Pack Ice Cover Research Articles

A review of aerial survey density estimates of bearded seals (Erignathus barbatus) in the Canadian Arctic highlights important knowledge gaps and research needs

AbstractThere has been significant sea ice loss in the Arctic as a result of climate change, with measurable impacts on available habitat for ice-obligate marine mammals. Bearded seals (Erignathus barbatus) have a circumpolar distribution and primarily inhabit coastal areas of shallower depths with seasonal pack ice cover. However, there is limited information and no formal estimates on population abundance and densities of bearded seals within Canada. Here, we review historic field reports and publications from aerial surveys conducted from 1974 to 2022 to compile the density and abundance estimates of bearded seals across Canadian waters. Aerial surveys for marine mammals have been flown in several areas across the Canadian Arctic, including the Beaufort Sea (1974–2006), Canadian High Arctic (1979–2022), Baffin Bay-Davis Strait (1979–2014), and the Hudson Complex (1994–2017). After reviewing all published data sources, we report that the density of bearded seals in Canada were highest (and highly variable) in the Beaufort Sea (0.01–8.68 seals*km−2), then in Baffin Bay-Davis Strait (0.004–8.3 seals*km−2), and lowest and less variable inter-annually in the Hudson Complex (0.02 to 0.12 seals*km−2) at the time of these surveys. We also determined that bearded seals are more often found in areas of patchy ice cover (50–75%) with shallow water depths ≤ 500 m. Further, this review identifies regions within the Canadian Arctic and sub-Arctic that require updated aerial survey information. Quantifying the abundance and density estimates of bearded seals in Canada is essential for monitoring population status over time to better understand how this species is responding to environmental variation from anthropogenic activity and climate change.

Spectral attenuation of ocean waves in pack ice and its application in calibrating viscoelastic wave-in-ice models

Abstract. We investigate a case of ocean waves through a pack ice cover captured by Sentinel-1A synthetic aperture radar (SAR) on 12 October 2015 in the Beaufort Sea. The study domain is 400 km by 300 km, adjacent to a marginal ice zone (MIZ). The wave spectra in this domain were reported in a previous study (Stopa et al., 2018b). In that study, the authors divided the domain into two regions delineated by the first appearance of leads (FAL) and reported a clear change of wave attenuation of the total energy between the two regions. In the present study, we use the same dataset to study the spectral attenuation in the domain. According to the quality of SAR-retrieved wave spectrum, we focus on a range of wave numbers corresponding to 9–15 s waves from the open-water dispersion relation. We first determine the apparent attenuation rates of each wave number by pairing the wave spectra from different locations. These attenuation rates slightly increase with increasing wave number before the FAL and become lower and more uniform against wave number in thicker ice after the FAL. The spectral attenuation due to the ice effect is then extracted from the measured apparent attenuation and used to calibrate two viscoelastic wave-in-ice models. For the Wang and Shen (2010b) model, the calibrated equivalent shear modulus and viscosity of the pack ice are roughly 1 order of magnitude greater than that in grease and pancake ice reported in Cheng et al. (2017). These parameters obtained for the extended Fox and Squire model are much greater, as found in Mosig et al. (2015) using data from the Antarctic MIZ. This study shows a promising way of using remote-sensing data with large spatial coverage to conduct model calibration for various types of ice cover.Highlights. Three key points: The spatial distribution of wave number and spectral attenuation in pack ice are analyzed from SAR-retrieved surface wave spectra. The spectral attenuation rate of 9–15 s waves varies around 10−5 m2 s−1, with lower values in thicker semicontinuous ice fields with leads. The calibrated viscoelastic parameters are greater than those found in pancake ice.

On the future navigability of Arctic sea routes: High-resolution projections of the Arctic Ocean and sea ice

The rapid Arctic summer sea ice reduction in the last decade has lead to debates in the maritime industries on the possibility of an increase in cargo transportation in the region. Average sailing times on the North Sea Route along the Siberian Coast have fallen from 20 days in the 1990s to 11 days in 2012–2013, attributed to easing sea ice conditions along the Siberian coast. However, the economic risk of exploiting the Arctic shipping routes is substantial. Here a detailed high-resolution projection of ocean and sea ice to the end of the 21st century forced with the RCP8.5 IPCC emission scenario is used to examine navigability of the Arctic sea routes. In summer, opening of large areas of the Arctic Ocean previously covered by pack ice to the wind and surface waves leads to Arctic pack ice cover evolving into the Marginal Ice Zone. The emerging state of the Arctic Ocean features more fragmented thinner sea ice, stronger winds, ocean currents and waves. By the mid 21st century, summer season sailing times along the route via the North Pole are estimated to be 13–17 days, which could make this route as fast as the North Sea Route.

Potential source regions and processes of aerosol in the summer Arctic

Abstract. Sub-micrometer particle size distributions measured during four summer cruises of the Swedish icebreaker Oden 1991, 1996, 2001, and 2008 were combined with dimethyl sulfide gas data, back trajectories, and daily maps of pack ice cover in order to investigate source areas and aerosol formation processes of the boundary layer aerosol in the central Arctic. With a clustering algorithm, potential aerosol source areas were explored. Clustering of particle size distributions together with back trajectories delineated five potential source regions and three different aerosol types that covered most of the Arctic Basin: marine, newly formed and aged particles over the pack ice. Most of the pack ice area with < 15% of open water under the trajectories exhibited the aged aerosol type with only one major mode around 40 nm. For newly formed particles to occur, two conditions had to be fulfilled over the pack ice: the air had spent 10 days while traveling over ever more contiguous ice and had traveled over less than 30% open water during the last 5 days. Additionally, the air had experienced more open water (at least twice as much as in the cases of aged aerosol) during the last 4 days before arrival in heavy ice conditions at Oden. Thus we hypothesize that these two conditions were essential factors for the formation of ultrafine particles over the central Arctic pack ice. In a comparison the Oden data with summer size distribution data from Alert, Nunavut, and Mt. Zeppelin, Spitsbergen, we confirmed the Oden findings with respect to particle sources over the central Arctic. Future more frequent broken-ice or open water patches in summer will spur biological activity in surface water promoting the formation of biological particles. Thereby low clouds and fogs and subsequently the surface energy balance and ice melt may be affected.

Antarctic penguin response to habitat change as Earth's troposphere reaches 2°C above preindustrial levels

We assess the response of pack ice penguins, Emperor (Aptenodytes forsteri) and Adélie (Pygoscelis adeliae), to habitat variability and, then, by modeling habitat alterations, the qualitative changes to their populations, size and distribution, as Earth's average tropospheric temperature reaches 2°C above preindustrial levels (ca. 1860), the benchmark set by the European Union in efforts to reduce greenhouse gases. First, we assessed models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) on penguin performance duplicating existing conditions in the Southern Ocean. We chose four models appropriate for gauging changes to penguin habitat: GFDL‐CM2.1, GFDL‐CM2.0, MIROC3.2(hi‐res), and MRI‐CGCM2.3.2a. Second, we analyzed the composited model ENSEMBLE to estimate the point of 2°C warming (2025–2052) and the projected changes to sea ice coverage (extent, persistence, and concentration), sea ice thickness, wind speeds, precipitation, and air temperatures. Third, we considered studies of ancient colonies and sediment cores and some recent modeling, which indicate the (space/time) large/centennial‐scale penguin response to habitat limits of all ice or no ice. Then we considered results of statistical modeling at the temporal interannual‐decadal scale in regard to penguin response over a continuum of rather complex, meso‐ to large‐scale habitat conditions, some of which have opposing and others interacting effects. The ENSEMBLE meso/decadal‐scale output projects a marked narrowing of penguins' zoogeographic range at the 2°C point. Colonies north of 70° S are projected to decrease or disappear: ∼50% of Emperor colonies (40% of breeding population) and ∼75% of Adélie colonies (70% of breeding population), but limited growth might occur south of 73° S. Net change would result largely from positive responses to increase in polynya persistence at high latitudes, overcome by decreases in pack ice cover at lower latitudes and, particularly for Emperors, ice thickness. Adélie Penguins might colonize new breeding habitat where concentrated pack ice diverges and/or disintegrating ice shelves expose coastline. Limiting increase will be decreased persistence of pack ice north of the Antarctic Circle, as this species requires daylight in its wintering areas. Adélies would be affected negatively by increasing snowfall, predicted to increase in certain areas owing to intrusions of warm, moist marine air due to changes in the Polar Jet Stream.

Gravity crustal models and heat flow measurements for the Eurasia Basin, Arctic Ocean

The Gakkel Ridge in the Arctic Ocean with its adjacent Nansen and Amundsen Basins is a key region for the study of mantle melting and crustal generation at ultraslow spreading rates. We use free-air gravity anomalies in combination with seismic reflection and wide-angle data to compute 2-D crustal models for the Nansen and Amundsen Basins in the Arctic Ocean. Despite the permanent pack-ice cover two geophysical transects cross both entire basins. This means that the complete basin geometry of the world’s slowest spreading system can be analysed in detail for the first time. Applying standard densities for the sediments and oceanic crystalline crust, the gravity models reveal an unexpected heterogeneous mantle with densities of 3.30 × 103, 3.20 × 103 and 3.10 × 103 kg/m3 near the Gakkel Ridge. We interpret that the upper mantle heterogeneity mainly results from serpentinisation and thermal effects. The thickness of the oceanic crust is highly variable throughout both transects. Crustal thickness of less than 1 km dominates in the oldest parts of both basins, increasing to a maximum value of 6 km near the Gakkel Ridge. Along-axis heat flow is highly variable and heat flow amplitudes resemble those observed at fast or intermediate spreading ridges. Unexpectedly, high heat flow along the Amundsen transect exceeds predicted values from global cooling curves by more than 100%.

Factors affecting fast-ice break-up frequency in Lützow-Holm Bay, Antarctica

AbstractAntarctic fast-ice variation is investigated using satellite images and ship’s ice navigation logs, focusing on break-up phenomena in Lützow-Holm Bay. Although spatio-temporal scales for breakup events vary somewhat for each event, their commencement is generally in autumn and almost always in the same region. Specifically, the 1997/98 break-up event occurred over a wide area and continued for a long time after the initial break-up. Since then, break-ups have recurred until 2004, and a total of 20 annual events have been detected and monitored since 1980. Moreover, information from icebreaker navigation logs shows that unstable fast-ice conditions occurred in the 1980s and after the late 1990s. From the analysis of surface meteorological data and the offshore pack-ice distribution, anomalously shallow snow-cover depths and a peculiar retreat pattern of the ice edge are found to be factors that favour fast-ice break-up. The pack-ice distribution controls the propagation of ocean swell inside the bay; encroaching swells are likely to mechanically disintegrate fast-ice during autumn prior to the annual formation of the protective pack-ice cover to the north. Less snow cover also leads to fast-ice weakening as the melt season progresses and broken floes are then transported offshore by prevailing southerly winds.

Physical control of chlorophyll a, POC, and TPN distributions in the pack ice of the Ross Sea, Antarctica

The pack ice ecosystem of the Ross Sea was investigated along a 1470‐km north‐south transect during the spring 1998 oceanographic program Research on Ocean‐Atmosphere Variability and Ecosystem Response in the Ross Sea (ROAVERRS). Snow and sea ice thickness along the transect varied latitudinally, with thinner snow and ice at the northern ice edge and thin new ice in the vicinity of the Ross Sea polynya. Relative to springtime observations in other sea ice regions, algal chlorophyll a (Chl a) concentrations were low. In contrast, particulate organic carbon (POC), total particulate nitrogen (TPN), and POC:Chl a were all high, indicating either that the ice contained substantial amounts of detritus or nonphotosynthetic organisms, or that the algae had a high POC:Chl a ratio. The abundance of Chl a, POC, and TPN in the sea ice was related to ice age and thickness, as well as to snow depth: older ice had thinner snow cover and contained higher algal biomass while new ice in the polynya had lower biomass. Older pack ice was dominated by diatoms (particularly Fragilariopsis cylindrus) and had vertical distributions of Chl a, POC, and TPN that were related to salinity, with higher biomass at the ice‐water interface. Fluorescence‐based measurements of photosynthetic competence (Fv/Fm) were higher at ice‐water interfaces, and photosynthesis‐irradiance characteristics measured for bottom ice algae were comparable to those measured in pack ice communities of other regions. Nutrient concentrations in extracted sea ice brines showed depletion of silicate and nitrate, depletion or regeneration of phosphate and nitrite, and production of ammonium when normalized to seawater salinity; however, concentrations of dissolved inorganic nitrogen, phosphorous, and silica were typically above levels likely to limit algal growth. In areas where pack ice and snow cover were thickest, light levels could be limiting to algal photosynthesis. Enrichment of δ13C‐POC in the sea ice was correlated with the accumulation of POC, suggesting that carbon sources for photosynthesis might shift in response to decreasing CO2 supply. Comparisons between new ice and underlying waters showed similar algal species dominance (Phaeocystis antarctica) implying incorporation of phytoplankton, with substantially higher POC and TPN concentrations in the ice.

Variability of epipelagic macrozooplankton/micronekton community structure in the NW Weddell Sea, Antarctica (1995–1996)

Macrozooplankton/micronekton in the epipelagic zone (0–100 m) of the NW Weddell Sea were sampled with a 10 m2 multiple opening-closing net environmental sampling system (MOCNESS) trawl during three cruises in 1995 and 1996. A total of 40 species were collected during all cruises. Community composition, as measured using Bray-Curtis analysis and multi-dimensional scaling, permitted separation into two groups: group 1 included samples collected during September–October 1995 and group 2 contained samples collected during April-May 1996 and November–December 1996. Zooplankton collections were dominated by Thysanoessa macrura in September–October 1995 (high pack-ice cover) and by both Euphausia superba and Salpa thompsoni in April–May 1996 (intermediate pack-ice cover) and November–December 1996 (open water). Copepods were not sampled quantitatively by the MOCNESS trawl and were not included in this analysis. Trends in measured parameters of community structure are discussed with respect to environmental characteristics during each sampling period.

Aerial observations of the evolution of ice surface conditions during summer

During spring and summer, the Arctic pack ice cover undergoes a dramatic change in surface conditions, evolving from a uniform, reflective surface to a heterogeneous mixture of bare ice, melt ponds, and leads. This transformation is accompanied by a significant decrease in areally averaged, integrated albedo. The key factors contributing to this reduction in albedo are the melting of the snow cover, the formation and growth of the melt ponds, and the increase in the open water fraction. To document these changes and enable quantification of the evolution of the ponds throughout the melt season, a program of aerial photography was carried out at the main site of the Surface Heat Budget of the Arctic Ocean (SHEBA) program. A modified square pattern, 50 km on a side, surrounding the SHEBA site was flown at altitudes ranging from 1220 to 1830 m. Twelve of these aerial survey photography flights were completed between 20 May and 4 October 1998. The flights took place at approximately weekly intervals at the height of the melt season, with occasional gaps as long as 3 weeks during August and September due to persistent low clouds and fog. In addition, flights on 17 May and 25 July were flown in a closely spaced pattern designed to provide complete photo coverage of a 10‐km square centered on the SHEBA main site. Images from all flights were scanned at high resolution and archived on CD‐ROMs. Using personal computer image processing software, we have measured ice concentration, melt pond coverage, statistics on size and shape of melt ponds, lead fraction, and lead perimeter for the summer melt season. The ponds began forming in early June, and by the height of the melt season in early August the pond fraction exceeded 0.20. The temporal evolution of pond fraction displayed a rapid increase in mid‐June, followed by a sharp decline 1 week later. After the decline, the pond fraction gradually increased until mid‐August when the ponds began to freeze. By mid‐September the surface of virtually all of the ponds had frozen. The open water fraction varied between 0.02 and 0.05 from May through the end of July. In early August the open water fraction jumped to 0.20 in just a few days owing to ice divergence. Melt ponds were ubiquitous during summer, with number densities increasing from 1000 to 5000 ponds per square kilometer between June and August.

Ectoenzymatic ratios in relation to particulate organic matter distribution (Ross Sea, Antarctica).

The results of a study on ectoenzymatic activity (the enzyme activity bound to particles larger than 0.2 micro m) and its relation to organic particle concentration are reported here. The sampling was carried out during the 1994 Antarctic spring, at a fixed station (Station 11) in the polynya of the Ross Sea, an area characterized by quick changes in sea ice cover. The sampling was repeated 4 times over a 20-day time period. The particulate organic matter distribution followed the physical structure of the water column, which depends on ice dynamics and is mainly determined by salinity. In the mixed-water surface layer (0-50 m) the concentrations were higher (on average 65.6 micro gC/L) than in the deeper water layer (50 m-bottom) (on average 19.1 micro gC/L). This distribution and quality, expressed by the protein:carbohydrate ratio, linked the particulate organic matter to the phytoplanktonic bloom which was in progress in the area. We determined the kinetic parameters of the glycolytic and proteolytic ectoenzymes and also the total activity for the proteolytic enzyme, in order to evaluate the contribution of the particle-bound activity. We observed higher values in the surface layer than in the deeper layer. b-Glucosidase activity ranged between 0.03 and 0.92 nmol L(-1) h(-1); b-N-acetylglucosaminidase activity was in the range of 0.04-0.58 nmol (L-1) (h-1). The total proteolytic activity (leucine aminopeptidase) ranged between 0.85 and 33.71 nmol L(-1) (h-1). The ectoproteolytic activity was about 35-60% of the total. The Km values were slightly higher for the proteolytic activity (on average 0.43 micro M for ectoproteolytic activity and 0.58 micro M for total proteolytic activity) than for the b-glucosidase (on average 0.36 micro M) and b-N-acetylglucosaminidase (on average 0.17 micro M), showing no remarkable variations in the water column. The ectoenzymatic ratios and their relationship with particulate organic substrates confirm the close link between organic substrate availability and degradation system response. The significant and positive correlations are not specific and suggest a prompt and efficient systemic response to the input of trophic resources. Nevertheless, changes in ectoenzyme activity and synthesis may act as adaptive responses to changing features of the ecosystem. In particular, variations in the proteolysis:glycolysis ratio depend on the functional features of the ecological system. In our study area this ratio is higher (about 10 or more) during production (particularly autotrophic) and lower (about 5 or less) during degradation/consumption events. The analysis of previous data, collected over a larger area characterized by different environmental conditions due to the changes of the pack ice cover, during the same cruise, confirms the existence of a significant relationship. Furthermore, the analysis of enzyme-uptake systems, expressed as Vmax:Km ratio, suggests that glycolytic ectoenzymes, although poorly expressed, may encourage microconsumers to grow rapidly on a wide range of organic substrates, including the refractory ones such as cellulose and chitin. However, low ectoenzyme potential exploitation rates of available organic substrates (on average about 5% for glycolytic and 12% for proteolytic ectoenzymes) would suggest that, during spring, zooplankton grazing or vertical and lateral transport are likely to play an important role in the removal of organic materials from the system.

A pseudo-Lagrangian study of the sulfur budget in the remote Arctic marine boundary layer

The atmospheric sulfur cycle of the remote Arctic marine boundary layer is studied using trajectories and measurements of sulfur compounds from the International Arctic Ocean Expedition 1991, along with a pseudo-Lagrangian approach and an analytical model. The dimethyl sulfide [DMS(g)] turnover time was 59⁺¹⁸ ^-9h. Only 25⁺¹¹ _-9% of DMS(g) followed reaction paths to sulfur dioxide [SO₂(g)], sub-micrometre aerosol non-seasalt sulfate (nss-SO₄ ²⁻) or methane sulfonate (MSA). During the first 3 d of transport over the pack ice, fog deposition and drizzle resulted in short turnover times; 16⁺⁸ _-6 h for SO₂(g), 18⁺⁴ _-4h for MSA and 18⁺² ^-3 h for nss-SO₄ ²⁻. Therefore, DMS(g) will, owing to its origin along or south of the ice edge and longer turnover time, survive the original sub-micrometre sulfur aerosol mass and gradually replace it with new biogenic sulfur aerosol mass. The advection of DMS(g) along with heat and moisture will influence the clouds and fogs over the Arctic pack ice through the formation of cloud condensation nuclei (CCN). If the pack ice cover were to decrease owing to a climate change, the total Arctic Ocean DMS production would change, and potentially there could be an ice–DMS–cloud–albedo climate feedback effect, but it would be accompanied by changes in the fog aerosol sink.

Ecological impact of a large Antarctic iceberg

Satellite imagery has been used to document for the first time the potential for large icebergs to substantially alter the dynamics of a marine ecosystem. The B‐15 iceberg (∼10,000 km2), which calved off the Ross Ice Shelf in the biologically productive southwestern Ross Sea, Antarctica, restricted the normal drift of pack ice, resulting in heavier spring/summer pack ice cover than previously recorded. Extensive ice cover reduced both the area suitable for phytoplankton growth and the length of the algal growing season. Consequently, primary productivity throughout the region was >40% below normal, which changed both the abundance and behavior of upper trophic level organisms.

A pseudo-Lagrangian study of the sulfur budget in the remote Arctic marine boundary layer

The atmospheric sulfur cycle of the remote Arctic marine boundary layer is studied using trajectories and measurements of sulfur compounds from the International Arctic Ocean Expedition 1991, along with a pseudo-Lagrangian approach and an analytical model. The dimethyl sulfide [DMS(g)] turnover time was 59+18-9h. Only 25+11-9% of DMS(g) followed reaction paths to sulfur dioxide [SO2(g)], sub-micrometre aerosol non-seasalt sulfate (nss-SO42−) or methane sulfonate (MSA). During the first 3 d of transport over the pack ice, fog deposition and drizzle resulted in short turnover times; 16+8-6 h for SO2(g), 18+4-4h for MSA and 18+2-3 h for nss-SO42−. Therefore, DMS(g) will, owing to its origin along or south of the ice edge and longer turnover time, survive the original sub-micrometre sulfur aerosol mass and gradually replace it with new biogenic sulfur aerosol mass. The advection of DMS(g) along with heat and moisture will influence the clouds and fogs over the Arctic pack ice through the formation of cloud condensation nuclei (CCN). If the pack ice cover were to decrease owing to a climate change, the total Arctic Ocean DMS production would change, and potentially there could be an ice–DMS–cloud–albedo climate feedback effect, but it would be accompanied by changes in the fog aerosol sink.

Moult of the emperor penguin: travel, location, and habitat selection

All penguins except emperors Aptenodytes forsteri and Adelies Pygoscelis adeliae moult on land, usually near the breeding colonies. These 2 Antarctic species typically moult some- where in the pack-ice. Emperor penguins begin their moult in early summer when the pack-ice cover of the Antarctic Ocean is receding. The origin of the few moulting birds seen by observers on pass- ing ships is unknown, and the locations are often far from any known colonies. We attached satellite transmitters to 12 breeding adult A. forsteri from western Ross Sea colonies before they departed the colony for the last time before moulting. In addition, we surveyed some remote areas of the Weddell Sea north and east of some large colonies that are located along the southern and western borders of this sea. The tracked birds moved at a rate of nearly 50 km d -1 for more than 1000 km over 30 d to reach areas of perennially consistent pack-ice. Almost all birds traveled to the eastern Ross Sea and western Amundsen Sea. Sea-ice conditions were observed directly in the Weddell Sea from ship and helicopter. Most floes selected for moulting were ridged, and usually >100 m 2 . From these observa- tions we predict where the most likely moult refuges are for emperor penguins from other colonies around the Antarctic continent.

Diet and foraging effort of Adélie penguins in relation to pack-ice conditions in the southern Ross Sea

We investigated the diet and aspects of for- aging eAort among Adelie penguins (Pygoscelis adeliae) breeding at three colonies on Ross Island, in the southwestern Ross Sea - Capes Royds, Bird and Crozier - during the chick-provisioning period of three austral summers, 1994-1995, 1995-1996 and 1996-1997. During the study period, pack-ice cover diAered in waters oA- shore of these colonies, by colony, seasons and year. Diet diAered among colonies only slightly. The fish Pleuragramma antarcticum was the most important prey, especially during years or periods within years when little pack ice was present. With respect to krill, which composed the remainder of diet, juvenile Euphausia crystallorophias were consumed predominantly in a year of heavy pack-ice cover; more adult krill were consumed in 2 years when pack ice was sparse. Foraging trip du- ration diAered by colony, season and year and was related directly to distance from the colony to the nearest pack ice. The amount of food brought to chicks increased as trip duration increased, to a point (2 days), but then decreased as duration increased further (up to 4 days). On the basis of data on mass of parents and of meal sizes to chicks, it appeared that on the longest trips more of the food gathered by parents was used for self maintenance; on the longest trips, parents lost body mass. Successful foraging during chick rearing, the pe- riod when adult foraging is most intense, appears to depend on the proximity of pack ice to nesting colonies for this penguin species.

Feeding habits of harp seals (Phoca groenlandica) during early summer and autumn in the northern Barents Sea

The feeding habits of harp seals (Phoca groenlandica) in the Barents Sea were examined in studies conducted during June 1991, September 1990 and 1991, and October 1992. Analyses of stomach and intestinal contents were carried out and concurrent estimates of prey abundance were made using trawl gear. Harp seals appeared to feed at low intensity in the pack ice belt during the first half of June. There was little potential prey in the water column, but prawns (Pandalus borealis), capelin (Mallotus villosus) and polar cod (Boreogadus saida) were abundant close to the bottom. In September, the seals sampled in the northern pack ice areas of the Barents Sea fed on the pelagic amphipod Parathemisto libellula, krill (Thysanoessa spp.), prawns and, to a lesser extent, on fish species such as polar cod, sculpins (Cottidae) and snailfish (Liparidae). Trawling revealed that large quantities of Parathemisto libellala were present in the upper layers of the water column. Fish, mainly capelin and polar cod, were less abundant and occurred in deeper waters. In mid-October, the diet of seals in the northern Barents Sea consisted mainly of amphipods (Parathemisto sp.). Later in October, when increasing pack ice cover forced the harp seals to move south, the diet seemed to change from amphipods to fish prey, predominantly capelin and polar cod.

A model of a drifting ice cover

Some theoretical ideas are developed concerning the dynamics of a drifting ice cover modelled by a four-phase two-dimensional continuous medium with elastic-plastic rheology under bulk and shearing deformations, including processes of hummocking and the formation of fractures [1]. Phase transition conditions are formulated in closed form, depending in general on eight functions that characterize the rheological properties of the sheet. New exact solutions are developed for the model equations, describing a diverging ice cover drifting in the field of an ocean eddy, the interaction of a diverging ice cover with surface waves, the plastic flow of a close pack ice cover under the influence of a localized sea current, and the collision of a close pack ice massif with a rigid, wedge-shaped wall.

Drag coefficients for winter Antarctic pack ice

This paper presents air‐ice and ice‐water drag coefficients referenced to 10‐m‐height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP‐86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS‐89). The optimal values of the air‐ice drag coefficients, made from turbulent flux measurements, are C10 = (1.79 ± 0.06) × 10−3 for WWSP‐86 and (1.45 ± 0.09) × 10−3 for WWGS‐89. Neutral drag coefficient values are CN10 = 1.68 × 10−3 for WWSP‐86 and 1.44 × 10−3 for WWGS‐89. The slightly lower values for WWGS‐89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median mm for WWSP‐86 and 0.27 mm for WWGS‐89). These values are consistent with Arctic measurements for 80–100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice‐water drag coefficient for both WWSP‐86 and WWGS‐89, estimated from periods of ice drift thought to represent free‐drift conditions (air‐ice stress balanced by ice‐water drag and Coriolis force), is (1.13 ± 0.26) × 10−3, and the ice‐water turning angle . This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first‐year Arctic ice. Consistent with previous findings for WWSP‐86, the free‐drift form of the momentum balance can be used to describe the observed WWGS‐89 ice drift observations by using an “effective” drag coefficient and turning angle that subsume the influence of ice‐ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by ∼33%.

Diatom Evidence for Late Holocene Climatic Events in Granite Harbor, Antarctica

Downcore diatom data from a deep basin in Granite Harbor, Antarctica, provide new, high‐resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years. The influence of global‐scale climatic change, during the time of the Medieval Warm Period and possibly the Little Ice Age, is revealed by long‐term changes in floral assemblages, as recorded by changes in the relative concentration of Nitzschia cylindrus. The diatom data suggest that Granite Harbor experienced less annual ice and pack ice cover during both these periods, possibly because of higher temperatures during the Medieval Warm Period and stronger offshore winds during the Little ice Age. The sedimentary record reveals several distinct, short‐lived events, in which nearly monospecific blooms of Corethron criophilum and Chaetoceros spp. occurred, possibly the result of strong stabilization of the upper water column and were most likely a response to regional, rather than global conditions. Although short‐lived, these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor.