance data. Specifically, F. curta and the morphological form T. antarctica T1 are representative of greater seasonal duration of sea ice coverage and lower productivity. Chaetoceros subg. Hyalochaete spp. and the morphological form T. antarctica T2 are associated with decreased seasonal duration of sea ice cover and greater surface water productivity, a setting that is further reflected in high absolute diatom abundances. These findings provide a basis for future paleoceanographic work in the region, especially as studies of surface sediment diatom assemblages are vital to establishing a link between downcore diatom assemblages and those presently found in the water column.