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; Freitag, Johannes; Mulvaney, Robert; Schneebeli, Martin; Lipenkov, Vladimir Ya.

doi:10.5194/tc-2019-89

Cited by 3 publications

(6 citation statements)

References 37 publications

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“…Observations of density profiles in Dome C and Vostok proposed by Bréant et al (2017) reveal densification rates around 2.2kg.m −4 for both sites. This value also applies to the Lock-In firn as inferred from high resolution density measurements (Fourteau et al, 2019). Therefore, the densification rate While this parametrization produces adequate results for most of the gas records studied here, it results in a clear underestimation of the layering artifacts for the DO events 7 and 8 in the EDC96 ice core ( Figure S4 of the Supplement).…”

Section: Modeling Layered Trapping Artifactsmentioning

confidence: 64%

Estimation of gas record alteration in very low accumulation ice cores

Fourteau¹,

Martinerie²,

Faïn³

et al. 2019

Preprint

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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 affect 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. Concentration measurements thus only measure the average value in the ice layer, which removes the fast variability from the record. 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. Our results suggest that the presence of layering artifacts in deep ice cores is linked with the chemical content of the ice. We use high-resolution methane data to parametrize a simple 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 ie yr−1(ice equivalent) 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 estimate their potential impacts on a million-and-a-half years 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 and carbon dioxide). However, these biases are small compared to the dampening experienced by the record due to firn smoothing.

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Section: Modeling Layered Trapping Artifactsmentioning

confidence: 64%

Estimation of gas record alteration in very low accumulation ice cores

Fourteau¹,

Martinerie²,

Faïn³

et al. 2019

Preprint

Self Cite

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“…These errors were then propagated to obtain an estimation of the uncertainty of the density and closed porosity of the samples. Fourteau et al (2019b) analysis indicates that the uncertainty on density is fairly constant over the entire range of measurements with a value of 0.0082 g cm −3 , that is to say an uncertainty of about 0.009 on density relative to pure ice. This represents a relative uncertainty of about 1% on the derived density.…”

Section: Uncertainty Analysismentioning

confidence: 93%

“…The pycnometry apparatus is composed of two airtight chambers of known volumes V 1 and V 2 with a valve between them allowing to either connect or isolate the chambers. A scheme is provided in the Supplementary Material of Fourteau et al (2019b). A pressure gauge is joined to the chamber V 1 to monitor its internal pressure.…”

Section: Open Accessmentioning

confidence: 99%

“…We are therefore not able to propagate the uncertainties of the raw measurements to the final derived quantities, such as the density or the closed porosity ratio. However, a recent pycnometry campaign was conducted for an East Antarctic firn core, including a dedicated uncertainty analysis (Section S1.3 of the Supplement in Fourteau et al, 2019b). The measurements were performed with the same pycnometry apparatus and with the same sample size as the data presented in this article.…”

Section: Uncertainty Analysismentioning

confidence: 99%

“…We can therefore expect the data of JM Barnola to be affected by similar uncertainties. Fourteau et al (2019b) quantified the errors associated with the measurements performed in the cold room and used to derived the density and the closed porosity of the samples. This includes the errors on the mass of the sample, its radius and height, the pressures of the chambers, and the volumes of the chambers.…”

Section: Uncertainty Analysismentioning

confidence: 99%

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Historical porosity data in polar firn

Fourteau¹,

Arnaud²,

Faïn³

et al. 2020

Preprint

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Abstract. In the 1990's, closed and open porosity volumes of firn samples have been measured by J.-M. Barnola using the technique of gas pycnometry, on firn from three different polar sites. They are the basis of a parameterization of closed porosity in polar firn, first introduced in Goujon et al. (2003) and used in several firn physics models (e.g. Buizert et al., 2012). However, these data and their processing have not been published in their own right yet. In this short article, we detail how they were processed by J.-M. Barnola, and how the closed porosity parameterization was obtained. We show that the original data processing only partially accounts for the presence of re-opened bubbles in the samples. Since the proper correction to apply for this effect is hard to estimate, we also processed the data without including a correction for re-opened bubbles. Finally, we intend to make these pycnometry data available, in order to be used by the glaciology community, notably for the study of polar ice formation and of the composition of gas records in ice cores. They are hosted on the PANGAEA database: https://doi.pangaea.de/10.1594/PANGAEA.907678 (Fourteau et al., 2019a).

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A 2000-year temperature reconstruction on the East Antarctic plateau, from argon-nitrogen and water stable isotopes in the Aurora Basin North ice core

Servettaz

Orsi

Curran

et al. 2022

Preprint

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Abstract. The temperature of the earth is one of the most important climate parameters. Proxy records of past climate changes, in particular temperature, are a fundamental tool for exploring internal climate processes and natural climate forcings. Despite the excellent information provided by ice core records in Antarctica, the temperature variability of the past 2000 years is difficult to evaluate from the low accumulation sites in the Antarctic continent interior. Here we present the results from the Aurora Basin North (ABN) ice core (71° S, 111° E, 2690 m a.s.l.) in the lower part of the East Antarctic plateau where accumulation is substantially higher than other ice core drilling sites on the plateau, and provide unprecedented insight in East Antarctic past temperature variability. We reconstructed the temperature of the last 2000 years using two independent methods: the widely used water stable isotopes (δ18O), and by inverse modelling of borehole temperature and past temperature gradients estimated from the inert gas stable isotopes (δ40Ar and δ15N). This second reconstruction is based on three independent measurement types: borehole temperature, firn thickness, and firn temperature gradient. The δ18O temperature reconstruction supports stable temperature conditions within 1 °C over the past 2000 years, in agreement with other ice core δ18O records in the region. However, the gas and borehole temperature reconstruction suggest that surface conditions 2 °C cooler than average prevailed in the 1000–1400 CE period, and support a 20th century warming of 1 °C. These changes are remarkably consistent with reconstructed Southern Annular Mode (SAM) variability, as it shows colder temperatures during the positive phase of the SAM in the beginning of the last millennium, with rapidly increasing temperature as the SAM changes to the negative phase. The transition to a negative SAM phase after 1400 CE is however not accompanied by a warming in West Antarctica, which suggests an influence of Pacific South American modes, inducing a cooling in West Antarctica while ABN is warming after this time. A precipitation hiatus during cold periods could explain why water isotope temperature reconstruction underestimates the temperature changes. Both reconstructions arguably record climate in their own way, with a focus on atmospheric and hydrologic cycles for water isotopes, as opposed to surface temperature for gases isotopes and borehole. This study demonstrates the importance of using a variety of sources for comprehensive paleoclimate reconstructions.

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

Cited by 3 publications

References 37 publications

Estimation of gas record alteration in very low accumulation ice cores

Estimation of gas record alteration in very low accumulation ice cores

Historical porosity data in polar firn

A 2000-year temperature reconstruction on the East Antarctic plateau, from argon-nitrogen and water stable isotopes in the Aurora Basin North ice core

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