Ice Ridges Research Articles

Wave interactions with circular ice ridge embedded in level ice

The present study examines the wave interactions with a circular elastic body trapped within a thin cover of sea ice of substantial extent. This configuration is an idealization of an ice ridge embedded in a field of level ice. The ice cover and ice ridge are both modeled as thin elastic plates. Two independent approaches are used. The first approach ignores the finite draft, and a closed form solution is obtained by taking the inner product with respect to the eigenfunctions for the ice-covered region. The second approach includes the finite draft, and a least square approximation method is adopted along with the matching of velocity and pressure between the ice cover and ice ridge. Using the second approach, the surge force on the ice ridge, its deflection and the bending moments over the surface are then computed for various ridge configurations and wave characteristics. The submergence of the ridge is found to have a considerable effect on the deflection as well as the surge force in the presence of the ice cover. The results show that the level ice significantly affects the ice ridge response to shorter waves. It is found that the ridge embedded in ice cover experiences larger bending moments compared to the open water case. The results from the present study can help estimate the response of ice ridges to incoming waves, as well as aid in the design of offshore cylindrical structures embedded in level ice.

Analysis of the joint application results of a borehole jack and thermal drilling in ice research

In last two decade, studies of ice ridge morphometry and strength properties have been actively carried out. Thermal drilling of ice and experiments to determine the local strength of ice using a borehole jack are performed. The paper discusses the issues of joint use of thermal drilling equipment and borehole jack for the ice cover research. Two approaches to the comparison of the results obtained by these two methods are considered. Average penetration rate versus local ice strength dependences are presented. With increasing ice strength and decreasing penetration rate the interval of changes in the ice strength increases and correspondence of the strength to the penetration rate decreases. Based on the results of ice ridges research, depth-wise distributions of local strength and thermal drill penetration rate are compared. Difference between the average thicknesses of the consolidated layer obtained from these distributions was 5 %.

Snow depth on Arctic sea ice from historical in situ data

Abstract. The snow data from the Soviet airborne expeditions Sever in the Arctic collected over several decades in March, April and May have been analyzed in this study. The Sever data included more measurements and covered a much wider area, particularly in the Eurasian marginal seas (Kara Sea, Laptev Sea, East Siberian Sea and Chukchi Sea), compared to the Soviet North Pole drifting stations. The latter collected data mainly in the central part of the Arctic Basin. The following snow parameters have been analyzed: average snow depth on the level ice (undisturbed snow) height and area of sastrugi, depth of snow dunes attached to ice ridges and depth of snow on hummocks. In the 1970s–1980s, in the central Arctic, the average depth of undisturbed snow was 21.2 cm, the depth of sastrugi (that occupied about 30 % of the ice surface) was 36.2 cm and the average depth of snow near hummocks and ridges was about 65 cm. For the marginal seas, the average depth of undisturbed snow on the level ice varied from 9.8 cm in the Laptev Sea to 15.3 cm in the East Siberian Sea, which had a larger fraction of multiyear ice. In the marginal seas the spatial variability of snow depth was characterized by standard deviation varying between 66 and 100 %. The average height of sastrugi varied from 23 cm to about 32 cm with standard deviation between 50 and 56 %. The average area covered by sastrugi in the marginal seas was estimated to be 36.5 % of the total ice area where sastrugi were observed. The main result of the study is a new snow depth climatology for the late winter using data from both the Sever expeditions and the North Pole drifting stations. The snow load on the ice observed by Sever expeditions has been described as a combination of the depth of undisturbed snow on the level ice and snow depth of sastrugi weighted in proportion to the sastrugi area. The height of snow accumulated near the ice ridges was not included in the calculations because there are no estimates of the area covered by those features from the Sever expeditions. The effect of not including that data can lead to some underestimation of the average snow depth. The new climatology refines the description of snow depth in the central Arctic compared to the results by Warren et al. (1999) and provides additional detailed data in the marginal seas. The snow depth climatology is based on 94 % Sever data and 6 % North Pole data. The new climatology shows lower snow depth in the central Arctic comparing to Warren climatology and more detailed data in the Eurasian seas.

Decay phase thermodynamics of ice ridges in the Arctic Ocean

Four ice ridges (R1, R2, R3, and R4) were studied during the spring of 2015 in the Arctic Ocean. During the Norwegian Young Sea Ice expedition (N-ICE2015) from January 11 to June 23, the R/V Lance was moored at four different ice floes (Floe 1, Floe 2, Floe 3, and Floe 4) and drifted along with them. Ice ridge studies were performed on Floe 3 (R1, R2) and Floe 4 (R3, R4). From May 21 to June 23, all ice ridges were drilled several times for structural measurements and cored for physical property measurements. In addition, ridges R1 and R4 were instrumented with Oceanetic thermistor buoys, model 908-20 (OTB 908-20), which remotely logged the vertical temperature profile through the sail and keel of the ridges from April 29 to June 28. After combining these datasets, we obtained thermodynamic properties and evaluated heat budgets of the ridge keels in R1 and R4. Ridge R1 was measured during the transition from the main phase to the decay phase, and ridge R4 was measured during the decay phase. In R1, a 3–4 W/m2 upwards vertical conductive heat flux through the keel was calculated, which caused cooling of the keel and the growth of new ice (i.e., 0.5 m over 22 days from May 5 to May 27, as observed by temperature readings). The total amount of heat extracted from the keel was spent on cooling the keel and growing new ice in fractions of 0.4 and 0.6, respectively. In R4, a downwards vertical conductive heat flux (up to 2 W/m2) transported energy into the keel through the top surface of the keel, while the bottom of the keel melted (i.e., 1.6 m over 12 days from June 12 to June 24) due to the oceanic heat flux.

Ice thickness measurements in Lake Erie during the winter of 2010–2011

Abstract Time series measurements of ice thicknesses were made at either 1 or 2 Hz at 6 locations in the western part of Lake Erie's central basin during the winter of 2010–2011. Ice was observed over approximately 80 days beginning in late December and continuing through mid-March. Deformation and ridging of ice occurred frequently and produced ice thicknesses of up to 10 m, and over 6 m at all stations. The measurements show considerable variability (up to several meters) between stations, even when the distance between them is

Toward a hybrid model of ship performance in ice suitable for route planning purpose

Practical knowledge about the performance of a ship while navigating in ice is crucial for the selection of safe and efficient route for a ship. Existing route finding tools estimate ship performance in ice adopting numerous approaches, ranging from model tests and engineering models to experts-based guidelines. Therein ship performance is usually understood as attainable ship speed or the average speed in given ice conditions; rarely the probability of besetting in ice is taken into account. Those models despite being fairly accurate in the theory share the same shortcoming in practice. The latter encompasses three main issues: (1) inaccurate information about prevailing ice conditions, (2) presence of ice conditions that goes beyond the scope of the models, and (3) the effect of operational patterns and traffic organization on the performance of an individual ship. To approach those issues, we propose a hybrid model of ship performance in ice-covered waters. The hybrid model combines two other sub-models: engineering and data-driven. The former determines ship speed and besetting probability in ridged ice field with ice concentration close to 100%. The latter sub-model provides information on ship’s speed in the actual ice conditions, where the speed is affected also by operational restrictions and icebreaker assistance. It is based on an extensive dataset combining ship data from automatic identification system and ice data from ice charts and ice forecast models. The presented hybrid model is valid for a specific ship type, which is ice going bulk carrier (IA Super ice class), operating within the Northern Baltic Sea winter navigation system. The obtained results reveal that the hybrid model in principle is capable of providing reliable information about the performance of a ship in a wide range of conditions accounting for environmental variability and existing operational conditions. The model is suitable for the purpose of safe route planning in ice for a single ship or group of similar ships, accounting for the economy and safety of a voyage.

On the decay of first-year ice ridges: Measurements and evolution of rubble macroporosity, ridge drilling resistance and consolidated layer strength

In this study, four first-year ice ridges (R1, R2, R3 and R4) were measured during the transition from the “main” phase to the “decay” phase. The measurements were conducted on two ice floes in the Arctic Ocean northwest of Svalbard during May and June 2015. Ice ridge R1 was approximately 13 m thick and 200 m long, R2 was 5 m thick and 500 m long, R3 was 6 m thick and 75 m long, and R4 was 9 m thick and 150 m long. The objective of this study was to investigate the rubble macroporosity evolution, ridge drilling resistance and consolidated layer small-scale strength in the decaying ridges. The ice rubble macroporosity and ridge drilling resistance values were obtained through mechanical drilling. The drilling resistance was measured by the drill operator, which was defined as hard, medium or soft. The small-scale strength was measured in the field via uniaxial compression with a nominal strain rate of 10−3 s−1. The rubble macroporosities in R1, R2, R3 and R4 ranged from 10% to 27%, and the temporal macroporosity variation was the result of seasonal developments. The rubble macroporosity in R2 decreased from 25% (27 days old) to 16% (34 days old, 4 days before breakup). In R1, which was larger, colder and older than R2, the rubble macroporosity remained constant (11%–10%) over a ten-day period. Because ridges R3 (22% rubble macroporosity) and R4 (27% rubble macroporosity) were only mapped once, no temporal development was measured. We suggest that the ice rubble macroporosity in saline, first-year ridges continuously decreases over time and that this decrease accelerates during the decay phase. Furthermore, both the consolidated layer uniaxial compressive strength (measured in R1, R2 and R3) and the ridge drilling resistance (measured in R1, R2, R3 and R4) decreased during the transition from the main phase to the decay phase, due to an increase in ice temperatures. After the ridges reached an isothermal state, the drilling resistance and strength remained constant, and the brine volume (microporosity) increased. The ice cores collected from the decaying ice exhibited ductile failure modes when subjected to uniaxial compression.

Environmental contours based on inverse SORM

It is well known that the full long-term response analysis is recognized as the most accurate and reliable method for evaluation of the extreme response in the design of ships and marine structures. However, such a method is time consuming for large and complex systems. To improve efficiency, the environmental contour method (ECM) is frequently used to approximate the long-term extreme response. The ECM is based on an environmental contour, which is traditionally obtained by the inverse first order reliability method (IFORM) with the assumption that the failure surface in the U space is approximated as a tangent plane at the design point. However, such an approximation underestimates the true failure probability if the failure surface in the U space is a concave set, and then the corresponding environmental contour would not be conservative for possible designs. In this work, a more conservative ISORM (inverse second order reliability method) contour is proposed. In this method, a specific second-order surface is applied to approximate the failure surface at the design point, and then the failure domain in the U space would always be overestimated since the corresponding safe domain is approximated and underestimated as a sphere, regardless of the shapes of the failure surfaces. Therefore, the generated environmental contour can be always conservative for design purposes. The differences of the environmental contours generated by different methods, i.e., the traditional IFORM and the proposed ISORM, are illustrated by relevant examples, such as the wave statistics, wind wave statistics, and first-year ice ridge statistics.

Evaluation of selected state-of-the-art methods for ship transit simulation in various ice conditions based on full-scale measurement

For the design and transit simulation of ice-going ships, a number of methods have been proposed for the prediction of ship resistance and transit speed in various ice conditions. In this paper, selected methods for ship performance in level ice, ridged ice and channel ice are evaluated based on full-scale measurement data of two ships. Uncertainties are identified and evaluated for a better understanding of the deviations in the results.Ice thickness in full-scale data was measured using multiple methods to minimize the uncertainty. The thickness of level ice was measured by a stereo camera system. The ridge profile was identified through measurement with an electromagnetic device. Visual observation was conducted for the description of encountered ice conditions. For a better estimation of ship net thrust through propulsive data, the net thrust model is revised in this paper to take the effect of power and propeller pitch into consideration.The results show that for transit simulation, the selected methods for level ice give acceptable prediction of ship speed with certain underestimation. For ridged ice, the method seems to underestimate the speed, especially when a ship needs to conduct ramming operation. Data acquisition is the most problematic for the investigation of channel ice. The uncertainties due to the modelling of sub-processes and ice properties lead to certain scatter in applying these methods for speed prediction. Possible improvements are given as the conclusion of this work.

Algal Hot Spots in a Changing Arctic Ocean: Sea-Ice Ridges and the Snow-Ice Interface

During the N-ICE2015 drift expedition north-west of Svalbard, we observed the establishment and development of algal communities in first-year ice (FYI) ridges and at the snow-ice interface. Despite some indications of being hot spots for biological activity, ridges are under-studied largely because they are complex structures that are difficult to sample. Snow infiltration communities can grow at the snow-ice interface when flooded. They have been commonly observed in the Antarctic, but rarely in the Arctic, where flooding is less common mainly due to a lower snow-to-ice thickness ratio. Combining biomass measurements and algal community analysis with under-ice irradiance and current measurements as well as light modeling, we comprehensively describe these two algal habitats in an Arctic pack ice environment. High biomass accumulation in ridges was facilitated by complex surfaces for algal deposition and attachment, increased light availability, and protection against strong under-ice currents. Notably, specific locations within the ridges were found to host distinct ice algal communities. The pennate diatoms Nitzschia frigida and Navicula species dominated the underside and inclined walls of submerged ice blocks, while the centric diatom Shionodiscus bioculatus dominated the top surfaces of the submerged ice blocks. Higher light levels than those in and below the sea ice, low mesozooplankton grazing, and physical concentration likely contributed to the high algal biomass at the snow-ice interface. These snow infiltration communities were dominated by Phaeocystis pouchetii and chain-forming pelagic diatoms (Fragilariopsis oceanica and Chaetoceros gelidus). Ridges are likely to form more frequently in a thinner and more dynamic ice pack, while the predicted increase in Arctic precipitation in some regions in combination with the thinning Arctic icescape might lead to larger areas of sea ice with negative freeboard and subsequent flooding during the melt season. Therefore, these two habitats are likely to become increasingly important in the new Arctic with implications for carbon export and transfer in the ice-associated ecosystem.

A Nonequilibrium Figure of Saturn’s Satellite Iapetus and the Origin of the Equatorial Ridge on Its Surface

The structure, dynamical equilibrium, and evolution of Saturn’s moon Iapetus are studied. It has been shown that, in the current epoch, the oblateness of the satellite e2 ≈ 0.046 does not correspond to its angular velocity of rotation, which causes the secular spherization behavior of the ice shell of Iapetus. To study this evolution, we apply a spheroidal model, containing a rock core and an ice shell with an external surface e2, to Iapetus. The model is based on the equilibrium finite-difference equation of the Clairaut theory, while the model parameters are taken from observations. The mean radius of the rock core and the oblateness of its level surface, e1 ≈ 0.028, were determined. It was found that Iapetus is covered with a thick ice shell, which is 56.6% of the mean radius of the figure. We analyze a role of the core in the evolution of the shape of a gravitating figure. It was determined that the rock core plays a key part in the settling of the ice masses of the equatorial bulge, which finally results in the formation of a large circular equatorial ridge on the surface of the satellite. From the known mean altitude of this ice ridge, it was found that, in the epoch of its formation, the rotation period of Iapetus was 166 times shorter than that at present, as little as T ≈ 11h27m. This is consistent with the fact that a driving force of the evolution of the satellite in our model was its substantial despinning. The model also predicts that the ice ridge should be formed more intensively in the leading (dark and, consequently, warmer) hemisphere of the satellite, where the ice is softer. This inference agrees with the observations: in the leading hemisphere of Iapetus, the ridge is actually high and continuous everywhere, while it degenerates into individual ice peaks in the opposite colder hemisphere.

Estimation of degree of sea ice ridging based on dual-polarized C-band SAR data

Abstract. For ship navigation in the Baltic Sea ice, parameters such as ice edge, ice concentration, ice thickness and degree of ridging are usually reported daily in manually prepared ice charts. These charts provide icebreakers with essential information for route optimization and fuel calculations. However, manual ice charting requires long analysis times, and detailed analysis of large areas (e.g. Arctic Ocean) is not feasible. Here, we propose a method for automatic estimation of the degree of ice ridging in the Baltic Sea region, based on RADARSAT-2 C-band dual-polarized (HH/HV channels) SAR texture features and sea ice concentration information extracted from Finnish ice charts. The SAR images were first segmented and then several texture features were extracted for each segment. Using the random forest method, we classified them into four classes of ridging intensity and compared them to the reference data extracted from the digitized ice charts. The overall agreement between the ice-chart-based degree of ice ridging and the automated results varied monthly, being 83, 63 and 81 % in January, February and March 2013, respectively. The correspondence between the degree of ice ridging reported in the ice charts and the actual ridge density was validated with data collected during a field campaign in March 2011. In principle the method can be applied to the seasonal sea ice regime in the Arctic Ocean.

A Multibeam-Based SLAM Algorithm for Iceberg Mapping Using AUVs

Using autonomous underwater vehicles (AUVs) for mapping underwater topography of sea-ice and icebergs, or detecting keels of ice ridges, is foreseen as enabling technology in future arctic marine operations. Wind, current, and Coriolis forces affect an iceberg’s trajectory, making automated mapping difficult. This paper presents a method aiming at enabling autonomous iceberg mapping using AUVs equipped with a multibeam echosounder by estimating the position and orientation of the iceberg. The method is based on a bathymetric simultaneous localization and mapping (SLAM) algorithm, namely, the bathymetric distributed particle filter SLAM (BPSLAM) algorithm. The proposed method estimates the AUV’s pose in an iceberg-fixed coordinate system. The relative states can be used for both guiding the vehicle to achieve complete coverage, as well as estimation of a consistent iceberg topography. The algorithm also provides an estimate of the iceberg’s drift velocity – an important parameter for the AUV trajectory planning as well as any related ice management (IM) operations. Two new weighting algorithms for the BPSLAM method are proposed, enabling batch processing of multibeam echosounder (MBE) measurements to ensure real-time operation without discarding information. The proposed method is demonstrated using a real iceberg topography taken from the PERD iceberg sightings database, with simulated AUV and MBE range measurements. The algorithm is also evaluated on a real world bathymetric dataset, collected using the HUGIN HUS AUV.

Numerical Study of Ice Loads on Ship Hulls in Ridged Ice Fields

Numerical Study of Ice Loads on Ship Hulls in Ridged Ice Fields

A field study of grounded ice features and associated seabed gouging in the Canadian Beaufort Sea

During the late summer of 1983, the southern Canadian Beaufort Sea was invaded by vast areas of thick sea ice features called hummock fields. Thousands of these features drifted into shallow water (10–27m water depth), became grounded and remained in place throughout the following winter. During the grounding event, it was assumed that the deeper ice ridge keels would have gouged the seabed. In April 1984, eight features with gouging potential were visited and investigated from the ice surface. Conditions allowed for measurement by sonar of the geometry of seabed gouges at two locations, named M1 and M2, as well as the geometry of the ice keels that caused the gouges. Volume and mass of the ice features were estimated from stereo air photography gathered earlier in the winter, from which elevation profiles were obtained. On-site elevation profiles confirmed elevations. The hummock field at site M1, estimated at 277,000tonnes, produced a gouge 2m in depth and a front mound about 3m high ahead of its keel. This feature had also been uplifted 2m at its leading edge. The deepest gouge was 3.5m, caused by hummock field M2, with an estimated mass of 1.16milliontonnes, which fragmented during the grounding process. A seabed gouging survey conducted in August 1984, in open water conditions, confirmed gouge depth at M2. Data on seabed soils were obtained at the M2 site one year later, in September 1985, as part of a geotechnical field survey. These cases of seabed gouging by massive sea ice features are unique in that they were found and documented with the ice features that made them still in place. All gouge, ice and soils data relevant to these two events are summarized herein, as a full-scale data set intended to increase understanding of seabed gouging and allow validation of models.

Characterizing Spatial Variability of Ice Algal Chlorophyll a and Net Primary Production between Sea Ice Habitats Using Horizontal Profiling Platforms

Assessing the role of sea ice algal biomass and primary production for polar ecosystems remains challenging due to the strong spatio-temporal variability of sea ice algae. Therefore, the spatial representativeness of sea ice algal biomass and primary production sampling remains a key issue in large-scale models and climate change predictions of polar ecosystems. To address this issue, we presented two novel approaches to up-scale ice algal chl a biomass and net primary production (NPP) estimates based on profiles covering distances of 100 to 1,000 s of meters. This was accomplished by combining ice core-based methods with horizontal under-ice spectral radiation profiling conducted in the central Arctic Ocean during summer 2012. We conducted a multi-scale comparison of ice-core based ice algal chl a biomass with two profiling platforms: a remotely operated vehicle and surface and under ice trawl (SUIT). NPP estimates were compared between ice cores and remotely operated vehicle surveys. Our results showed that ice core-based estimates of ice algal chl a biomass and NPP do not representatively capture the spatial variability compared to the remotely operated vehicle-based estimates, implying considerable uncertainties for pan-Arctic estimates based on ice core observations alone. Grouping sea ice cores based on region or ice type improved the representativeness. With only a small sample size, however, a high risk of obtaining non-representative estimates remains. Sea ice algal chl a biomass estimates based on the dominant ice class alone showed a better agreement between ice core and remotely operated vehicle estimates. Grouping ice core measurements yielded no improvement in NPP estimates, highlighting the importance of accounting for the spatial variability of both the chl a biomass and bottom-ice light in order to get representative estimates. Profile-based measurements of ice algae chl a biomass identified sea ice ridges as an underappreciated component of the Arctic ecosystem because chl a biomass was significantly greater in this unique habitat. Sea ice ridges are not easily captured with ice coring methods and thus require more attention in future studies. Based on our results, we provide recommendations for designing an efficient and effective sea ice algal sampling program for the summer season.

Ice bridges and ridges in the Maxwell-EB sea ice rheology

Abstract. This paper presents a first implementation of a new rheological model for sea ice on geophysical scales. This continuum model, called Maxwell elasto-brittle (Maxwell-EB), is based on a Maxwell constitutive law, a progressive damage mechanism that is coupled to both the elastic modulus and apparent viscosity of the ice cover and a Mohr–Coulomb damage criterion that allows for pure (uniaxial and biaxial) tensile strength. The model is tested on the basis of its capability to reproduce the complex mechanical and dynamical behaviour of sea ice drifting through a narrow passage. Idealized as well as realistic simulations of the flow of ice through Nares Strait are presented. These demonstrate that the model reproduces the formation of stable ice bridges as well as the stoppage of the flow, a phenomenon occurring within numerous channels of the Arctic. In agreement with observations, the model captures the propagation of damage along narrow arch-like kinematic features, the discontinuities in the velocity field across these features dividing the ice cover into floes, the strong spatial localization of the thickest, ridged ice, the presence of landfast ice in bays and fjords and the opening of polynyas downstream of the strait. The model represents various dynamical behaviours linked to an overall weakening of the ice cover and to the shorter lifespan of ice bridges, with implications in terms of increased ice export through narrow outflow pathways of the Arctic.

Modeling Arctic sea‐ice algae: Physical drivers of spatial distribution and algae phenology

AbstractAlgae growing in sea ice represent a source of carbon for sympagic and pelagic ecosystems and contribute to the biological carbon pump. The biophysical habitat of sea ice on large scales and the physical drivers of algae phenology are key to understanding Arctic ecosystem dynamics and for predicting its response to ongoing Arctic climate change. In addition, quantifying potential feedback mechanisms between algae and physical processes is particularly important during a time of great change. These mechanisms include a shading effect due to the presence of algae and increased basal ice melt. The present study shows pan‐Arctic results obtained from a new Sea Ice Model for Bottom Algae (SIMBA) coupled with a 3‐D sea‐ice–ocean model. The model is evaluated with data collected during a ship‐based campaign to the Eastern Central Arctic in summer 2012. The algal bloom is triggered by light and shows a latitudinal dependency. Snow and ice also play a key role in ice algal growth. Simulations show that after the spring bloom, algae are nutrient limited before the end of summer and finally they leave the ice habitat during ice melt. The spatial distribution of ice algae at the end of summer agrees with available observations, and it emphasizes the importance of thicker sea‐ice regions for hosting biomass. Particular attention is given to the distinction between level ice and ridged ice. Ridge‐associated algae are strongly light limited, but they can thrive toward the end of summer, and represent an additional carbon source during the transition into polar night.

Sea ice local surface topography from single-pass satellite InSAR measurements: a feasibility study

Abstract. Quantitative parameters characterizing the sea ice surface topography are needed in geophysical investigations such as studies on atmosphere–ice interactions or sea ice mechanics. Recently, the use of space-borne single-pass interferometric synthetic aperture radar (InSAR) for retrieving the ice surface topography has attracted notice among geophysicists. In this paper the potential of InSAR measurements is examined for several satellite configurations and radar frequencies, considering statistics of heights and widths of ice ridges as well as possible magnitudes of ice drift. It is shown that, theoretically, surface height variations can be retrieved with relative errors ≤ 0.5 m. In practice, however, the sea ice drift and open water leads may contribute significantly to the measured interferometric phase. Another essential factor is the dependence of the achievable interferometric baseline on the satellite orbit configurations. Possibilities to assess the influence of different factors on the measurement accuracy are demonstrated: signal-to-noise ratio, presence of a snow layer, and the penetration depth into the ice. Practical examples of sea surface height retrievals from bistatic SAR images collected during the TanDEM-X Science Phase are presented.

Iceberg and ice-keel ploughmarks on the Gdansk-Gotland Sill (south-eastern Baltic Sea)

New interpretation of the undulating moraine relief of the Gdansk-Gotland Sill, Baltic Sea is proposed. Relict iceberg and ice-keel ploughmarks were observed based on the integration of recently acquired side-scan sonar, multi-beam, single-beam and lithological data. The most likely time of their formation is the period of fast Scandinavian sheet retreat occurring from approximately 13.2 to 11.7 ka. Weak erosional-accumulative processes on the sill from 11.7 ka until the present favoured preservation of the iceberg ploughmarks. The predominant directions of the ploughmarks (north–south and northwest–southeast) coincide with the major iceberg (ice) drift direction from the Scandinavian ice sheet. Furrow width varies from 1 to 300 m with a main width of 20–60 m in a depth range of 1 to 10 m (mostly 2–4 m depth). The ploughmarks are flanked by side ridges 0.5–2 m high, and there is a push mound at the end of some furrows. Three types of cross-sectional furrow profiles have been distinguished: V-shaped cross-section profiles would have been formed by a peaked iceberg keel, U-shaped profiles by a flat keel, and W-shaped profiles by double-keel icebergs (ice ridges). The wide local depressions at the end of ploughmarks could have been formed during periods of fast falling of the Baltic Ice Lake water level, when the ice ridges (stamukhi) or icebergs could ground into the seafloor.