Hostname: page-component-586b7cd67f-gb8f7 Total loading time: 0 Render date: 2024-11-22T04:56:36.099Z Has data issue: false hasContentIssue false

Differentiating bubble-free layers from melt layers in ice cores using noble gases

Published online by Cambridge University Press:  10 July 2017

Anais J. Orsi*
Affiliation:
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA Laboratoire des Sciences du Climat et de l’Environnement, CEA Saclay, Gif-sur-Yvette, France
Kenji Kawamura
Affiliation:
National Institute of Polar Research, Tachikawa, Tokyo, Japan Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tachikawa, Tokyo, Japan Institute of Biogeosciences, Japan Agency for Marine–Earth Science and Technology, Yokosuka, Japan
John M. Fegyveresi
Affiliation:
Department of Geosciences, The Pennsylvania State University, University Park, PA, USA
Melissa A. Headly
Affiliation:
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA
Richard B. Alley
Affiliation:
Department of Geosciences, The Pennsylvania State University, University Park, PA, USA
Jeffrey P. Severinghaus
Affiliation:
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA, USA
*
Anais J. Orsi <[email protected]>
Rights & Permissions [Opens in a new window]

Abstract

Melt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.

Type
Research Article
Copyright
Copyright © International Glaciological Society 2015

Introduction

Melt layers in ice sheets form during exceptionally warm conditions. Records of melt layers in ice cores can therefore give insight into changes in summertime peak temperatures, recording the frequency of extremely warm summers (Reference Herron, Herron and LangwayHerron and others, 1981; Reference Langway and ShojiLangway and Shoji, 1990; Reference Alley and AnandakrishnanAlley and Anandakrishnan, 1995; Reference Kameda, Narita, Shoji, Nishio, Fujii and WanatabeKameda and others, 1995; Reference Das and AlleyDas and Alley, 2008).

Melt also significantly decreases the albedo of the snow surface, which triggers a positive feedback. As a result, it is useful to understand the history of surface melting in order to successfully model the radiative balance of an ice sheet (e.g. Reference Tedesco, Serreze and FettweisTedesco and others, 2008, Reference Tedesco2011; Reference Box, Fettweis, Stroeve, Tedesco, Hall and SteffenBox and others, 2012). In addition, the study of recent melt events is important for estimating the mass balance of the ice sheet, because refrozen meltwater is not lost from the ice sheet, and affects firn density and thus mass-balance estimates from repeated altimetry (e.g. Reference Pfeffer, Illangasekare and MeierPfeffer and others, 1990, Reference Pfeffer, Meier and Illangasekare1991; Reference Harper, Humphrey, Pfeffer, Brown and FettweisHarper and others, 2012; Reference ForsterForster and others, 2014; Reference Morris, Mair, Nienow, Bell, Burgess and WrightMorris and others, 2014). The observed correlation of melt layers between different ice-core sites shows that melt layers often arise from widespread meteorological events, and can reflect regional climate conditions (Reference Kameda, Narita, Shoji, Nishio, Fujii and WanatabeKameda and others, 1995; Reference Das and AlleyDas and Alley, 2008). Satellite imaging of recent melt events that extended over much of Greenland (Reference Tedesco, Serreze and FettweisTedesco and others, 2008; Reference NghiemNghiem and others, 2012; Reference Hall, Comiso, DiGirolamo, Shuman, Box and KoenigHall and others, 2013; Reference McGrath, Colgan, Bayou, Muto and SteffenMcGrath and others, 2013) supports this view.

When the surface or near-surface snow melts, the liquid water percolates down into the firn and spreads along thin, near-surface crusts, subsequently refreezing into a melt layer when the air temperature lowers, or incoming radiation drops (Reference Langway and ShojiLangway and Shoji, 1990; Reference Pfeffer and HumphreyPfeffer and Humphrey, 1998; Reference Das and AlleyDas and Alley, 2005; Reference Wong, Hawley, Lutz and OsterbergWong and others, 2013). Melt layers appear in bubbly ice cores as near-horizontal, typically irregular or discontinuous bands of ice with few, oddly shaped or sized, air bubbles, typically 1–100 mm thick (Fig. 1; Reference Langway and ShojiLangway and Shoji, 1990; Reference Das and AlleyDas and Alley, 2005). The visual identification of melt layers can be complicated by another type of bubble-free feature: thin crusts, usually a single grain in thickness ( 1 mm), which are observed in all seasons (Reference Fujii and KusunokiFujii and Kusunoki, 1982; Reference Alley and AnandakrishnanAlley and Anandakrishnan, 1995). The origin of these crusts is poorly understood, and they probably have multiple origins (Reference AlleyAlley, 1988), including glazing by wind action (Reference Albert, Shuman, Courville, Bauer, Fahnestock and ScambosAlbert and others, 2004), vapor transfer (Reference Fujii and KusunokiFujii and Kusunoki, 1982) or even some slight melting (Reference Alley and AnandakrishnanAlley and Anandakrishnan, 1995). Prior interpretations of melt features generally have been restricted to thicker, nearly bubble-free layers, which include other indications of melt, such as large grains, anomalously large, elongated bubbles, and lenticular shape (Reference Langway and ShojiLangway and Shoji, 1990; Reference Alley and AnandakrishnanAlley and Anandakrishnan, 1995; Reference Das and AlleyDas and Alley, 2005).

Fig. 1. Melt layers from the NEEM (North Greenland) ice core at 44.3 m. This melt event is dated to AD 1888. Photo courtesy of Kaitlin Keegan.

As we explore further back in time using deeper ice, melt layers are harder to identify due to thinning of the layers with depth and air clathrate formation. Air clathrates form when the pressure is sufficient to change the air bubble within the ice into a solid phase, in which the air is surrounded by water molecules, forming a lattice (Reference MillerMiller, 1969).

At sufficiently great depth, typically below 1250 m (Reference FitzpatrickFitzpatrick and others, 2014), the air bubbles have completely changed to air clathrates, thus precluding visual identification of melt layers. As a result, a chemical identification of melt layers is essential to study melt events during warm periods beyond the Holocene, for instance the last interglacial (NEEM community members, 2013).

Melt layers have a strong influence on the gas composition of the ice core: soluble gases (e.g. carbon dioxide, methane, nitrous oxide, krypton and xenon) can dissolve in the liquid water before it freezes, and induce very high concentrations of these gases in the ice core (Reference Ahn, Headly, Wahlen, Brook, Mayewski and TaylorAhn and others, 2008; NEEM community members, 2013). While CH4 and N2O are clearly elevated in melt layers, these gases may also be produced by microbes in ice, so they cannot serve as reliable indicators of refrozen meltwater (NEEM community members, 2013).

We present here a method to use noble gases as a reliable test of the presence of refrozen meltwater in ice-core samples, and describe two applications: testing for the presence of melt events during the Eemian at the NEEM (North Greenland Eemian Ice Drilling) ice-core site, and analyzing the composition of the pervasive bubble-free layers present in the West Antarctic Ice Sheet (WAIS) Divide ice-core site.

Melt-Layer Detection using Krypton and Xenon in Ice Cores

Krypton and xenon are highly soluble in liquid water, and both are more soluble than argon. Therefore, Kr/Ar and Xe/ Ar ratios are higher in melted and refrozen ice than within air bubbles in ice that has not melted. We show that these noble gas ratios may be used as additional, independent indicators of melt layers in ice cores.

Theory

Fractionation due to melt

Xenon is approximately four times, and krypton two times as soluble as argon (Table 1; Reference Wood and CaputiWood and Caputi, 1966; Reference WeissWeiss, 1970; Reference Weiss and KyserWeiss and Kyser, 1978). As a result, Kr/Ar and Xe/Ar ratios are higher in melted and refrozen ice than within air bubbles in ice that has not melted, and exhibit a specific relationship.

Table 1. Parameter values. Solubility coefficients (β) are calculated at 0°C (Reference Weiss and KyserWeiss and Kyser, 1978; Reference Hamme and EmersonHamme and Emerson, 2004). Gas transfer velocities (kc ) are from Reference Nicholson, Emerson, Caillon, Jouzel and HammeNicholson and others (2010) and Reference Cole, Bade, Bastviken, Pace and Van de BogertCole and others (2010), with Schmidt numbers from Reference Jähne, Heinz and DietrichJähne and others (1987). p, β, S and kc are given for the bulk Kr and Xe rather than for a specific isotope

The solubility equilibrium is expressed in terms of the solubility coefficient β, expressed in mol L−1 of liquid water:

(1)

We report the ratios (R) in delta notation, δ = (R/R standard − 1) 1000, and use modern atmospheric air as our standard. In order to compare the meltwater ratio to the atmospheric ratio, we define S Ar = β Ar/[Ar]atm, the solubility in liters of air per atmosphere per liters of liquid, with [Ar]atm being the concentration of argon in the atmosphere (mol L−1 atm−1).

The solubility ratios of Xe and Kr with Ar are therefore

(2)

(3)

We can also express these as δ(Xe/Armelt) = (4.36 − 1) 1000 = 3360 ‰, and similarly, δ(Kr/Armelt) = 1004‰.

Therefore, it appears that not only are samples containing refrozen melt enriched in Kr/Ar and Xe/Ar in comparison to non-melted ice, but a pure melt end-member will also show a unique relationship between δ(Kr/Ar) and δ(Xe/Ar), with a slope of ∼3.3.

The refreezing process contributes to the exclusion of atoms and molecules whose size is larger than that of the crystal spacing of the ice, which includes Ar, Kr and Xe (Reference Top, Martin and BeckerTop and others, 1988). Experiments done on the growth of frazil ice (Reference Top, Martin and BeckerTop and others, 1988) and on the freezing of lake water show that 1–50% of the noble gases are retained, likely depending on the freezing rate, and that there is no clear indication of fractionation of Kr/Ar or Xe/Ar during refreezing (Reference Malone, Castro, Hall, Doran, Kenig and McKayMalone and others, 2010).

Gravitational fractionation

In non-melted ice, measured Kr/Ar and Xe/Ar are primarily affected by gravitational fractionation (Reference Craig and WiensCraig and Wiens, 1996). Gravitational fractionation is a process in which heavier isotopes are preferentially concentrated with increasing depth in firn. According to Reference Craig, Horibe and SowersCraig and others (1988), gravitational fractionation is described by the barometric equation modified for a gas pair:

(4)

where δgrav is the gravitational fractionation of an isotope pair, g is the local gravitational acceleration, Z is the thickness of the firn, Δm is the mass difference between the two isotopes, R 0 is the gas constant and T (K) is the temperature of the firn column. The mass difference Δm = 96 for δ(Xe/Ar), and it is Δm = 48 for δ(Kr/Ar). As a result, we expect a δ(Xe/Ar) vs δ(Kr/Ar) slope of 2 in gas samples from non-melted ice.

Gas loss

Kr/Ar and Xe/Ar ratios can also be affected by gas loss during air bubble close-off and during core storage. Gas loss affects molecules with a diameter smaller than 3.6 Å (3.6 × 10−10 m), including Ar and O2 but not N2, Kr and Xe (Reference Bender, Sowers and LipenkovBender and others, 1995; Reference Severinghaus and BattleSeveringhaus and Battle, 2006). As a result, gas loss will cause Kr/Ar and Xe/Ar to be slightly higher than expected. In order to remove the effects of gas loss, we can compute δ(Xe/Kr), which is independent of Ar:

(5)

We expect that for meltwater, δ(Xe/Kr) ≈ 1177‰. A sample with refrozen melt will have elevated δ(Xe/Kr).

Analysis of Kr/Ar and Xe/Ar in known melt layers

The ice-core samples analyzed in the first part of this study were from the Holocene, taken from the Dye 3 ice core. Dye 3 is a site in South Greenland (65° N, 43° W; 2480 m a.s.l.), with a mean annual temperature of −21°C and an annual accumulation rate of 0.49 m a−1 (Reference Dahl-Jensen and JohnsenDahl-Jensen and Johnsen, 1986). The core contains numerous visible melt layers, identified as horizontal bands of ice with few air bubbles, a few mm to a few cm thick (Reference Langway and ShojiLangway and Shoji, 1990). We analyzed samples containing melt layers at 144 m. We also sampled ice that did not contain visible melt layers at 143–145 m depth, and refer to it as non-melted ice. The ice-core Kr/Ar and Xe/Ar are measured following the technique outlined by Reference Severinghaus, Grachev, Luz and CaillonSeveringhaus and others (2003) and Reference Headly and SeveringhausHeadly and Severinghaus (2007). Ice samples are typically 50–60 g each, measured in duplicate or triplicate for a given depth. The standard used in these measurements is air collected from the Scripps pier in La Jolla, CA, USA. The method used to collect the air is described in Reference Headly and SeveringhausHeadly and Severinghaus (2007).

The δ(Kr/Ar) and δ(Xe/Ar) measured in Holocene ice from the Dye 3 ice core are shown in Table 2 and Figure 2. The data show different trends in melted versus non-melted ice. The δ(Kr/Ar) and δ(Xe/Ar) in melt layers are clearly enriched compared with those of non-melted ice. The melt-layer values range from 30‰ to 63‰ for δ(Kr/Ar), and 69‰ to 168‰ for δ(Xe/Ar). Non-melted ice samples from Dye 3, in contrast, have δ(Kr/Ar) values of 13.0 ± 2.0‰, and δ(Xe/Ar) of 24.5 ± 3.8‰. There are large variations of δ(Kr/Ar) and δ(Xe/Ar) for replicate samples of melt layers from the same depth, which shows that δ(Kr/Ar) and δ(Xe/Ar) are not quantitative indicators of melt intensity, or that the fraction of refrozen meltwater varies strongly over short distances (Table 2).

Table 2. Measurements of the Dye 3 ice core. Samples with visible melt layers are shown in bold font (144.2–144.31 m and 144.47–144.6 m). Replicates from the same depth are labeled a, b, c, etc. The air content (column 4) is expressed as the ratio of the sample with respect to the mean of all non-melt samples. The rightmost three columns show the calculated diffusive column height Z, the gas loss fraction λ and the melt fraction γ. The mean and standard deviation of all non-melt samples give an idea of the expected sample-to-sample variability. The pooled standard deviation σ pooled describes the measurement error

Fig. 2. Melt layer identification. When δ(Kr/Ar) and δ(Xe/Ar) are high, they indicate the presence of melt, as can be seen in Dye 3 melt layer samples (solid diamonds), compared to non-melt samples (+). NEEM Eemian samples are shown in large squares. The analytical precision is better than 0.8‰ for δ(Kr/Ar) and 2.9‰ for δ(Xe/Ar), which is approximately the size of the markers. WAIS Divide bubble-free layers (filled circles) do not have anomalously high δ(Kr/Ar) and δ(Xe/Ar), which indicates that they do not contain a significant amount of melt. The box shows the range of values found in the WDC05A ice core. The dashed line indicates gravitational fractionation, and the solid lines indicate the fractionation due to melt.

We also find that the slope of δ(Xe/Ar) vs δ(Kr/Ar) is 3.01, which is in agreement with that predicted from theory for gases at solubility equilibrium, and confirms that the possible expulsion of gases during refreezing does not alter the melt signature in the ice. The slope of δ(Xe/Ar) vs δ(Kr/Ar) for non-melt samples is 1.77, which is slightly lower than the expected slope (2) due to gravitational fractionation, possibly due to an incomplete degassing of Xe in the sample.

Quantification of the amount of melt

A typical ice-core sample contains both air from bubbles (called Arair in the following) and from dissolved gas (called Armelt). In order to quantify the amount of refrozen water in a sample, we model the respective contributions of (1) gravitational fractionation, (2) melt and (3) gas loss as follows (units are moles of gas):

(6)

The ‘air’ terms (e.g. n(Ar)air) in Eqns (6) represent the abundance of each gas in the air bubble within the ice core. The abundance in the air bubbles reflects both the atmospheric abundance at the time the air was trapped (e.g. Aratm), as well as the effects of gravitational fractionation on the gases in the firn. The concentration of argon in the air bubbles [Ar]air is related to the atmospheric concentration [Ar]atm as

(7)

If we denote V b the volume of the air bubbles, the number of moles of argon in the air bubbles is n(Arair) = V b[Ar]air.

The ‘melt’ terms reflect the enrichment of these gases due to the presence of melt layers. Argon, krypton and xenon are partially expelled during freezing; thus, a portion of the refrozen melt will be gas-free, and will not contribute to anomalies in ice-core gas composition. We quantify the volume of refrozen water that retained the melt signature V l and find that the quantity of dissolved gas follows

(8)

The 36Arloss term represents Ar loss due to gas loss from the ice core. Gas loss typically causes δ40 Ar/36 Ar in the air that remains in an air bubble to be positively enriched by 0.007‰ per 1‰ δ(Kr/Ar) enrichment (Reference Severinghaus, Grachev, Luz and CaillonSeveringhaus and others, 2003). Therefore, 40Ar is slightly less depleted by gas loss than 36Ar: n(40Ar)loss = 0.993(n(36Ar)loss). We denote λ as the fraction of gas lost, so we can write

(9)

We use atmospheric air as our standard gas. If we denote V the total volume of gas in the sample, the number of moles of argon in the reference gas is n(Arstandard) = V[Ar]atm. This implies that the ratio of sample to standard is

We note γ = V l/V b the ratio of the volume of liquid water to the volume of gas in the bubbles. The equation becomes

(10)

Following the same pattern for the other observed gases, we can rewrite the system of Eqn (6) in terms of our observations (δ40 Ar/36 Ar, δKr/Ar and δXe/Ar) and the three parameters

(11)

To solve the system of equations above, we calculate Z, γ and λ by iterated linearized inversion. The results from six different melt-layer samples from shallow ice from the Dye 3 ice core are shown in Table 2. The calculated firn depth Z and gas loss λ are consistent in all replicates, but the wide range of values for γ, the volume ratio of refrozen meltwater to bubble air, shows that the extent of melt is not a spatially uniform parameter in an ice-core sample.

Melt Events during the Last Interglacial at Neem

The last interglacial, the Eemian (130–115 ka BP), was warmer than the current interglacial in many places including Greenland, owing to higher insolation forcing (Reference Francis, Wolfe, Walker and MillerFrancis and others, 2006; Reference Otto-BliesnerOtto-Bliesner, 2006; Reference Axford, Briner, Francis, Miller, Walker and WolfeAxford and others, 2011; Reference Masson-DelmotteMasson-Delmotte and others, 2011; Reference LuntLunt and others, 2012). A recent study at the NEEM site in North Greenland (77.45° N, 51.06° W; 2450 m a.s.l.; −29°C) shows that the temperature may have been 8 ± 4°C warmer than present at constant elevation (NEEM community members, 2013). The analysis of CH4 and N2O in NEEM ice shows brief intervals of very high concentrations: up to 2000 ppb in CH4 for a background of ∼750 ppb, and up to 400 ppb in N2O for a background of 250 ppb. However, these values are higher than can be explained by solubility alone, and point towards a significant contribution of in situ production of CH4 and N2O by microbes present in the ice (NEEM community members, 2013).

We measured δ40 Ar, δ(Kr/Ar) and δ(Xe/Ar) in NEEM samples from the Eemian, between 2398 and 2403 m, and found δ(Kr/Ar) values between 17.73‰ and 56.47‰, and δ(Xe/Ar) values between 22.60‰ and 166.94‰ (Fig. 3). In order to save precious ice, we used 30 g, 3.5 cm samples, and diluted them 20 times with pure N2. The rest of the analytical method is the same as for Dye 3 samples (Reference Severinghaus, Grachev, Luz and CaillonSeveringhaus and others, 2003). We calculated the melt fraction (γ) of all samples, and found that 21 out of 39 samples had γ > 0.1, corresponding to at least 1% of the mass of the sample being refrozen water (1 mL of air is typically found in a 10 g ice-core sample). These measurements confirm the presence of melt layers in the interval 127–118.3 ka BP.

Fig. 3. Measurements of δ40 Ar, δ(Kr/Ar) and δ(Xe/Ar) in the Eemian section of the NEEM ice core. The dashed line on top shows the modeled melt fraction, inferred from the measurements. Elevated δ(Kr/Ar) and δ(Xe/Ar) are clear evidence of the presence of melt in the ice core.

Very few melt layers have been observed at the NEEM ice-coring site above the clathrate transition, although the extreme heatwave of 12–15 July 2012 produced melt at NEEM, as well as on >98.6% of the Greenland ice sheet (Reference NghiemNghiem and others, 2012; Reference McGrath, Colgan, Bayou, Muto and SteffenMcGrath and others, 2013). Contrary to the Dye 3 samples (65° N, 44° W; −21°C), which contained melt from a single event, the NEEM Eemian samples each cover about a decade. Yet the melt fractions are comparable, with three samples above γ = 0.5. This suggests that the magnitude and/or frequency of Eemian melt events was larger than in the Holocene at Dye 3, which is consistent with up to 8 ± 4°C higher temperature, and high summer insolation (NEEM community members, 2013).

The presence of melt layers at NEEM during the Eemian confirms that the additional warmth had a strong summer component, which is consistent with the fact that the eccentricity was much higher during the last interglacial than during the Holocene, enhancing the impact of precession and seasonal contrasts at that time.

Analysis of Bubble-Free Layers in the WAIS Divide Ice Core

The WAIS Divide ice core provides the highest-resolution gas record of the last 60 000 years (e.g. Reference MarcottMarcott and others, 2014). The WAIS Divide site is located in Antarctica (79° S, 112° W; −29°C, 0.22 m ice a−1), where dust fluxes are minimal, and in situ production of greenhouse gases is much smaller than in Greenland. It is also in a high-accumulation region, which allows for a small gas-age–ice-age difference (∼500 years at the Last Glacial Maximum (Reference BuizertBuizert and others, 2014)), and a small uncertainty in the gas chronology, including during Termination 1, unlike existing East Antarctic deep ice cores such as Dome C or Vostok.

Upon physical inspection of the core, we noticed a very large number of bubble-free layers, ∼1–2 mm thick (Fig. 4), at a rate of ∼15 (m (ice))−1, or 4a−1. They are visible in layers deposited during all seasons, with a higher occurrence in the late summer, as measured using the position relative to the summer peak of the annual-layer counted timescale (WAIS Divide Project Members, 2013; Fegyveresi, 590 2014). They can only be detected in the first 600 m of the core: the bubbles disappear in the deeper part of the core, due to the formation of air hydrates under high hydrostatic pressure, making the ice clear, and the bubble-free layers visibly indistinguishable from the rest of the ice (Reference FitzpatrickFitzpatrick and others, 2014).

Fig. 4. A pair of bubble-free layers in the WDC05A core. The layers are 1 mm thick. Black in this image represents clear ice, and bubbles appear white. Photo by John Fegyveresi.

Similar bubble-free layers have been documented in other ice cores (Reference Fujii and KusunokiFujii and Kusunoki, 1982; Reference AlleyAlley, 1988; Reference Alley and AnandakrishnanAlley and Anandakrishnan, 1995) and have been interpreted as the remnants of surface crusts, which can be several mm thick, but typically much thinner. These crusts can have multiple origins, and current understanding includes the possibility that some crusts may contain refrozen meltwater.

It is important for the accuracy of the gas record to know whether these layers contain sufficient melt to compromise the soluble gas record, and whether they affected gas diffusion through the firn, thereby complicating the gas chronology.

Influence on gas composition

To test for the presence of significant melt, we analyzed two samples containing bubble-free layers, and three control samples, taken between 114 and 147 m in the WDC05A ice core, which was collected at the site the year before the main deep core (Reference AhnAhn and others, 2012). All of these samples were taken from mature ice, well below the bubble-closure depth, which is at ∼78 m at WAIS Divide (Reference BattleBattle and others, 2011). Each sample was prepared using a bandsaw to cut across the core, extracting a slab ∼5 mm thick containing the 1 mm crust. Several slabs were pooled into one 600 g sample. Control samples consisted of the same number of slabs of bubbly ice, taken from neighboring ice (Table 3). The samples were melted under vacuum to release the trapped air. The air sample was then exposed to a Zr/Al getter at 900°C to remove N2, O2 and other reactive gases, and the resulting gas was expanded into a MAT 252 mass spectrometer to measure δ40 Ar, δ86/82 Kr, δ(Kr/Ar), δ(Xe/Ar) and δ(Ne/Ar) (Reference OrsiOrsi, 2013).

Table 3. Analysis of bubble-free layers in the WDC05A ice core. The δ values are given in ‰ with respect to modern air. Z, λ and γ are the outputs of the melt layer model. The bottom two rows show the mean and standard deviation of 73 WDC05A samples measured from 38 depths between 78 and 300 m. The uncertainty in the modeled Z, λ and γ and is calculated by a Monte Carlo perturbation with variances corresponding to the bottom row of the table, and is also shown in the bottom row

The first bubble-free sample is slightly elevated in Kr and Xe: when compared to the standard deviation of all ice-core measurements taken together (noted σ all), δ(Kr/Ar) is 5.4σ all higher than the control samples, and δ(Xe/Ar) is 4.5σ all all higher than the control samples (Table 3). However, we have used very thin samples, which can be subject to the loss of argon through cut bubbles and permeation (Reference Severinghaus and BattleSeveringhaus and Battle, 2006). We calculated the respective contributions of gravitational fractionation, gas loss and melt for all samples (Table 3), and found more gas loss than other samples (λ = 0.007 ± 0.0004), and a melt contribution that is not statistically significant (γ = 0.0026 ± 0.0024).

We thus conclude that the 1 mm bubble-free layers in the WAIS Divide ice core are unlikely to contain any significant amount of melt, and do not alter the gas composition of the ice core. This observation is confirmed by the studies of CO2 in the same WDC05A core, which show no sensitivity to the presence of a bubble-free layer in a sample (Reference Ahn, Brook and HowellAhn and others, 2009). The absence of melt is also supported by the observation that the bubble-free layers occur in winter as well as in summer (Reference FegyveresiFegyveresi, 2014). A possible mechanism for the formation of a bubble-free layer is the infilling of pores by condensation on a surface crust during times of subsurface temperature inversion (Reference Fujii and KusunokiFujii and Kusunoki, 1982). This is consistent with the prevalence of these layers in late summer, when the surface is cooler than the firn (Reference FegyveresiFegyveresi, 2014), and suggests that we may be able to use the frequency and thickness of bubble-free layers as a proxy for the occurrence of surface temperature inversions. The formation of these layers could also involve other mechanisms and is the subject of another paper (Reference FegyveresiFegyveresi, 2014).

Influence on gas diffusion

Gases diffuse through the firn before getting locked in within a zone in which the air is separated from the atmosphere, at ∼66 m depth at WAIS Divide (Reference BattleBattle and others, 2011). The firn matrix determines the effective gas diffusivity, which impacts the width of the age distribution of bubble air, and the lock-in horizon, which determines the age difference between gases and ice, and ultimately the gas chronology. The presence of an impermeable layer can thus sharpen the gas age distribution by preventing diffusion, and create an abrupt change in the gas chronology.

When they are in the firn, bubble-free layers are likely to have lower permeability than the surrounding firn. We were actually unable to find ice lenses in 2 m snow pits dug in 2008, 2009 or 2010, but we found evidence for numerous hard ‘crusts’ (Fig. 5c). In addition, these crusts typically have a limited spatial extent, on the order of 3 m × 10 m, and they can be broken by what looks like thermal-contraction cracks, through which gases can flow (Fig. 5b).

Fig. 5. Hard surfaces (‘crusts’) seen at WAIS Divide (a). These crusts are hard enough that the weight of a person does not puncture through. They can be laced by cracks, likely formed by thermal contraction (b). The thickness of these crusts can be up to 5 mm (c). They are common, but their horizontal extent is limited.

Wind crusts are composed of well-sintered but clearly separate snow grains (e.g. Reference AlleyAlley, 1988). Permeability measurements on samples with wind crusts at Greenland Summit confirm that they are permeable. For example, samples of firn containing crusts from ∼7m depth in the firn have permeability ranging from 25 to 33.8 × 10−10 m2, which is well within the range of permeability commonly found in near-surface firn (e.g. Reference Albert and ShultzAlbert and Shultz, 2002).

In fact, even melt layers have been measured to have nonzero permeability. The permeability of ice layers resulting from melt has been measured from the NEEM core; these layers (Reference Keegan, Albert and BakerKeegan and others, 2014), and also ice layers measured in seasonal snowpacks (Reference Albert and PerronAlbert and Perron, 2000), all show that bulk samples containing naturally formed melt layers have a permeability above 3 × 10−10 m2.

Although there are numerous fine-grained, sintered crusts in the WAIS Divide firn, there is no evidence of any discontinuity or impedance in gas diffusion (Reference BattleBattle and others, 2011). The lack of discontinuity in the CO2 record, a gas with changing atmospheric history, has also shown that there is no stratigraphic disturbance over a bubble-free layer (Reference Ahn, Brook and HowellAhn and others, 2009). This evidence illustrates the fact that gases are sensitive to bulk permeability at a scale larger than the diameter of a typical ice core, and that the presence of low-permeability layers of limited spatial extent does not have a measurable impact on gas diffusion in the open firn.

We conclude from these observations that the presence of bubble-free layers does not compromise the integrity of the gas record at WAIS Divide.

Conclusion

Melt layers in ice cores have been identified visually by their absence of bubbles, by the lower air content and by elevated CH4 or N2O, but each of these methods has limitations: elevated CH4 or N2O can be due to the presence of microbes in the ice (NEEM community members, 2013), air content is not necessarily lowered in a sample containing a melt layer (Table 3) and bubble-free layers are not easily visually recognizable in deep ice. In this paper, we have shown that the analysis of Kr/Ar and Xe/Ar is a robust method to identify melt layers, and can be used to detect warm-summer events during the Eemian at NEEM. The presence of a large amount of melt in NEEM Eemian ice confirms that Greenland was warmer than at present, and that the additional warmth had a strong summer component.

The WAIS Divide ice core contains many 1 mm thick bubble-free layers. These layers do not contain any significant amount of refrozen meltwater, and do not measurably impede gas flow in the firn. In this respect, they do not interfere with the gas record. The presence of these layers during winter, and the absence in them of anomalously high concentrations of soluble gases, suggests that they can be formed by processes other than melt, perhaps by the infilling of pores by condensation on a surface crust during periods of subsurface temperature inversion. These results point out that the visual identification of the absence of bubbles in thin crusts is not in itself proof of a melt event.

Acknowledgements

This work was supported by US National Science Foundation (NSF) grants 0440701, 0538630, 0521642, 0944343 and 0806377 to J.P.S, 1142085 and 1043528 to R.B.A., and Japan Society for the Promotion of Science (JSPS) KAKENHI grants 22221002, 21671001 and 26241011 to K.K. Joan Fitzpatrick assisted with the imaging of bubble sections. We appreciate the support of the WAIS Divide Science Coordination Office at the Desert Research Institute, Reno NE, USA, and University of New Hampshire, USA, for the collection and distribution of the WAIS Divide ice core and related tasks (NSF grants 0230396, 0440817, 0944348 and 0944266). Kendrick Taylor led the field effort that collected the samples. The NSF Office of Polar Programs also funded the Ice Drilling Program Office (IDPO) and Ice Drilling Design and Operations (IDDO) group for coring activities; the National Ice Core Laboratory for curation of the core; Raytheon Polar Services for logistics support in Antarctica; and the 109th New York Air National Guard for airlift in Antarctica. NEEM is directed and organized by the Center of Ice and Climate at the Niels Bohr Institute and US NSF Office of Polar Programs. It is supported by funding agencies and institutions in Belgium (FNRS-CFB and FWO), Canada (GSC), China (CAS), Denmark (FIST), France (IPEV and INSU/CNRS), Germany (AWI), Iceland (RannIs), Japan (NIPR), Korea (KOPRI), the Netherlands (NWO/ALW), Sweden (VR), Switzerland (SNF), United Kingdom (NERC) and the United States (US NSF Office of Polar Programs).

Appendix: Lower Detection Limit

Thin melt layers are measured in a sample containing mostly bubbly ice. Here we quantify the minimum amount of melt that can be detected by this method. (The symbols used are explained in Table 4.) We estimate the uncertainty in the reconstructed (Z, λ, γ) by a Monte Carlo perturbation experiment: We build a set of 1000 virtual measurements (δ40 Ar, δ(Kr/Ar), δ(Xe/Ar)) by adding random numbers of zero mean, and σ pooled variance to an initial set of (δ40 Ar, δ (Kr/Ar), δ(Xe/Ar)) corresponding to Z = 45, λ = 0, γ = 0. We used the pooled standard deviation of repeat measurements of ice cores from the same depth as an estimate of the uncertainty in the measurements (Reference Severinghaus, Grachev, Luz and CaillonSeveringhaus and others, 2003): σ pooled40 Ar) = 0.054‰, σ pooled (δ(Kr/Ar)) = 0.74‰ and σ pooled (δ(Xe/Ar)) = 2.87‰. For each set of virtual measurements, we calculate (Z, λ, γ), and estimate the uncertainty as the standard deviation of the 1000 values of (Z, λ, γ). We find σ(Z) = 0.83 m, σ(γ) = 0.0004 and σ(γ) = 0.0058. We can establish that a sample contains melt at the 99% confidence level if γ > 3 σ(γ) = 0.017.

Table 4. List of symbols

For a thin layer of melt that had time to come to full equilibrium before refreezing, and with no loss of gas during refreezing, γ = V liquid/V air. For a 50 g sample, there is ∼5 Ml of air, so we are able to detect V liquid = γV air = 0.017 × 5 = 0.085 mL of liquid water. For an ice core of 10 cm diameter, this corresponds to a 10 μm thick ice lens.

Alternatively, it is possible that a thin film of water melted and refroze before reaching equilibrium. In this case, we can calculate the time-varying flux of gas between the air and the liquid water following

(A1)

where F is the flux into the water, kc is the gas transfer velocity, A is the surface area considered, C sat is the concentration of gas at saturation, and C is the concentration of gas in the liquid. The time-varying concentration of gas in the water follows

(A2)

which has a solution of the form

(A3)

where C 0 is the initial concentration, C sat is the concentration at saturation and h is the thickness of the water layer. The gas transfer velocity is 3.7 ± 1 × 10−6 m s−1 for Ar, and 3.1 ± 1 × 10−6 m s−1 for Kr at 0°C (Table 4; Reference Jähne, Heinz and DietrichJähne and others, 1987; Reference Cole, Bade, Bastviken, Pace and Van de BogertCole and others, 2010; Reference Nicholson, Emerson, Caillon, Jouzel and HammeNicholson and others, 2010). The e-folding time to reach equilibrium for a 1mm thick layer is τ = h/kc = 1 × 10−3 /(3.1 × 10−6) = 320 s = 5.3 min. Starting from C 0 = 0, and for t small, we can approximate

(A4)

We can adapt Eqn (11) by replacing S by Skct/h:

(A5)

We can substitute γ with γ(t) = V l/V b kt/h. For a layer of thickness h = 1 mm, with k Kr = 3.1 × 10−6m s−1, we will reach γ(t) = 0.017 after t = γV b/V l h/k = (0.017 × 5)/(π × 5 × 5 × 0.1) × 1 × 10−3/(3.1 × 10−6) = 3.5 s.

We conclude from this analysis that a thin film of liquid is very likely to produce a measurable signal, even if it only stays melted for a few seconds. We note that a thin film of liquid is more likely to efficiently expel all gases during refreezing, and it is possible that micro-melt coating snow grains or sitting in interstices would not be detected by δ(Kr/ Ar) and δ(Xe/Ar) measurements. However, such a layer would also not alter the gas composition of the ice core, and likely not be an indicator of widespread summer warmth.

References

Ahn, J, Headly, M, Wahlen, M, Brook, EJ, Mayewski, PA and Taylor, KC (2008) CO2 diffusion in polar ice: observations from naturally formed CO2 spikes in the Siple Dome (Antarctica) ice core. J. Glaciol., 54(187), 685695 (doi: 10.3189/002214308786570764)CrossRefGoogle Scholar
Ahn, J, Brook, EJ and Howell, K (2009) A high-precision method for measurement of paleoatmospheric CO2 in small polar ice samples. J. Glaciol., 55(191), 499506 (doi: 10.3189/002214309788816731)CrossRefGoogle Scholar
Ahn, J and 7 others (2012) Atmospheric CO2 over the last 1000 years: a high-resolution record from the West Antarctic Ice Sheet (WAIS) Divide ice core. Global Biogeochem. Cycles, 26(2), GB2027 (doi: 10.1029/2011GB004247)Google Scholar
Albert, MR and Perron, F (2000) Ice layer and surface crust permeability in a seasonal snowpack. Hydrol. Process., 14(18), 32073214 (doi: 10.1002/1099-1085(20001230) 14:183.0.CO;2-C)Google Scholar
Albert, MR and Shultz, E (2002) Snow and firn properties and air–snow transport processes at Summit, Greenland. Atmos. Environ., 36, 27892797 (doi: 10.1016/S1352-2310(02)00119-X)Google Scholar
Albert, M, Shuman, C, Courville, Z, Bauer, R, Fahnestock, M and Scambos, T (2004) Extreme firn metamorphism: impact of decades of vapor transport on near-surface firn at a low-accumulation glazed site on the East Antarctic plateau. Ann. Glaciol., 39(1), 7378 (doi: 10.3189/172756404781814041)Google Scholar
Alley, RB (1988) Concerning the deposition and diagenesis of strata in polar firn. J. Glaciol., 34(118), 283290 Google Scholar
Alley, RB and Anandakrishnan, S (1995) Variations in melt-layer frequency in the GISP2 ice core: implications for Holocene summer temperatures in central Greenland. Ann. Glaciol., 21, 6770 Google Scholar
Axford, Y, Briner, JP, Francis, DR, Miller, GH, Walker, IR and Wolfe, AP (2011) Chironomids record terrestrial temperature changes throughout Arctic interglacials of the past 200,000 yr. Geol. Soc. Am. Bull., 123(7–8), 12751287 (doi: 10.1130/B30329.1)Google Scholar
Battle, MO and 8 others (2011) Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide. Atmos. Chem. Phys., 11(21), 11 00711 021 (doi: 10.5194/acp-11-11007-2011)Google Scholar
Bender, ML, Sowers, T and Lipenkov, V (1995) On the concentrations of O2, N2, and Ar in trapped gases from ice cores. J. Geophys. Res., 100(D9), 18 65118 660 (doi: 10.1029/94JD02212)Google Scholar
Box, JE, Fettweis, X, Stroeve, JC, Tedesco, M, Hall, DK and Steffen, K (2012) Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers. Cryosphere, 6(4), 821839 (doi: 10.5194/tc-6-821-2012)Google Scholar
Buizert, C and 9 others (2014) The WAIS-Divide deep ice core WD2014 chronology – Part 2: Methane synchronization (68–31 ka BP) and the gas age–ice age difference. Climate Past Discuss., 10(4), 35373584 Google Scholar
Cole, JJ, Bade, DL, Bastviken, D, Pace, ML and Van de Bogert, M (2010) Multiple approaches to estimating air-water gas exchange in small lakes. Limnol. Oceanogr., 8, 285293 (doi: 10.4319/lom.2010.8.285)CrossRefGoogle Scholar
Craig, H and Wiens, RC (1996) Gravitational enrichment of 84Kr/36 Ar ratios in polar ice caps: a measure of firn thickness and accumulation temperature. Science, 271(5256), 17081710 (doi: 10.1126/science.271.5256.1708)Google Scholar
Craig, H, Horibe, Y and Sowers, T (1988) Gravitational separation of gases and isotopes in polar ice caps. Science, 242(4886), 16751678 CrossRefGoogle ScholarPubMed
Dahl-Jensen, D and Johnsen, SJ (1986) Palaeotemperatures still exist in the Greenland ice sheet. Nature, 320(6059), 250252 (doi: 10.1038/320250a0)Google Scholar
Das, S and Alley, RB (2005) Characterization and formation of melt layers in polar snow: observations and experiments from West Antarctica. J. Glaciol., 51(173), 307313 (doi: 10.3189/ 172756505781829395)Google Scholar
Das, S and Alley, RB (2008) Rise in frequency of surface melting at Siple Dome through the Holocene: evidence for increasing marine influence on the climate of West Antarctica. J. Geophys. Res., 113(D2), D02112 (doi: 10.1029/ 2007JD008790)Google Scholar
Fegyveresi, JM (2014) Physical properties of the West Antarctic Ice Sheet (WAIS) Divide deep ice core: development, evolution and interpretation. (PhD thesis, The Pennsylvania State University)Google Scholar
Fitzpatrick, JJ and 9 others (2014) Physical properties of the WAIS Divide ice core. J. Glaciol., 60(224), 11811198 (doi: 10.3189/2014JoG14J100)Google Scholar
Forster, RR and 12 others (2014) Extensive liquid meltwater storage in firn within the Greenland ice sheet. Nature Geosci., 7(2), 9598 (doi: 10.1038/ngeo2043)Google Scholar
Francis, DR, Wolfe, AP, Walker, IR and Miller, GF (2006) Interglacial and Holocene temperature reconstructions based on midge remains in sediments of two lakes from Baffin Island, Nunavut, Arctic Canada. Palaeogeogr. Palaeoclimatol. Palaeoecol., 236, 107124 (doi: 10.1016/j.palaeo.2006.01.005)Google Scholar
Fujii, Y and Kusunoki, K (1982) The role of sublimation and condensation in the formation of ice sheet surface at Mizuho Station, Antarctica. J. Geophys. Res., 87(C6), 42934300 (doi: 10.1029/JC087iC06p04293)Google Scholar
Hall, DK, Comiso, JC, DiGirolamo, NE, Shuman, CA, Box, JE and Koenig, LS (2013) Variability in the surface temperature and melt extent of the Greenland ice sheet from MODIS. Geophys. Res. Lett., 40(10), 21142120 (doi: 10.1002/grl.50240)Google Scholar
Hamme, RC and Emerson, SR (2004) The solubility of neon, nitrogen and argon in distilled water and seawater. Deep Sea Res. I, 51(11), 15171528 (doi: 10.1016/j.dsr.2004.06.009)Google Scholar
Harper, J, Humphrey, N, Pfeffer, WT, Brown, J and Fettweis, X (2012) Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature, 491(7423), 240243 (doi: 10.1038/nature11566)Google Scholar
Headly, MA and Severinghaus, JP (2007) A method to measure Kr/N2 ratios in air bubbles trapped in ice cores, and its application in reconstructing past mean ocean temperature. J. Geophys. Res., 112(D19), D19105 (doi: 10.1029/2006JD008317)Google Scholar
Herron, MM, Herron, SL and Langway, CC Jr (1981) Climatic signal of ice melt features in Southern Greenland. Nature, 293(5831), 389391 (doi: 10.1038/293389a0)Google Scholar
Jähne, B, Heinz, G and Dietrich, W (1987) Measurement of the diffusion coefficients of sparingly soluble gases in water. J. Geophys. Res., 92(C10), 10 76710 776 (10.1029/JC092iC10p10767)Google Scholar
Kameda, T, Narita, H, Shoji, H, Nishio, F, Fujii, Y and Wanatabe, O (1995) Melt features in ice cores from Site J, southern Greenland: some implications for summer climate since AD 1550. Ann. Glaciol., 21, 5158 Google Scholar
Keegan, K, Albert, MR and Baker, I (2014) The impact of ice layers on gas transport through firn at the North Greenland Eemian Ice Drilling (NEEM) site, Greenland. Cryosphere, 8(5), 18011806 (doi: 10.5194/tc-8-1801-2014)Google Scholar
Langway, CC Jr and Shoji, H (1990) Past temperature record from the analysis of melt features in the Dye 3, Greenland, Ice Core. Ann. Glaciol., 14, 343344 CrossRefGoogle Scholar
Lunt, DJ and 9 others (2012) A multi model assessment of last interglacial temperatures. Climate Past, 8, 36573691 (doi: 10.5194/cp-9-699-2013)Google Scholar
Malone, JL, Castro, M, Hall, CM, Doran, PT, Kenig, F and McKay, CP (2010) New insights into the origin and evolution of Lake Vida, McMurdo Dry Valleys, Antarctica – a noble gas study in ice and brines. Earth Planet. Sci. Lett., 289(1), 112122 (doi: 10.1016/j. epsl.2009.10.034)Google Scholar
Marcott, SA and 9 others (2014) Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature, 514(7524), 616619 (doi: 10.1038/nature13799)Google Scholar
Masson-Delmotte, V and 18 others (2011). A comparison of the present and last interglacial periods in six Antarctic ice cores. Climate Past, 7(2), 397423 (doi: 10.5194/cp-7-397-2011)Google Scholar
McGrath, D, Colgan, W, Bayou, N, Muto, A and Steffen, K (2013) Recent warming at Summit, Greenland: global context and implications. Geophys. Res. Lett., 40(10), 20912096 (doi: 10.1002/grl.50456)Google Scholar
Miller, SL (1969) Clathrate hydrates of air in Antarctic ice. Science, 165(3892), 489490 (doi: 10.1126/science.165.3892.489)Google Scholar
Morris, RM, Mair, DWF, Nienow, PW, Bell, C, Burgess, DO and Wright, AP (2014) Field-calibrated model of melt, refreezing, and runoff for polar ice caps: application to Devon Ice Cap. J. Geophys. Res.: Earth Surf. (doi: 10.1002/2014JF003100)Google Scholar
NEEM community members ( 2013) Eemian interglacial reconstructed from a Greenland folded ice core. Nature, 493(7433), 489494 (doi: 10.1038/nature11789)Google Scholar
Nghiem, SV and 8 others (2012) The extreme melt across the Greenland ice sheet in 2012. Geophys. Res. Lett., 39(20), L20502 (doi: 10.1029/2012GL053611)Google Scholar
Nicholson, D, Emerson, S, Caillon, N, Jouzel, J and Hamme, RC (2010) Constraining ventilation during deepwater formation using deep ocean measurements of the dissolved gas ratios40 Ar/36 Ar, N2/ Ar, and Kr/Ar. J. Geophys. Res.: Oceans, 115(C11), C11015 (doi: 10.1029/2010JC006152)Google Scholar
Orsi, AJ (2013) Temperature reconstruction at the West Antarctic Ice Sheet Divide, for the last millennium, from the combination of borehole temperature and inert gas isotope measurements. (PhD thesis, University of California, San Diego)Google Scholar
Otto-Bliesner, BL (2006) Simulating arctic climate warmth and icefield retreat in the last interglaciation. Science, 311(5768), 17511753 (doi: 10.1126/science.1120808)CrossRefGoogle ScholarPubMed
Pfeffer, WT and Humphrey, NF (1998) Formation of ice layers by infiltration and refreezing of meltwater. Ann. Glaciol., 26, 8391 Google Scholar
Pfeffer, WT, Illangasekare, TH and Meier, MF (1990) Analysis and modeling of meltwater refreezing in dry snow. J. Glaciol, 36(123), 238246 Google Scholar
Pfeffer, WT, Meier, MF and Illangasekare, TH (1991) Retention of Greenland runoff by refreezing: implications for projected future sea level change. J. Geophys. Res.: Oceans (1978–2012), 96(C12), 22 11722 124 (doi: 10.1029/91JC02502)Google Scholar
Severinghaus, JP and Battle, MO (2006) Fractionation of gases in polar ice during bubble close-off: new constraints from firn air Ne, Kr and Xe observations. Earth Planet. Sci. Lett., 244(1–2), 474500 (doi: 10.1016/j.epsl.2006.01.032)Google Scholar
Severinghaus, JP, Grachev, A, Luz, B and Caillon, AN (2003) A method for precise measurement of argon 40/36 and krypton/argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in Greenland and at Siple Dome, Antarctica. Geochim. Cosmochim. Acta, 67, 325343 (doi: 10.1016/S0016-7037(02)00965-1)Google Scholar
Tedesco, M, Serreze, M and Fettweis, X (2008) Diagnosing the extreme surface melt event over southwestern Greenland in 2007. Cryosphere, 2(2), 159166 (doi: 10.5194/tc-2-159-2008)Google Scholar
Tedesco, M and 7 others (2011) The role of albedo and accumulation in the 2010 melting record in Greenland. Envir. Res. Lett., 6(1), 014005 (doi: 10.1088/1748-9326/6/1/014005)Google Scholar
Top, Z, Martin, S and Becker, P (1988) A laboratory study of dissolved noble gas anomaly due to ice formation. Geophys. Res. Lett., 15(8), 796799 (doi: 10.1029/GL015i008p00796)Google Scholar
WAIS Divide Project Members ( 2013) Onset of deglacial warming in West Antarctica driven by local orbital forcing. Nature, 500(7463), 440444 (doi: 10.1016/0011-7471(70)90037-9)Google Scholar
Weiss, RF (1970) The solubility of nitrogen, oxygen, and argon in water and seawater. Deep Sea Res., 17, 721735 (doi: 10.1021/ je60076a014)Google Scholar
Weiss, RF and Kyser, TK (1978) Solubility of krypton in water and seawater. J. Chem Eng. Data, 23(1), 6972 Google Scholar
Wong, GJ, Hawley, RL, Lutz, ER and Osterberg, EC (2013) Trace-element and physical response to melt percolation in Summit (Greenland) snow. Ann. Glaciol., 54(63), 5262 (doi: 10.3189/ 2013AoG63A602)Google Scholar
Wood, D and Caputi, R (1966) Solubilities of Kr and Xe in fresh and seawater. (Tech. Rep. USNRDL-TR-988) US Naval Radiological Defense Laboratory, San Francisco, CA Google Scholar
Figure 0

Fig. 1. Melt layers from the NEEM (North Greenland) ice core at 44.3 m. This melt event is dated to AD 1888. Photo courtesy of Kaitlin Keegan.

Figure 1

Table 1. Parameter values. Solubility coefficients (β) are calculated at 0°C (Weiss and Kyser, 1978; Hamme and Emerson, 2004). Gas transfer velocities (kc) are from Nicholson and others (2010) and Cole and others (2010), with Schmidt numbers from Jähne and others (1987). p, β, S and kc are given for the bulk Kr and Xe rather than for a specific isotope

Figure 2

Table 2. Measurements of the Dye 3 ice core. Samples with visible melt layers are shown in bold font (144.2–144.31 m and 144.47–144.6 m). Replicates from the same depth are labeled a, b, c, etc. The air content (column 4) is expressed as the ratio of the sample with respect to the mean of all non-melt samples. The rightmost three columns show the calculated diffusive column height Z, the gas loss fraction λ and the melt fraction γ. The mean and standard deviation of all non-melt samples give an idea of the expected sample-to-sample variability. The pooled standard deviation σpooled describes the measurement error

Figure 3

Fig. 2. Melt layer identification. When δ(Kr/Ar) and δ(Xe/Ar) are high, they indicate the presence of melt, as can be seen in Dye 3 melt layer samples (solid diamonds), compared to non-melt samples (+). NEEM Eemian samples are shown in large squares. The analytical precision is better than 0.8‰ for δ(Kr/Ar) and 2.9‰ for δ(Xe/Ar), which is approximately the size of the markers. WAIS Divide bubble-free layers (filled circles) do not have anomalously high δ(Kr/Ar) and δ(Xe/Ar), which indicates that they do not contain a significant amount of melt. The box shows the range of values found in the WDC05A ice core. The dashed line indicates gravitational fractionation, and the solid lines indicate the fractionation due to melt.

Figure 4

Fig. 3. Measurements of δ40 Ar, δ(Kr/Ar) and δ(Xe/Ar) in the Eemian section of the NEEM ice core. The dashed line on top shows the modeled melt fraction, inferred from the measurements. Elevated δ(Kr/Ar) and δ(Xe/Ar) are clear evidence of the presence of melt in the ice core.

Figure 5

Fig. 4. A pair of bubble-free layers in the WDC05A core. The layers are 1 mm thick. Black in this image represents clear ice, and bubbles appear white. Photo by John Fegyveresi.

Figure 6

Table 3. Analysis of bubble-free layers in the WDC05A ice core. The δ values are given in ‰ with respect to modern air. Z, λ and γ are the outputs of the melt layer model. The bottom two rows show the mean and standard deviation of 73 WDC05A samples measured from 38 depths between 78 and 300 m. The uncertainty in the modeled Z, λ and γ and is calculated by a Monte Carlo perturbation with variances corresponding to the bottom row of the table, and is also shown in the bottom row

Figure 7

Fig. 5. Hard surfaces (‘crusts’) seen at WAIS Divide (a). These crusts are hard enough that the weight of a person does not puncture through. They can be laced by cracks, likely formed by thermal contraction (b). The thickness of these crusts can be up to 5 mm (c). They are common, but their horizontal extent is limited.

Figure 8

Table 4. List of symbols