Improved methodologies for continuous-flow analysis of stable water isotopes in ice cores

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

Onset of deglacial warming in West Antarctica driven by local orbital forcing

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate[superscript 1,2]. 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[superscript 3,4]. An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently[superscript 2,5]. 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[superscript 6] associated with an abrupt decrease in Atlantic meridional overturning circulation[superscript 7]. 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.Keywords: Last glacial period, Carbon Dioxide, High resolution, Chronology, Ice core, Circulation, Abrupt climate change, Atmospheric Co2, Greenland, Polar ic

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives¹. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa[superscript 2,3], suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw[superscript 4–6]. 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[superscript 7–9]. 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[superscript 2,3,10]. 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

Fundamental controls on triple oxygen-isotope ratios in Antarctic precipitation and ice cores

Thesis (Ph.D.)--University of Washington, 2015Stable isotope ratios of water (δD and δ18O) in polar precipitation and ice cores have long been used to study past climate variations and the hydrological cycle. Recently-developed methods permit the precise measurement of δ17O and the 17O-excess, relative to the δ17O vs. δ18O meteoric water line. The novel isotope parameter "17Oexcess" provides an additional tool for investigating the global hydrological cycle. Early experimental and modeling studies showed that 17Oexcess in atmospheric water vapor is sensitive to relative humidity during evaporation from the ocean surface, and suggested that there was little fractionation during condensation. It was therefore expected that 17Oexcess in polar snow could be used as an indicator for humidity in the ocean source regions where polar moisture originates. Later work shows that the magnitude of 17Oexcess change between the last glacial period and the Holocene warm period, measured in Antarctic ice cores, increases from the Antarctic coast towards the interior, suggested significant fractionation during transport. Full interpretation of these conflicting results has been challenging, hindered in part by the labor-intensive nature of making 17Oexcess measurements and by the lack of an accepted standard for reporting 17Oexcess values. This thesis provides a new, comprehensive assessment of the 17Oexcess of Antarctic precipitation and ice core data. The contributions from this work also include improvements to 17Oexcess measurement techniques, using both isotope-ratio mass spectrometry and collaborative developments in laser spectroscopy, and a formal calibration of international water standards for 17Oexcess. It further addresses both the spatial and temporal variations observed in Antarctic 17Oexcess values, providing a coherent explanation for both. New Antarctic 17Oexcess measurements from this work show that there is a strong negative spatial gradient of 17Oexcess in snowfall towards the interior of Antarctica, a similar spatial pattern to the glacial-interglacial change in 17Oexcess, and a smaller-amplitude seasonal cycle in West Antarctica than in the interior of East Antarctica. These measurements, when combined with earlier published work, provide the most complete view of the spatial distribution and temporal variability of 17Oexcess to date. Model studies, using both an intermediate complexity isotope model (ICM) and an isotope-enabled general circulation model (GCM), have permitted a thorough investigation of the most relevant and important processes affecting 17Oexcess in Antarctica. The model simulations show that changes in source relative humidity have only a modest effect on 17Oexcess in polar precipitation, and can not account for the full seasonal cycle amplitude, nor the large glacial-interglacial 17Oexcess changes observed in Antarctic ice cores. The spatial gradient of 17Oexcess in modern precipitation, along with the large amplitude seasonal cycle in East Antarctica and the greater magnitude of 17Oexcess change for interior sites between glacial and interglacial periods, can be explained by kinetic isotope fractionation during snow formation under supersaturated conditions. The model experiments further show that the influence of moisture recharge is important to the evolution of 17Oexcess in poleward-moving air masses. The seasonal presence of sea ice is also a significant factor affecting 17Oexcess. Greater sea ice concentration or extent reduces evaporative recharge and increases the spatial area over which kinetic fractionation processes are important; both these factors tend to lower 17Oexcess

Water stable isotope record of ice core WAIS divide

Changes in atmospheric circulation over the past five decades have enhanced the wind-driven inflow of warm ocean water onto the Antarctic continental shelf, where it melts ice shelves from below. Atmospheric circulation changes have also caused rapid warming over the West Antarctic Ice Sheet, and contributed to declining sea-ice cover in the adjacent Amundsen-Bellingshausen seas. It is unknown whether these changes are part of a longer-term trend. Here, we use water-isotope (d18O) data from an array of ice-core records to place recent West Antarctic climate changes in the context of the past two millennia. We find that the d18O of West Antarctic precipitation has increased significantly in the past 50 years, in parallel with the trend in temperature, and was probably more elevated during the 1990s than at any other time during the past 200 years. However, d18O anomalies comparable to those of recent decades occur about 1% of the time over the past 2,000 years. General circulation model simulations suggest that recent trends in d18O and climate in West Antarctica cannot be distinguished from decadal variability that originates in the tropics. We conclude that the uncertain trajectory of tropical climate variability represents a significant source of uncertainty in projections of West Antarctic climate and ice-sheet change

Recent climate and ice-sheet changes in West Antarctica compared with the past 2,000 years

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