ALBERT

Beschreibung: This thesis deals with the analysis and interpretation of ice core data from northern Greenland, one of the least studied areas in the Arctic. The focus of the thesis is the study of δ18O values which can be used as a proxy for temperature. The 13 ice cores from the North Greenland Traverse (NGT 93/95) are between 100 and 175 m long and were cored between 1993 and 1995. These cores were dated in this thesis and the high- resolution values of δ18O and accumulation rate averaged to annual mean values. The time series spans the last 500 – 1000 years. The topography of the ice sheet is confirmed as the main influencing factor on the long-term annual mean values of δ18O and snow accumulation rate. The predominant southwest/west winds ensure that the northeast of Greenland is located in the precipitation shadow area of the main ice divide, which divides northern Greenland into east and west regions. The lowest δ18O ratios are found east of the divide. In addition, the eastern ice cores are characterized by small variations in δ18O and thickness of accumulation horizons. The correlation between the time series is low due to the large distance between the ice cores and the influence of stratigraphic local noise. By stacking all individual records the signal-to-noise ratio was improved. The stack is representative for a large area in northern Greenland and allows for conclusions regarding the climate during the last thousand years (ending 1994). Globally-known results, such as the Medieval Climate Anomaly that was followed by the Little Ice Age, could be detected in the averaged δ18O time series. The time series is also sensitive to decadal variations such as the warm signal between 1920 and 1930, which cannot be broken down in climate models. Another warm period around 1420 A.D. was observed, which until now has not been seen in any other data set. It is suspected that changes in the local climate dynamic as a result of changed sea ice coverage in the Arctic Sea are possible reasons. Because there was no clear trend in the δ18O NGT 93/95 data in recent years, additional ice cores were drilled in 2011 and 2012 in northern Greenland at comparable positions, in order to extend the series into more recent times. Five cores, each about 30 m long, were analyzed at the AWI Bremerhaven for this purpose. The cores were yearly dated, and the high-resolution δ18O values were averaged for each year. An overlap of about 100 years made it possible to use the time series as an extension of the NGT 93/95 time series. For the first time, an estimation of the actual air temperatures in Greenland over the last thousand years was possible with only one proxy. Since 1980, there has been a constant warming trend. The gradient of the warming is not unusual when compared with other values from the last thousand years, but the level of the values is unusual high.

Beschreibung: Precise measurements of snow structural parameters are crucial to understand snow stratigraphy and its formation by deposition and metamorphism. However, most snow measurements are limited in spatial and temporal resolution, and an important limiting factor is often the time needed to make measurements. We demonstrate the potential of a high-resolution penetrometer by the stratigraphy of a 25 m long and 1.1 m deep transect through the snow and firn at Kohnen Station, Antarctica. We used a statistical model that extracts three major snow structural parameters, density (rho), correlation length (lex) and specific surface area (SSA). We could show using independent measurements from the site that the statistical model is indeed sufficient. The two-dimensional plots reveal the depositional and metamorphic events clearly. Based on these data, we are able to make a much more complete interpretation of the stratigraphic evolution at Kohnen Station. SnowMicroPen derived density of a 1.10 m deep and 25 m long snow and firn transect at Kohnen Station, Antarctica, measured in December 2012.

Beschreibung: Knowledge of snow accumulation rates of the large polar ice sheets and their variability over time is crucial for mass budget studies and sea level predictions. Here we present mean long-term snow accumulation rates of 12 shallow ice cores drilled by the North Greenland traverse in the northern part of Greenland. The ice core records cover the last 500 to 1000 years. We find a trend of decreasing accumulation rate from the southwest (~180 mmWE/a) to northeast (~95 mmWE/a). Ice divide sites show higher accumulation rates but also higher variability (up to 20%) than sites off the ice divides (less than 10%). Unlike a recent modeling study our results indicate no change in the accumulation in the north of Greenland during the last 400 years

Beschreibung: Precise measurements of snow structural parameters and stratigraphy are essential to understand and model the radiative properties of the snow cover. However, most snow measurements are limited in spatial resolution and by extensive measurement times, which particularly constrains their applicability in harsh polar environments. For this reason, we developed a statistical model to derive three major snow structural parameters, density, correlation length and specific surface area solely from a portable, high-resolution penetrometer. We demonstrate the potential of the method by a 25 m long and 1.1 m deep transect through Antarctic firn at Kohnen Station, Antarctica, which reveals the stratigraphic features of the firn clearly. Based on these data, we run the Microwave Emission Model of Layered Snowpacks (MEMLS) and analyze the influence of the spatial variability of the firn around Kohnen Station on the microwave emission of the snowpack. We discuss the potential and limitations of the method and highlight the need for spatially distributed, quantitative measurements for modeling the microwave emission from polar snow and firn.

Beschreibung: Paleoclimatic records from polar ice cores provide unique information about past atmospheric conditions, like temperature from stable water isotopes and greenhouse gas concentrations. To investigate leads and lags of temperature, measured in the ice phase, with gas concentration the exact dating of gas and ice is important. However, the age difference between the air and the surrounding ice complicates the exact dating. After the deposition the age of the snow increases with depth, whereas the pores only close in a certain depth, so called close-off depth (COD). Thus, the knowledge of the COD is crucial for dating the gas. Snow accumulation rate and temperature are known as the main factors of firn densification. Recently, a possible influence of impurities enclosed in the ice was found. Here, we present a case study to investigate which impurities could have an influence on the densification of polar firn. We have analyzed 80 cm from an unbroken firn core from north-west Greenland from a depth of ~54 m, next to the firn-ice transition. Here the density is of very high variability. We can differentiate between layers of different densities in spite of the same temperatures and snow accumulation rates and its deep enough to analyse the influence of trace elements. We measured the density using a new full core X-ray computer tomograph with a resolution of 113 µm. We analysed major ions (SO42-, CH3SO3-, NH4+, NO3-, F-, Br-, Na+, Cl-, K+, Ca2+ and Mg2+) and dust particles on discrete samples in 3-4 mm resolution and compared the obtained concentration profiles with the density of the firn. The uncertainty in depth correspondence is less than 1 mm, thus a comparison on this level is possible for the first time. We found the highest correlations between the dust proxies (particle concentration and Ca2+-concentration), r = 0.6, indicating an influence of these impurities on the densification of firn. Correlation coefficients between other ion concentrations and the density range from r = -0.04 (F-) to r = 0.27 (NO3-). Only sulphur components (sulphate and MSA) show also higher correlations to the density of the firn (r~0.5). For the first time in a direct comparison in a resolution of 1 mm or less it was possible to investigate the influence of impurities on firn densification. The correlation between the density and the dust proxies indicates that the dust particle or some other process linked to the dust proxies, have an influence on the densification of firn. However, the mechanism of densification is not clearly understood yet.

Beschreibung: Snow stratigraphy in Antarctica is strongly influenced by wind. This results in a complex stratigraphy. Yearly accumulation is often masked by intermediate erosion and deposition events. Kohnen-Station has a higher accu- mulation than the very low accumulation areas of East Antarctica, and therefore is suited to investigate complex stratigraphy. Here we use very high frequency GPR at 1.6 GHz to resolve stratigraphy. We compared the GPR mea- surements with detailed snow profiles measured using near-infrared photography and translucent profiles. Detailed depositional features, as dunes and cross-bedding are visible. First results show very complex patterns, which are not easily correlated to other features, and not necessarily to yearly accumulation.

Beschreibung: 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.).

Beschreibung: We present for the first time all 12 δ18O records obtained from ice cores drilled in the framework of the North Greenland Traverse (NGT) between 1993 and 1995 in northern Greenland. The cores cover an area of 680 km  ×  317 km, 10 % of the Greenland ice sheet. Depending on core length (100–175 m) and accumulation rate (90–200 kg m−2 a−1) the single records reflect an isotope–temperature history over the last 500–1100 years. Lowest δ18O mean values occur north of the summit and east of the main divide as a consequence of Greenland's topography. In general, ice cores drilled on the main ice divide show different results than those drilled east of the main ice divide that might be influenced by secondary regional moisture sources. A stack of all NGT records and the NGRIP record is presented with improved signal-to-noise ratio. Compared to single records, this stack represents the mean δ18O signal for northern Greenland that is interpreted as proxy for temperature. Our northern Greenland δ18O stack indicates distinctly enriched δ18O values during medieval times, about AD 1420 ± 20 and from AD 1870 onwards. The period between AD 1420 and AD 1850 has depleted δ18O values compared to the average for the entire millennium and represents the Little Ice Age. The δ18O values of the 20th century are comparable to the medieval period but are lower than that about AD 1420.

Beschreibung: To reconstruct climate changes it is neccessary to have regionally representative data set. Stable water isotope records from the NGT cores are used to better understand the spatial variability of the climate in Northern Greenland. Questions: • How representative is a single record? • What ist the correlation between the sites in Northern Greenland?

Beschreibung: Die in den Luftblasen der Eiskerne eingeschlossene Luft ist ein wichtiges Klimaarchiv. Hieraus kann die Zusammensetzung der Paläoatmosphäre bestimmt werden. Dabei ist es von Nöten das exakte Alter der Luft bestimmen zu können, um eine zeitliche Zuordnung zu garantieren. Das Alter des die Luftblasen umgebenden Eises ist dabei stets größer als jenes der Luft selber, da eine gewissen Zeit von Nöten ist, bis die Blasen vollständig abgeschnürt und vom Austausch zur Atmosphäre abgeschnitten sind. Bislang gibt es noch Unsicherheiten bei der genauen Bestimmung dieses Zeitpunktes. In den Studien von Hörhold (2010) wurde erstmals eine Synchronität des Verlaufes des Dichteprofils und dem der Kalziumkonzentration gefunden. Hieraus entstand die Idee, dass Spurenstoffe, wie Kalzium, einen Einfluss auf die Verdichtung und somit auf den Zeitpunkt der Separation der Luftblasen haben könnten. Ziel dieser Arbeit war es nun, diesen Zusammenhang zu bestätigen. Durch die hohe Auflösung, sollte gezeigt werden, dass die Synchronität der Verläufe auch in den kleinen Schwankungen der Firnporosität zu finden ist. Erstmals wurde in diesem Zusammenhang ein großes Ionenspektrum an diskreten Proben quantifiziert. Dies sollte einen Hinweis darauf geben, welche Ionenspezies den größten Einfluss auf die Verdichtung hat. Es wurde hierzu eine Probe aus Grönland (B22), nahe des Firn-Eis-Überganges bearbeitet. Durch das Schneiden diskreter Proben war eine besonders hohe Auflösung möglich. Es konnte der vermutete Zusammenhang bestätigt werden. Spurenstoffe beeinflussen die Verdichtung des Firns. Kalzium und Staub zeigen hierbei die höchste Korrelation. Je höher der Spurenstoffkonzentration, desto größer die Dichte des Firn. Zum Beispiel. Ca2+ =71,18 ng/g bei einer Dichte von 804,7 kg/m³ und Ca2+ = 36,56 ng/g mit einer Dichte von 743,7 kg/m³. Ob sich das für Grölland zu erwartende saisonale Spurenstoffsignal auch in der Porosität wiederfindet blieb unklar, da keine Saisonalität im Spurenstoffeintrag sichtbar wurde. Aus der Arbeit kann geschlussfolgert werden, dass Firnverdichtungsmodelle, welche die Konzentration der Spurenstoffe nicht beachten, falsche Alter der eingeschlossne Luft berechnen. Besonders für glazialen Firn wurden die Alter unterschätzt.