Core handling and processing for the WAIS Divide ice-core project

On 1 December 2011 the West Antarctic Ice Sheet (WAIS) Divide ice-core project reached its final depth of 3405 m. The WAIS Divide ice core is not only the longest US ice core to date, but is also the highest-quality deep ice core, including ice from the brittle ice zone, that the US has ever recovered. The methods used at WAIS Divide to handle and log the drilled ice, the procedures used to safely retrograde the ice back to the US National Ice Core Laboratory (NICL) and the methods used to process and sample the ice at the NICL are described and discussed

Core handling and processing for the WAIS Divide ice-core project

On 1 December 2011 the West Antarctic Ice Sheet (WAIS) Divide ice-core project reached its final depth of 3405 m. The WAIS Divide ice core is not only the longest US ice core to date, but is also the highest-quality deep ice core, including ice from the brittle ice zone, that the US has ever recovered. The methods used at WAIS Divide to handle and log the drilled ice, the procedures used to safely retrograde the ice back to the US National Ice Core Laboratory (NICL) and the methods used to process and sample the ice at the NICL are described and discussed

Observing and modeling the influence of layering on bubble trapping in polar firn

Interpretation of ice core trace gas records depends on an accurate understanding of the processes that smooth the atmospheric signal in the firn. Much work has been done to understand the processes affecting air transport in the open pores of the firn, but a paucity of data from air trapped in bubbles in the firn-ice transition region has limited the ability to constrain the effect of bubble closure processes. Here we present high-resolution measurements of firn density, methane concentrations, nitrogen isotopes, and total air content that show layering in the firn-ice transition region at the West Antarctic Ice Sheet (WAIS) Divide ice core site. Using the notion that bubble trapping is a stochastic process, we derive a new parameterization for closed porosity that incorporates the effects of layering in a steady state firn modeling approach. We include the process of bubble trapping into an open-porosity firn air transport model and obtain a good fit to the firn core data. We find that layering broadens the depth range over which bubbles are trapped, widens the modeled gas age distribution of air in closed bubbles, reduces the mean gas age of air in closed bubbles, and introduces stratigraphic irregularities in the gas age scale that have a peak-to-peak variability of ~10 years at WAIS Divide. For a more complete understanding of gas occlusion and its impact on ice core records, we suggest that this experiment be repeated at sites climatically different from WAIS Divide, for example, on the East Antarctic plateau.This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/%28ISSN%292169-8996/.Keywords: ice core, firn density, layering, firn, total air content, methan

The Iso2k Database: A global compilation of paleo-δ18O and δ2H records to aid understanding of Common Era climate

Reconstructions of global hydroclimate during the Common Era (CE; the past ~ 2000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ2H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.6084/m9.figshare.11553162 (McKay and Konecky, 2020)

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

The Iso2k database: a global compilation of paleo-δ18O and δ2H records to aid understanding of common era climate

Reconstructions of global hydroclimate during the Common Era (CE; the past ~2,000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ²H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.25921/57j8-vs18 (Konecky and McKay, 2020) and is also accessible via through the NOAA/WDS Paleo Data landing page: https://www.ncdc.noaa.gov/paleo/study/29593

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

Temperature reconstruction at the West Antarctic Ice Sheet Divide, for the last millennium, from the combination of borehole temperature and inert gas isotope measurements /

The study of past climates informs us on the causes, amplitude and mechanisms of climate change, which is necessary to our ability to predict future changes, and build the necessary infrastructure to ensure the resilience of our society to these changes. Local climate records contain both large scale and local signals, from both external forcing, such as radiative forcing, and internal climate variability. In order to understand the response of the climate system to global drivers, we must average out the local signals into hemispheric or global variables. This process has been difficult because we lack the spatial coverage in large areas of the Earth, including the oceans, and much of the Southern Hemisphere. The work presented here aims at improving our knowledge of the climate by producing a new temperature time series from the center of West Antarctica for the last 1000 years, a region previously unexplored. This temperature reconstruction is based on a new method, combining borehole temperature measurements with inert gas isotopes from the WAIS-Divide ice cores into a single inverse problem. Borehole temperature measurements constrain the long term changes in the climate, while inert gas isotopes record decadal to centennial scale changes. Together, they produce a temperature estimate that is independent of the traditional water isotope proxy [Delta]¹⁸O of ice, and provide a way to calibrate it. WAIS-Divide experienced a long term cooling trend from 950 to 1850 A.D., which ended abruptly by warming by 2.3°C in 30 years. More recently, WAIS-Divide has been warming by more than 1.5°C since 1957, which refutes the idea that Antarctica would not experience the current warming seen elsewhere. The long term cooling trend was superimposed on centennial scale variations in the climate, including two warming events, notably between 1315 and 1395 A.D., 1596 and 1626 A.D., with a warming rate of 0.24 and 0.32°C/decade. This evidence shows that the current rate of warming at WAIS- Divide, of 0.23°C/decade for the last 50 years is rare but not unprecedented. This record is consistent with the idea that the decrease in solar radiation from 1400 to 1800 A.D. induced widespread cooling in mid and high latitudes of both hemisphere

A database of Isotope time-averaged values and standard deviations from precipitations, snow and firn/ice cores

The present data consists in a database of isotope (δO18, δD and deuterium excess) data from precipitations, snow and firn/ice cores, gathering the following data: - the isotopic surface snow data from Masson et al. (2008, doi:10.1175/2007JCLI2139.1) - the Antarctica2k database from Stenni et al. (2017, doi:10.5194/cp-13-1609-2017), available on https://www.ncdc.noaa.gov/paleo-search/study/22589 - the data from Fernandoy et al. (2012, doi:10.5194/tc-6-313-2012) - the precipitation data from Rozanski et al. (1993, doi:10.1029/GM078p0001) and available on the IAEA/GNIP platform -data personnally communicated See below for full references of articles and datasets. The "averages" xls file give necessary informations to retrieve the data a its original temperoral scale, as well as time-averaged, standard deviations and extremum values. They are completed isotope time-averages and standard deviations from the ECHAM5-wiso model forced to the ERA-interim reanalysis and run at the daily scale over the period 1979-2013 (Werner et al., 2011; doi:10.1029/2011JD015681). The "seasonal_snowfall.xls" file give the seasonal cycles of precipitation, temperature, δO18 and deuterium excecss of snowfall data, as used in the associated manuscript