Layer Of Sea Ice Research Articles

Surface ice and gap layers in Antarctic sea ice: highly productive habitats

Biogeochemical investigations of the upper layers of sea ice were made on layered sum- mer ice floes collected from the Weddell Sea, Antarctica, from mid-February to March 1997. The sur- face layers had a clearly defined bottom layer immediately overlying a gap filled with seawater. Gen- erally the gap covered rotten sea ice below. Using differences in algal biomass, mostly in the bottom layer of the surface ice overlying the gap, the floes were classified as low, moderate or high biomass. In addition, a floe with a re-frozen gap layer was studied. In the floes with the highest biomass, par- ticulate organic carbon (POC) and nitrogen (PON) reached concentrations of up to 6000 µMC and 600 µM N in the bottom layer. In the upper part of the surface ice layer and the gap water, particulate and dissolved organic matter concentrations (POM, DOM) were clearly lower. High concentrations of POM were generally accompanied by high values of DOM although POM values generally exceeded DOM. All C and N contents of organic matter were significantly correlated. In gap waters, POM was low but still clearly higher than in the surrounding seawater, whereas DOM was in the range of sea- water concentrations. Most POC/PON and C/chlorophyll a ratios pointed to an actively growing algae community, whereas the higher and more variable DOC/DON ratios reflected the various sources influencing DOM composition. Nitrate and silicate closely followed the signature of salinity, reaching in some gap water samples values similar to seawater concentrations. In some samples, in particular from the upper part of the surface ice layer, nitrate was totally exhausted. The distribution of the regenerated nutrients ammonium and phosphate was totally different from that of nitrate and silicate, reaching values of up to 15.9 and 9.08 µM, respectively. The bottom ice layer of the floe with the re-frozen gap layer had a high biomass similar to that of the high-biomass ice floe. DOC concen- trations were lower, and DON maximum was not clearly linked with DOC maximum, but instead was associated with high ammonium and phosphate concentrations. The significant correlations between POM and DOM as well as between nitrate and silicate and between the regenerated nutrients ammo- nium and phosphate indicate that the gap-layer floes are semi-enclosed, highly productive habitats that still maintain high biomass during freezing. They are ubiquitous in the Antarctic pack-ice zone and important features that support high algae standing stocks.

Anaerobic N2 production in Arctic sea ice

We quantified anaerobic N2 production through bacterial denitrification and anaerobic NH4+ oxidation (anammox) in first‐year ice from Young Sound (74°N) and in an ice floe off Northeast Greenland (79°N). Bacterial denitrification activity (100–300 nmol N L−1 sea ice d−1) occurred in the lower 0.5 m of the sea ice, which had high concentrations of NO3−, NH4+, and dissolved organic carbon (DOC). Despite sea‐ice algal production in the lower sea‐ice layers, heterotrophic activity resulted in a net O2 consumption of 13 µmol O2 L−1 sea ice d−1 in the lower 0.5‐m ice layers. Together with melting of deoxygenated ice crystals, this led to anoxic conditions in the brine system favoring conditions for anaerobic NO3− reduction. Numbers of anaerobic NO3−‐reducing bacteria in the same ice layers were high (1.1 × 105 cells ml−1 sea ice, corresponding to 1.2 × 106 cells ml−1 brine). Area‐integrated denitrification rates were 10–45 µmol N m−2 sea ice d−1, which corresponds to 7–50% of the sediment activity in the area. Although the proportion of anammox to total N2 production was up to 19% in layers of the ice floe from the Greenland Sea, the integrated rate only accounted for 0–5% of total NO3− reduction at the investigated localities.

The effect of flow at Maud Rise on the sea-ice cover – numerical experiments

The role of seamounts in the formation and evolution of sea ice is investigated in a series of numerical experiments with a coupled sea ice–ocean model. Bottom topography, stratification and forcing are configured for the Maud Rise region in the Weddell Sea. The specific flow regime that develops at the seamount as the combined response to steady and tidal forcing consists of free and trapped waves and a vortex cap, which is caused by mean flow and tidal flow rectification. The enhanced variability through tidal motion in particular modifies the mixed layer above the seamount enough to delay and reduce sea-ice formation throughout the winter. The induced sea-ice anomaly spreads and moves westward and affects an area of several 100 000 km2. Process studies reveal the complex interaction between wind, steady and periodic ocean currents: all three are required in the process of generation of the sea ice and mixed layer anomalies (mainly through tidal flow), their detachment from the topography (caused by steady oceanic flow) and the westward translation of the sea-ice anomaly (driven by the time-mean wind).

Meiofauna in sea ice of the Weddell Sea (Antarctica)

Sea-ice meiofauna was studied during various cruises to the Weddell Sea. Foraminifers dominate (75%) the sea-ice community in terms of numerical abundance while turbellarians dominate (45%) in terms of biomass. Distribution of organisms is patchy and varies considerably between cruises but also between sampling sites within one cruise. The bulk of the meiofauna is concentrated in the lowest parts of the sea ice, especially during winter and autumn. However, in porous summer sea ice, sympagic organisms also occur in high densities in upper and intermediate layers of sea ice. Proto- and metazoans associated with Antarctic sea ice include organisms actually living in sea ice, as well as those on the underside of floes and in the underlying water. The sea-ice habitat serves as a feeding ground, as well as an important nursery for juveniles, providing energy-rich food resources. The ice also constitutes a shelter from predators.

Correlated Observations of Radarsat SAR and DMSP SSM/I of Sea Ice in China's Bohai Sea

AbstractUsing the image data from active RADARSAT SAR and passive DMSP SSM/I measurements of the ice in the Bohai Sea on January 23, 1999, this paper presents a correlated analysis for sea ice detection and classification. Numerical modeling of scattering and radiative transfer of a layer of sea ice for characteristic variations of the Bohai Sea shoreice is studied. The scattering index (SI), polarization index (PI), and the polarization ratio (PR) from passive observation of the brightness temperature (TB) are defined to detect the existence of sea ice. Further, using the co‐horizontally polarized back‐scattering coefficient (σhh) measured by the RADARSAT high resolution SAR, the sea ice in the Bohai Sea can be further classified. Temporal and spatial variations of these indices can be used to monitor the sea ice in the Bohai Sea.

Evidence for active microbial nitrogen transformations in sea ice (Gulf of Bothnia, Baltic Sea) in midwinter

Nutrient concentrations, chlorophyll-a, bacterial biomass and relative activity of denitrifying organisms were investigated from ice-core, brine and underlying water samples in February 1998 in the Gulf of Bothnia, Baltic Sea. Examined sea ice was typical for the Baltic Sea; ice bulk salinity varied from 0.1 to 1.6 psu, and in underlying water salinity was from 4.2 to 4.7 psu. In 2- to 3-months-old sea ice (thickness 0.4–0.6 m), sea-ice communities were at the winter stage; chl-a concentrations were generally below 1 mg m−3 and heterotrophic organisms composed 7–20% of organism assemblage. In 1-month-old ice (thickness 0.2–0.25 m), an ice spring bloom was already developing and chl-a concentrations were up to 5.6 mg m−3. In relation to low salinity, high concentrations of NH+ 4, NO− 2, PO3+ 4 and SiOH4 were found in the ice column. The results suggest that the upper part of ice accumulates atmospheric nutrient load during the ice season, and nutrients in the upper 10–20 cm of ice are mainly of atmospheric origin. The most important biological processes controlling the sea-ice nutrient status are nutrient regeneration, nutrient uptake and nitrogen transformations. Nutrient regeneration is specially active in the middle parts of the 50- to 60-cm-thick ice and subsequent accumulation of nutrients probably enhances the ice spring bloom. Nitrite accumulation and denitrifying activity were located in the same ice layers with nutrient regeneration, which together with the observed significant correlation between the concentrations of nitrogenous nutrients points to active nitrogen transformations occurring in the interior layers of sea ice in the Baltic Sea.

Distribution and partial characterization of ice-active molecules associated with sea-ice diatoms

Macromolecular ice-active substances (IASs) that cause pitting of ice-crystal surfaces were previously shown to be associated with two species of Antarctic sea-ice diatom. Here it is shown that IASs are associated with many, if not all, sea-ice diatoms of McMurdo Sound, Antarctica, are present in the bottom layer of sea ice in rough proportion to the density of cells, and exist in winter-grown, algae-containing sea ice in different parts of the Southern Ocean and in at least one location in the Arctic. The IASs are retained to varying degrees by dialysis tubing with 50-, 100- and 300-kDa molecular weight cut-offs, suggesting a large molecular weight range, and have a solute requirement (∼≥200 mOsm/kg) for activity. The IASs occur as smears on both native and denaturing electrophoretic gels, which further suggests a heterogeneous nature. One apparently pure sample contained carbohydrate and protein (at an approximate 3:2 ratio on a weight basis), suggesting that the IASs are glycoproteins. The IASs preferentially bind to ice crystals, as they are concentrated in the ice phase of partially frozen solutions.

Feeding plasticity of Trematomus newnesi (Pisces, Nototheniidae) in Terra Nova Bay, Ross Sea, in relation to environmental conditions

Forty-three and 49 specimens of the Antarctic fish Trematomus newnesi were collected in the coastal waters of Terra Nova Bay, Ross Sea, during December 1994 and February 1998, respectively. The dietary composition of the two fish samples was determined by means of stomach content analysis and then compared. In general, as reported in most of the previous studies, planktivory appeared to be the main feeding habit of T. newnesi. However, the different environmental conditions occurring in the study area in the two periods investigated, such as the degree of the sea-ice coverage and the related amount of light available below the ice, influenced the food composition of T. newnesi. In December 1994, the whole area investigated was covered by a thick layer of sea ice and the diet of T. newnesi consisted of few prey taxa. Some species that characterize the so-called “cryopelagic habitat”, such as the euphausiid Euphausia crystallorophias and the copepod Metridia gerlachei, were by far the most important prey, followed by amphipods and the pteropod Limacina helicina. Conversely, the ostracod Alacia belgicae and larval stages of fish (Pleuragramma antarcticum) were consumed occasionally. In late summer (February 1998), the lack of sea-ice coverage and changes in the associated fauna influenced the feeding habit of T. newnesi. In this period the food spectrum appeared to be more diverse, revealing an active feeding search in the water column. The bulk of food was composed of several prey groups, such as E. crystallorophias, hyperiid amphipods (Hyperiella dilatata), copepods, L. helicina and several species of fish larvae. Present data provide evidence of a marked feeding plasticity of T. newnesi, in response to diverse environmental conditions that characterize the High-Antarctic Zone.

Recent Environmental Changes in ihe Arctic: A Review

Numerous recent observations indicate that the Arctic is undergoing a significant change. In the last decade, the hydrography of the Arctic Ocean has shifted, and the atmospheric circulation has undergone a change from the lower stratosphere to the surface. Typically the eastern Arctic Ocean, on the European side of the Lomonosov Ridge, is dominated by water of Atlantic origin. A cold halocline of varying thickness overlies the warmer Atlantic water and isolates it from the sea ice and surface mixed layer. The western Arctic Ocean, on the North American side of the Lomonosov Ridge, is characterized by an added layer of water from the Pacific immediately below the surface mixed layer. Data collected during several cruises from 1991 to 1995 indicate that in the 1990s the boundary between these eastern and western halocline types shifted from a position roughly parallel to the Lomonosov Ridge to near alignment with the Alpha and Mendeleyev Ridges. The Atlantic Water temperature has also increased, and the cold halocline has become thinner. The change has resulted in increased surface salinity in the Makarov Basin. Recent results suggest that the change also includes decreased surface salinity and greater summer ice melt in the Beaufort Sea. Atmospheric pressure fields and ice drift data show that the whole patterns of atmospheric pressure and ice drift for the early 1990s were shifted counterclockwise 40°-60° from earlier patterns. The shift in atmospheric circulation seems related to the Arctic Oscillation in the Northern Hemisphere atmospheric pressure pattern. The changes in the ocean circulation, ice drift, air temperatures, and permafrost can be explained as responses to the Arctic Oscillation, as can changes in air temperatures over the Russian Arctic.

Polar Water Column Stability

Abstract An expression is derived for the surface salt input needed to induce complete convective overturning of a polar water column consisting of 1) a layer of sea ice, 2) a freezing temperature mixed layer, 3) a pycnocline with linearly varying temperature and salinity, and 4) deep water with fixed temperature and salinity. This quantity has been termed the bulk stability by Martinson. The bulk stability is found to consist of three components. The first two make up Martinson’s salt deficit and are the salt input needed to increase the density of the mixed layer and the pycnocline layer to that of the deep water (the mixed layer stability and pycnocline layer stability, respectively). The third component is Martinson’s thermal barrier: the potential for pycnocline heat to melt ice, reducing the surface salinity. It is found that when the pycnocline density gradient due to temperature offsets more than one half of that due to salinity, the pycnocline layer stability is negative. Consequently, it is poss...

Formation and Maintenance of a Polynya in the Weddell Sea

A dynamic‐thermodynamic sea ice‐mixed layer model for the Weddell Sea is complemented by a simple, diagnostic model to account for local sea ice‐atmosphere interaction. To consider the atmospheric influence on the oceanic mixed layer, the pycnocline upwelling velocity is calculated using the theory of Ekman pumping. In several experiments, formation and conservation of a polynya in the Weddell Sea are investigated. Intrusion of heat into the lower atmosphere above the polynya area is assumed to cause a thermal perturbation and a cyclonic thermal wind field. Superposed with daily ECMWF surface winds, this modified atmospheric forcing field intensifies oceanic upwelling and induces divergent ice drift. Simulation results indicate that in case of a weak atmospheric cross-polynya flow the formation of a thermal wind field can significantly extend the lifetime of a large polynya. The repeated occurrence of the Weddell polynya in the years 1974‐76 thus appears to be an effect of feedback mechanisms between sea ice, atmosphere, and oceanic mixed layer.

Heat budget of snow‐covered sea ice at North Pole 4

The Russian drifting station North Pole 4 (NP‐4) was within 5° latitude of the North Pole from April 1956 to April 1957. We use a wide‐ranging set of snow and meteorological data collected at 3‐hourly intervals on NP‐4 during this period to investigate energy and mass transfer in the snow, sea ice, and atmospheric surface layer in the central Arctic. SNTHERM, a one‐dimensional energy and mass balance model, synthesizes these diverse NP‐4 data and thereby yields energetically consistent time series of the components of the surface heat budget. To parameterize the sensible heat flux during extremely stable stratification, we replace the usual log‐linear stability function with the “Dutch” formulation and introduce a windless coefficient in the bulk parameterization. This coefficient provides sensible heat transfer at the surface, even when the mean wind speed is near zero, and thereby prevents the surface temperature from falling to unrealistically low values, a common modeling problem when the stratification is very stable. Several other modifications to SNTHERM introduce procedures for creating a realistic snowpack that has continuously variable density and is subject to erosion and wind packing. The NP‐4 data provide for two distinct simulations: one on 2‐year ice and one on multiyear ice. We validate our modeling by comparing simulated and observed temperatures at various depths in the snow and sea ice. Simulations for both sites show the same tendencies. During the summer, the shortwave radiation is the main term in the surface heat budget. Shortwave radiation also penetrates into the snow and causes a subsurface temperature maximum that both the data and the model capture. During the winter, the net longwave balance is the main term in the surface heat budget. The snow and sea ice cool in response to longwave losses, but the flux of sensible heat from the air to the surface mitigates these losses and is thus nearly a mirror image of the emitted longwave flux.

Monitoring regional sea ice of China's Bohai Sea by SSM/I scattering indexes

The Specific Sensor Microwave/Imager (SSM/I) data over China's Bohai Sea from 1993 to 1996 has been studied. Some empirical algorithms of data calibration/validation for sea ice, such as the method of the Atmospheric Environment Science/York University temperature brightness index (AES/YORK TBI) and ocean scattering index (OSI), are not well applicable to detection of regional sea ice such as the midlatitude Bohai Sea area. This paper presents a method of the scattering index (SI) and polarization index (PI) to identify regional Bohai Sea ice. Numerical results of vector radiative transfer for a model of a layer of sea ice over ocean simulate the relationship between SIs/PIs and sea ice depth and other variations. It is employed to categorize the graph of SIs/PIs for detection of sea ice.

Natural Convection, Solute Trapping, and Channel Formation during Solidification of Saltwater

We present new experimental results relating to the growth and evolution of sea ice. These show, in particular, that brine initially remains trapped in the interstices of the sea ice, only draining into the underlying ocean once the depth of the sea-ice layer exceeds a critical value. A general theory for convection within mushy layers is applied to develop a hypothesis for when brine drainage occurs, which is tested against the experimental results.

High-frequency reflection coefficient of sea ice

This article describes a simple one-parameter model of the high-frequency reflection coefficient of sea ice. Sea ice is modeled as a layered Biot Stoll medium over a viscoelastic half-space. The viscoelastic half-space represents the columnar zone of sea ice, and the layered Biot Stoll layer represents the dendritic layer of sea ice. The model uses APL/UW’s Biot model of the elastic moduli of sea ice to parametrize the sound velocity in the columnar zone as a function of the temperature and salinity of the sea ice in the columnar zone. The model also uses APL/UW’s Biot model of the elastic moduli of sea ice to parametrize the sound velocities in the dendritic layer as a function of the porosity of the dendritic layer. The dendritic layer is assumed to have a linear porosity profile. The model predicts that the sound velocity transitions from 1800 m/s at the ice water interface to 3700 m/s at the ice–ice interface at the columnar zone.

Ice‐ocean interaction on Ronne Ice Shelf, Antarctica

Detailed glaciological studies have been completed at 28 sites lying on an approximate flow line, extending 760 km across Ronne Ice Shelf. Parameters measured at each location include ice velocity, thickness, principal strain rates, surface elevation, temperature, and accumulation rate. The data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site, from the principles of mass and energy conservation. These calculations indicate basal melting in excess of 1 m yr−1 over the first 100 km of the flow line downstream of the grounding line, where the ice shelf is between 1200 m and 1600 m thick. The maximum melt rates in this region occur near the inland margin and exceed 4 m yr−1. Melting continues at a lesser rate over the next 200 km before freezing commences. Freezing then dominates up to the final 100 km before the ice front, resulting in the accumulation of a layer of basal sea ice up to 50 m thick. This is rapidly removed as melt rates increase to over 6 m yr−1 at the ice front. The cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85% of the mass discharged across the grounding line before it reaches the ice front. Supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections, observed during radio echo sounding of ice thickness. In the region where mass balance calculations suggest an accretion of basal sea ice, reflection coefficients are consistently low, ranging from −6 dB to −38 dB. Most of these weak reflections are believed to originate from the true base of the ice shelf, the additional energy loss being the result of increased attenuation of the radar signal within the saline layer. The derived pattern of basal melting and freezing is consistent with a simple model of sub‐ice‐shelf oceanic circulation, involving a deep thermohaline convection cell. Dense, saline water, which is formed over the continental shelf during winter when the sea surface freezes, drains into the deepest parts of the sub‐ice‐shelf cavity. At the inland margin, where this water mass comes into contact with the ice shelf, its temperature is 1°C above the local pressure freezing point. Melting of ice results, producing a buoyant outflow of cold, relatively fresh water, along the ice shelf base. Basal freezing occurs towards the ice front, where the ascending water becomes supercooled. This circulation has important implications for the production of Antarctic Bottom Water and for the response of the ice shelf to driving stresses, through the temperature dependent viscosity of ice.

Massive, ancient sea-ice strata and preserved physical-structural characteristics in the Ward Hunt Ice Shelf

Two sea-ice layers, one measured as 9 m thick, the other at least 12 m thick and estimated to be 24.5 m thick, have been located by ice core drilling in the west Ward Hunt Ice Shelf. To examine the preservation of physical-structural characteristics over long time intervals, the crystal structure and brine volumes in the sea ice, which possibly dates back to about 3000 BP, have been studied. The structural characteristics are immediately recognizable as those of undeformed congelation sea ice accreted by Stefan growth. Brine volumes in the ancient sea ice are higher than those in modern multi-year ice at the same temperature. The preservation of brine over a time span of hundreds to thousands of years is attributed to an absence of surface meltwater to effect brine flushing and the very slow, even negligible action of gravity drainage, brine pocket migration and brine expulsion. The congelation structures indicate that sea ice can grow by the Stefan accretion mechanism to thicknesses exceeding the equilibrium thickness (2.5–5 m) of most undeformed multi-year ice. The observed physical-structural characteristics of the Ward Hunt sea ice strongly suggest that many of the properties attained by sea ice are permanent and not affected by slow-acting physical processes.

Massive, ancient sea-ice strata and preserved physical-structural characteristics in the Ward Hunt Ice Shelf

Two sea-ice layers, one measured as 9 m thick, the other at least 12 m thick and estimated to be 24.5 m thick, have been located by ice core drilling in the west Ward Hunt Ice Shelf. To examine the preservation of physical-structural characteristics over long time intervals, the crystal structure and brine volumes in the sea ice, which possibly dates back to about 3000 BP, have been studied. The structural characteristics are immediately recognizable as those of undeformed congelation sea ice accreted by Stefan growth. Brine volumes in the ancient sea ice are higher than those in modern multi-year ice at the same temperature. The preservation of brine over a time span of hundreds to thousands of years is attributed to an absence of surface meltwater to effect brine flushing and the very slow, even negligible action of gravity drainage, brine pocket migration and brine expulsion. The congelation structures indicate that sea ice can grow by the Stefan accretion mechanism to thicknesses exceeding the equilibrium thickness (2.5–5 m) of most undeformed multi-year ice. The observed physical-structural characteristics of the Ward Hunt sea ice strongly suggest that many of the properties attained by sea ice are permanent and not affected by slow-acting physical processes.

Vertical Fine Structure of Particulate Matter and Nutrients in Sea Ice of the High Arctic

The vertical fine structure of particulate matter and inorganic nutrients through the bottom layers of sea ice was determined at a site in the Canadian high arctic. Intense vertical gradients of chlorophyll a, nitrate, ammonium, nitrite, phosphate, and silicate developed in the lower 6 cm of the ice as ice algae attained standing crops of 250 mg∙m−2 (up to 60 mg∙L−1) of chlorophyll a. Pigment and inorganic nutrient concentrations were closely correlated, and pools of inorganic nutrient were shown to exist in the particulate matter, suggesting that the extremely high dissolved nutrient concentrations in the bottom ice (e.g. up to 400 μmol∙L−1 nitrate) were derived at least in part from leakage of algal intracellular pools. Nitrogen and phosphorus were present in excess of algal needs, but silicon may not have been. The ratios of particulate organic carbon to chlorophyll a and to particulate organic nitrogen increased from the ice–water interface upwards, consistent with a physiological response of the ice algae to the strong corresponding gradient of light availability. Although spatially compact, the assemblage of algae in the bottom ice inhabits a highly stratified environment.

Some measurements and observations of very old sea ice and brackish ice, Ward Hunt Ice Shelf, N.W.T.

Abstract Using 18O/16O ratio measurements, sea ice and brackish ice have been identified in a 10‐m ice core from Ward Hunt Ice Shelf. Brackish ice constitutes 62% of the core, and sea ice the remainder. The sea ice and brackish ice occur in alternating layers of 2–4 m thickness. The mean salinity of brackish ice (0.22) is an order of magnitude lower than that of the sea ice (1.26). The discrete sea and brackish ice layers and their individual salinity populations have been maintained apparently while the ice has aged and been raised about 40–50 m from the bottom of the ice shelf to its surface, a process taking roughly 400–500 years. Thin sections of the brackish ice reveal variable textures and an almost complete absence of cellular substructure that is associated with brine inclusion and retention in modern sea ice. Thin sections of the old sea ice show evidence of the former cellular substructure that appears to have been altered from the original. The discrete salinity populations and variable texture...