CO(2) Diffusion in Polar Ice: Observations from Naturally Formed CO(2) Spikes in the Siple Dome (Antarctica) Ice Core

One common assumption in interpreting ice-core CO(2) records is that diffusion in the ice does not affect the concentration profile. However, this assumption remains untested because the extremely small CO(2) diffusion coefficient in ice has not been accurately determined in the laboratory. In this study we take advantage of high levels of CO(2) associated with refrozen layers in an ice core from Siple Dome, Antarctica, to study CO(2) diffusion rates. We use noble gases (Xe/Ar and Kr/Ar), electrical conductivity and Ca(2+) ion concentrations to show that substantial CO(2) diffusion may occur in ice on timescales of thousands of years. We estimate the permeation coefficient for CO(2) in ice is similar to 4 x 10(-21) mol m(-1) s(-1) Pa(-1) at -23 degrees C in the top 287 m (corresponding to 2.74 kyr). Smoothing of the CO(2) record by diffusion at this depth/age is one or two orders of magnitude smaller than the smoothing in the firn. However, simulations for depths of similar to 930-950m (similar to 60-70 kyr) indicate that smoothing of the CO(2) record by diffusion in deep ice is comparable to smoothing in the firn. Other types of diffusion (e.g. via liquid in ice grain boundaries or veins) may also be important but their influence has not been quantified

Instruments and Methods: A novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon

We present techniques for obtaining large (∼100 L STP) samples of ancient air for analysis of ¹⁴C of methane (¹⁴CH₄) and other trace constituents. Paleoatmospheric ¹⁴CH₄ measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ([CH₄]) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bølling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of δ¹⁵N of N₂, δ¹⁸O of O₂, δ¹⁸O[subscript ice] and [CH₄] measurements. The [CH₄] blank of the melt extractions was <4 ppb. Measurements of δ¹⁸O and δ¹⁵N indicated no significant gas isotopic fractionation from handling. Measured Ar/N₂, CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar/N₂, Kr/Ar and Xe/Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample [CH₄]. Corrected [CH₄] is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH₄ production in ice at this site.This is the publisher’s final pdf. The published article is copyrighted by the International Glaciological Society and can be found at: http://www.igsoc.org/journal

Krypton and xenon in air trapped in polar ice cores : paleo-atmospheric measurements for estimating past mean ocean temperature and summer snowmelt frequency

Krypton and xenon are highly soluble noble gases. Because they are inert, they do not react biologically or chemically, and therefore can trace purely physical processes. By taking advantage of both the inert nature of these gases and their high solubilities, krypton and xenon can be used to reconstruct past ocean temperature variations and summer snow melt frequency. Ocean temperature is a fundamental parameter of the climate system. It plays a vital role in the transport and storage of heat, and may play a role in regulating atmospheric CO₂ , but its past variations are poorly constrained. This is due to the ambiguous nature of the benthic [delta]¹⁸O record in ocean sediments, which reflects both deep water temperature and the [delta]¹⁸O of the water itself (which depends on the extent of ice sheets on land). Recent studies have better constrained localized ocean temperature, but there is still need for global mean ocean temperature reconstructions. Krypton (Kr) and xenon (Xe) are highly soluble and more soluble in colder water. The total amount of Kr and Xe in the atmosphere and ocean together are essentially constant through time, so variations in mean ocean temperature would therefore modulate atmospheric Kr and Xe abundances. Kr and Xe, measured as ratios to nitrogen (N₂), are measured in air bubbles in ice cores to reconstruct atmospheric Kr/N₂ and Xe/N₂ histories, which can then be interpreted in terms of past mean ocean temperature. These Kr/N₂ and Xe/N₂ data and their derived mean ocean temperature (noble gas temperature index, NGTI) reconstructions are presented in Chapters 2 and 3. In Chapter 2, the initial Kr/N₂ data from the LGM indicate that mean ocean temperatures were 2̃.7°C colder at that time, which is consistent with other estimates of local deep ocean temperatures. In Chapter 3, [delta]Kr/N₂ and [delta]Xe/N₂ time series during the last glacial termination and inception are presented. The reconstructed mean ocean temperatures (NGTI's) are consistent with our earlier measurement and those of other studies. Additionally, these mean ocean temperature reconstructions appear to vary in step with atmospheric CO₂. Because Kr and Xe are highly soluble, they can also be used as an indicator of ice that has melted and refrozen. Visual identification of melt layers is been used as a proxy for exceptionally warm summers temperatures, but this type of melt layer identification becomes difficult as air bubbles form air clathrates at deeper depths. The use of Kr and Xe, measured as ratios to argon (Ar), is examined in Chapter 4. Seasononality may play a role in climate change, so a proxy of summer temperatures may prove to be a powerful constraint on climate change mechanisms that invoke seasonalit

CO₂ diffusion in polar ice: observations from naturally formed CO₂ spikes in the Siple Dome (Antarctica) ice core

One common assumption in interpreting ice-core CO₂ records is that diffusion in the ice does not affect the concentration profile. However, this assumption remains untested because the extremely small CO₂ diffusion coefficient in ice has not been accurately determined in the laboratory. In this study we take advantage of high levels of CO₂ associated with refrozen layers in an ice core from Siple Dome, Antarctica, to study CO₂ diffusion rates. We use noble gases (Xe/Ar and Kr/Ar), electrical conductivity and Ca²⁺ ion concentrations to show that substantial CO₂ diffusion may occur in ice on timescales of thousands of years. We estimate the permeation coefficient for CO₂ in ice is ~4 × 10⁻²¹ mol m⁻¹ s⁻¹: Pa⁻¹ at -23°C in the top 287 m (corresponding to 2.74 kyr). Smoothing of the CO₂ record by diffusion at this depth/age is one or two orders of magnitude smaller than the smoothing in the firn. However, simulations for depths of ~930-950 m (~60-70 kyr) indicate that smoothing of the CO₂ record by diffusion in deep ice is comparable to smoothing in the firn. Other types of diffusion (e.g. via liquid in ice grain boundaries or veins) may also be important but their influence has not been quantified

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

International audienceMelt 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 even

Instruments and Methods - A novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon

This is the publisher’s final pdf. The published article is copyrighted by the International Glaciological Society and can be found at: http://www.igsoc.org/journal/.We present techniques for obtaining large (∼100 L STP) samples of ancient air for analysis of ¹⁴C of methane (¹⁴CH₄) and other trace constituents. Paleoatmospheric ¹⁴CH₄ measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ([CH₄]) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bølling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of δ¹⁵N of N₂, δ¹⁸O of O₂, δ¹⁸O[subscript ice] and [CH₄] measurements. The [CH₄] blank of the melt extractions was <4 ppb. Measurements of δ¹⁸O and δ¹⁵N indicated no significant gas isotopic fractionation from handling. Measured Ar/N₂, CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar/N₂, Kr/Ar and Xe/Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample [CH₄]. Corrected [CH₄] is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH₄ production in ice at this site