Iceberg Grounding Research Articles

Tracking ablation and movement of icebergs with time-lapse photography at an alpine proglacial lake in Austria

ABSTRACT Little is known about the evolution and dynamics of icebergs in alpine lakes. We analyzed the movement and ablation patterns of icebergs at an ice-contact lake at Pasterze Glacier, Austria, using time-lapse images. Iceberg evolution was quantified for two timescales and related to meteorological as well as glacier ablation data from the adjacent glacier tongue. On a multiyear scale, ablation and movement of one iceberg (IB1) was monitored during a twenty-five-month period. On a single-day scale, the movement paths of eighty-four icebergs were tracked over 16 hours. Results for IB1 revealed an average iceberg ablation of 72 mm d−1 from June to September and no winter ablation. Iceberg ablation rates rose over time, explained by a rising surface area-to-volume ratio. Monitoring lake-wide iceberg movement for one day shows that a persistent katabatic glacier wind and a valley wind are the main influences on horizontal iceberg movement. Iceberg velocity is roughly 0.6 percent of the wind velocity. The existence of a wind-driven current on the lake surface is proposed. Sudden changes in movement rates, which are not explained by wind data, suggest that iceberg grounding is common. This study provides insight into iceberg melt rates in the absence of wave erosion.

Variability and Formation Mechanism of Polynyas in Eastern Prydz Bay, Antarctica

Based on satellite remote sensing, several polynyas have been found in Prydz Bay, East Antarctica. Compared with the Mackenzie Bay Polynya, the only polynya in the west, the polynyas in eastern Prydz Bay have a larger area and higher ice production, but have never been studied individually. In this study, four recurrent polynyas were identified in eastern Prydz Bay from sea ice concentration data during 2002–2011. Their areas generally exhibit synchronous temporal variations and have good correlation with wind speed, which indicates that they are primarily wind-driven polynyas that need at least one stationary ice barrier to block the inflow of drifting sea ice. The components of the ice barriers of these four polynyas were identified through comparison of satellite remote sensing visible images and synthetic aperture radar images. All types of fast ice, including landfast ice, offshore fast ice and ice fingers serving as ice barriers for these polynyas are anchored by an assemblage of small icebergs and have an approximately year-round period of variations that also regulates the variability of polynyas. The movement and grounding of giant icebergs near the polynyas significantly affects the development of the polynyas. The results of this study illustrate the important impact of icebergs on Antarctic wind-driven polynyas and the formation of dense shelf water.

Glacially influenced morphodynamic features – examples from the north Faroe margin

High resolution reflection seismic data, subbottom profiler and side scan sonar information, together with results from sediment core studies, have been used to study various glacially influenced morphodynamic features on the northern Faroese continental margin. On the shelf and upper slope a thick, buried turbate complex has been found, which we estimate to have been formed between ca 470,000 and 120,000yearsBP. We interpret this turbate to be the result of episodic, extensive iceberg grounding during extreme glaciation within the period MIS 12–MIS 6 (Elster-Saale complex). Late MIS 6 is found to be the most likely age of the last episode of turbate formation as suggested by age/depth correlation with dated sediment cores from the nearby area. The origin of the iceberg turbate may be attributed to deglacial drift of deep-draft icebergs from icestream sources in the Arctic, East Greenland and/or northern Iceland. At the base of slope and adjacent deep-water area a complex of mud diapir features has been observed. Our data show that activation of these features are the result of density inversion processes caused by fine-grained, unconsolidated hemipelagic deposits being overlain by dense, glacigenic North Sea Fan (NSF) deposits. Due to sudden and fast accumulation of the fan sediments, a normal and regular compaction and dewatering of the underlying hemipelagic sediments could not occur; instead, sub-seabed sediment mobilisation took place resulting in enhanced diapir formation. A general intensification of the diapiric processes is likely related to the MIS 6 (Saalian) glacial period, as NSF-derived glacigenic debris flows reached the northern outlet of the Faroe-Shetland Channel. The most recent diapir activity is concluded to have occurred during the last (Weichselian) glaciation.

The effect of seafloor topography in the Southern Ocean on tabular iceberg drifting and grounding

Antarctic tabular icebergs are important active components in the ice sheet-ice shelf-ocean system. Seafloor topography is the key factor that affects the drifting and grounding of icebergs, but it has not been fully investigated. This study analyzes the impact of seafloor topography on the drifting and grounding of Antarctic tabular icebergs using Bedmap-2 datasets and iceberg route tracking data from Brigham Young University. The results highlight the following points. (1) The quantitative distributions of iceberg grounding events and the tracking points of grounded icebergs are mainly affected by iceberg draft and reach their peak values in sea water with depths between 200 m and 300 m. The peak tracking point number and linear velocity of free-drifting icebergs are found in the Antarctic Slope Front (water depth of approximately 500 m). (2) The area of possible grounding regions of small-scale icebergs calved from ice shelf fronts accounts for 28% of the sea area at water depths less than 2000 m outside the Antarctic coastline periphery (3.62 million km2). Their spatial distribution is mainly around East Antarctica and the Antarctic Peninsula. The area of possible grounding regions of large tabular icebergs with long axes larger than 18.5 km (in water depths of less than 800 m) accounts for 74% of the sea area. (3) The iceberg drifting velocity is positively correlated with ocean depth in areas where the depth is less than 2000 m (R=0.85, P<0.01). This result confirms the effect of water depth variations induced by seafloor topography fluctuations on iceberg drifting velocity.

Iceberg ploughmarks and associated sediment ridges on the southern Weddell Sea margin

Ploughing by deep keels of floating icebergs is a common feature of high-latitude continental margins. Icebergs that have calved from glaciers or ice sheets produce a range of seafloor signatures, including linear to curvilinear grooves, rounded pits and ploughmarks terminating in sediment ridges. The dimensions and patterns of iceberg ploughmarks vary with iceberg size, water depth, local current, tide and wind conditions, seafloor sediment and past glacial history. The outer shelf and upper continental slope of the southern Weddell Sea is extensively ploughed by icebergs, and three different types of iceberg ploughmarks are recognized. The first signature is small grounding pits which occur over a wide area of the upper slope to water depths of c. 720 m (white arrow in Fig. 1a). The grounding pits have a mean depth of 8 m and a mean diameter of 280 m. The pits appear to be distributed randomly, with the highest intensity occurring in water depths of 410–670 m. Fig. 1. Multibeam bathymetry and cross-shelf profiles of iceberg ploughmarks on the southern Weddell Sea margin. ( a ) Sun-illuminated multibeam-bathymetric image showing different iceberg ploughmark signatures. White arrow indicates an iceberg grounding pit. Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 30 m. ( b ) Location of the study area (red box; map from IBCSO v. 3.0). FT, Filchner Trough; …

Sources and levels of ambient ocean sound near the Antarctic Peninsula.

Arrays of hydrophones were deployed within the Bransfield Strait and Scotia Sea (Antarctic Peninsula region) from 2005 to 2009 to record ambient ocean sound at frequencies of up to 125 and 500 Hz. Icequakes, which are broadband, short duration signals derived from fracturing of large free-floating icebergs, are a prominent feature of the ocean soundscape. Icequake activity peaks during austral summer and is minimum during winter, likely following freeze-thaw cycles. Iceberg grounding and rapid disintegration also releases significant acoustic energy, equivalent to large-scale geophysical events. Overall ambient sound levels can be as much as ~10–20 dB higher in the open, deep ocean of the Scotia Sea compared to the relatively shallow Bransfield Strait. Noise levels become lowest during the austral winter, as sea-ice cover suppresses wind and wave noise. Ambient noise levels are highest during austral spring and summer, as surface noise, ice cracking and biological activity intensifies. Vocalizations of blue (Balaenoptera musculus) and fin (B. physalus) whales also dominate the long-term spectra records in the 15–28 and 89 Hz bands. Blue whale call energy is a maximum during austral summer-fall in the Drake Passage and Bransfield Strait when ambient noise levels are a maximum and sea-ice cover is a minimum. Fin whale vocalizations were also most common during austral summer-early fall months in both the Bransfield Strait and Scotia Sea. The hydrophone data overall do not show sustained anthropogenic sources (ships and airguns), likely due to low coastal traffic and the typically rough weather and sea conditions of the Southern Ocean.

Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high‐resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long‐term change in Pine Island Glacier. Corrugations and ploughmarks on a sub‐ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed‐shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding‐line configuration throughout the Holocene.

Benthic changes at McMurdo Station, Antarctica following local sewage treatment and regional iceberg-mediated productivity decline

McMurdo Station, the largest research station in Antarctica, ceased on-site garbage dumping in 1988 and initiated sewage treatment in 2003. In 2003–2004 its sea-ice regime was altered by the massive B-15A and C-19 iceberg groundings in the Ross Sea, approximately 100km distant. Here we follow macrofaunal response to these changes relative to a baseline sampled since 1988. In the submarine garbage dump, surface contaminants levels have declined but associated macrofaunal recolonization is not yet evident. Although sewage-associated macrofauna were still abundant around the outfall nearly 2yr after initiation of treatment, small changes downcurrent as far as 434m from the outfall suggest some community recovery. Widespread community changes in 2003–2004, not seen in the decade previously, suggests that the benthos collectively responded to major changes in sea-ice regime and phytoplankton production caused by the iceberg groundings.

Persistent iceberg groundings in the western Weddell Sea, Antarctica

We use the time-average of a series of satellite synthetic aperture radar (SAR) images from 2005 to 2007 to identify grounded icebergs in the western Weddell Sea. Satellite laser altimetry along repeated ground tracks confirms that regions of high mean backscatter are associated with stationary, tabular icebergs. The altimeter data also provide iceberg freeboard, from which we infer maximum ice draft assuming the lightly-grounded limit and a two-layer density model consistent with a source at Filchner–Ronne Ice Shelf. The maximum iceberg draft agrees very well with the ocean depth at a point where the bathymetry is well constrained by ship soundings. However, for an extensive region near 71°S, south of Larsen C Ice Shelf, the maximum grounding depth for several icebergs reveals the ocean to be locally more than 200 m shallower than in bathymetry products. This previously uncharted bank is expected to have a profound effect on the northward flow of the western Weddell Gyre and the cross-slope transport of offshore warm deep water towards the eastern Antarctic Peninsula ice shelves. Passive microwave data also show that sea-ice concentration is reduced in the vicinity of this group of grounded icebergs, indicating an iceberg influence on ocean/atmosphere heat and freshwater exchanges.

Benthic community response to iceberg scouring at an intensely disturbed shallow water site at Adelaide Island, Antarctica

Disturbance is a key structuring force influencing shallow water communities at all latitudes. Polar nearshore communities are intensely disturbed by ice, yet little is known about benthic recovery following iceberg groundings. Understanding patterns of recovery following ice scour may be particularly important in the West Antarctic Peninsula region, one of the most rapidly changing marine systems on Earth. Here we present the first observations from within the Antarctic Circle of community recovery following iceberg scouring. Three grounded icebergs were marked at a highly disturbed site at Adelaide Island (-67 degrees S) and the resultant scours were sampled at <1, 3, 6, 12, 18 and 30 to 32 mo following formation. Each iceberg impact was catastrophic in that it resulted in a 92 to 96% decrease in abundance compared with reference zones, but all post-scoured communities increased in similarity towards 'undisturbed' assemblages over time. Taxa recovered at differing rates, probably due to varying mechanisms of return to scoured areas. By the end of the study, we found no differences in abundance between scoured and reference samples for 6 out of 9 major taxonomic groups. Five pioneer species had consistently elevated abundances in scours compared with reference zones. Variability between the reference zones was high and scour assemblages became comparable to those at reference zones after just 12 mo of recovery. The study site is intensely disturbed by icebergs and the relatively simple infaunal community is well adapted to recover rapidly from disturbance events. Water movements induced by wave action and iceberg groundings at the site are likely to advect small fauna into newly formed scours and promote recovery.

RISK ASSESSMENT OF SCIENTIFIC SONARS

RISK ASSESSMENT OF SCIENTIFIC SONARSELKE BURKHARDT1, OLAF BOEBEL1, HORST BORNEMANN1 AND CHRISTOPH RUHOLL11 Alfred Wegener Institute for Polar and Marine Research, P.O. Box 12016, 27515 Bremerhaven, GERMANY. Elke.Burkhardt@awi.de1. Introduction Scientific sonars form an important asset to conduct oceanographic, geophysical and biological research and are hence installed in many research vessels. Multibeam deep-sea echosounder map the sea-floor topography at high resolution, enabling researchers to identify sites of environmental importances such as cold water coral reefs and sea mounts, to locates suitable sites for oceanographic, geophysical and biological studies such as deep water passages and ice-berg grounding sites, and to develop navigational charts, also in the context of defining marine protected areas. Sediment echosounder explore the upper sediment layer stratification to determine location and thickness of sediment layers for coring sites for paleooceangraphic and sedimentological studies and to map the sediment distribution for paleo-oceanographic and paleo-biodiversity studies. Scientific fish finders are used to map the fish and krill distribution across large areas and at resolutions unachievable by catch data. This study analyses potential risks of these echosounders' usage with special emphasis on cetaceans present in the Antarctic Treaty region, i.e. the ocean south of 60° South. 2. Methods The study first presents a comprehensive description of the echosounders' technical parameters such as sound pressure level, effective pulse length and beam pattern, and continues by analysing three possible impact scenarios:a) Risk of direct damage due to immediate effects of sound;b) Risk of direct damage due to behavioural response;c) Risk of indirect damage due to changes to the habitat.3. Results For cruising ships (R/V Polarstern particularly), the study quantifies that the cumulative risk of type (a) (i.e. the risk of causing a temporary threshold shift, TTS) during simultaneous operation of Hydrosweep and Parasound is estimated to be less then 2 % of the risk of a collision between ship and whale. The risk of causing a permanent threshold shift (PTS) will be smaller. For both, cruising ships and ships on station, the risk of behaviourally induced damage (type b) appears unlikely for cetaceans in the Antarctic Treaty region. This is due to fundamental hydroacoustic differences between the scientific echosounders and the naval mid-frequency sonars associated with recent atypical stranding events elsewhere, and certain physiological and behavioural adaptations of the species involved. Similarly, the risk of indirect damage due to changes to the habitat (type c) appears unlikely. However, while we consider these risks to be significantly less likely than the risk of type (a), a quantification of these latter risks is outside the scope of this study. Figure 1: Critical volume of Hydrosweep Fan-Beam echosounder4. Discussion For ships on station (zero velocity), the non-zero risk of ensonifying a marine mammal at TTS levels obviously exceeds the risk of collision, as the latter becomes zero.The study revealed that only for ships on station, a finite risk of enfonifying a whale with critical sound levels. To mitigate possible negative effects, the Alfred Wegener Institute implemented the following mitigation policy: - shut off of scientific echosounders when on station, unless scientifically need - in later case, implementation of observer, shut-off if whales within 100m radius 5. Acknowledgments This research was supported by Atlas Electronics and WTD 71.6. ReferencesNoise exposure Criteria Group, et al. (2004), Noise Exposure Criteria, paper presented at Second Plenary Meeting of the Advisory Committee on Acoustic Impacts on Marine Mammals, Marine Mammal Commission, Arlington, Virginia, 28- 30 April 2004.Wartzok, D. (2004), Marine Mammal Populations and Ocean Noise: Determining when noise causes biologically signifficant effects, in 4th Plenary Meeting of the Advisory Committee on Acoustic Impacts on Marine Mammals, edited by M. M. Commission, Marine Mammal Commission, New Orleans, Louisianna. http://www.mmc.gov/sound/plenary4/pdf/wartzok.pdf

Deposition of Polar Carbonates During Interglacial Highstands on an Early Permian Shelf, Tasmania

Abstract The early Permian Lower Parmeener Supergroup contains two bioclastic limestones that accumulated during the period of prolonged deglaciation of southern Gondwana as the region moved equatorward from 80° S to 70° S. The Darlington and Berriedale limestones formed in neritic environments, in areas where abundant ice-rafted debris testifies to numerous icebergs and glendonites indicating near-freezing seawater. These limestones consist of argillaceous and clean bioclastic floatstone, rudstone, and grainstone that contain a high-abundance, low-diversity heterozoan assemblage of calcareous invertebrates. The components are dominated by large, robust brachiopods, bryozoans, and Eurydesma bivalves. Sponge spicules and crinoids are also common, whereas coralline algae and conodonts are conspicuously absent. Carbonates were deposited on the middle shelf during sea-level highstands, below the iceberg grounding line, where the faunal communities remained undisturbed. In this setting the organisms were outboard of significant terrigenous sediment influx and brackish water, which were trapped on the inner shelf by bathymetry and icebergs. Strong bottom currents also prevented the deposition of fine-grained siliciclastics by continuous winnowing of the pure limestone facies. Upwelling of nutrient-rich water, inferred from the distribution of phosphate, promoted high primary productivity, which fueled this carbonate factory and inhibited dissolution of the biogenic carbonate.

Subglacial imprints associated with the isolation and decay of an ice mass in the Lower Lough Erne basin, Co. Fermanagh, NW Ireland

High-resolution Chirp sub-bottom profiler and side-scan sonar data record the final phases of ice margin activity in the Lower Lough Erne basin at the end of the last cycle of Quaternary glaciations in Ireland. Relative to the terrestrial glacial landforms the features in the Lough are smaller in scale and are considered to represent local ice mass dynamics that followed regional-scale events. Four phases are identified. (1) After the last ice sheet-wide readvance associated with the Killard Point Stadial (between 15.0 and 14.1 14 C ka bp), stagnation zone retreat resulted in isolation of a residual ice block in the Lower Lough Erne basin. (2) Proglacial waters developed coeval with retreat of the western margin of the ice block. Drawdown induced localized surging and the generation of push features and lineations. (3) Squeeze-up features, reflecting a heavily crevassed ice margin, mark the quiescent phase of the local surge cycle. (4) Iceberg grounding pits and keel marks record calving and rapid disintegration of the Lough Erne ice margin. The well-preserved glacigenic features observed in the lake basin suggest formation by a succession of mechanisms that were too short lived to obliterate the underlying evidence of ice margin dynamics.

Scale-dependent impact on diversity of Antarctic benthos caused by grounding of icebergs

Analysis of seabed video-strips on the Weddell Sea shelf provided evidence that the effects of iceberg scouring on megabenthic assemblages differ, depending on spatial scale. At a local scale (1 to 100 m), undisturbed glass sponge-associated habitats were significantly more diverse than disturbed ones; at a regional scale (1 to 100 km), increased habitat heterogeneity caused by iceberg scouring enhanced species diversity. The pace and succession of the recolonization exhibits high variability and is difficult to predict. The results not only underpin the concept that disturbance events are essential factors in the regulation of ecosystem dynamics, but also emphasize the rele- vance of scale in the evaluation and explanation of diversity patterns.

On the direct impact of ice on marine benthic communities, a review

Ice has a significant impact on the polar and sub-polar benthos, but relationships between corresponding physical and biological processes are not yet sufficiently understood. Sea ice contributes to a vertical zonation in shallow waters, which also experience other important disturbances. Due to the length of the non-glaciated coastline, sea ice is of greater relevance in the north than in the south. Scouring by icebergs and ridged sea ice causes an increased diversity when different recolonisation stages coexist. Frequently scoured areas do not recover, especially in the Antarctic, due to slow growth rates of the fauna. Iceberg grounding in the Arctic is mainly restricted to the western Eurasian and northeastern American shelf, including Greenland. Around Antarctica, scouring is more evenly distributed. Glacier termini prevent sessile animals from settling in their proximity where only few motile species occur.

Bottom currents, sedimentation and ice-sheet retreat facies successions on the Mac Robertson shelf, East Antarctica

Measurements of water turbidity, currents, seafloor sediment samples and geophysical data document the sedimentary processes and the Late Quaternary sedimentary history of a continental shelf valley system on the East Antarctic continental margin. The valley is up to 1200 m in depth and strikes across the shelf; it is interpreted as having formed by glacial erosion processes. On the outer-shelf sill of the valley, northwestward (offshore) currents with speeds of up to 0.47 m/s referenced to 100 cm above the seabed were recorded over a 10-month period. Such currents are competent to initiate bedload transport of medium sand and formation of small (ripple-sized) bedforms which explains the occurrence of ripple cross-bedding observed in some X-radiographs of cores. A nepheloid layer with concentrations of up to 3.5 mg/l was noted at two stations and the available evidence suggests that density flows resulting from the sea-ice brine-rejection mechanism are competent to entrain shelf bottom sediments episodically. Sediment cores taken from this environment document the succession of facies resulting from retreat of glacial ice from the shelf during the Holocene transgression. Under low sea-level, glacial conditions, the ice sheet is interpreted to have been grounded on the outer shelf, where it deposited a grounding line moraine. Rising sea level and global warming caused the ice shelf to retreat leaving a sub-ice shelf, glacial-marine mud. This is overlain by a laminated to massively bedded ice-rafted debris-rich facies, interpreted as being deposited at the glacial calving front. Above this is a siliceous mud and ooze facies having a basal age of about 10,000 years BP, characterising modern, open-marine conditions. Two types of iceberg keel marks on the outer part of the shelf valley are recognised from sidescan sonographs: one relict, arcuate type formed during the retreat of the ice shelf prior to 10,000 years BP and a second modern, elongate type that is presently being formed near the shelf break by iceberg grounding processes. This facies succession has been described from other Antarctic shelf environments and is probably representative of facies deposited during Holocene glacial retreat from the Antarctic shelf.

Impact of iceberg scouring on polar benthic habitats

In s j t u photographs and videos demonstrate that iceberg grounding in both polar regions causes considerable damage to benthic communities. Sessile organisms are eradicated and pioneer species begin to grow in high abundances on the devastated substratum. A preliminary quantitative analysis shows that the sea floor in the Antarctic and Arctic areas of investigation is disturbed by icebergs statistically once every 230 and 53 yr, respectively. Due to the extreme slow growth of many species, particularly in Antarctica, areas frequently disturbed in this manner are likely to be characterised by a continuous natural fluctuation between destruction and recovery. Increased perturbation by iceberg groundings through predicted global warming will result in considerable impairment of this environment.

Sub-bottom profiling and sedimentological studies in the southern Weddell Sea, Antarctica: evidence for large-scale erosional/depositional processes

Low-frequency echo-sounder profiles (3.5 kHz and Parasound systems), surface sediments, and sediment cores were taken from the continental margin of the southern Weddell Sea during four cruises of the R.V. Polarstern between 1985/86 and 1988/89. Nine sediment echo types were classified, mapped and interpreted and verified using sedimentological data. The results provide evidence of large-scale erosional and depositional processes on the continental margin of the southern Weddell Sea. During the last glacial maximum, the Antarctic ice sheet advanced to the continental shelf edge directly north of the Crary Trough, where it overdeepened the shelf by erosion. The supply of eroded material presumably initiated sediment gravity transport processes on the slope in the extension of the Crary Trough. A combination of high energy gravitational transport, such as by turbidity currents, and contour current activity probably caused westward migration of a zone of erosion along the steep, 200–400 m high western flank of an asymmetrical channel extending down the slope. Within the channel, predominantly coarse-grained sediments were deposited, whereas to the west of it, fine-grained levee sedimentation occurred on a sediment ridge. Probably after decoupling of the ice masses, large amounts of sediment were deposited at the sill north of the Crary Trough and at the shallow shelf edge further to the west. Grounding of icebergs resulted in redeposition and compaction of glaciomarine sediments on the continental shelf. Following the retreat of the ice from the shelf edge, sediment gravity transport is inferred to have ceased at the slope. The modern situation is characterized by the flow of high velocity Ice Shelf Water across the shelf edge and at the upper continental slope to the west across the channel-levee complex. On the lower slope today, the current velocities of the remaining Ice Shelf Water are probably not strong enough to cause erosion along the steep western channel flank.

The physical processes of seabed disturbance during iceberg grounding and scouring

Icebergs drifting in ocean currents over the Labrador continental shelf may contact and move through seabed sediments to create characteristically curvilinear features, commonly referred to as scours, which average 1–2 m in depth and about 30–40 m in width. Such scours may continue for several hundreds of metres or several kilometres. The period of seabed interaction may last from a few minutes to several months and results in modification of both the seabed sediments and the iceberg keel. When observed from submersible soon after their formation, scours exhibit morphological characteristics not seen in old, degraded scours. The scour trough, between two berm ridges, is generally flat-bottomed but is characterized by the presence of ridge-and-groove microtopography, with amplitudes up to 0.3 m developed parallel to the scour axis. These features are formed at the trailing edge of the keel by clastic material embedded in the ice and by open fissures in the ice. In places along the inner berm margins, ridges and grooves may be developed at an angle to the scour axis, reflecting lateral displacement of material towards the berm as the iceberg moves forward. Voids up to 1 m deep and 2 m wide occasionally truncate the ridges and grooves. Voids are formed following the dissolution of small (a few m 3) masses of debris-laden ice that are mechanically broken off from the base of the keel and pressed into the seabed by the scouring iceberg. Initially low areas within the scour trough may preserve seafloor that has not been affected by ice-seabed interaction. In these regions deposition of bulldozed sediment from the surcharge at the leading edge of the keel may partially fill the narrow spaces beneath the keel in areas of initially low seabed elevation. Scour berms consist of in situ fractured but intact blocks of material on the inner flanks, and disarticulated blocks 1–2 m high along the berm crest. The outer berm slopes generally consist of pieces of larger blocks spalled from the berm crest resting in relatively finely comminuted, reworked material. The reworked material originates in the leading edge surcharge before being displaced to either side of the keel. Scour berms have irregular topography ranging in height from a few centimetres to as much as 6 m above the seabed. Isolated irregularly-shaped mounds beyond the outer berm may represent pieces of cohesive sediment that originated at the berm crest as overhangs and which subsequently collapsed.

Iceberg/Seabed Interaction Events Observed During the DIGS Experiment

Abstract The Dynamics of Iceberg Grounding and Scouring (DIGS) experiment was conducted in the Labrador Sea during August 1985. The objectives of the experiment were to obtain full-scale data sets documenting iceberg/seabed interactions, and to obtain by direct observation new information regarding the processes of iceberg scour formation and degradation. Utilizing a vessel and a helicopter, measurements were made of icebergs’ above and below-water shapes, plus local winds, waves, currents and tides. Special self-contained motion monitoring packages were deployed by helicopter on icebergs thought to be good grounding candidates. Seabed observations were made directly using the submersible Pisces IV, and extensive side-scan sonar data were collected. This paper describes two dynamic iceberg/seabed interaction events documented during DIGS: the roll/pitting behavior of the 1.2-million-ton domed iceberg “Bertha,” and the split/grounding behavior of the 7.7-million-ton tabular iceberg “Gladys.” This latter event is particularly interesting due to its very energetic nature, and the fact that it represents the only full-scale observation of any iceberg impact with sufficient documentation to yield estimates of the interaction forces. Subsequent to the experiment, the recorded above and below-water shapes were used to obtain hydrostatic stability maps for these icebergs. A time stepping procedure was also developed to re-create these two dynamic events, and comparisons between the observed and simulated motions are provided in this paper.