Firn-air δ15N in modern polar sites and glacial–interglacial ice: a model-data mismatch during glacial periods in Antarctica?

Landais, A.; Barnola, Jean-Marc; Kawamura, Kenji; Caillon, Nicolas; Delmotte, Marc; Ommen, T. D. van; Dreyfus, G.; Jouzel, Jean; Masson‐Delmotte, Valérie; Minster, Bénédicte; Freitag, Johannes; Leuenberger, Markus; Schwander, Jakob; Huber, Christof; Etheridge, D. M.; Morgan, Vin

doi:10.1016/j.quascirev.2005.06.007

Cited by 109 publications

(138 citation statements)

References 44 publications

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“…Our new results would suggest that we can expect softening and faster densification than assumed in the model. However, δ 15 N 2 measurements performed on glacial Greenland ice, which reflect the changes in the diffusive column length of the firn, agree rather well with the firn depth predicted by firnification models (Landais et al, 2006). This apparently limits the influence of Ca ++ concentrations on glacial densification in Greenland.…”

Section: Discussionsupporting

confidence: 59%

“…The observed development of seasonal cycles in density, which increase with depth and which follow the Ca++ seasonality, shows that strong seasonal cycles in impurities can induce a seasonal cycle in firn density unrelated to the temperature. This contradicts the hypothesis that the deep firn seasonal density cycle is induced by temperature at the surface (for example Landais et al, 2006;Zwally and Li, 2002). The relative maximum in density variability in deeper firn (Freitag et al, 2004;Gerland et al, 1999;Hörhold et al, 2011) can likely be attributed to the impurity effect.…”

Section: Discussionmentioning

confidence: 75%

“…high temperature and high accumulation rate with low dust content and vice versa) may explain why the impurity effect has not been observed earlier in the data and why the model results agree with the observations. However, at Antarctic low accumulation sites, where Ca++ concentrations also increase by 1-2 orders of magnitude, the glacial bubble close-off depth is notoriously over-predicted by firn models (Landais et al, 2006). The impurity effect may therefore contribute to solving the problem of the mismatch between predicted and observed depths of firn-ice transitions determined from gas records (Dreyfus et al, 2010;Sowas et al, 1992).…”

Section: Discussionmentioning

confidence: 99%

“…However, for several ice cores located in very low accumulation rate areas (such as Dome C or Vostok), a systematic mismatch between the observed nitrogen isotopic composition during glacial periods and the predictions from firn densification models was found (Landais et al, 2006;Sowas et al, 1992). For example, the comparison of two EPICA gas records from Dome C and Dronning Maud Land (Loulergue et al, 2007) revealed, that the firn densification models overestimate the age difference between the ice and the gas phase at Dome C during the last glacial period.…”

Section: Introductionmentioning

confidence: 94%

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On the impact of impurities on the densification of polar firn

Hörhold

Laepple

Freitag

et al. 2012

Earth and Planetary Science Letters

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Editor: P. DeMenocalKeywords: polar firn density densification impurity density variability Understanding polar firn densification is crucial for reconstructing the age of greenhouse gas concentrations extracted from ice cores, and for the interpretation of air in ice as a dating tool or as a climate proxy. Firn densification is generally modeled as a steady burial and sintering process of defined layers, where the structure of the layering is maintained along the whole firn and ice column. However, available high-resolution density data, as well as firn air samples, question this picture and point to a lack of understanding of firn densification. Based on analysis of high-resolution density and calcium concentration records from Antarctic and Greenland ice cores, we show for the first time that also impurities may have a significant impact on the densification. Analysis of firn cores shows a correlation between density and the calcium ion (Ca++) concentration, and this correlation increases with depth. The existence of this relationship is independent of the local climatic conditions at the core sites analyzed. The strong positive correlation between the density and the logarithm of Ca++ concentration indicates that impurities induce softening and lead to faster densification over a wide range of concentrations. In one core, the impurity effect manifests itself so strongly that the density develops a seasonal cycle closely following the seasonal cycle of Ca++. Our results clearly show that the structure of the firn layering changes with depth and suggest that the increased variability in density observed in deep firn, recently described as a universal feature of polar firn, may arise from the influence of Ca++ and/or other impurities. The impurity effect is likely to have direct implications on our understanding of glacial firn densification and on glacial gas age estimates.

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Section: Discussionsupporting

confidence: 59%

Section: Discussionmentioning

confidence: 75%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 94%

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On the impact of impurities on the densification of polar firn

Hörhold

Laepple

Freitag

et al. 2012

Earth and Planetary Science Letters

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153

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“…δ 15 N of N 2 ). Our precision for the gravitational correction of 0.05 ‰ per mass unit obtained from δ 136 Xe is lower than the results from a dedicated system for δ 15 N (Landais et al, 2006), yet sufficient for our purpose here. Note that using δ 136 Xe to correct for gravitational effects of any gas species in the firn column assumes that the gravitational enrichment of the gases in the firn column is only dependent on the absolute mass difference of two species.…”

Section: Xenon-based Mixing Ratiosmentioning

confidence: 69%

Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

et al. 2014

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Abstract. Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15 ‰ for δ 13 C-CH 4 , 0.22 ‰ for δ 15 N-N 2 O, 0.34 ‰ for δ 18 O-N 2 O, and 0.05 ‰ per mass difference for δ 136 Xe for typical concentrations of glacial ice. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichlorodifluoromethane with reproducibilities of 7 ppb for CH 4 , 3 ppb for N 2 O, 70 ppt for C 2 H 6 , 70 ppt for C 3 H 8 , 20 ppt for CH 3 Cl, and 2 ppt for CCl 2 F 2 . However, the blank contribution for C 2 H 6 and C 3 H 8 is large in view of the measured values for Antarctic ice samples. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH 4 is combusted to CO 2 prior to detection while we bypass the oven for all other species. The highly automated system uses only ∼ 160 g of ice, equivalent to ∼ 16 mL air, which is less than previous methods. The measurement of this large suite of parameters on a single ice sample is new and key to understanding phase relationships of parameters which are usually not measured together. A multi-parameter data set is also key to understand in situ production processes of organic species in the ice, a critical issue observed in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ 136 Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn diffusion effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.

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Layering of surface snow and firn at Kohnen Station, Antarctica: Noise or seasonal signal?

Laepple

Hörhold

Münch

et al. 2016

J. Geophys. Res. Earth Surf.

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The density of firn is an important property for monitoring and modeling the ice sheets as well as to model the pore close‐off and thus to interpret ice core‐based greenhouse gas records. One feature, which is still in debate, is the potential existence of an annual cycle of firn density in low‐accumulation regions. Several studies describe or assume seasonally successive density layers, horizontally evenly distributed, as seen in radar data. On the other hand, high‐resolution density measurements on firn cores in Antarctica and Greenland show no clear seasonal cycle in the top few meters. A major caveat of most existing snow‐pit and firn‐core‐based studies is that they represent one vertical profile from a laterally heterogeneous density field. To overcome this, we created an extensive data set of horizontal and vertical density data at Kohnen Station, Dronning Maud Land, on the East Antarctic Plateau. We drilled and analyzed three 90 m long firn cores as well as 143 one‐meter‐long vertical profiles from two elongated snow trenches to obtain a two‐dimensional view of the density variations. The analysis of the 45 m wide and 1 m deep density fields reveals a seasonal cycle in density. However, the seasonality is overprinted by strong stratigraphic noise, making it invisible when analyzing single firn cores. Our density data set extends the view from the local ice core perspective to a hundred meter scale and thus supports linking spatially integrating methods such as radar and seismic studies to ice and firn cores.

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Firn-air δ15N in modern polar sites and glacial–interglacial ice: a model-data mismatch during glacial periods in Antarctica?

Cited by 109 publications

References 44 publications

On the impact of impurities on the densification of polar firn

On the impact of impurities on the densification of polar firn

Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

Layering of surface snow and firn at Kohnen Station, Antarctica: Noise or seasonal signal?

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Firn-air δ15N in modern polar sites and glacial–interglacial ice: a model-data mismatch during glacial periods in Antarctica?

Cited by 109 publications

References 44 publications

On the impact of impurities on the densification of polar firn

On the impact of impurities on the densification of polar firn

Online technique for isotope and mixing ratios of CH&lt;sub&gt;4&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O, Xe and mixing ratios of organic trace gases on a single ice core sample

Layering of surface snow and firn at Kohnen Station, Antarctica: Noise or seasonal signal?

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Product

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Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample