Multi-tracer study of gas trapping in an East Antarctic ice core

Fourteau, Kévin; Martinerie, Patricia; Faïn, Xavier; Schaller, Christoph Florian; Tuckwell, Rebecca; Löwe, Henning; Arnaud, Laurent; Magand, Olivier; Thomas, Elizabeth R.; Freitag, Johannes; Mulvaney, Robert; Schneebeli, Martin; Lipenkov, Vladimir Ya.

doi:10.5194/tc-13-3383-2019

Cited by 34 publications

(49 citation statements)

References 65 publications

(169 reference statements)

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“…The high-resolution Modern Dome C, Holocene Dome C, DO6-9 Dome C, and DO21 Vostok methane datasets are available on the World Data Center for Paleoclimatology: https://www.ncdc.noaa.gov/paleo-search/study/28831 (last access: 2 March 2020). The high-resolution Modern Lock-In dataset is available on the PANGAEA database: https://doi.org/10.1594/PANGAEA.909627 (Fourteau et al, 2019b).…”

Section: Discussionmentioning

confidence: 99%

Estimation of gas record alteration in very low-accumulation ice cores

et al. 2020

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Abstract. We measured the methane mixing ratios of enclosed air in five ice core sections drilled on the East Antarctic Plateau. Our work aims to study two effects that alter the recorded gas concentrations in ice cores: layered gas trapping artifacts and firn smoothing. Layered gas trapping artifacts are due to the heterogeneous nature of polar firn, where some strata might close early and trap abnormally old gases that appear as spurious values during measurements. The smoothing is due to the combined effects of diffusive mixing in the firn and the progressive closure of bubbles at the bottom of the firn. Consequently, the gases trapped in a given ice layer span a distribution of ages. This means that the gas concentration in an ice layer is the average value over a certain period of time, which removes the fast variability from the record. Here, we focus on the study of East Antarctic Plateau ice cores, as these low-accumulation ice cores are particularly affected by both layering and smoothing. We use high-resolution methane data to test a simple trapping model reproducing the layered gas trapping artifacts for different accumulation conditions typical of the East Antarctic Plateau. We also use the high-resolution methane measurements to estimate the gas age distributions of the enclosed air in the five newly measured ice core sections. It appears that for accumulations below 2 cm ice equivalent yr−1 the gas records experience nearly the same degree of smoothing. We therefore propose to use a single gas age distribution to represent the firn smoothing observed in the glacial ice cores of the East Antarctic Plateau. Finally, we used the layered gas trapping model and the estimation of glacial firn smoothing to quantify their potential impacts on a hypothetical 1.5-million-year-old ice core from the East Antarctic Plateau. Our results indicate that layering artifacts are no longer individually resolved in the case of very thinned ice near the bedrock. They nonetheless contribute to slight biases of the measured signal (less than 10 ppbv and 0.5 ppmv in the case of methane using our currently established continuous CH4 analysis and carbon dioxide, respectively). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

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Section: Discussionmentioning

confidence: 99%

Estimation of gas record alteration in very low-accumulation ice cores

et al. 2020

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“…Within the pore closure zone (t ≤ 500 years), the compression speed with increasing pressure is reduced on average by 10%. The discrepancy between modeled and measured air content values in ice cores reported by Fourteau et al (2019) cannot therefore be explained solely by the reduced compressibility of closed pores.…”

Section: Effect Of Internal Pressure On Pore Compressionmentioning

confidence: 78%

“…In real firn, the increased pressure in the already closed bubbles might slow their compression compared to open pores. A recent study by Fourteau et al (2019) has shown that reducing the compressibility of closed pores by an amount of 50% in gas trapping models could explain a discrepancy between model results and direct measurements of air content in ice cores. It is however not clear if this reduced compressibility of 50% is physically sound or not.…”

Section: Effect Of Internal Pressure On Pore Compressionmentioning

confidence: 99%

“…At that point, the building pressure in the closed pores reaches 0.16 MPa, more than twice the initial value. The measured thickness of the pore closure zone at Lock-In is of about 20 m (Fourteau et al, 2019). With an accumulation of 3.9 cm per year, a given firn stratum takes about 500 yr to cross the pore closure zone.…”

Section: Effect Of Internal Pressure On Pore Compressionmentioning

confidence: 99%

“…To answer this question, we simulate the densification of a layer of the Lock-In firn, on the East Antarctic plateau. This site is characterized by an accumulation rate of 3.9 cm yr −1 (ice equivalent), a surface temperature of −53 • C, and a surface pressure evaluated at 0.0645 MPa (645 mbar) (Fourteau et al, 2019). The model is initialized with the microstructure obtained with a 2.5 × 2.5 × 2.5 mm tomography image, and selected at 80 m depth, near the start of the pore closure zone.…”

Section: Effect Of Internal Pressure On Pore Compressionmentioning

confidence: 99%

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A Micro-Mechanical Model for the Transformation of Dry Polar Firn Into Ice Using the Level-Set Method

et al. 2020

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Interpretation of greenhouse gas records in polar ice cores requires a good understanding of the mechanisms controlling gas trapping in polar ice, and therefore of the processes of densification and pore closure in firn (compacted snow). Current firn densification models are based on a macroscopic description of the firn and rely on empirical laws and/or idealized geometries to obtain the equations governing the densification and pore closure. Here, we propose a physically-based methodology explicitly representing the porous structure and its evolution over time. In order to handle the complex geometry and topological changes that occur during firn densification, we rely on a Level-Set representation of the interface between the ice and the pores. Two mechanisms are considered for the displacement of the interface: (i) mass surface diffusion driven by local pore curvature and (ii) ice dislocation creep. For the latter, ice is modeled as a viscous material and the flow velocities are solutions of the Stokes equations. First applications show that the model is able to densify firn and split pores. Using the model in cold and arid conditions of the Antarctic plateau, we show that gas trapping models do not have to consider the reduced compressibility of closed pores compared to open pores in the deepest part of firns. Our results also suggest that the mechanism of curvature-driven surface diffusion does not result in pore splitting, and that ice creep has to be taken into account for pores to close. Future applications of this type of model could help quantify the evolution and closure of firn porous networks for various accumulation and temperature conditions.

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Microstructural characterization of depth hoar and ice‐crust layers using a micro‐CT, and hypothesis of ice‐crust formation under a thunderstorm

Li,

Fu,

Keegan

et al. 2023

Hydrological Processes

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The microstructural features typical of depth hoar and ice‐crust layers in both blocks of snow and a firn core that were extracted at Summit, Greenland (72°35′ N, 38°25′ W) in June, 2017 have been characterized using x‐ray microcomputed tomography (micro‐CT). In the depth hoar, the density is much lower, and the porosity, pore sizes, and specific surface area (SSA) are greater than those in adjacent layers. In the ice‐crusts, the density and the particle size are greater, and the porosity, pore size, and SSA are less than those in adjacent layers. Note that the mean structure thickness in the depth hoar was greater than that in adjacent layers, but that increase was simply related to the one‐ or two‐dimension ice crystals, that is, needle‐like or plate‐like structures, being included in the measurements for depth hoar. Using related microstructural parameters derived from the micro‐CT data, we propose a model based on refreezing of pre‐melted water (PMW) droplets electrostatically‐transported by the electric field between thunderclouds and the ice sheet created by a thunderstorm that describes the processes of the ice‐crust formation (ICF). Whether the ice‐crust forms with depth hoar depends on both the kinetic energy from the PMW droplets and the latent heat liberated from the freezing of the PMW. This work is the first to build the relationship between the atmosphere and ice sheets by a thunderstorm. Finally, we provide an experimental geophysics‐based method through the ICF under laboratory conditions to learn more about the interaction between atmospheric electrodynamics and thermodynamics.

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Multi-tracer study of gas trapping in an East Antarctic ice core

Cited by 34 publications

References 65 publications

Estimation of gas record alteration in very low-accumulation ice cores

Estimation of gas record alteration in very low-accumulation ice cores

A Micro-Mechanical Model for the Transformation of Dry Polar Firn Into Ice Using the Level-Set Method

Microstructural characterization of depth hoar and ice‐crust layers using a micro‐CT, and hypothesis of ice‐crust formation under a thunderstorm

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