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We here describe, document, and make available a wide range of data sets used for annual-layer identification in ice cores from DYE-3, GRIP, NGRIP, NEEM, and EGRIP. The data stem from detailed measurements performed both on the main deep cores and shallow cores over more than 40 years using many different setups developed by research groups in several countries and comprise both discrete measurements from cut ice samples and continuous-flow analysis data. The data series were used for counting annual layers 60 000 years back in time during the construction of the Greenland Ice-Core Chronology 2005 (GICC05) and/or the revised GICC21, which currently only reaches 3800 years back. Now that the underlying data are made available (listed in Table 1) we also release the individual annual-layer positions of the GICC05 timescale which are based on these data sets. We hope that the release of the data sets will stimulate further studies of the past climate taking advantage of these highly resolved data series covering a large part of the interior of the Greenland ice sheet. [ABSTRACT FROM AUTHOR]

Because the total air content (TAC) of polar ice is directly affected by the atmospheric pressure and temperature, its record in polar ice cores was initially considered as a proxy for past ice sheet elevation changes. However, the Antarctic ice core TAC record is known to also contain an insolation signature, although the underlying physical mechanisms are still a matter of debate. Here we present a high-resolution TAC record over the whole North Greenland Ice Core Project ice core, covering the last 120 000 years, which independently supports an insolation signature in Greenland. Wavelet analysis reveals a clear precession and obliquity signal similar to previous findings on Antarctic TAC, with a different insolation history. In our high-resolution record we also find a decrease of 4-6% (4-5mL kg-1) in TAC as a response to Dansgaard-Oeschger events (DO events). TAC starts to decrease in parallel to increasing Greenland surface temperature and slightly before CH4 reacts to the warming but also shows a two-step decline that lasts for several centuries into the warm interstadial. The TAC response is larger than expected considering only changes in air density by local temperature and atmospheric pressure as a driver, pointing to a transient firnification response caused by the accumulation-induced increase in the load on the firn at bubble close-off, while temperature changes deeper in the firn are still small. [ABSTRACT FROM AUTHOR]

Perfluorocarbons (PFCs) are very potent and longlived greenhouse gases in the atmosphere, released predominantly during aluminium production and semiconductor manufacture. They have been targeted for emission controls under the United Nations Framework Convention on Climate Change. Here we present the first continuous records of the atmospheric abundance of CF4 (PFC-14), C2F6 (PFC-116) and C3F8 (PFC-218) from 1800 to 2014. The records are derived from high-precision measurements of PFCs in air extracted from polar firn or ice at six sites (DE08, DE08-2, DSSW20K, EDML, NEEM and South Pole) and air archive tanks and atmospheric air sampled from both hemispheres. We take account of the age characteristics of the firn and ice core air samples and demonstrate excellent consistency between the ice core, firn and atmospheric measurements. We present an inversion for global emissions from 1900 to 2014. We also formulate the inversion to directly infer emission factors for PFC emissions due to aluminium production prior to the 1980s. We show that 19th century atmospheric levels, before significant anthropogenic influence, were stable at 34:1±0:3 ppt for CF4 and below detection limits of 0.002 and 0.01 ppt for C2F6 and C3F8, respectively. We find a significant peak in CF4 and C2F6 emissions around 1940, most likely due to the high demand for aluminium during World War II, for example for construction of aircraft, but these emissions were nevertheless much lower than in recent years. The PFC emission factors for aluminium production in the early 20th century were significantly higher than today but have decreased since then due to improvements and better control of the smelting process. Mitigation efforts have led to decreases in emissions from peaks in 1980 (CF4) or early-to-mid-2000s (C2F6 and C3F8) despite the continued increase in global aluminium production; however, these decreases in emissions appear to have recently halted. We see a temporary reduction of around 15% in CF4 emissions in 2009, presumably associated with the impact of the global financial crisis on aluminium and semiconductor production. [ABSTRACT FROM AUTHOR]

After the NEEM (Greenland) deep ice-core drilling was declared terminated with respect to developing stratigraphic climate reconstructions, efforts were turned toward collecting basal icesheet debris and, if possible, drilling into the bedrock itself. In 2010, several meters of banded debrisrich ice were obtained under normal ice-drilling operations with the NEEM version of the Hans Tausen (HT) drill, but further penetration was obstructed by a rock in the path of the drill head at 2537.36 m. During short campaigns in 2011 and 2012, attempts were made to penetrate further using various reinforced ice cutters mounted on the HT drill head, tailored to cut through rock. These had some success in penetrating coarse material, but produced severely damaged cutters. Additionally a 51 mm diameter diamond cutting tipped rock drill was adapted to fit the NEEM drill. With this device, several additional meters of core containing subglacial sediments, rocks and rock fragments were collected. With these tools 1.39 m of additional material were obtained during the 2011 field season, and 7.1 m during 2012. Subglacial water refreezing into the newly formed borehole hindered further penetration, and the bedrock interface was not reached before final closure of the NEEM Camp. [ABSTRACT FROM AUTHOR]

The mechanisms involved in the glacial inception are still poorly constrained due to a lack of high resolution and cross-dated climate records at various locations. Using air isotopic measurements in the recently drilled NorthGRIP ice core, we show that no evidence exists for stratigraphic disturbance of the climate record of the last glacial inception (∼123–100 kyears BP) encompassing Dansgaard–Oeschger events (DO) 25, 24 and 23, even if we lack sufficient resolution to completely rule out disturbance over DO 25. We quantify the rapid surface temperature variability over DO 23 and 24 with associated warmings of 10±2.5 and 16±2.5°C, amplitudes which mimic those observed in full glacial conditions. We use records of δ18O of O2 to propose a common timescale for the NorthGRIP and the Antarctic Vostok ice cores, with a maximum uncertainty of 2,500 years, and to examine the interhemispheric sequence of events over this period. After a synchronous North–South temperature decrease, the onset of rapid events is triggered in the North through DO 25. As for later events, DO 24 and 23 have a clear Antarctic counterpart which does not seem to be the case for the very first abrupt warming (DO 25). This information, when added to intermediate levels of CO2 and to the absence of clear ice rafting associated with DO 25, highlights the uniqueness of this first event, while DO 24 and 23 appear similar to typical full glacial DO events. [ABSTRACT FROM AUTHOR]

Abstract: Ice sheets and deep ice cores have yielded a wealth of paleoclimate information based on continuous dating methods while independent radiometric ages of ice have remained elusive. Here we demonstrate the application of (234U/238U) measurements to dating the EPICA Dome C ice core based on the accumulation of 234U in the ice matrix from recoil during 238U decay out of dust bound within the ice. Measured (234U/238U) activity ratios within the ice generally increase with depth while the surface areas of the dust grains are relatively constant. Using a newly designed device for measuring surface area for small samples, we were able to estimate reliably the recoil efficiency of nuclides from dust to ice. The resulting calculated radiometric ages range between 80ka and 870ka. Measured samples in the upper 3100m fall on the previously published age-depth profile. Samples in the 3200–3255m section show a marked change from 723–870ka to 85ka indicating homogenization of the deep ice prior to resetting of the (234U/238U) age in the basal layers. The mechanism for homogenization is likely enhanced lateral ice flow due to high basal melting and geothermal heat flux. [Copyright &y& Elsevier]

Abstract: CO2 measurements on the EPICA (European Project for Ice Coring in Antarctica) DML ice core in depth levels just below the bubble ice–clathrate ice transformation zone (1230–2240m depth) were performed. In the youngest part (1230–1600m), they reveal variations of up to 25ppmv around the mean atmospheric concentration within centimetres, corresponding to a snow deposition interval of a few years. Similar results are found at corresponding depth regions of the Dome C and the Talos Dome ice cores. Since we can exclude all hitherto known processes altering the concentration of CO2 in ice cores, we present a hypothesis about spatial fractionation of air components related to episodically increasing clathrate formation followed by diffusion processes from bubbles to clathrates. This hypothesis is supported by optical line-scan observations and by O2/N2 measurements at the same depth where strong CO2 variations are detected. Below the clathrate formation zone, this small-scale fractionation process is slowly smoothed out, most likely by diffusion, regaining the initial mean atmospheric concentration. Although this process compromises the representativeness of a single CO2 measurement on small ice samples in the clathrate formation zone of an ice core, it does not affect the mean atmospheric CO2 concentration if CO2 values are averaged over a sufficiently long depth scale (>10cm in case of the EPICA DML ice core). [ABSTRACT FROM AUTHOR]

Abstract: We present records of atmospheric nitrous oxide obtained from the ice cores of the European Project for Ice Coring in Antarctica (EPICA) Dome C and Dronning Maud Land sites shedding light on the concentration of this greenhouse gas on glacial–interglacial and millennial time scales. The extended EPICA Dome C record covers now all interglacials of the last 800,000 years and reveals nitrous oxide variations in concert with climate. Highest mean interglacial nitrous oxide concentrations of 280 parts per billion by volume are observed during the interglacial corresponding to Marine Isotope Stage 11 around 400,000 years before present, at the same time when carbon dioxide and methane reach maximum mean interglacial concentrations. The temperature reconstruction at Dome C indicates colder interglacials between 800,000 and 440,000 years before present compared to the interglacials of the last 440,000 years. In contrast to carbon dioxide and methane, which both respond with lower concentrations at lower temperatures, nitrous oxide shows mean interglacial concentrations of 4–19 parts per billion by volume higher than the preindustrial Holocene value during the interglacials corresponding to Marine Isotope Stage 9–19. At the end of most interglacials, nitrous oxide remains substantially longer on interglacial levels than methane. Nevertheless, nitrous oxide shows millennial-scale variations at the same time as methane throughout the last 800,000 years. We suggest that these millennial-scale variations have been driven by a similar mechanism as the Dansgaard/Oeschger events known from the last glacial. Our data lead to the hypothesis that emissions from the low latitudes drive past variations of the atmospheric nitrous oxide concentration. [Copyright &y& Elsevier]