A 4.5 Year‐Long Record of Svalbard Water Vapor Isotopic Composition Documents Winter Air Mass Origin

From May 2014 to September 2018, a laser spectrometer analyzer provided a 4.5 years continuous record of water vapor isotopic composition at Ny-Ålesund (8 m above sea level, a.s.l.), Svalbard. It corresponds to the longest data set published in polar regions. A comparison of this data set with a parallel similar data set obtained during 20 days by a second laser spectrometer installed near Mount Zeppelin (474 m a.s.l.) shows that this data set is representative of a regional signal. In addition, the observation of insignificant diurnal cycles in the isotopic signal compared to the strong isotopic signature of synoptic events and the comparison of simultaneous measurements in the vapor and in rain or snow samples lead to the conclusion that our record reflects a large part of the regional dynamics of the atmospheric water cycle driven by large-scale variability. This study focuses on winters dominated by the occurrence of synoptic events associated with humidity peaks. Using statistics and back trajectories calculations, we link high humidity peaks characterized by an anticorrelation between δ18O and d-excess in the water vapor to a rapid shift of air mass source regions from the Arctic to the North Atlantic Ocean below 60°N. On the other hand, correlation between δ18O and d-excess may be associated with a shift of air mass sources within the Arctic. These results demonstrate the added value of long-term water vapor isotopic monitoring to better understand the moisture origin in the Arctic and the atmospheric dynamics.publishedVersio

A modular field system for near-surface, vertical profiling of the atmospheric composition in harsh environments using cavity ring-down spectroscopy

Cavity ring-down spectroscopy (CRDS) has allowed for increasingly widespread, in situ observations of trace gases, including the stable isotopic composition of water vapor. However, gathering observations in harsh environments still poses challenges, particularly in regard to observing the small-scale exchanges taking place between the surface and atmosphere. It is especially important to resolve the vertical structure of these processes. We have designed the ISE-CUBE system as a modular CRDS deployment system for profiling stable water isotopes in the surface layer, specifically the lowermost 2 m above the surface. We tested the system during a 2-week field campaign during February–March 2020 in Ny-Ålesund, Svalbard, Norway, with ambient temperatures down to −30 ∘C. The system functioned suitably throughout the campaign, with field periods exhibiting only a marginal increase in isotopic measurement uncertainty (30 %) as compared to optimal laboratory operation. Over the 2 m profiling range, we have been able to measure and resolve gradients on the temporal and spatial scales needed in an Arctic environment.publishedVersio

A low-cost autonomous rover for polar science

We present the developmental considerations, design, and deployment of an autonomous modular terrestrial rover for ice-sheet exploration that is inexpensive, easy to construct, and allows for instrumentation customization. The total construction cost for this rover is less than USD 3000, approximately one-tenth the cost of existing platforms, and it can be built using facilities frequently available at academic institutions (machine shop, 3-D printer, open-source hardware and software). Instrumentation deployed on this rover can be customized; the rover presented in this study was equipped with a dual-frequency GPS receiver and a digital SLR camera for constructing digital elevation models using structure-from-motion (SfM) photogrammetry. We deployed this prototype rover on the Northeast Greenland Ice Stream to map local variations in snow accumulation and surface topography. The rover conducted four autonomous missions based out of the East Greenland Ice-Core Project (EastGRIP) camp during July 2017, measuring surface elevation transects across the hazardous ice-stream shear margins. During these missions, the rover proved capable of driving over 20 km on a single charge with a drawbar pull of 250 N, sufficient to tow instrumentation of up to 100 kg. The rover also acquired photographs that were subsequently used to construct digital elevation models of a site monitored for spatiotemporal variability in snow accumulation, demonstrating adequate stability for high-resolution imaging applications. Due to its low cost, low-power requirements, and simple modular design, mass deployments of this rover design are practicable. Operation of the rover in hazardous areas circumvents the substantial expense and risk to personnel associated with conventional, crewed deployments. Thus, this rover is an investigatory platform that enables direct exploration of polar environments considered too hazardous for conventional field expeditions.publishedVersio

Estimation of moisture fluxes in East Antarctica and their impact on the isotopic composition of the snow surface

The ability to infer past temperatures from ice core records has in the past relied on the assumption that after precipitation, the stable water isotopic composition of the snow surface layer is not modified before being buried deeper into the snowpack and transformed into ice. However, in extremely dry environments, such as the East Antarctic plateau, the precipitation is so sparse that the surface is exposed to the atmosphere for significant time before burial. During that exposure, several processes have been recently identified as impacting the snow isotopic composition after snowfall: (1) exchanges with the atmosphere (i.e. sublimation/condensation cycles), (2) wind effects (i.e. redistribution and pumping) and (3) exchanges with the firn below (i.e. metamorphism and diffusion). Here we present the data over several seasons and years of the atmospheric water vapor and snow surface isotopic composition at Dome C, East Antarctica. To understand the link between these two elements, we investigate the moisture fluxes at the surface of the ice sheet, at the snowair interface. No eddy-covariance measurements are available for the recent years, we therefore make use of the available primary meteorological parameters measured continuously on site to estimate the surface moisture fluxes using the bulk method. We estimate that the cumulative effect of the moisture fluxes is positive: about 12% of the mean annual accumulation is sublimated away. Alongside, we see an enrichment in d18O in the snow surface during the summer months, when most of the moisture fluxes are taking place. The snow d-excess is also affected and evolving in anti-phase with d18O. This indicates occurrence of fractionation during sublimation in line with previous field and laboratory studies. The moisture fluxes could be a key driver of changes in the snow isotopic composition between precipitation events influencing the climate signal stored in the isotopic record of ice cores

From precipitation to ice core: On the importance of surface processes for stable water-isotope records in East Antarctica

Stable water-isotope records from Antarctic ice cores allow the reconstruction of past temperature variability. However, accurate interpretation of the isotopic signal requires comprehensive understanding of the processes leading to its archiving in snow and ice, which can be documented by in situ measurements

On the importance of the humidity flux for the surface mass balance in the accumulation zone of the Greenland Ice Sheet

It is highly uncertain how the humidity flux between the snow surface and the atmosphere contributes to the surface mass balance (SMB) of the interior Greenland Ice Sheet (GrIS). Due to sparse observations, evaluations of the simulated humidity flux are limited. Model-based estimates of the humidity flux contribution to the SMB are, therefore, unconstrained and even disagree in magnitude and sign. In this study, we evaluate the regional climate model MAR at the EGRIP (East Greenland Ice-Core Project) site in the accumulation zone of the GrIS. We use a combined dataset of continuous one-level bulk estimates of the humidity flux covering the period 05/2016&ndash;08/2019 and direct eddy-covariance humidity flux measurements from all four summer seasons. In summer, we document a bias of too little sublimation (-1.3 W m&minus;2) caused by a cold bias in both air and surface temperature leading to a reduced humidity gradient. In winter, MAR overestimates deposition by about one order of magnitude. This is a consequence of an overestimated temperature gradient in too stable atmospheric conditions compared to observations. Both systematic errors cause a large discrepancy in the annual net humidity flux between the model and observations of -9 mm w. eq. yr&minus;1. Remarkably, the simulated net annual humidity flux contributes positively to the SMB, contrary to observations documenting a net sublimation flux. We correct the systematic errors by applying a simple but effective correction function to the simulated latent heat flux. Using this correction, we find that 5.1 % of the annual mass gain at the EGRIP site sublimates again, and 4.3 % of the total mass gain is deposited vapor from the near-surface air. The estimated net humidity flux contribution to the annual SMB is about -1 % (net sublimation) compared to +5.6 % for the uncorrected simulation. In summer, the corrected MAR simulation shows that deposition accounts for 9.6 % of the total mass gain and that 31 % of the total mass gain at the EGRIP site sublimates again. The net fluxes contribute to -32 % of the summer SMB. These results demonstrate that the humidity flux is a major driver of the summer SMB in the accumulation zone of the GrIS and highlight that even small changes could increase its importance for the annual SMB in a warming climate.</p

Non-Equilibrium Fractionation Factors for D/H and 18O/16O During Oceanic Evaporation in the North-West Atlantic Region

Ocean isotopic evaporation models, such as the Craig-Gordon model, rely on the description of nonequilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient-diffusion type measurements have been performed in ocean settings to test the validity of the commonly used parametrization of nonequilibrium isotopic fractionation during ocean evaporation. In this work, we present 6 months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating nonequilibrium fractionation factors (k, ‰) for ocean evaporation and their wind speed dependency. The Keeling Plot method and Craig-Gordon model combination were sensitive enough to resolve nonequilibrium fractionation factors during evaporation resulting into mean values of k18 = 5.2 ± 0.6‰ and k2 = 4.3 ± 3.4‰. Furthermore, we evaluate the relationship between k and 10-m wind speed over the ocean. Such a relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (a) in the observed wind speed range [0–10 m s−1], the sensitivity of k to wind speed is small, in the order of −0.2‰ m−1 s for k18, and (b) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. The water vapor d-excess variability predicted under the closure assumption using the k values estimated in this study is in agreement with observations over the Atlantic Ocean.publishedVersio

A Snapshot on the Buildup of the Stable Water Isotopic Signal in the Upper Snowpack at EastGRIP on the Greenland Ice Sheet

The stable water isotopic composition in firn and ice cores provides valuable information on past climatic conditions. Because of uneven accumulation and post-depositional modifications on local spatial scales up to hundreds of meters, time series derived from adjacent cores differ significantly and do not directly reflect the temporal evolution of the precipitated snow isotopic signal. Hence, a characterization of how the isotopic profile in the snow develops is needed to reliably interpret the isotopic variability in firn and ice cores. By combining digital elevation models of the snow surface and repeated high-resolution snow sampling for stable water isotope measurements of a transect at the East Greenland Ice-core Project campsite on the Greenland Ice Sheet, we are able to visualize the buildup and post-depositional changes of the upper snowpack across one summer season. To this end, 30 cm deep snow profiles were sampled on six dates at 20 adjacent locations along a 40 m transect. Near-daily photogrammetry provided snow height information for the same transect. Our data shows that erosion and redeposition of the original snowfall lead to a complex stratification in the δ18O signature. Post-depositional processes through vapor-snow exchange affect the near surface snow with d-excess showing a decrease in surface and near-surface layers. Our data suggests that the interplay of stratigraphic noise, accumulation intermittency, and local post-depositional processes form the proxy signal in the upper snowpack.publishedVersio