ALBERT

Beschreibung: Whereas ice cores from high-accumulation sites in coastal Antarctica clearly demonstrate annual layering, it is debated whether a seasonal signal is also preserved in ice cores from lower-accumulation sites further inland and particularly on the East Antarctic Plateau. In this study, we examine 5 m of early Holocene ice from the Dome Fuji (DF) ice core at a high temporal resolution by continuous flow analysis. The ice was continuously analysed for concentrations of dust, sodium, ammonium, liquid conductivity, and water isotopic composition. Furthermore, a dielectric profiling was performed on the solid ice. In most of the analysed ice, the multi-parameter impurity data set appears to resolve the seasonal variability although the identification of annual layers is not always unambiguous. The study thus provides information on the snow accumulation process in central East Antarctica. A layer counting based on the same principles as those previously applied to the NGRIP (North Greenland Ice core Project) and the Antarctic EPICA (European Project for Ice Coring in Antarctica) Dronning Maud Land (EDML) ice cores leads to a mean annual layer thickness for the DF ice of 3.0 ± 0.3 cm that compares well to existing estimates. The measured DF section is linked to the EDML ice core through a characteristic pattern of three significant acidity peaks that are present in both cores. The corresponding section of the EDML ice core has recently been dated by annual layer counting and the number of years identified independently in the two cores agree within error estimates. We therefore conclude that, to first order, the annual signal is preserved in this section of the DF core. This case study demonstrates the feasibility of determining annually deposited strata on the central East Antarctic Plateau. It also opens the possibility of resolving annual layers in the Eemian section of Antarctic ice cores where the accumulation is estimated to have been greater than in the Holocene. © Author(s) 2015.

Beschreibung: The widely used detailed SNOWPACK model has undergone constant development over the years. A notable recent extension is the introduction of a Richards equation (RE) solver as an alternative for the bucket-type approach for describing water transport in the snow and soil layers. In addition, continuous updates of snow settling and new snow density parameterizations have changed model behavior. This study presents a detailed evaluation of model performance against a comprehensive multiyear data set from Weissfluhjoch near Davos, Switzerland. The data set is collected by automatic meteorological and snowpack measurements and manual snow profiles. During the main winter season, snow height (RMSE: 〈 4.2 cm), snow water equivalent (SWE, RMSE: 〈 40 mm w.e.), snow temperature distributions (typical deviation with measurements: 〈 1.0 °C) and snow density (typical deviation with observations: 〈 50 kg m−3) as well as their temporal evolution are well simulated in the model and the influence of the two water transport schemes is small. The RE approach reproduces internal differences over capillary barriers but fails to predict enough grain growth since the growth routines have been calibrated using the bucket scheme in the original SNOWPACK model. However, the agreement in both density and grain size is sufficient to parameterize the hydraulic properties successfully. In the melt season, a pronounced underestimation of typically 200 mm w.e. in SWE is found. The discrepancies between the simulations and the field data are generally larger than the differences between the two water transport schemes. Nevertheless, the detailed comparison of the internal snowpack structure shows that the timing of internal temperature and water dynamics is adequately and better represented with the new RE approach when compared to the conventional bucket scheme. On the contrary, the progress of the meltwater front in the snowpack as detected by radar and the temporal evolution of the vertical distribution of melt forms in manually observed snow profiles do not support this conclusion. This discrepancy suggests that the implementation of RE partly mimics preferential flow effects.

Beschreibung: A still open question is how equilibrium warming in response to increasing radiative forcing (equilibrium climate sensitivity S) is depending on background climate. We here bring paleo-data based evidence on the state-dependency of S by using CO2 proxy data together with model-based reconstruction of land ice albedo over the last 5 million years. We find that the land-ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcings depends on the CO2 data set used. Over the last 2 million years the combined S_[CO2,LI] from CO2 and land-ice albedo forcing is state-dependent and during interglacials at least twice as high as during glacials, thus CO2 doubling leads to an interglacial warming of 5 K. In the Pliocene data uncertainties prevents a well-supported calculation, but our analysis suggests that S_[CO2,LI] during a land-ice free northern hemisphere was smaller than during interglacials of the Pleistocene.

Beschreibung: Thermokarst lakes are typical features of the northern permafrost ecosystems, and play an important role in the thermal exchange between atmosphere and subsurface. The objective of this study is to describe the main thermal processes of the lakes and to quantify the heat exchange with the underlying sediments. The thermal regimes of five lakes located within the continuous permafrost zone of northern Siberia (Lena River Delta) were investigated using hourly water temperature and water level records covering a 3-year period (2009–2012), together with bathymetric survey data. The lakes included thermokarst lakes located on Holocene river terraces that may be connected to Lena River water during spring flooding, and a thermokarst lake located on deposits of the Pleistocene Ice Complex. Lakes were covered by ice up to 2 m thick that persisted for more than 7 months of the year, from October until about mid-June. Lake-bottom temperatures increased at the start of the ice-covered period due to upward-directed heat flux from the underlying thawed sediment. Prior to ice break-up, solar radiation effectively warmed the water beneath the ice cover and induced convective mixing. Ice break-up started at the beginning of June and lasted until the middle or end of June. Mixing occurred within the entire water column from the start of ice break-up and continued during the ice-free periods, as confirmed by the Wedderburn numbers, a quantitative measure of the balance between wind mixing and stratification that is important for describing the biogeochemical cycles of lakes. The lake thermal regime was modeled numerically using the FLake model. The model demonstrated good agreement with observations with regard to the mean lake temperature, with a good reproduction of the summer stratification during the ice-free period, but poor agreement during the ice-covered period. Modeled sensitivity to lake depth demonstrated that lakes in this climatic zone with mean depths 〉 5 m develop continuous stratification in summer for at least 1 month. The modeled vertical heat flux across the bottom sediment tends towards an annual mean of zero, with maximum downward fluxes of about 5 W m−2 in summer and with heat released back into the water column at a rate of less than 1 W m−2 during the ice-covered period. The lakes are shown to be efficient heat absorbers and effectively distribute the heat through mixing. Monthly bottom water temperatures during the ice-free period range up to 15 °C and are therefore higher than the associated monthly air or ground temperatures in the surrounding frozen permafrost landscape. The investigated lakes remain unfrozen at depth, with mean annual lake-bottom temperatures of between 2.7 and 4 °C.

Beschreibung: Deformation of ice in continental sized ice sheets determines the flow behavior of ice towards the sea. Basal dislocation glide is assumed to be the dominant deformation mechanism in the creep deformation of natural ice, but non-basal glide is active as well. Knowledge of what types of deformation mechanisms are active in polar ice is critical in predicting the response of ice sheets in future warmer climates and its contribution to sea level rise, because the activity of deformation mechanisms depends critically on deformation conditions (such as temperature) as well as on the material properties (such as grain size). One of the methods to study the deformation mechanisms in natural materials is Electron Backscattered Diffraction (EBSD). We obtained ca. 50 EBSD maps of five different depths from a Greenlandic ice core (NEEM). The step size varied between 8 and 25 micron depending on the size of the deformation features. The size of the maps varied from 2000 to 10000 grid point. Indexing rates were up to 95%, partially by saving and reanalyzing the EBSP patterns. With this method we can characterize subgrain boundaries and determine the lattice rotation configurations of each individual subgrain. Combining these observations with arrangement/geometry of subgrain boundaries the dislocation types can be determined, which form these boundaries. Three main types of subgrain boundaries have been recognized in Antarctic (EDML) ice core (Weikusat et al. 2010, 2011). Here, we present the first results obtained from EBSD measurements performed on the NEEM ice core samples from the last glacial period, focusing on the relevance of dislocation activity of the possible slip systems. Preliminary results show that all three subgrain types, recognized in the EDML core, occur in the NEEM samples. In addition to the classical boundaries made up of basal dislocations, subgrain boundaries made of non-basal dislocations are also common.

Beschreibung: Ice is a common mineral at the Earth’s surface. How much ice is stored in the Greenland and Antarctic ice sheets depends on its mechanical properties. Therefore properties of ice directly impact on human society through its role in controlling sea level. The bulk behaviour of large ice masses is the result of the behaviour of the ensemble of individual ice grains. This is strongly influenced by the viscoplastic anisotropy of these grains and their lattice orientation. Numerical modelling provides a better insight into the mechanics of ice from the micro to the ice sheet scale. We present numerical simulations that predict the microstructural evolution of an aggregate of pure ice grains at different strain rates. We simulate co-axial deformation and dynamic recrystallization up to large strain using a full-field approach. The crystal plasticity code (Lebensohn et al., 2009) is used to calculate the response of a polycrystalline aggregate that deforms by purely dislocation glide, applying a Fast Fourier Transform (FFT). This code is coupled with the ELLE microstructural modelling platform to include intracrystalline recovery, as well as grain boundary migration driven by the reduction of surface and strain energies. The results show a strong effect of recrystallization on the final microstructure, producing larger and more equiaxed grains, with smooth boundaries. This effect does not significantly modify the single-maximum pattern of c-axes that are distributed at a low angle to the shortening direction. However, in experiments with significant recrystallization the a-axes rotate towards the elongation axis at the same time as the c-axes rotate towards the compression axis. If slip systems on prismatic and/or pyramidal planes are active, it is thought that a-axes gradually concentrate with depth (Miyamoto, 2005). The bulk activity of the slip systems is different depending on the relative activity of deformation versus recrystallization: the non-basal slip systems are more active at high strain in experiments with dynamic recrystallization compared to those experiments with low recrystallization activity.

Beschreibung: The ice sheets in Greenland and Antarctica contain a significant amount of air within their upper approximately thousand meters and air hydrates below. While this air is still in exchange with the atmosphere in the permeable firn, the gas is entrapped at the firn-ice transition at 60 – 120 m depth. Understanding the dominant deformation mechanisms is essential to interpret paleo-atmosphere records and to allow a more realistic model of ice sheet dynamics. Recent research shows how the presence of air bubbles can significantly influence microdynamical processes such as grain growth and grain boundary migration (Azuma et al., 2012, Roessiger et al., 2014). Therefore, numerical modelling was performed focussing on the mechanical properties of ice with air inclusions and the implications of the presence of bubbles on recrystallisation. The full-field crystal plasticity code of Lebensohn (2001), using a Fast Fourier Transform (FFT), was coupled to the 2D numerical microstructural modelling platform Elle, following the approach by Griera et al. (2013), and used to simulate dynamic recrystallization of pure ice (Montagnat et al., 2013). FFT calculates the viscoplastic response of polycrystalline and polyphase materials that deform by dislocation glide, takes into account the mechanical anisotropy of ice and calculates dislocation densities using the local gradient of the strain-rate field. Incorporating a code for polyphase grain boundary migration driven by surface and internal strain energy reduction, based on the methodology of Becker et al. (2008) and Roessiger et al. (2014), now also enables us to model deformation of ice with air bubbles. The presence of bubbles leads to an increase in strain localization, which reduces the bulk strength of the bubbly ice. In the absence of dynamic recrystallisation, air bubbles quickly collapse at low strains and spherical to elliptical bubble shapes are only maintained when recrystallisation is activated. Our modelling confirms that strain-induced grain boundary migration already occurs in the uppermost levels of ice sheets (Kipfstuhl et al. 2009, Weikusat et al. 2009).

Beschreibung: Ice cores are the only climate archives incorporating paleo-atmosphere as individual gas inclusions, enabling the extraction and analysis of the contained gasses. A firm understanding of the processes involved is mandatory for a reliable interpretation of the gas records. One prominent process is the transition from air bubbles to crystalline air hydrates, which is known to influence, at least temporarily, the gas mixing ratios by diffusion and fractionation. This transition is still not understood completely and the existing theories do not explain the large diversity of observed hydrate morphologies. Raman tomographic measurements using the AWI cryo-Raman system provide 3D reconstructions of air hydrate morphologies. The results show complex growth structures that emphasize the importance of crystallography, microstructure and ice rheology for the hydrate formation process. Accurate hydrate volumes can be calculated from the 3D objects, improving the estimates of total gas contents.

Beschreibung: The Asian monsoon system is an important tipping element in Earth's climate with a large impact on human societies in the past and present. In light of the potentially severe impacts of present and future anthropogenic climate change on Asian hydrology, it is vital to understand the forcing mechanisms of past climatic regime shifts in the Asian monsoon domain. Here we use novel recurrence network analysis techniques for detecting episodes with pronounced non-linear changes in Holocene Asian monsoon dynamics recorded in speleothems from caves distributed throughout the major branches of the Asian monsoon system. A newly developed multi-proxy methodology explicitly considers dating uncertainties with the COPRA (COnstructing Proxy Records from Age models) approach and allows for detection of continental-scale regime shifts in the complexity of monsoon dynamics. Several epochs are characterised by non-linear regime shifts in Asian monsoon variability, including the periods around 8.5–7.9, 5.7–5.0, 4.1–3.7, and 3.0–2.4 ka BP. The timing of these regime shifts is consistent with known episodes of Holocene rapid climate change (RCC) and high-latitude Bond events. Additionally, we observe a previously rarely reported non-linear regime shift around 7.3 ka BP, a timing that matches the typical 1.0–1.5 ky return intervals of Bond events. A detailed review of previously suggested links between Holocene climatic changes in the Asian monsoon domain and the archaeological record indicates that, in addition to previously considered longer-term changes in mean monsoon intensity and other climatic parameters, regime shifts in monsoon complexity might have played an important role as drivers of migration, pronounced cultural changes, and the collapse of ancient human societies.

Beschreibung: Thermokarst lakes are important emitters of methane, a potent greenhouse gas. However, accurate estimation of methane flux from thermokarst lakes is difficult due to their remoteness and observational challenges associated with the heterogeneous nature of ebullition (bubbling). We used multi-temporal high-resolution (9–11 cm) aerial images of an interior Alaskan thermokarst lake, Goldstream Lake, acquired 2 and 4 days following freeze-up in 2011 and 2012, respectively, to characterize methane ebullition seeps and to estimate whole-lake ebullition. Bubbles impeded by the lake ice sheet form distinct white patches as a function of bubbling rate vs. time as ice thickens. Our aerial imagery thus captured in a single snapshot the ebullition events that occurred before the image acquisition. Image analysis showed that low-flux A- and B-type seeps are associated with low brightness patches and are statistically distinct from high-flux C-type and Hotspot seeps associated with high brightness patches. Mean whole-lake ebullition based on optical image analysis in combination with bubble-trap flux measurements was estimated to be 174 ± 28 and 216 ± 33 mL gas m−2 d−1 for the years 2011 and 2012, respectively. A large number of seeps demonstrated spatio-temporal stability over our two-year study period. A strong inverse exponential relationship (R2 ≥ 0.79) was found between percent surface area of lake ice covered with bubble patches and distance from the active thermokarst lake margin. Our study shows that optical remote sensing is a powerful tool to map ebullition seeps on lake ice, to identify their relative strength of ebullition and to assess their spatio-temporal variability.