Variability of sulfate signal in ice core records based on five replicate cores

International audienceCurrent volcanic reconstructions based on ice core analysis have significantly improved over the past few decades by incorporating multiple-core analyses with a high temporal resolution from different parts of the polar regions into a composite common volcanic eruption record. Regional patterns of volcanic deposition are based on composite records, built from cores taken at both poles. However, in many cases only a single record at a given site is used for these reconstructions. This assumes that transport and regional meteorological patterns are the only source of the dispersion of the volcanic products. Here we evaluate the local-scale variability of a sulfate profile in a low-accumulation site (Dome C, Antarctica), in order to assess the representativeness of one core for such a reconstruction. We evaluate the variability with depth, statistical occurrence, and sulfate flux deposition variability of volcanic eruptions detected in five ice cores, drilled 1 m apart from each other. Local-scale variability, essentially attributed to snow drift and surface roughness at Dome C, can lead to a non-exhaustive record of volcanic events when a single core is used as the site reference , with a bulk probability of 30 % of missing volcanic events and close to 65 % uncertainty on one volcanic flux measurement (based on the standard deviation obtained from a five-core comparison). Averaging n records reduces the uncertainty of the deposited flux mean significantly (by a factor 1/ √ n); in the case of five cores, the uncertainty of the mean flux can therefore be reduced to 29 %

The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin

Reactive nitrogen (Nr = NO, NO2, HONO) and volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140 ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic flux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and midlatitude snow, but snow-sourced reactive nitrogen fluxes in mid-latitude regions have not yet been quantified in the field. Here we present vertical profiles of snow nitrate concentration and nitrogen isotopes (δ15N) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic flux in snow and the production of Nr from the photolysis of snow nitrate. The observed δ15N(NO−3) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of Nr to the overlying boundary layer. Snow-surface δ15N(NO−3) measurements range from −5 to 10 ‰ and suggest that the local nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NOx sources from beyond the basin are dominant. Modeled daily averaged snow-sourced Nr fluxes range from 5.6 to 71 × 107 molec cm−2s−1 over the course of the field campaign, with a maximum noontime value of 3.1 × 109 molec cm−2s−1. The top-down emission estimate of primary, anthropogenic NOx in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NOx emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen fluxes are minor contributors to the Nr boundary layer budget in the highly polluted Uintah Basin boundary layer during winter 2014

Contraintes isotopiques sur l'interprétation de l'enregistrement en nitrate dans la carotte de glace de Vostok

Nitrate ions (NO3-) found in Antarctic snows stem from the degradation of nitrogen oxydes (NOx = NO + NO2) in the atmosphere. At sites with low snow accumulation rates such as Vostok or Dome C (East Antarctic plateau), nitrate deposition to the snow is not irreversible and this strongly hampers the interpretation of nitrate concentration records in ice cores. Nitrogen stable isotopic (δ15N) as high as +339‰ were measured in nitrate in the upper firn at Dome C and have been attributed to nitrate photolysis initiating a strong recycling at the snow surface. The oxygen isotopic anomaly (Δ17O) reflects the activity of ozone (O3) in nitrate formation. We present the first comprehensive isotopic analysis of nitrate (δ15N, Δ17O and δ18O) in a deep ice core. 64 samples of nitrate from the Vostok ice core have been analyzed and cover the last 150 000 years. This dataset has been completed with 313 samples recently collected in the atmosphere/surface hoar/snow continuum at Dome C as well as in several snowpits from various sites covering most of the East Antarctica. Those present-day samples are used to evaluate a conceptual model (named TRANSITS) developped during this PhD and which aims at representing nitrate recycling at the snow/atmosphere interface and at modelling its impact on the isotopic composition of the archived nitrate. High positive δ15N values measured in the Vostok ice core reveal that nitrate recycling has always occurred at the surface of the Antarctic plateau over this period. Past variations of the primary flux of nitrate to the Vostok site have been estimated using the TRANSITS model. They show that glacials are characterized by higher inputs which may be linked to a greater stratospheric denitrification. The Δ17O values indicate that intrusions of stratospheric air masses to the troposphere may have been more frequent in glacials thus incorporating significant amounts of stratospheric ozone to the lower atmosphere. Last, we suggest that this study may have some relevance to the coastal nitrogen budget in Antarctica and to the interpretation of ice cores retrieved from high accumulation sites (e.g. in Greenland).L'ion nitrate (NO3-) présent dans les neiges Antarctiques est issus de l'oxydation des oxydes d'azote (NOx = NO + NO2) dans l'atmosphère. Aux sites de faible accumulation de neige tels que Vostok et Dôme C sur le plateau de l'Est de l'Antarctique, le nitrate n'est pas piégé de manière ultime dans la neige, ce qui limite fortement l'interprétation des enregistrements en nitrate dans les carottes de glace. Des mesures récentes de la composition isotopique en azote du nitrate (δ15N) montrent des valeurs extrêmement élevées (+339‰) dans les premiers décimètres de la neige de Dôme C. Ces valeurs ont été attribuées à la photolyse du nitrate et au recyclage important qui en résulte. Le nitrate possède, par ailleurs, une anomalie isotopique en oxygène (Δ17O) qui permet de tracer l'activité de l'ozone (O3) au cours de sa formation. Ce travail de thèse présente le premier enregistrement de la composition isotopique complète du nitrate (δ15N, Δ17O et δ18O) dans une carotte de glace : la carotte de glace de Vostok dont les 64 échantillons analysés couvrent une période de 150 000 ans. Ce jeu de données a été complété par 313 échantillons collectés entre 2007 et 2010 dans le continuum atmosphère/givre/neige au Dôme C ainsi que dans 21 puits de neige prélevés dans une zone couvrant l'essentiel de l'Est de l'Antarctique. L'analyse isotopique de ces échantillons modernes a permis de contraindre le modèle conceptuel TRANSITS développé au cours de cette thèse et dont le but est de représenter le recyclage du nitrate à l'interface entre l'atmosphère et la neige ainsi que son impact sur la composition isotopique du nitrate archivé. Les valeurs positives et élevées du δ15N du nitrate piégé dans la carotte de glace de Vostok montrent que le recyclage du nitrate a toujours eu lieu sur le plateau Antarctique au cours de la période étudiée. Les variations du flux primaire de nitrate reçu au site de Vostok estimées à l'aide du modèle TRANSITS montrent, en périodes glaciaires, un flux primaire plus important qui pourrait être lié à une dénitrification stratosphérique plus conséquente. Les valeurs de Δ17O du nitrate montrent que l'incursion d'ozone d'origine stratosphérique dans la troposphère était plus fréquente en périodes glaciaires. Nous proposons enfin que les résultats acquis dans le cadre de cette thèse pourraient permettre de mieux contraindre le cycle de l'azote sur la côte Antarctique et d'apporter des éléments d'interprétation des enregistrements en nitrate dans les carottes de glace de sites de plus forte accumulation de neige (au Groenland par exemple)

Isotopic constrains on the interpretation of the nitrate record in the Vostok ice core

L'ion nitrate (NO3-) présent dans les neiges Antarctiques est issus de l'oxydation des oxydes d'azote (NOx = NO + NO2) dans l'atmosphère. Aux sites de faible accumulation de neige tels que Vostok et Dôme C sur le plateau de l'Est de l'Antarctique, le nitrate n'est pas piégé de manière ultime dans la neige, ce qui limite fortement l'interprétation des enregistrements en nitrate dans les carottes de glace. Des mesures récentes de la composition isotopique en azote du nitrate (δ15N) montrent des valeurs extrêmement élevées (+339‰) dans les premiers décimètres de la neige de Dôme C. Ces valeurs ont été attribuées à la photolyse du nitrate et au recyclage important qui en résulte. Le nitrate possède, par ailleurs, une anomalie isotopique en oxygène (Δ17O) qui permet de tracer l'activité de l'ozone (O3) au cours de sa formation. Ce travail de thèse présente le premier enregistrement de la composition isotopique complète du nitrate (δ15N, Δ17O et δ18O) dans une carotte de glace : la carotte de glace de Vostok dont les 64 échantillons analysés couvrent une période de 150 000 ans. Ce jeu de données a été complété par 313 échantillons collectés entre 2007 et 2010 dans le continuum atmosphère/givre/neige au Dôme C ainsi que dans 21 puits de neige prélevés dans une zone couvrant l'essentiel de l'Est de l'Antarctique. L'analyse isotopique de ces échantillons modernes a permis de contraindre le modèle conceptuel TRANSITS développé au cours de cette thèse et dont le but est de représenter le recyclage du nitrate à l'interface entre l'atmosphère et la neige ainsi que son impact sur la composition isotopique du nitrate archivé. Les valeurs positives et élevées du δ15N du nitrate piégé dans la carotte de glace de Vostok montrent que le recyclage du nitrate a toujours eu lieu sur le plateau Antarctique au cours de la période étudiée. Les variations du flux primaire de nitrate reçu au site de Vostok estimées à l'aide du modèle TRANSITS montrent, en périodes glaciaires, un flux primaire plus important qui pourrait être lié à une dénitrification stratosphérique plus conséquente. Les valeurs de Δ17O du nitrate montrent que l'incursion d'ozone d'origine stratosphérique dans la troposphère était plus fréquente en périodes glaciaires. Nous proposons enfin que les résultats acquis dans le cadre de cette thèse pourraient permettre de mieux contraindre le cycle de l'azote sur la côte Antarctique et d'apporter des éléments d'interprétation des enregistrements en nitrate dans les carottes de glace de sites de plus forte accumulation de neige (au Groenland par exemple).Nitrate ions (NO3-) found in Antarctic snows stem from the degradation of nitrogen oxydes (NOx = NO + NO2) in the atmosphere. At sites with low snow accumulation rates such as Vostok or Dome C (East Antarctic plateau), nitrate deposition to the snow is not irreversible and this strongly hampers the interpretation of nitrate concentration records in ice cores. Nitrogen stable isotopic (δ15N) as high as +339‰ were measured in nitrate in the upper firn at Dome C and have been attributed to nitrate photolysis initiating a strong recycling at the snow surface. The oxygen isotopic anomaly (Δ17O) reflects the activity of ozone (O3) in nitrate formation. We present the first comprehensive isotopic analysis of nitrate (δ15N, Δ17O and δ18O) in a deep ice core. 64 samples of nitrate from the Vostok ice core have been analyzed and cover the last 150 000 years. This dataset has been completed with 313 samples recently collected in the atmosphere/surface hoar/snow continuum at Dome C as well as in several snowpits from various sites covering most of the East Antarctica. Those present-day samples are used to evaluate a conceptual model (named TRANSITS) developped during this PhD and which aims at representing nitrate recycling at the snow/atmosphere interface and at modelling its impact on the isotopic composition of the archived nitrate. High positive δ15N values measured in the Vostok ice core reveal that nitrate recycling has always occurred at the surface of the Antarctic plateau over this period. Past variations of the primary flux of nitrate to the Vostok site have been estimated using the TRANSITS model. They show that glacials are characterized by higher inputs which may be linked to a greater stratospheric denitrification. The Δ17O values indicate that intrusions of stratospheric air masses to the troposphere may have been more frequent in glacials thus incorporating significant amounts of stratospheric ozone to the lower atmosphere. Last, we suggest that this study may have some relevance to the coastal nitrogen budget in Antarctica and to the interpretation of ice cores retrieved from high accumulation sites (e.g. in Greenland)

Contraintes isotopiques sur l'interprétation de l'enregistrement en nitrate dans la carotte de glace de Vostok

L'ion nitrate (NO3-) présent dans les neiges Antarctiques est issus de l'oxydation des oxydes d'azote (NOx = NO + NO2) dans l'atmosphère. Aux sites de faible accumulation de neige tels que Vostok et Dôme C sur le plateau de l'Est de l'Antarctique, le nitrate n'est pas piégé de manière ultime dans la neige, ce qui limite fortement l'interprétation des enregistrements en nitrate dans les carottes de glace. Des mesures récentes de la composition isotopique en azote du nitrate ( 15N) montrent des valeurs extrêmement élevées (+339 ) dans les premiers décimètres de la neige de Dôme C. Ces valeurs ont été attribuées à la photolyse du nitrate et au recyclage important qui en résulte. Le nitrate possède, par ailleurs, une anomalie isotopique en oxygène ( 17O) qui permet de tracer l'activité de l'ozone (O3) au cours de sa formation. Ce travail de thèse présente le premier enregistrement de la composition isotopique complète du nitrate ( 15N, 17O et 18O) dans une carotte de glace : la carotte de glace de Vostok dont les 64 échantillons analysés couvrent une période de 150 000 ans. Ce jeu de données a été complété par 313 échantillons collectés entre 2007 et 2010 dans le continuum atmosphère/givre/neige au Dôme C ainsi que dans 21 puits de neige prélevés dans une zone couvrant l'essentiel de l'Est de l'Antarctique. L'analyse isotopique de ces échantillons modernes a permis de contraindre le modèle conceptuel TRANSITS développé au cours de cette thèse et dont le but est de représenter le recyclage du nitrate à l'interface entre l'atmosphère et la neige ainsi que son impact sur la composition isotopique du nitrate archivé. Les valeurs positives et élevées du 15N du nitrate piégé dans la carotte de glace de Vostok montrent que le recyclage du nitrate a toujours eu lieu sur le plateau Antarctique au cours de la période étudiée. Les variations du flux primaire de nitrate reçu au site de Vostok estimées à l'aide du modèle TRANSITS montrent, en périodes glaciaires, un flux primaire plus important qui pourrait être lié à une dénitrification stratosphérique plus conséquente. Les valeurs de 17O du nitrate montrent que l'incursion d'ozone d'origine stratosphérique dans la troposphère était plus fréquente en périodes glaciaires. Nous proposons enfin que les résultats acquis dans le cadre de cette thèse pourraient permettre de mieux contraindre le cycle de l'azote sur la côte Antarctique et d'apporter des éléments d'interprétation des enregistrements en nitrate dans les carottes de glace de sites de plus forte accumulation de neige (au Groenland par exemple).Nitrate ions (NO3-) found in Antarctic snows stem from the degradation of nitrogen oxydes (NOx = NO + NO2) in the atmosphere. At sites with low snow accumulation rates such as Vostok or Dome C (East Antarctic plateau), nitrate deposition to the snow is not irreversible and this strongly hampers the interpretation of nitrate concentration records in ice cores. Nitrogen stable isotopic ( 15N) as high as +339 were measured in nitrate in the upper firn at Dome C and have been attributed to nitrate photolysis initiating a strong recycling at the snow surface. The oxygen isotopic anomaly ( 17O) reflects the activity of ozone (O3) in nitrate formation. We present the first comprehensive isotopic analysis of nitrate ( 15N, 17O and 18O) in a deep ice core. 64 samples of nitrate from the Vostok ice core have been analyzed and cover the last 150 000 years. This dataset has been completed with 313 samples recently collected in the atmosphere/surface hoar/snow continuum at Dome C as well as in several snowpits from various sites covering most of the East Antarctica. Those present-day samples are used to evaluate a conceptual model (named TRANSITS) developped during this PhD and which aims at representing nitrate recycling at the snow/atmosphere interface and at modelling its impact on the isotopic composition of the archived nitrate. High positive 15N values measured in the Vostok ice core reveal that nitrate recycling has always occurred at the surface of the Antarctic plateau over this period. Past variations of the primary flux of nitrate to the Vostok site have been estimated using the TRANSITS model. They show that glacials are characterized by higher inputs which may be linked to a greater stratospheric denitrification. The 17O values indicate that intrusions of stratospheric air masses to the troposphere may have been more frequent in glacials thus incorporating significant amounts of stratospheric ozone to the lower atmosphere. Last, we suggest that this study may have some relevance to the coastal nitrogen budget in Antarctica and to the interpretation of ice cores retrieved from high accumulation sites (e.g. in Greenland).SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

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

International audienceAbstract. 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

Impacts of post-depositional processing on nitrate isotopes in the snow and the overlying atmosphere at Summit, Greenland

International audienceAbstract. The effect of post-depositional processing on the preservation of snow nitrate isotopes at Summit, Greenland, remains a subject of debate and is relevant to the quantitative interpretation of ice-core nitrate (isotopic) records at high snow accumulation sites. Here we present the first year-round observations of atmospheric nitrate and its isotopic compositions at Summit and compare them with published surface snow and snowpack observations. The atmospheric δ15N(NO3-) remained negative throughout the year, ranging from −3.1 ‰ to −47.9 ‰ with a mean of (−14.8 ± 7.3) ‰ (n=54), and displayed minima in spring which are distinct from the observed spring δ15N(NO3-) maxima in snowpack. The spring average atmospheric δ15N(NO3-) was (−17.9 ± 8.3) ‰ (n=21), significantly depleted compared to the snowpack spring average of (4.6 ± 2.1) ‰, while the surface snow δ15N(NO3-) of (−6.8 ± 0.5) ‰ was in between the atmosphere and the snowpack. The differences in atmospheric, surface snow and snowpack δ15N(NO3-) are best explained by the photo-driven post-depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ15N(NO3-), the atmospheric Δ17O(NO3-) was of a similar seasonal pattern and magnitude of change to that in the snowpack, suggesting little to no changes in Δ17O(NO3-) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO3-) varied similarly to atmospheric Δ17O(NO3-), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO3-) was larger than that of Δ17O(NO3-). We found a strong correlation between atmospheric δ18O(NO3-) and Δ17O(NO3-) that is very similar to previous measurements for surface snow at Summit, suggesting that atmospheric δ18O(NO3-) versus Δ17O(NO3-) relationships were conserved during deposition. However, we found the linear relationships between δ18O and Δ17O(NO3-) were significantly different for snowpack compared to atmospheric samples. This likely suggests the oxygen isotopes are also affected before preservation in the snow at Summit, but the degree of change for δ18O(NO3-) should be larger than that of Δ17O(NO3-). This is because photolysis is a mass-dependent process that would directly affect δ18O(NO3-) in snow but not Δ17O(NO3-) as the latter is a mass-independent signal. Although there were uncertainties associated with the complied dataset, the results suggested that post-depositional processing at Summit can induce changes in nitrate isotopes, especially δ15N(NO3-), consistent with a previous modeling study. This reinforces the importance of understanding the effects of post-depositional processing before ice-core nitrate isotope interpretation, even for sites with relatively high snow accumulation rates

On the potential fingerprint of the Antarctic ozone hole in ice-core nitrate isotopes: a case study based on a South Pole ice core

International audienceColumn ozone variability has important implications for surface photochemistry and the climate. Ice-core nitrate isotopes are suspected to be influenced by column ozone variability and δ15N(NO3-) has been sought to serve as a proxy of column ozone variability. In this study, we examined the ability of ice-core nitrate isotopes to reflect column ozone variability by measuring δ15N(NO3-) and Δ17O(NO3-) in a shallow ice core drilled at the South Pole. The ice core covers the period 1944–2005, and during this period δ15N(NO3-) showed large annual variability ((59.2 ± 29.3) ‰ ), but with no apparent response to the Antarctic ozone hole. Utilizing a snow photochemical model, we estimated 6.9 ‰ additional enrichments in δ15N(NO3-) could be caused by the development of the ozone hole. Nevertheless, this enrichment is small and masked by the effects of the snow accumulation rate at the South Pole over the same period of the ozone hole. The Δ17O(NO3-) record has displayed a decreasing trend by ∼ 3.4 ‰ since 1976. This magnitude of change cannot be caused by enhanced post-depositional processing related to the ozone hole. Instead, the Δ17O(NO3-) decrease was more likely due to the proposed decreases in the O3 / HOx ratio in the extratropical Southern Hemisphere. Our results suggest ice-core δ15N(NO3-) is more sensitive to snow accumulation rate than to column ozone, but at sites with a relatively constant snow accumulation rate, information of column ozone variability embedded in δ15N(NO3-) should be retrievable