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While the processes controlling pore closure are broadly understood, the physical mechanisms driving the associated elemental fractionation remains ambiguous. Previous studies have shown that the pore closure process leads to a depletion in small-sized molecules (e.g. H2 , O2 , Ar, Ne, He) in ice core bubbles relative to larger-sized molecules like N2. This size-dependent fractionation, identified using ice core δ(O2/N2) records, exhibits a clear anti-correlation with local summer solstice insolation, making δ(O2/N2) a valuable ice core dating tool. Mechanisms controlling this relationship are attributed to the physical properties of deep firn. In this study, we compile δ(O2/N2) records from 15 polar ice cores and show a new additional link between δ(O2/N2) and local surface temperature and/or accumulation rate. Using the Crocus snowpack model, we perform sensitivity tests to identify the response of near-surface snow properties to changes in insolation intensity, accumulation rate, and air temperature. These tests support a mechanism linked to firn grain size, such that the larger the grain size for a given density, the stronger the pore closure fractionation and, hence, the lower the δ(O2/N2) values archived in the ice. Based on both snowpack model outputs and data compilation, our findings suggest that local accumulation rate and temperature should be considered when interpreting δ(O2/N2) as a local insolation proxy. [ABSTRACT FROM AUTHOR]

Water stable isotope records in polar ice cores have been largely used to reconstruct past local temperatures and other climatic information such as evaporative source region conditions of the precipitation reaching the ice core sites. However, recent studies have identified post-depositional processes taking place at the ice sheet's surface modifying the original precipitation signal and challenging the traditional interpretation of ice core isotopic records. In this study, we use a combination of existing and new datasets of the precipitation, snow surface and subsurface isotopic compositions (δ18O and d-excess), meteorological parameters, ERA5 reanalyses, outputs from the isotope-enabled climate model ECHAM6-wiso, and a simple modelling approach to investigate the transfer function of water stable isotopes from precipitation to the snow surface and subsurface at Dome C, in East Antarctica. We first show that water vapor fluxes at the surface of the ice sheet result in a net annual sublimation of snow, from 3.1 to 3.7 mm water equivalent per year between 2018 and 2020, corresponding to 12 to 15 % of the annual surface mass balance. We find that the precipitation isotopic signal cannot fully explain the mean, nor the variability of the isotopic composition observed in the snow, from annual to intra-monthly timescales. We observe that the mean effect of post-depositional processes over the study period enriches the snow surface in δ18O by 3.3 ‰ to 6.6 ‰ and lowers the snow surface d-excess by 3.5 ‰ to 7.6 ‰ compared to the incoming precipitation isotopic signal. We also show that the mean isotopic composition of the subsurface snow is not statistically different from that of the surface snow, indicating the preservation of the mean isotopic composition of the surface snow in the top centimetres of the snowpack. This study confirms previous findings about the complex interpretation of the water stable isotopic signal in the snow and provides the first quantitative estimation of the impact of post-depositional processes on the snow isotopic composition at Dome C, a crucial step for the accurate interpretation of isotopic records from ice cores. [ABSTRACT FROM AUTHOR]

Past climate and environmental changes can be reconstructed using paleoclimate archives such as ice cores, lake and marine sediment cores, speleothems, tree rings and corals. The dating of these natural archives is crucial for deciphering the temporal sequence of events and rates of change during past climate changes. It is also essential to provide quantified estimates of the absolute and relative errors associated with the inferred chronologies. However, this task is complex since it involves combining different dating approaches at different paleoclimatic sites and often on different types of archives. Here we present Paleochrono-1.1 , a new probabilistic model to derive a common and optimised chronology for several paleoclimatic sites with potentially different types of archives. Paleochrono-1.1 is based on the inversion of an archiving model: a varying deposition rate (also named growth rate, sedimentation rate or accumulation rate) and also, for ice cores, a lock-in-depth of air (since, in the absence of significant surface melt, the air is trapped in the ice at about 50–120 m below the surface) and a thinning function (since glacier ice undergoes flow). Paleochrono-1.1 integrates several types of chronological information: prior knowledge of the archiving process, independently dated horizons, depth intervals of known duration, undated stratigraphic links between records, and, for ice cores, Δdepth observations (depth differences between events recorded synchronously in the gas and solid phases of a certain core). The optimization is formulated as a least-squares problem, assuming that all probability densities are near-Gaussian and that the model is nearly linear in the vicinity of the best solution. Paleochrono-1.1 is the successor of IceChrono, which produces common and optimized chronologies for ice-cores. Paleochrono1.1 outperforms IceChrono in terms of computational efficiency, ease of use, and accuracy. We demonstrate the ability of Paleochrono-1.1 in a new ice-core–speleothem dating experiment, which combines the Antarctic Ice Core Chronology 2023 dating experiment, based on records from five polar ice cores, with data from two speleothems dated using uranium/thorium radiometric techniques from Hulu Cave (China). We analyse the performance of Paleochrono-1.1 in terms of computing time and memory usage in various dating experiments. Paleochrono-1.1 is freely available under the MIT open-source license. [ABSTRACT FROM AUTHOR]

Processes controlling pore closure are broadly understood yet defining the physical mechanisms controlling associated elemental fractionation remains ambiguous. Previous studies have shown that the pore closure process leads to a decrease in concentration of small-size molecules (e.g., H2, O2, Ar, Ne, He) in the trapped bubbles. Ice core δ(O2/N2) records – the ratio of O2 to N2 molecules in bubbles trapped in ice cores relative to the atmosphere – are therefore depleted owing to this O2 loss and show a clear link with local summer solstice insolation making it a useful dating tool. In this study, we compile δ(O2/N2) records from 14 polar ice cores and show a new link between δ(O2/N2) and local surface temperature and/or accumulation rate, in addition to the influence of the summer solstice insolation. We argue that both local climate-driven and insolation forcings are linked to the modulation of snow physical properties near the surface. Using the Crocus snowpack model, we perform sensitivity tests to identify the response of near-surface snow properties to changes in insolation, accumulation rate, and air temperature. These tests support a mechanisms linked to snow grain size, such that the larger the grain size for a given density, the stronger the pore closure fractionation, and hence, lower δ(O2/N2) values. Our findings suggest that local accumulation rate and temperature should be considered when interpreting δ(O2/N2) as an insolation proxy. [ABSTRACT FROM AUTHOR]

The EPICA (European Project for Ice Coring in Antarctica) Dome C (EDC) ice core drilling in East Antarctica reaches a depth of 3260 m. The reference EDC chronology, the AICC2012 (Antarctic Ice Core Chronology 2012), provides an age vs. depth relationship covering the last 800 kyr (thousands of years), with an absolute uncertainty rising up to 8000 years at the bottom of the ice core. The origins of this relatively large uncertainty are twofold: (1) the δ18 O atm , δO2/N2 and total air content (TAC) records are poorly resolved and show large gaps over the last 800 kyr, and (2) large uncertainties are associated with their orbital targets. Here, we present new highly resolved δ18 O atm , δO2/N2 and δ15 N measurements for the EDC ice core covering the last five glacial–interglacial transitions; a new low-resolution TAC record over the period 440–800 ka BP (ka: 1000 years before 1950); and novel absolute 81 Kr ages. We have compiled chronological and glaciological information including novel orbital age markers from new data on the EDC ice core as well as accurate firn modeling estimates in a Bayesian dating tool to construct the new AICC2023 chronology. For the first time, three orbital tools are used simultaneously. Hence, it is possible to observe that they are consistent with each other and with the other age markers over most of the last 800 kyr (70 %). This, in turn, gives us confidence in the new AICC2023 chronology. The average uncertainty in the ice chronology is reduced from 1700 to 900 years in AICC2023 over the last 800 kyr (1σ). The new timescale diverges from AICC2012 and suggests age shifts reaching 3800 years towards older ages over marine isotope stages (MISs) 5, 11 and 19. But the coherency between the new AICC2023 timescale and independent chronologies of other archives (Italian Lacustrine succession from Sulmona Basin, Dome Fuji ice core and northern Alpine speleothems) is improved by 1000 to 2000 years over these time intervals. [ABSTRACT FROM AUTHOR]

We report high resolution measurements of the stable isotope ratios of ancient ice (δ18O, δD) from the North Greenland Eemian deep ice core (NEEM, 77.45° N, 51.06° E). The record covers the period 8–130 ky b2k (y before 2000) with a temporal resolution of ≈0.5 and 7 y at the top and the bottom of the core respectively and contains important climate events such as the 8.2 ky event, the last glacial termination and a series of glacial stadials and interstadials. At its bottom part the record contains ice from the Eemian interglacial. Isotope ratios are calibrated on the SMOW/SLAP scale and reported on the GICC05 (Greenland Ice Core Chronology 2005) and AICC2012 (Antarctic Ice Core Chronology 2012) time scales interpolated accordingly. We also provide estimates for measurement precision and accuracy for both δ18O and δD. Measurement(s) isotope analysis • water ice core Technology Type(s) cavity ring-down spectroscopy Factor Type(s) δ18O • δD Sample Characteristic - Location Greenland Ice Sheet Machine-accessible metadata file describing the reported data: https://doi.org/10.6084/m9.figshare.14216441 [ABSTRACT FROM AUTHOR]

Each glacial – interglacial transition of the Quaternary occurs in a different orbital context leading to various timing for the deglaciation and sequence of high vs low latitudes events. Termination 3, 250 kyears before present (ka), is an unusual deglaciation in the context of the last 9 deglaciations recorded in the old EPICA Dome C (EDC) Antarctic ice core: it exhibits a three-phase sequence, two warming phases separated by a small cooling, the last phase suggesting a particularly rapid temperature increase. We present here new high resolution δ15N and deuterium excess (d-excess) data from the EDC ice core to provide a detailed temperature change estimate during this termination. Then, we combined the δD and δ18O to discuss the relationship between high and low latitude changes through the d-excess. We also provide the high vs low latitude sequence of events over this deglaciation without chronological uncertainty using low latitude ice core proxies. In agreement with previous studies based on speleothem analyses, we show that the first phase of Termination 3 (256–249 ka) is associated with small Heinrich like events linked to changes in ITCZ position, monsoon activity and teleconnections with Antarctica. In a context of minimum Northern Hemisphere insolation, this leads to a rather strong Antarctic warming, as observed in the δ15N record in contrast to the relatively small δD increase. The second warming phase occurs during the rise of the Northern hemisphere insolation, with a large Heinrich like event leading to the characteristic Antarctic warming observed in the δ15N and δD increase as for the more recent terminations. • New high resolution δ 15N and d-excess measurements over Termination 3 provide a quantification of temperature evolution over this period. • Three-phase high vs low latitude sequence of events over Termination 3 in a context of enhanced millennial scale variability. • Impurity contents does not play a major role on the δ 15N evolution over deglaciations. [ABSTRACT FROM AUTHOR]