Description: Combined records of snow accumulation rate, δ18O and deuterium excess were produced from several shallow ice cores and snow pits at NEEM (North Greenland Eemian Ice Drilling), covering the period from 1724 to 2007. They are used to investigate recent climate variability and characterise the isotope–temperature relationship. We find that NEEM records are only weakly affected by inter-annual changes in the North Atlantic Oscillation. Decadal δ18O and accumulation variability is related to North Atlantic sea surface temperature and is enhanced at the beginning of the 19th century. No long-term trend is observed in the accumulation record. By contrast, NEEM δ18O shows multidecadal increasing trends in the late 19th century and since the 1980s. The strongest annual positive δ18O values are recorded at NEEM in 1928 and 2010, while maximum accumulation occurs in 1933. The last decade is the most enriched in δ18O (warmest), while the 11-year periods with the strongest depletion (coldest) are depicted at NEEM in 1815–1825 and 1836–1846, which are also the driest 11-year periods. The NEEM accumulation and δ18O records are strongly correlated with outputs from atmospheric models, nudged to atmospheric reanalyses. Best performance is observed for ERA reanalyses. Gridded temperature reconstructions, instrumental data and model outputs at NEEM are used to estimate the multidecadal accumulation–temperature and δ18O–temperature relationships for the strong warming period in 1979–2007. The accumulation sensitivity to temperature is estimated at 11 ± 2 % °C−1 and the δ18O–temperature slope at 1.1 ± 0.2 ‰ °C−1, about twice as large as previously used to estimate last interglacial temperature change from the bottom part of the NEEM deep ice core.

Description: Für die Rekonstruktion der Klimageschichte spielen Eisbohrkerne aus den Polarregionen und den Hochgebirgsgletschern eine herausragende Rolle unter den Klimaarchiven auf unserer Erde. Nur im Eis ist alte atmosphärische Luft, mit Einschlüssen von z.B. Kohlendioxid und Methan, in bestimmbarer zeitlicher Abfolge gespeichert. Tiefe Eiskerne aus der Ostantarktis dokumentieren bislang etwa die letzten 800.000 Jahre. Die letzte Kaltzeit und die vorletzte Zwischeneiszeit, also etwa die letzten 100.000-130.000 Jahre, werden mit Eisbohrkernen aus der Westantarktis und von Grönland erfasst, die letzten 100-2.000 Jahre sind auch in Eisbohrkernen aus Hochgebirgsgletschern und den arktischen Eiskappen enthalten. Die Entwicklung der Messtechnik zur Bestimmung von Gehalten an stabilen Wasserisotopen und chemischen Substanzen im Eis in den letzen zwei Jahrzehnten hat die zeitliche Auflösung der gewonnenen Information deutlich erhöht. Ice cores from the polar regions and high alpine glaciers play an important role for the reconstruction of the climate history on earth. Old atmospheric gases like carbon dioxide or methane have been stored only in the climate archive ice where their concentrations and ages can be determined. Until now deep ice cores from East Antarctica provide information about the past 800,000 years. The last ice age and the previous interglacial, approximately the past 100,000 to 130,000 years, are included in ice cores from West Antarctica and Greenland. In addition, ice cores from high alpine areas and arctic ice caps provide information about the past 100 to 2,000 years. During the past two decades technical improvements to measure the content of stable isotopes in the water molecule and trace elements included in ice enabled us to get a much higher time resolution than it was possible in earlier investigations.

Description: Ice cores from polar ice sheets provide a unique archive for past climatic variations. But due to their remoteness Greenland and Antarctica are up to now still to large extent unstudied areas. Deep ice cores represent single spots. To get an estimate for the regional representativeness of one ice core and to set the results from deep cores into a wider regional picture more drill sites are necessary covering a larger area. A set of 13 shallow inter-mediate depth (100-150 m) ice cores were drilled during the AWI-North Greenland traverse (NGT) in the 1990ies. It covers 500 to 1000 years back in time and offers the possibility to assess regional representativeness. These 13 single records were analyzed for their water isotopic composition (delta18O) and have been averaged to produce an isotope stack for North Greenland. The main objectives of this study are 1) to analyse this new dataset for its spatial variability and to evaluate the impact of isotopic noise, 2) to assess whether stable water isotope records from sites with very low accumulation rates can also be interpreted as climate signals, 3) to present a new stacked isotope record and 4) to interpret this in terms of paleoclimate (temporal variability, relation to large scale climate information from other ice-core records etc.).

Description: Palaeoclimatic information can be retrieved from the diffusion of the stable water isotope signal during firnification of snow. The diffusion length, a measure for the amount of diffusion a layer has experienced, depends on the firn temperature and the accumulation rate. We show that the estimation of the diffusion length using power spectral densities (PSDs) of the record of a single isotope species can be biased by uncertainties in spectral properties of the isotope signal prior to diffusion. By using a second water isotope and calculating the difference in diffusion lengths between the two isotopes, this problem is circumvented. We study the PSD method applied to two isotopes in detail and additionally present a new forward diffusion method for retrieving the differential diffusion length based on the Pearson correlation between the two isotope signals. The two methods are discussed and extensively tested on synthetic data which are generated in a Monte Carlo manner. We show that calibration of the PSD method with this synthetic data is necessary to be able to objectively determine the differential diffusion length. The correlation-based method proves to be a good alternative for the PSD method as it yields precision equal to or somewhat higher than the PSD method. The use of synthetic data also allows us to estimate the accuracy and precision of the two methods and to choose the best sampling strategy to obtain past temperatures with the required precision. In addition to application to synthetic data the two methods are tested on stable-isotope records from the EPICA (European Project for Ice Coring in Antarctica) ice core drilled in Dronning Maud Land, Antarctica, showing that reliable firn temperatures can be reconstructed with a typical uncertainty of 1.5 and 2 °C for the Holocene period and 2 and 2.5 °C for the last glacial period for the correlation and PSD method, respectively.