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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]

Atmospheric dioxygen concentration and isotopic composition are closely linked to the carbon cycle through anthropic CO2 emissions and biological processes such as photosynthesis and respiration. Measurement of isotopic ratio of atmospheric dioxygen, trapped in ice core bubbles, bring information about past variation in the hydrological cycle at low latitudes, as well as past productivity. Currently, the interpretation of those variations could be drastically improved with a better (i.e. quantitative) knowledge of the oxygen fractionation that occur during photosynthesis and respiration processes. This could be achieved, for example, during experiment using closed-biological chambers. In order to estimate the fractionation coefficient with a good precision, one of the principal limitations is the need of high frequency on-line measurements of δ18O of O2 and O2 concentration. To address this issue, we developed a new instrument, based on the OF-CEAS (Optical-Feedback Cavity-Enhanced Absorption Spectroscopy) technique, enabling high temporal resolution and continuous measurements of dioxygen concentration as well as δ18O of O2, both simultaneously. Minimum of Allan deviation occurred between 10 and 20 minutes while precision reached 0.002 % for O2 concentration and 0.06 - for δ18O of O2, which correspond to the optimal integration time and analytical precision before drift start degrading the measurements. While humidity did not affect much the measured values, O2 concentration had an influence on δ18O of O2, which should hence be quantified. To ensure good quality of O2 concentration and δ18O of O2 measurements we eventually proposed to measure calibration standard every 20 minutes. [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 (AICC2012) provides an age vs depth relationship covering the last 800 kyr (thousands of years) with an absolute uncertainty rising up to 8,000 years at the bottom of the ice core. The origins of this relatively large uncertainty are threefold: (1) the d18Oatm, dO2/N2 and total air content (TAC) records are poorly resolved and discontinuous over the last 800 kyr, (2) the three orbital tools are not used simultaneously and (3) large uncertainties are associated with their orbital targets. Here, we present new highly resolved d18Oatm, dO2/N2 and d15N measurements for EDC ice core covering the last five glacial - interglacial transitions as well as novel absolute 81Kr ages. We have compiled chronological and glaciological information including novel orbital age markers from new data on EDC ice core as well as accurate firn modeling estimates in a Bayesian dating tool to construct the new AICC2023 chronology. The average uncertainty of the ice chronology is reduced from 2,500 years to 1,800 years in AICC2023 over the last 800 kyr. The new timescale diverges from AICC2012 and suggests age shifts reaching 3,800 years towards older ages over Marine Isotopes Stages (MIS) 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 1,000 to 2,000 years over these time intervals. [ABSTRACT FROM AUTHOR]

Obtaining precise continuous measurements of water vapor isotopic composition in dry places (polar or high-altitude regions) is an important challenge. The current limitation is the strong influence of humidity on the measured water isotopic composition by laser spectroscopy instruments for low humidity levels (below 3000 ppmv). This problem is addressed by determining the relationships between humidity and measured δ18O and δD of known water standards. We present here the development of a robust field instrument able to generate water vapor, down to 70 ppmv, at very stable humidity levels (average 1σ lower than 10 ppmv). This instrument, operated by a Raspberry interface, can be coupled to a commercial laser spectroscopy instrument: it turned to be very stable in an autonomous mode during more than one year at the East Antarctic Concordia station. [ABSTRACT FROM AUTHOR]

To evaluate the impact of vapor diffusion onto isotopic composition variations in the snow pits and then in ice cores, we introduced water isotopes in the detailed snowpack model Crocus. The isotopes routine is run with a 1 s resolution. At each step and for each snow layer, 1) the initial isotopic composition of vapor is taken at equilibrium with solid phase, 2) kinetic fractionation is applied during transport, and 3) condensation is realized. We study the different effects of temperature gradient, compaction, wind compaction and precipitation on the final vertical isotopic profiles. We also run complete simulations of vapor and isotopic diffusion at GRIP, Greenland and at Dome C, Antarctica over periods of 1 or 10 years. The vapor diffusion tends to smooth the original seasonal signal, with an attenuation of 9.5 % of the original signal over 10 years at GRIP. This is smaller than the observed attenuation in ice cores, indicating that the model underestimates attenuation due to diffusion or that other processes, such as ventilation, also contribute to the observed attenuation. At Dome C, the attenuation is stronger (14 %), probably because of the lower accumulation and stronger δ18O gradients. Because vapor diffusion is not the only process responsible of the signal attenuation, it would be useful to implement in the model ventilation of the snowpack and exchanges with the atmosphere to evaluate their contribution. [ABSTRACT FROM AUTHOR]

The transformation of snow into ice is a complex phenomenon difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger that the surrounding ice. The resulting gas-ice age difference is essential to document the phasing between CO2 and temperature changes especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using Î'15N of air measured in ice cores. All firn densification models applied to deglaciations show a large disagreement with Î'15N measurements in several sites of East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas Î'15N suggests a reduced firn thickness compared to the Holocene. We present here modifications of the LGGE firn densification model, which significantly reduce the model-data mismatch for the gas trapping depth evolution over the last deglaciation, while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the activation energy to temperature and impurities in the firn densification rate calculation. The temperature influence reflects the existence of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured Î'15N evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. However, the inclusion of impurities effects deteriorates the agreement between modelled and measured Î'15N evolution in Greenland and Antarctic sites with high accumulation. [ABSTRACT FROM AUTHOR]

The article presents the research on the first 800,000-year record of triple isotopic ratios of atmospheric oxygen, which reflects past global biosphere productivity. Global biosphere productivity in the past eight glacial intervals was found to be lower than that in the preindustrial era, and it has started to increase millennia before deglaciations. The dominant control of carbon dioxide on global biosphere productivity supports pervasive negative feedback under the glacial climate.