The role of sublimation as a driver of climate signals in the water isotope content of surface snow: Laboratory and field experimental results

Ice core water isotope records from Greenland and Antarctica are a valuable proxy for paleoclimate reconstruction, yet the processes influencing the climate signal stored in the isotopic composition of the snow are being challenged and revisited. Apart from precipitation input, post-depositional processes such as wind-driven redistribution and vapor–snow exchange processes at and below the surface are hypothesized to contribute to the isotope climate signal subsequently stored in the ice. Recent field studies have shown that surface snow isotopes vary between precipitation events and co-vary with vapor isotopes, which demonstrates that vapor–snow exchange is an important driving mechanism. Here we investigate how vapor–snow exchange processes influence the isotopic composition of the snowpack. Controlled laboratory experiments under forced sublimation show an increase in snow isotopic composition of up to 8 ‰ δ18O in the uppermost layer due to sublimation, with an attenuated signal down to 3 cm snow depth over the course of 4–6 d. This enrichment is accompanied by a decrease in the second-order parameter d-excess, indicating kinetic fractionation processes. Our observations confirm that sublimation alone can lead to a strong enrichment of stable water isotopes in surface snow and subsequent enrichment in the layers below. To compare laboratory experiments with realistic polar conditions, we completed four 2–3 d field experiments at the East Greenland Ice Core Project site (northeast Greenland) in summer 2019. High-resolution temporal sampling of both natural and isolated snow was conducted under clear-sky conditions and demonstrated that the snow isotopic composition changes on hourly timescales. A change of snow isotope content associated with sublimation is currently not implemented in isotope-enabled climate models and is not taken into account when interpreting ice core isotopic records. However, our results demonstrate that post-depositional processes such as sublimation contribute to the climate signal recorded in the water isotopes in surface snow, in both laboratory and field settings. This suggests that the ice core water isotope signal may effectively integrate across multiple parameters, and the ice core climate record should be interpreted as such, particularly in regions of low accumulation.publishedVersio

Impurities throughout the EGRIP ice core – a microstructural perspective

Impurities in polar ice cores are analyzed for various reasons, ranging from the reconstruction of the climate of the past to the absolute positioning of age markers. In particular, microstructural impurity research provides insights into the internal deformation of ice and post-depositional stratigraphy changes. However, most stud- ies offer limited snapshots of impurity characteristics at a few specific depth regimes, highlighting the need to determine the localization and chemistry of impurities throughout one ice core with complementary methods. We report a detailed investigation of solid and dissolved impurities throughout the 2120 m long East Green- land Ice Core Project (EGRIP) ice core. Using microstructure mapping and confocal Cryo-Raman spectroscopy, we analyzed solid micro-inclusions inside 25 solid ice samples covering the last 50 ka. Micro-inclusions are heterogeneously distributed inside the ice matrix and in Holocene ice, as an upper limit assumption, between 22.3 and 42.4% are located in the vicinity of grain boundaries. We identified the mineralogy of more than 1600 solid inclusions. Most are terrestrial dust minerals, such as quartz, feldspar, mica, carbonaceous particles, and sulfate minerals, such as gypsum. Less common minerals are e.g., dolomite, hematite, nitrates, rutile, and anatase. However, the upper 900 m are characterized by various sulfate minerals, while gypsum is the domi- nant sulfate species below. In the deepest 400 m of the core, we expose the mineralogy inside and surrounding distinct cloudy bands. Aiming at a holistic picture of soluble and insoluble impurities, we combined two meth- ods for the first time: We further analyzed most samples with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Due to recent adaptions, LA-ICP-MS now enables us to image the 2D distribution of elements, such as Na, Mg, Al, and Fe, with a resolution of up to 10 microns showing element-depended dif- ferences in localization. For example, Na is primarily located at grain boundaries, and Al indicates dispersed particle clusters. Mg, and to some extent also Fe, are found in both regimes. Our results illustrate the merit of combining cryo-Raman spectroscopy and LA-ICP-MS to obtain new insights into small-scale deformation, chemical stratigraphy, and processes in deep ice and the future potential to enhance our understanding of impurities by exploiting such a multi-method approach

Evidence of Isotopic Fractionation During Vapor Exchange Between the Atmosphere and the Snow Surface in Greenland

Several recent studies from both Greenland and Antarctica have reported significant changes in the water isotopic composition of near‐surface snow between precipitation events. These changes have been linked to isotopic exchange with atmospheric water vapor and sublimation‐induced fractionation, but the processes are poorly constrained by observations. Understanding and quantifying these processes are crucial to both the interpretation of ice core climate proxies and the formulation of isotope‐enabled general circulation models. Here, we present continuous measurements of the water isotopic composition in surface snow and atmospheric vapor together with near‐surface atmospheric turbulence and snow‐air latent and sensible heat fluxes, obtained at the East Greenland Ice‐Core Project drilling site in summer 2016. For two 4‐day‐long time periods, significant diurnal variations in atmospheric water isotopologues are observed. A model is developed to explore the impact of this variability on the surface snow isotopic composition. Our model suggests that the snow isotopic composition in the upper subcentimeter of the snow exhibits a diurnal variation with amplitudes in δ18O and δD of ~2.5‰ and ~13‰, respectively. As comparison, such changes correspond to 10–20% of the magnitude of seasonal changes in interior Greenland snow pack isotopes and of the change across a glacial‐interglacial transition. Importantly, our observation and model results suggest, that sublimation‐induced fractionation needs to be included in simulations of exchanges between the vapor and the snow surface on diurnal timescales during summer cloud‐free conditions in northeast Greenland

Near-infrared sensitivity enhancement of photorefractive polymer composites by pre-illumination

Among the various applications for reversible holographic storage media, a particularly interesting one is time-gated holographic imaging (TGHI). This technique could provide a noninvasive medical diagnosis tool, related to optical coherence tomography. In this technique, biological samples are illuminated within their transparency windowwith near-infrared light, and information about subsurface features is obtained by a detection method that distinguishes between reflected photons originating from a certain depth and those scattered from various depths. Such an application requires reversible holographic storage media with very high sensitivity in the near-infrared. Photorefractive materials, in particular certain amorphous organic systems, are in principle promising candidate media, but their sensitivity has so far been too low, mainly owing to their long response times in the near-infrared. Here we introduce an organic photorefractive material—a composite based on the poly(arylene vinylene) copolymer TPD-PPV—that exhibits favourable near-infrared characteristics. We show that pre-illumination of this material at a shorter wavelength before holographic recording improves the response time by a factor of 40. This process was found to be reversible. We demonstrate multiple holographic recording with this technique at video rate under practical conditions

Comparison Of New Photorefractive Composites Based On A Poly(Phenylene Vinylene) Derivative With Traditional Poly(n-vinylcarbazole) Composites

The performances of two classes of photorefractive polymer composites with low glass-transition temperatures (about 10-5 °C) are compared. One is based on the commonly used photoconductor poly(N-vinylcarbazole) (PVK), i.e., containing isolated charge-transport moieties for hopping. The other is based on the π-conjugated poly[1,4-phenylene-1,2-di(4-benzyloxyphenyl)vinylene] (DBOP-PPV), promising faster response times. The steady-state performance of the DBOP-PPV-based composites was found to be superior owing to (i) the larger internal free volume, allowing more efficient poling of the chromophores, and (ii) the slightly stronger space-charge field as a result of an increased trap density. By contrast, the dynamic response in a four-wave mixing experiment was similar to that of PVK-based composites despite the higher hole-drift mobility in conjugated PPV homopolymers than PVK. It was demonstrated that this is mainly a result of the poor charge-carrier generation efficiency.