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

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.

Surface formation, preservation, and history of low-porosity crusts at the WAIS Divide site, West Antarctica

“…We thus trust the basic result, with agreement between model and measured temperatures, that the energy balance generates sufficient heat fluxes to drive the vapor mass transport needed for glazed-crust development (Pinzer et al, 2012). We are confident that a more comprehensive study extending this work would be instructive, with additional sampling and modeling addressing the overall radiative and SEB responses, boundary-layer stability responses, cloud forcings, and vapor mass flux rates.…”

“…Crusts form during days when atmospheric humidity is low, however, and thus when mass is not being added from above. We have not observed bulk melting at the site (with the one possible exception noted above), nor do the gas measurements of Orsi et al (2015) indicate bulk melting, so the density increase must arise from some combination of vapor diffusion from below and surface or volume mass transfer likely involving pseudo-liquid layers (Dash et al, 2006), as discussed next.…”

“…The general elongated shape of the bubbles is due the proximity of this sample to the bubble close-off depth at WAIS Divide of ∼ 75 m. This sample is from the secondary WDC05A core at the WAIS Divide site. Image modified from Orsi et al (2015). served continually by one of us (John M. Fegyveresi) during the five field seasons extending from 2008-2009 to 2012-2013 (Table 1).…”

“…Work by Orsi et al (2015) and Mitchell et al (2015) showed that no significant artifacts are introduced to paleoclimatic records by the WAIS Divide crusts. Here, we report additional studies showing that summertime crusts form under specific conditions linked to persistent high-pressure systems, so the time series of crusts likely contains paleoclimatic information; however, many additional issues must be addressed before useful climate histories can be constructed confidently.…”

“…These are of interest as paleoclimatic records but have the potential to anomalously distort records of trapped gases or other components of ice cores. Refrozen meltwater can be identified by an excess of trapped heavy noble gases, so Orsi et al (2015) analyzed WAIS Divide samples containing bubble-free crusts, finding that not enough meltwater was involved to significantly perturb records of other trace gases. Additionally, crusts might greatly modify gas trapping in the firn, but measured nitrogen-isotopic ratios at WAIS Divide show that gravitational fractionation occurs down to the normal trapping depth where normal amounts of air are trapped, demonstrating that the crusts are not both impermeable and laterally extensive at shallow depth (Mitchell et al, 2015;Battle et al, 2011).…”