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

The isotopic composition of water vapour in the Central Arctic during the MOSAiC campaign: local versus distant-moisture sources.

The Arctic atmosphere has undergone a process of moistening during the past decades. The loss of sea ice has led to enhanced transfer of heat and moisture from the ocean to the lower atmosphere, while strengthening of cyclonic events has enhanced the poleward transport of moisture from lower latitudes. Eventually, the increased humidity of the Arctic air masses serves today as a new, increasingly important source of moisture for the northern hemisphere. Still, to date, the relative contributions of local evaporation versus distant-moisture sources remains uncertain, as well as the processes responsible for exchanges within and between the hydrological compartments of the Arctic. Such uncertainties limit our ability to understand the importance of the Arctic water cycle to global climate change and to project its future. In this study we use atmospheric water vapour isotopes to investigate the origin of the Arctic moisture and assess whether and which relevant changes occur within the coupled ocean-sea ice-atmosphere system (i.e., sea ice, sea water, snow, melt ponds). Stable isotopologues of water (HDO, H218O) have different saturation vapour pressures and molecular diffusivity coefficients in air. These differences lead to isotopic fractionation during each phase change of water, making water isotopes a powerful tracer of the Arctic hydrological cycle. Water vapour humidity, delta-18O, and delta-D have been measured continuously by a Picarro L2140i Cavity Ringdown Spectrometer installed onboard research vessel Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition, which took place in the Central Arctic Ocean from October 2019 to September 2020. Our measurements depict a clear seasonal cycle and a strong and significant covariance of delta-18O and delta-D with air temperature and specific humidity. At the synoptic time scale the dataset is characterized by the occurrence of events associated with humidity peaks and abrupt isotopic excursions. We use statistical analysis and backwards trajectories to i) identify the origin of the air masses and the relative contributions of distant vs. locally sourced moisture, and ii) illustrate the isotopic fingerprint of these two distinct moisture contributors and discuss on the source-to-sink processes leading to their differences. Further, the MOSAiC observations are compared to an ECHAM6 simulation, nudged to ERA5 reanalysis data and enabled for water isotope diagnostics. The model-data comparison makes it possible to explore the spatial representativeness of our observations and assess whether the model can correctly simulate the observed isotopic changes. In the future, our observations may serve as a benchmark to test the parametrization of under(mis-)represented fractionation processes such as snow sublimation, evaporation from leads and melt ponds. Our study provides the very first isotopic characterization of the Central Arctic moisture throughout an entire year and contributes to disentangling the influence of local evaporative processes versus large-scale vapour transport on the Arctic moistening

A ‘warm path’ for Gulf Stream–troposphere interactions

Warm advection by the Gulf Stream creates a characteristic ‘tongue’ of warm water leaving a strong imprint on the sea surface temperature (SST) distribution in the western North Atlantic. This study aims at quantifying the climatological impact of this feature on cyclones travelling across this region in winter using a combination of reanalysis data and numerical experiments. It is suggested that the Gulf Stream ‘warm tongue’ is conducive to enhanced upward motion in cyclones because (i) it helps maintain a high equivalent potential temperature of air parcels at low levels which favors deep ascent in the warm conveyor belt of cyclones and (ii) because the large SST gradients to the north of the warm tongue drive a thermally direct circulation reinforcing and, possibly, destabilizing, the transverse circulation embedded in cyclones. This hypothesis is confirmed by comparing simulations at 12 km resolution from the Met Office Unified Model forced with realistic SST distribution to simulations with an SST distribution from which the Gulf Stream warm tongue was artificially removed or made colder by . It is also supported by a dynamical diagnostic applied to the ERA interim data-set over the wintertime period (1979–2012). The mechanism of oceanic forcing highlighted in this study is associated with near thermal equilibration of low level air masses with SST in the warm sector of cyclones passing over the Gulf Stream warm tongue, which is in sharp contrast to what occurs in their cold sector. It is suggested that this ‘warm path’ for the climatic impact of the Gulf Stream on the North Atlantic storm-track is not currently represented in climate models because of their coarse horizontal resolution

On the similarity and apparent cycles of isotopic variations in East Antarctic snow and ice cores

Oxygen and hydrogen isotope ratios in polar ice typically show variations over a large range of timescales. Since the isotope ratios are interpreted as a proxy for atmospheric temperatures, their variations can provide essential information about the natural climate variability and cycles. Nowadays high-resolution isotope samplings corresponding to depth intervals below or around the local accumulation of snow per year are routinely performed, and observed variations in the isotopic composition at a given site have frequently been interpreted as the reflection of the seasonal cycle in temperature and also to indicate multi-year quasi-periodic climatic cycles. However, studies from strongly different accumulation conditions in East Antarctica reported similar isotopic variability and comparable apparent cycles in isotope profiles with typical wavelengths of around 20 cm, which is inconsistent with a climatically driven origin. Here we show, based on spectral analysis, that these features do not correspond to truly or quasi-periodic cycles. In addition, the typical wavelengths increase with depth for most East Antarctic sites, which is inconsistent with the effect of burial and compression on a climatic cyclic signal. We explain these results by isotopic diffusion acting on a noise-dominated isotope signal. The firn diffusion length is rather stable across the Antarctic Plateau, leading to similar power spectral densities of the isotopic variations, and increases with depth in the near-surface firn. Since the first moments of the spectral density govern the characteristic spacing of the extrema of a time series – a fundamental relationship known as Rice’s law – the similar isotope spectra in turn imply similar average distances between the isotopic minima and maxima that get larger with increasing depth. Our results bear important implications for the interpretation of isotope records in terms of cyclical climate variability. They underline that simply counting isotopic extrema is not sufficient to detect periodicities, instead robust spectral analyses have to be applied in order to differentiate between true climate cycles and the apparent cycles created in the diffusion process. This has consequences for the dating of ice-core records, which is often based on or underpinned by counting isotopic maxima, but also for the detection and interpretation of quasi-periodic climate phenomena on longer timescales. Finally, the general implications of our findings are not restricted to ice cores but likely also apply to other paleo-climate archives, as other smoothing processes, e.g. the bioturbational smoothing of proxy records from marine sediments, might lead to similar apparent cycles

The spatial variability in isotopic composition of surface snow and snowpits on the East Antarctic Ice Sheet

The water isotope composition of snow precipitations, archived in the Antarctic ice sheet every year, is an important proxy of climatic conditions. This signal depends on several parameters such as local temperature, altitude, moisture source areas and air mass pathways. However, especially in areas where snow accumulation is very low (as on the East Antarctic Plateau), the isotopic composition is affected by additional spatial variability induced by the interactions between the atmosphere and snow surface, and the pristine signal may be modified through isotopic exchanges, sublimation processes and mechanical mixing originated from wind action. Here, we present the isotopic composition (D and 18O) and the second-order parameter d-excess of surface snow and snowpit samples collected during the Italian-French campaign in Antarctica (2019-2020). The sampling sites cover the area from Dumont D'Urville to Concordia Station and from Concordia Station towards the South Pole (EAIIST – East Antarctic International Ice Sheet Traverse). These data, compared with a previous dataset of Antarctic surface snow isotopic composition (Masson-Delmotte et al. 2008), are analyzed to determine the variability of the spatial relationship between precipitation isotopic composition and local temperature in relation to geographical parameters (latitude, distance from the coast and elevation). The interpretation of these factors determining the isotope signature is the base to better define the amount of the effects caused by subsequent interaction between atmosphere and surface snow, and by the wind action. Understanding the spatial variability of this proxy, which strongly decreases the signal-to-noise ratio, could permit to improve the use of the “isotopic thermometer” to quantify past changes in temperature based on the stable isotopic record of deep ice cores

A Sephin1-insensitive tripartite holophosphatase dephosphorylates translation initiation factor 2α.

The integrated stress response (ISR) is regulated by kinases that phosphorylate the α subunit of translation initiation factor 2 and phosphatases that dephosphorylate it. Genetic and biochemical observations indicate that the eIF2αP-directed holophosphatase, a therapeutic target in diseases of protein misfolding, is comprised of a regulatory subunit, PPP1R15, and a catalytic subunit, protein phosphatase 1 (PP1). In mammals, there are two isoforms of the regulatory subunit, PPP1R15A and PPP1R15B, with overlapping roles in the essential function of eIF2αP dephosphorylation. However, conflicting reports have appeared regarding the requirement for an additional co-factor, G-actin, in enabling substrate-specific dephosphorylation by PPP1R15-containing PP1 holoenzymes. An additional concern relates to the sensitivity of the holoenzyme to the [(o-chlorobenzylidene)amino]guanidines Sephin1 or guanabenz, putative small-molecule proteostasis modulators. It has been suggested that the source and method of purification of the PP1 catalytic subunit and the presence or absence of an N-terminal repeat-containing region in the PPP1R15A regulatory subunit might influence the requirement for G-actin and sensitivity of the holoenzyme to inhibitors. We found that eIF2αP dephosphorylation by PP1 was moderately stimulated by repeat-containing PPP1R15A in an unphysiological low ionic strength buffer, whereas stimulation imparted by the co-presence of PPP1R15A and G-actin was observed under a broad range of conditions, low and physiological ionic strength, regardless of whether the PPP1R15A regulatory subunit had or lacked the N-terminal repeat-containing region and whether it was paired with native PP1 purified from rabbit muscle or recombinant PP1 purified from bacteria. Furthermore, none of the PPP1R15A-containing holophosphatases tested were inhibited by Sephin1 or guanabenz.Supported by a Wellcome Trust Principal Research Fellowship to D.R. (Wellcome 200848/Z/16/Z) and a Wellcome Trust Strategic Award to the Cambridge Institute for Medical Research (Wellcome 100140). M.B. was supported by a Flemish Concerted Research Action (GOA15/016). W.P. was supported by National Institute of Health R01NS091336 and the American Diabetes Association Pathway to Stop Diabetes Grant 1-14-ACN-31. Z.C. is a PhD fellow of the Fund for Scientific Research - Flanders

Sub-millennial climate variability from high-resolution water isotopes in the EPICA Dome C ice core

The EPICA Dome C (EDC) ice core provides the longest continuous climatic record, covering the last 800 000 years (800 kyr). A unique opportunity to investigate decadal to millennial variability during past glacial and interglacial periods is provided by the high-resolution water isotopic record (δ18O and δD) available for the EDC ice core. We present here a continuous compilation of the EDC water isotopic record at a sample resolution of 11 cm, which consists of 27 000 δ18O measurements and 7920 δD measurements (covering, respectively, 94 % and 27 % of the whole EDC record), including published and new measurements (2900 for both δ18O and δD) for the last 800 kyr. Here, we demonstrate that repeated water isotope measurements of the same EDC samples from different depth intervals obtained using different analytical methods are comparable within analytical uncertainty. We thus combine all available EDC water isotope measurements to generate a high-resolution (11 cm) dataset for the past 800 kyr. A frequency decomposition of the most complete δ18O record and a simple assessment of the possible influence of diffusion on the measured profile shows that the variability at the multi-decadal to multi-centennial timescale is higher during glacial than during interglacial periods and higher during early interglacial isotopic maxima than during the Holocene. This analysis shows as well that during interglacial periods characterized by a temperature optimum at the beginning, the multi-centennial variability is strongest over this temperature optimum.publishedVersio

Stacked or folded? impact of chelate cooperativity on the self-assembly pathway to helical nanotubes from dinucleobase monomers

Self-assembled nanotubes exhibit impressive biological functions that have always inspired supramolecular scientists in their efforts to develop strategies to build such structures from small molecules through a bottom-up approach. One of these strategies employs molecules endowed with self-recognizing motifs at the edges, which can undergo either cyclization-stacking or folding-polymerization processes that lead to tubular architectures. Which of these self-assembly pathways is ultimately selected by these molecules is, however, often difficult to predict and even to evaluate experimentally. We show here a unique example of two structurally related molecules substituted with complementary nucleobases at the edges (i.e., G:C and A:U) for which the supramolecular pathway taken is determined by chelate cooperativity, that is, by their propensity to assemble in specific cyclic structures through Watson-Crick pairing. Because of chelate cooperativities that differ in several orders of magnitude, these molecules exhibit distinct supramolecular scenarios prior to their polymerization that generate self-assembled nanotubes with different internal monomer arrangements, either stacked or coiled, which lead at the same time to opposite helicities and chiroptical propertiesFunding from the European Research Council (ERC-Starting Grant 279548 PROGRAM-NANO) MCIN (RED2018- 102331-T, PID2020-116921GB-I00, and TED2021-132602BI00), the Italian Ministry of Education, University and Research (PRIN project prot. 2017A4XRCA_003), the Ministry of Education, Youth, and Sports of the Czech Republic (CZ.02.1.01/0.0/0.0/16_019/0000754, e-INFRA CZ (ID:90254)), the Swedish Research Council (2018- 4343), and the Swedish e-Science Research Centre (SeRC) is gratefully acknowledged. The authors also acknowledge the provision of supercomputer resources from the Swedish National Infrastructure for Computing (SNIC). F.A. is grateful to MCIN and Next Generation EU funding for a “Ramon-yCajal” fellowship (RyC-2021-031538-I). A.dJ. is grateful to EU funding from a MSCA-IEF action (897507-SuprAlloCat