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Stable water isotopic observations in surface snow over Antarctica provide a foundation for validating isotopic models and interpreting Antarctic ice core records. Here, we present a new compilation of Antarctic surface snow isotopic dataset with strict quality control from published and unpublished sources including measurements from snow pits, snow cores, ice cores, deep surface snow, and precipitation (multi-year average values). The dataset contains a total of 1867 data points, including 1604 locations for oxygen isotope ratio (δ18O) and 1278 locations for deuterium isotope ratio (δ2H). 1204 locations have both δ18O and δ2H, from which d-excess (d-excess = δ2H − 8 × δ18O) can be calculated. The dataset also contains geographic and climate information. The database has a wide range of potential applications, such as the study of the spatial distribution of water isotopes in Antarctica, the evaluation of climate models, and the reconstruction and interpretation of Antarctic ice core records. As an example of model evaluation, the compiled isotopic dataset is used to assess the performance of isotope-enabled atmospheric general circulation models (AGCMs) on simulating the spatial distribution of water isotopes over Antarctica. This dataset is the most comprehensive compilation so far of observed water isotope records at multi-year average scale from multiple sources for Antarctica. It is available for download at https://doi.org/10.5281/zenodo.7294183 (Wang et al., 2022).

Water isotopes in ice cores are used as a climate proxy for local temperature and regional atmospheric circulation as well as evaporative conditions in moisture source regions. Traditional measurements of water isotopes have been achieved using magnetic sector isotope ratio mass spectrometry (IRMS). However, a number of recent studies have shown that laser absorption spectrometry (LAS) performs as well or better than IRMS. The new LAS technology has been combined with continuous-flow analysis (CFA) to improve data density and sample throughput in numerous prior ice coring projects. Here, we present a comparable semi-automated LAS-CFA system for measuring high-resolution water isotopes of ice cores. We outline new methods for partitioning both system precision and mixing length into liquid and vapor components – useful measures for defining and improving the overall performance of the system. Critically, these methods take into account the uncertainty of depth registration that is not present in IRMS nor fully accounted for in other CFA studies. These analyses are achieved using samples from a South Pole firn core, a Greenland ice core, and the West Antarctic Ice Sheet (WAIS) Divide ice core. The measurement system utilizes a 16-position carousel contained in a freezer to consecutively deliver ∼ 1 m × 1.3 cm2 ice sticks to a temperature-controlled melt head, where the ice is converted to a continuous liquid stream and eventually vaporized using a concentric nebulizer for isotopic analysis. An integrated delivery system for water isotope standards is used for calibration to the Vienna Standard Mean Ocean Water (VSMOW) scale, and depth registration is achieved using a precise overhead laser distance device with an uncertainty of ±0.2 mm. As an added check on the system, we perform inter-lab LAS comparisons using WAIS Divide ice samples, a corroboratory step not taken in prior CFA studies. The overall results are important for substantiating data obtained from LAS-CFA systems, including optimizing liquid and vapor mixing lengths, determining melt rates for ice cores with different accumulation and thinning histories, and removing system-wide mixing effects that are convolved with the natural diffusional signal that results primarily from water molecule diffusion in the firn column.

Eastern Beringia is one of the few Western Arctic regions where full Holocene climate reconstructions are possible. However, most full Holocene reconstructions in Eastern Beringia are based either on pollen or midges, which show conflicting early Holocene summer temperature histories. This discrepancy precludes understanding the factors that drove past (and potentially future) climate change and calls for independent proxies to advance the debate. We present a ~13.6 ka summer temperature reconstruction in central Yukon, part of Eastern Beringia, using precipitation isotopes in syngenetic permafrost. The reconstruction shows that early Holocene summers were consistently warmer than the Holocene mean, as supported by midges, and a thermal maximum at ~7.6–6.6 ka BP. This maximum was followed by a ~6 ka cooling, and later abruptly reversed by industrial-era warming leading to a modern climate that is unprecedented in the Holocene context and exceeds the Holocene thermal maximum by +1.7 ± 0.7 °C., Traditional precipitation isotope archives (e.g., ice cores) are fundamental to our knowledge of past climate but limited to glaciated locales. Here the authors show that pore ice in relict permafrost holds equal promise as a proxy and use it to provide insights on the Holocene summer climate history of northwestern Canada.

During the last glacial period, the North Atlantic region experienced a series of Dansgaard–Oeschger cycles in which climate abruptly alternated between warm and cold periods. Corresponding variations in Antarctic surface temperature were out of phase with their Northern Hemisphere counterparts. The temperature relationship between the hemispheres is commonly attributed to an interhemispheric redistribution of heat by the ocean overturning circulation. Changes in ocean heat transport should be accompanied by changes in atmospheric circulation to satisfy global energy budget constraints. Although changes in tropical atmospheric circulation linked to abrupt events in the Northern Hemisphere are well documented, evidence for predicted changes in the Southern Hemisphere’s atmospheric circulation during Dansgaard–Oeschger cycles is lacking. Here we use a high-resolution deuterium-excess record from West Antarctica to show that the latitude of the mean moisture source for Antarctic precipitation changed in phase with abrupt shifts in Northern Hemisphere climate, and significantly before Antarctic temperature change. This provides direct evidence that Southern Hemisphere mid-latitude storm tracks shifted within decades of abrupt changes in the North Atlantic, in parallel with meridional migrations of the intertropical convergence zone. We conclude that both oceanic and atmospheric processes, operating on different timescales, link the hemispheres during abrupt climate change. Abrupt glacial climate changes were slowly communicated between hemispheres by oceanic heat transport. Ice core data point to more rapid atmospheric teleconnections linking the North Atlantic, tropics, and southern storm track.

Rationale Water isotope analysis for δ(2) H and δ(18) O values via laser spectroscopy is routine for many laboratories. While recent work has added the δ(17) O value to the high-precision suite, it does not follow that researchers will routinely obtain high precision (17) O excess (Δ(17) O). We demonstrate the routine acquisition of high-precision δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values using a commercially available laser spectroscopy instrument. Methods We use a Picarro L2140-i cavity ring-down spectroscopy analyzer with discrete liquid injections into an A0211 vaporization module by a Leap Technologies LC PAL autosampler. The instrument is run in two modes: (1) as recommended by the manufacturer (default mode) and (2) after modifying select default settings and using alternative data types (advanced mode). Reference waters analyzed over the course of 15 months while running unknown samples are used to assess system performance. Results The default mode provides precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values that may be sufficient for many applications. When using the advanced mode, we reach a higher level of precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values (0.4 mUr, 0.04 mUr, 0.07 mUr, 0.5 mUr, and 8 μUr, respectively, where mUr = 0.001 = ‰, and μUr = 10(-6) ) in a shorter amount of time and with fewer syringe actuations than in the default mode. The improved performance results from an increase in the total integration time for each injected water pulse. Conclusions Our recommended approach for routine δ(2) H, δ(17) O, δ(18) O, d and Δ(17) O measurements with the Picarro L2140-i is to make use of conditioning vials, use fewer injections (5 per vial) with greater pulse duration (520 seconds (s) per injection) and use only the first 120 s for δ(2) H measurements and all 520 s for δ(17) O and δ(18) O measurements. Although the sample throughput is 10 unknowns per day, our optimal approach reduces the number of syringe actuations, the effect of memory, and the total analysis time, while improving precision relative to the default approach. Copyright © 2016 John Wiley & Sons, Ltd.

High-precision analysis of the 17O / 16O isotope ratio in water and water vapor is of interest in hydrological, paleoclimate, and atmospheric science applications. Of specific interest is the parameter 17O excess (Δ17O), a measure of the deviation from a~linear relationship between 17O / 16O and 18O / 16O ratios. Conventional analyses of Δ17O of water are obtained by fluorination of H2O to O2 that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for high-precision Δ17O measurements. The new instrument uses cavity ring-down spectroscopy (CRDS) with laser-current-tuned cavity resonance to achieve reduced measurement drift compared with previous-generation instruments. Liquid water and water-vapor samples can be analyzed with a better than 8 per meg precision for Δ17O using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy of Δ17O are competitive with conventional IRMS methods. The new instrument also achieves simultaneous analysis of δ18O, Δ17O and δD with precision of < 0.03‰, < 0.02 and < 0.2‰, respectively, based on repeated calibrated measurements.

Measurements of the 17Oexcess of H2O were obtained from ice cores in West and East Antarctica. Combined with previously published results from East Antarctica, the new data provide the most complete spatial and temporal view of Antarctic 17Oexcess to date. There is a steep spatial gradient of 17Oexcess in present-day precipitation across Antarctica, with higher values in marine-influenced regions and lower values in the East Antarctic interior. There is also a spatial pattern to the change in 17Oexcess between the Last Glacial Maximum (LGM) and Holocene periods. At coastal locations, there is no significant change in 17Oexcess. At both the West Antarctic Ice Sheet Divide site and at Vostok, East Antarctica, the LGM to Early Holocene change in 17Oexcess is about 20 per meg. Atmospheric general circulation model (GCM) experiments show that both the observed spatial gradient of 17Oexcess in modern precipitation, and the spatial pattern of LGM to Early Holocene change, can be explained by kinetic isotope effects during snow formation under supersaturated conditions, requiring a high sensitivity of supersaturation to temperature. The results suggest that fractionation during snow formation is the primary control on 17Oexcess in Antarctic precipitation. Variations in moisture source relative humidity play a negligible role in determining the glacial-interglacial 17Oexcess changes observed in Antarctic ice cores. Additional GCM experiments show that sea ice expansion increases the area over which supersaturating conditions occur, amplifying the effect of colder temperatures. Temperature and sea ice changes alone are sufficient to explain the observed 17Oexcess glacial-interglacial changes across Antarctica.

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate. Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier. Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

West Antarctic temperature and pressure observations begin onlyin 1957, and reliable satellite observations of Antarctic sea icedate to 1979. Comprehensive observations of glacier dynamicsin the most rapidly changing areas were initiated in the 1990s.Borehole temperature data from the WAIS, although confirmingthe recent rapid rise in temperature, do not resolve decadal-scalevariability in the past

RATIONALE The absence of an agreed-upon δ(17)O value for the primary reference water SLAP leads to significant discrepancies in the reported values of δ(17)O and the parameter (17)O(excess). The accuracy of δ(17)O and (17)O(excess) values is significantly improved if the measurements are normalized using a two-point calibration, following the convention for δ(2)H and δ(18)O values. METHODS New measurements of the δ(17)O values of SLAP2 and GISP are presented and compared with published data. Water samples were fluorinated with CoF(3). Helium carried the O(2) product to a 5A (4.2 to 4.4 A) molecular sieve trap submerged in liquid nitrogen. The O(2) sample was introduced into a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer for measurement of m/z 32, 33, and 34. The δ(18)O and δ(17) values were calculated after 90 comparisons with an O(2) reference gas. RESULTS We propose that the accepted δ(17)O value of SLAP be defined in terms of δ(18) O = -55.5 ‰ and (17)O(excess) = 0, yielding a δ(17)O value of approximately -29.6986 ‰ [corrected]. Using this definition for SLAP and the recommended normalization procedure, the δ(17)O value of GISP is -13.16 ± 0.05 ‰ and the (17)O(excess) value of GISP is 22 ± 11 per meg. Correcting previous published values of GISP δ(17)O to both VSMOW and SLAP improves the inter-laboratory precision by about 10 per meg. CONCLUSIONS The data generated here and compiled from previous studies provide a substantial volume of evidence to evaluate the various normalization techniques currently used for triple oxygen isotope measurements. We recommend that reported δ(17) O and (17)O(excess) values be normalized to the VSMOW-SLAP scale, using a definition of SLAP such that its (17)O(excess) is exactly zero.

An intermediate complexity isotope model (ICM) is used to investigate the sensitivity of water isotope ratios in precipitation, including 17Oexcess, to climate variations in the Southern Hemisphere. The ICM is forced with boundary conditions from seasonal NCEP/DOE II reanalysis data. Perturbations to the surface temperature and humidity fields are used to investigate the isotopic sensitivity. The response of 17Oexcess to a uniform temperature change is insignificant over the ocean, while there is a large magnitude response over the ice sheet, particularly in East Antarctica. A decrease of ocean surface relative humidity produces increased 17Oexcess and dexcess, with a coherent response over both the ocean and Antarctica. For interior East Antarctica, the model simulates a seasonal cycle in 17Oexcess that is positively correlated with δ18O and of large magnitude (~50 per meg), consistent with the observations from Vostok. The seasonal cycle in 17Oexcess for interior West Antarctica is predicted to be considerably smaller in magnitude (12 per meg), and is negatively correlated with δ18O, consistent with new data from a firn core near the West Antarctic Ice Sheet Divide site. Over the ocean, the ICM predicts much smaller seasonal cycles in 17Oexcess. Oceanic source changes (i.e., humidity) are insufficient to explain the amplitude of the simulated seasonal cycle over the Antarctic continent. Spatial differences in the seasonal response of 17Oexcess to local temperature reflect the balance of equilibrium and kinetic fractionation during snow formation.

Water isotopes in ice cores are used as a climate proxy for local temperature and regional atmospheric circulation as well as evaporative conditions in moisture source regions. Traditional measurements of water isotopes have been achieved using magnetic sector isotope ratio mass spectrometry (IRMS). However, a number of recent studies have shown that laser absorption spectrometers (LAS) perform as well or better than IRMS. The new LAS technology has been combined with continuous flow analysis (CFA) to improve data density and sample throughput in numerous prior ice coring projects. Here, we present a comparable semi-automated LAS-CFA system for measuring high-resolution water isotopes of ice cores. We outline new methods for partitioning both system uncertainty and system mixing length into liquid and vapor components – useful measures for defining and improving the overall performance of the system. Critically, our methods take into account the uncertainty of depth registration that is not present in IRMS nor fully accounted for in other CFA studies. We also explain a method for introducing consecutive sections of isotopically distinct ice at the melt head to define the system-wide mixing length. These analyses are achieved using samples from a South Pole firn core, a Greenland ice core, and the WAIS Divide ice core. The measurement system utilizes a 16-position carousel contained in a freezer to consecutively deliver ~ 1 m × 1.3 cm2 ice sticks to a temperature controlled melt head, where the ice is converted to a continuous liquid stream, and eventually vaporized using a concentric nebulizer for isotopic analysis. An integrated delivery system for water isotope standards is used for calibration to the VSMOW-SLAP scale and depth registration is achieved using a precise overhead laser distance device with an uncertainty of ±0.2 mm. As an added check on our system, we perform inter-lab LAS comparisons using WAIS Divide ice samples, a corroboratory step not taken in prior CFA studies. The overall results are important for substantiating data obtained from LAS-CFA systems, including optimizing liquid and vapor mixing lengths, determining melt rates for ice cores with different accumulation and thinning histories, and removing system-wide mixing effects that are convolved with the natural diffusional signal that results primarily from water molecule diffusion in the firn column.

Water isotope analysis for δ(2) H and δ(18) O values via laser spectroscopy is routine for many laboratories. While recent work has added the δ(17) O value to the high-precision suite, it does not follow that researchers will routinely obtain high precision (17) O excess (Δ(17) O). We demonstrate the routine acquisition of high-precision δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values using a commercially available laser spectroscopy instrument.We use a Picarro L2140-i cavity ring-down spectroscopy analyzer with discrete liquid injections into an A0211 vaporization module by a Leap Technologies LC PAL autosampler. The instrument is run in two modes: (1) as recommended by the manufacturer (default mode) and (2) after modifying select default settings and using alternative data types (advanced mode). Reference waters analyzed over the course of 15 months while running unknown samples are used to assess system performance.The default mode provides precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values that may be sufficient for many applications. When using the advanced mode, we reach a higher level of precision for δ(2) H, δ(17) O, δ(18) O, d, and Δ(17) O values (0.4 mUr, 0.04 mUr, 0.07 mUr, 0.5 mUr, and 8 μUr, respectively, where mUr = 0.001 = ‰, and μUr = 10(-6) ) in a shorter amount of time and with fewer syringe actuations than in the default mode. The improved performance results from an increase in the total integration time for each injected water pulse.Our recommended approach for routine δ(2) H, δ(17) O, δ(18) O, d and Δ(17) O measurements with the Picarro L2140-i is to make use of conditioning vials, use fewer injections (5 per vial) with greater pulse duration (520 seconds (s) per injection) and use only the first 120 s for δ(2) H measurements and all 520 s for δ(17) O and δ(18) O measurements. Although the sample throughput is 10 unknowns per day, our optimal approach reduces the number of syringe actuations, the effect of memory, and the total analysis time, while improving precision relative to the default approach. Copyright © 2016 John WileySons, Ltd.

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

High precision analysis of the 17O/16O isotope ratio in water and water vapor is of interest in hydrological, paleoclimate, and atmospheric science applications. Of specific interest is the parameter 17Oexcess, a measure of the deviation from a linear relationship between 17O/16O and 18O/16O ratios. Conventional analyses of 17Oexcess are obtained by fluorination of H2O to O2 that is analyzed by dual-inlet isotope ratio mass spectrometry (IRMS). We describe a new laser spectroscopy instrument for high-precision 17Oexcess measurements. The new instrument uses cavity ring-down spectroscopy (CRDS) with laser-current tuning to achieve reduced measurement drift compared with previous-generation instruments. Liquid water and water vapor samples can be analyzed with better than 8 per meg precision for 17Oexcess using integration times of less than 30 min. Calibration with respect to accepted water standards demonstrates that both the precision and the accuracy are competitive with conventional IRMS methods. The new instrument also achieves simultaneous measurements of δ18O, 17Oand δD with precision < 0.03‰, < 0.02‰ and < 0.2‰, respectively.

The absence of an agreed-upon δ(17)O value for the primary reference water SLAP leads to significant discrepancies in the reported values of δ(17)O and the parameter (17)O(excess). The accuracy of δ(17)O and (17)O(excess) values is significantly improved if the measurements are normalized using a two-point calibration, following the convention for δ(2)H and δ(18)O values.New measurements of the δ(17)O values of SLAP2 and GISP are presented and compared with published data. Water samples were fluorinated with CoF(3). Helium carried the O(2) product to a 5A (4.2 to 4.4 Å) molecular sieve trap submerged in liquid nitrogen. The O(2) sample was introduced into a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer for measurement of m/z 32, 33, and 34. The δ(18)O and δ(17) values were calculated after 90 comparisons with an O(2) reference gas.We propose that the accepted δ(17)O value of SLAP be defined in terms of δ(18) O = -55.5 ‰ and (17)O(excess) = 0, yielding a δ(17)O value of approximately -29.6986 ‰ [corrected]. Using this definition for SLAP and the recommended normalization procedure, the δ(17)O value of GISP is -13.16 ± 0.05 ‰ and the (17)O(excess) value of GISP is 22 ± 11 per meg. Correcting previous published values of GISP δ(17)O to both VSMOW and SLAP improves the inter-laboratory precision by about 10 per meg.The data generated here and compiled from previous studies provide a substantial volume of evidence to evaluate the various normalization techniques currently used for triple oxygen isotope measurements. We recommend that reported δ(17) O and (17)O(excess) values be normalized to the VSMOW-SLAP scale, using a definition of SLAP such that its (17)O(excess) is exactly zero.