Sea Ice Brine Research Articles

Acclimation of Antarctic bottom-ice algal communities to lowered salinities during melting

Sea-ice brine algal communities were exposed to salinities between 30 and 10‰ during melting. There was a progressive decline in maximum quantum yield, relative electron transfer rate (rETRmax) and photosynthetic efficiency (α) with decreasing salinity of the final melted sample. While all species showed a drop in these parameters, Fragilariopsis curta and Entomoneis kjellmannii showed the least inhibition. There was a steady increase in rETRmax and α over 5 days after melting, especially in the samples melted into the highest salinities. In addition, the samples melted from the ice without added filtered seawater showed no photosynthetic activity after 2 days. Our results suggest that for experimental work using sea-ice microalgae, the final salinity of the melted sample should be greater than 28‰ (i.e. the ratio of sea ice to filtered seawater should be at least 1:2).

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.

Abundance, biomass and composition of biota in Baltic sea ice and underlying water (March 2000)

Abstract Physical, chemical and biological properties of sea ice and underlying brackish water (0–10 m water depth) were investigated in the Bothnian Bay and at a coastal station at the entrance of the Gulf of Finland in March 2000. Sea-ice texture was analysed and vertical profiles for δ18O, inorganic nutrient concentrations [NO3 –, NO2 –, NH4 +, PO4 3+, Si(OH)4] and chlorophyll a and pheopigment concentrations were obtained with a vertical resolution of 1–10 cm. Biomass estimates of bacteria, protists and metazoans were based on microscopical counts. Due to low ice bulk salinities, sea-ice brine volumes were small with a median value of only 3%. Ice thickness varied between 22 and 27 cm. Refrozen snow and granular ice dominated in the surface parts of the sea ice; columnar ice formed the bottom layers. The total biomass in the sea ice and under-ice water ranged from 2.7 mg C m–2 to 45.1 mg C m–2 and 36.8 mg C m–2 to 3,874.3 mg C m–2, respectively. Mean integrated ice values were dominated by pennate diatoms (33%), followed by centric diatoms (29%), autotrophic flagellates (23%), bacteria (8%) and heterotrophic flagellates (7%). The low integrated biomass of ice-associated metazoans (1%) consumed on average only 1.5% of the algal standing stock per day. The combined data suggest that abiotic factors (particularly nutrient availability as reflected by small brine volumes), rather than the heterotrophic component of the sympagic community, controlled autotrophic biomass in Baltic sea ice during March 2000.

Dissolved organic matter in Antarctic sea ice

AbstractIt has been hypothesized that there are significant dissolved organic matter (DOM) pools in sea-ice systems, although measurements of DOM in sea ice have only rarely been made. The significance of DOM for ice-based productivity and carbon turnover therefore remains highly speculative. DOM within sea ice from the Amundsen and Bellingshausen Seas, Antarctica, in 1994 and the Weddell Sea, Antarctica, in 1992 and 1997 was investigated. Measurements were made on melted sea-ice sections in 1994 and 1997 and in sea-ice brines in 1992. Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations in melted ice cores were up to 1.8 and 0.78 mM, respectively, or 30 and 8 times higher than those in surface water concentrations, respectively. However, when concentrations within the brine channel/pore space were calculated from estimated brine volumes, actual concentrations of DOC in brines were up to 23.3 mM and DON up to 2.2 mM, although mean values were 1.8 and 0.15 mM, respectively. There were higher concentrations of DOM in warm, porous summer second-year sea ice compared with colder autumn first-year ice, consistent with the different biological activity supported within the various ice types. However, in general there was poor correlation between DOC and DON with algal biomass and numbers of bacteria within the ice. The mean DOC/DON ratio was 11, although again values were highly variable, ranging from 3 to highly carbon-enriched samples of 95. Measurements made on a limited dataset showed that carbohydrates constitute on average 35% of the DOC pool, with highly variable contributions of 1−99%.

Lifetime estimation for a land-fast ice sheet subjected to ocean swell

AbstractIt is well known that an incoming ocean swell produces a strain field in a land-fast ice sheet. The attenuation and spectral content of this strain field can be calculated and has been measured. The response of the sea ice to this type of cyclic forcing has also been measured, and in particular we are able to estimate the number of cycles to failure for sea ice loaded at constant amplitude. In this paper we consider the response of the land-fast ice sheet or vast floe to a measured ice-coupled wave field of variable amplitude. We use the Palmgren-Miner cumulative damage law and stress-lifetime curves taken from field experiments to predict the lifetime of the sea-ice sheet as a function of significant wave height and sea-ice brine fraction. Calculations are performed to account for the swell entering a land-fast sea-ice sheet at arbitrary angle, and the influence of c-axis alignment and the presence of pre-existing cracks are discussed.

Magnetic resonance imaging of sea-ice pore fluids: methods and thermal evolution of pore microstructure

Microstructure and thermal evolution of sea-ice brine inclusions were investigated with magnetic resonance imaging (MRI) techniques. Ice samples were kept at temperatures between −2°C and −25°C during 1H imaging in a 4.7-T magnet at 200 MHz. Measurements were completed in a 20-cm diameter cylindrical probe and actively shielded gradient coils (max. 50 mT m −1, pixel dimensions >0.2 mm, slice thicknesses >1 mm), and for higher resolution in a mini-imaging unit with a 9-cm diameter probe with gradient coils of 200 mT m −1 (pixel dimensions <0.1 mm, slice thickness <0.4 mm). Absorption of radio-frequency (RF) signals in the dielectrically lossy brine resulted in degraded signals and was alleviated by use of a contrast agent (decane). MRI data and sea-ice thin section images agree very well (<5% deviation for pore microstructural parameters). Analysis of ice grown under different current speeds indicates that pores are smaller and pore number densities larger at higher current speeds. The thermal evolution of fluid inclusions was studied on cold first-year ice samples, maintained at close to in-situ temperatures prior to experiments. Warming from −21°C to −10°C to −6°C is associated with a distinct increase in pore size (from 1.5 to 1.7 to 2.6 mm for the upper 10-percentile in the vertical) and elongation (4.0 to 4.2 to 6.2 for ratio of major to minor pore axes in the vertical) and a decrease in number densities (0.75 to 0.62 to 0.58 mm −3 in the vertical). Aspect ratios increased from 4:2:1 to 6:2:1 (upper 10-percentile), indicating expansion and merging of pores in the vertical, possibly promoted by microscopic residual brine inclusions.

A nuclear magnetic resonance study of Antarctic sea ice brine diffusivity

We have measured the diffusive motion of water molecules in the brine inclusions of Antarctic sea ice using a specially constructed nuclear magnetic resonance (NMR) apparatus. The method relies on the use of pulsed magnetic field gradients in precise analogy to well established laboratory procedures. One version of the apparatus utilised core samples extracted from the ice sheet which were subsequently analysed on site while a later version utilised a probehead which was inserted into the ice sheet, thus minimising any sample perturbation. The diffusive motion of water molecules in the brine inclusions is found to be strongly anisotropic, and, over short length scales, exhibits a rapidity greatly in excess of that expected for thermal equilibrium Brownian behaviour, an effect which we attribute to convective transport.

Linkages between salinity and brine channel distribution in young sea ice

A high‐resolution study of bulk salinity was undertaken on laboratory‐grown sea ice to determine the extent of spatial variability of salinity and whether this was associated with brine channel structures. Ice samples at two different stages of development were compared with respect to physical state, brine distribution, and brine channel structures. Cold, growing sea ice is shown to have steep bulk salinity gradients and a highly variable brine distribution that is closely linked to the location and morphology of brine channels. Areas of high bulk salinity are found to correspond directly to the positions of the brine channels, and these areas are surrounded by ice that shows significant brine depletion. It is observed that redistribution of the brine occurs as the ice cover goes through warming and melting phases. The redistribution is attributed to an increase in porosity and pore connectivity permitting migration of brine through the ice. The distribution of brine in “warm” ice is shown to be more homogeneous and independent of brine channel structures. A number of mechanisms exploiting the enhanced mobility of brine in warm sea ice are used to explain the changes in the salinity profiles and brine distribution.

Arthrobacter, Brachybacterium and Planococcus Isolates Identified from Antarctic Sea Ice Brine. Description of Planococcus mcmeekinii, sp. nov.

Three facultative psychrophilic Gram-positive bacterial strains were isolated from brine samples from the sea ice community in Antarctica. All strains were coccoid to rod-shaped and exhibited broad salinity and temperature ranges for growth. The three strains were subjected to 16S rDNA sequencing and subsequent phylogenetic analysis. All possess unique 16S rDNA sequences indicating they are new, previously unreported organisms. Phylogenetic analyses coupled with phenotypic characterization indicated that one of the strains is most closely related to the low mol% G + C genus Planococcus for which a new species, P. mcmeekinii, is proposed. The two other strains are members of the high mol% G + C Gram-positive bacteria and most closely related to the genera Arthrobacter and Brachybacterium. This study reports the first phylogenetic evidence that Gram-positive bacteria reside in the marine sea ice brine.

C-band backscatter measurements of winter sea-ice in the Weddell Sea, Antarctica

During the 1992 Winter Weddell Gyre Study, a C-band scatterometer was used from the German ice-breaker R/V Polarstern to obtain detailed shipborne measurement scans of Antarctic sea-ice. The frequency-modulated continuous-wave (FM-CW) radar operated at 4-3 GHz and acquired like- (VV) and cross polarization (HV) data at a variety of incidence angles (10-75 deg). Calibrated backscatter data were recorded for several ice types as the icebreaker crossed the Weddell Sea and detailed measurements were made of corresponding snow and sea-ice characteristics at each measurement site, together with meteorological information, radiation budget and oceanographic data. The primary scattering contributions under cold winter conditions arise from the air/snow and snow/ice interfaces. Observations indicate so e similarities with Arctic sea-ice scattering signatures, although the main difference is generally lower mean backscattering coefficients in the Weddell Sea. This is due to the younger mean ice age and thickness, and correspondingly higher mean salinities. In particular, smooth white ice found in 1992 in divergent areas within the Weddell Gyre ice pack was generally extremely smooth and undeformed. Comparisons of field scatterometer data with calibrated 20-26 deg incidence ERS-1 radar image data show close correspondence, and indicate that rough Antarctic first-year and older second-year ice forms do not produce as distinctively different scattering signatures as observed in the Arctic. Thick deformed first-year and second-year ice on the other hand are clearly discriminated from younger undeformed ice. thereby allowing successful separation of thick and thin ice. Time-series data also indicate that C-band is sensitive to changes in snow and ice conditions resulting from atmospheric and oceanographic forcing and the local heat flux environment. Variations of several dB in 45 deg incidence backscatter occur in response to a combination of thermally-regulated parameters including sea-ice brine volume, snow and ice complex dielectric properties, and snow physical properties.

Thickness, structure, and properties of level summer multiyear ice in the Eurasian sector of the Arctic Ocean

In August and September 1991, thickness, structure, and properties of level multiyear ice were studied at 66 locations in the Eurasian sector of the Arctic Ocean. The mean ice thickness was 2.86 m, with 0.31 m of freeboard (including 0.08 m deteriorated ice of mean density 370 kg m−3). On the basis of the study of ice cores, 61% of the ice cover consisted of undeformed columnar ice, the remaining 39% consisted of a mixture of ice types including frazil (18%) and deformed ice (9%). Through microstructural studies, six main classes of pores could be identified. The mean density of the ice cover increased from 720 kg m−3 at the top to &gt;880 kg m−3 below 0.4 m depth. Sea ice salinities (mean value 2.1‰) correlate with ice thickness. On average, salinity profiles exhibit a linear increase from values close to 0‰ at the top to 2‰ at 1 m depth, with less steep salinity gradients below. Sampling from different depths within the ice cover indicates that the brine in summer sea ice is strongly stratified. The influence of meltwater percolation is evident, with salinities around 5‰ and pH values &lt;8 at the top and &gt;15‰ and &gt;8 at greater depths. Brine volumes (ranging from 80 to 150‰) are controlled by the thermodynamic equilibrium between the solid and liquid phases. Gas volumes decrease from &gt;200‰ at the top to &lt;50‰ below 1 m depth. Pore microstructure is highly variable even on small scales. Salinity and other ice properties do not vary to a large degree between different regions. The evolution of level multiyear sea ice is discussed with particular reference to “hidden” occurrence of deformed ice and the importance of ablation processes.

Comparison of summer and winter inorganic carbon, oxygen and nutrient concentrations in Antarctic sea ice brine

Abstract During summer (January 1991) and winter (April 1992) cruises to the southern Weddell Sea (Antarctica), brine samples were collected from first year sea ice and analysed for salinity, temperature, dissolved oxygen and major nutrient concentrations. Additionally, the carbonate system was determined from measurements of pH and total alkalinity. During winter, brine chemical composition was largely determined by seawater concentration in the course of freezing. Brine temperatures ranged from −1.9 to −6.7 °C. Precipitation of calcium carbonate was not observed at the corresponding salinity range of 34 to 108. Removal of carbon from the total inorganic carbon pool (up to 500 μmol C t kg −1 ) was related to reduced nutrient concentrations, indicating the presence of photosynthetically active ice algal assemblages in the winter sea ice. However, nutrient and inorganic carbon concentrations did generally not reach growth limiting levels for phytoplankton. The combined effect of photosynthesis and physical concentration resulted in O 2 concentrations of up to 650 μmol kg −1 . During summer, brine salinities ranged from 21 to 41 with most values > 28, showing that the net effect of freezing and melting on brine chemical composition was generally slight. Opposite to the winter situation, brine chemical composition was strongly influenced by biological activity. Photosynthetic carbon assimilation resulted in a C t depletion of up to 1200 μmol kg −1 , which was associated with CO 2 (aq) exhaustion and O 2 concentrations as high as 933 μmol kg −1 . The concurrent depletion of major nutrients generally corresponded to uptake ratios predicted from phytoplankton biochemical composition. Primary productivity in summer sea ice is apparently sustained until inorganic resources are fully exhausted, resulting in brine chemical compositions that differ profoundly from those of surface waters. This may have important implications for pathways of ice algal carbon acquisition, carbon isotope fractionation as well as for species distribution in the open water phytoplankton.

Refractive-index measurements in freezing sea-ice and sodium chloride brines

Sea ice contains numerous pockets of brine and precipitated salts whose size and number distributions change dramatically with temperature. Theoretical treatment of scattering produced by these inclusions requires information on refractive-index differences among the brine, salts, and surrounding ice. Lacking specific data on refractive-index variations in the brine, we carried out laboratory measurements in freezing-equilibrium solutions between -2 and -32 °C. Index values at 589 nm increased from 1.341 to 1.397 over this temperature range, corresponding to salinities of 35 and 240 parts per thousand (ppt). Spectral data were also taken at 50-nm intervals between 400 and 700 nm in nonequilibrium solutions with salinities ranging up to 300 ppt. Spectral gradients increased slightly with salinity but showed no measurable dependence on temperature between +12 and -16 °C. The Lorentz-Lorenz equation, combined with data on density, molar refractivities, and brine composition, yielded temperature-dependent index predictions in excellent agreement with the experimental data. Similar index and density measurements in freezing sodium chloride brines yielded values nearly identical to those in the sea-ice brines. The absence of mirabilite crystals in sodium chloride ice, however, will cause it to have higher transmissivity and lower reflectivity than sea ice above -22 °C.

A simulated Antarctic ast ice ecosystem

A simple two‐dimensional (z,t) model of first year sea ice structure and dynamics is coupled to a high resolution, time‐dependent model of microalgal growth in which simulated physiological responses are determined by ambient temperature, spectral irradiance, nutrient concentration, and salinity. The physical component utilizes atmospheric data to simulate congelation ice growth, initial brine entrapment, desalination, and nutrient flux. Temperature gradient, sea ice salinity, brine salinity, and brine volume are also computed. The biological component is based on the concept of a maximum temperature‐dependent algal growth rate which is reduced by limitations imposed from insufficient light or nutrients, as well as suboptimal salinity. Estimated gross primary productivity is reduced by respiration and grazing terms. Preliminary simulations indicate that, during a bloom, microalgae are able to maintain their vertical position relative to the lower congelation ice margin and are not incorporated into the crystal matrix as the ice sheet thickens. Model results imply that land fast sea ice contains numerous microhabitats that are functionally distinct based upon the unique suite of processes that control microalgal growth and accumulation within each. In the early stages of the spring bloom, high brine salinity inhibits microalgal growth at all depths within the congelation ice, except near the skeletal layer. Light is predicted to be the limiting resource throughout the congelation ice and platelet ice at this time. Later in the bloom when environmental conditions are more favorable for algal growth, model results suggest that biomass accumulation in the upper congelation ice is controlled by microzooplankton grazing, Microalgae in the skeletal layer and upper platelet ice are susceptible to nutrient limitation at this time due to diminished flux and high nutrient demand. Light limits microalgal growth in the lower platelet ice throughout the bloom. Results indicate that land fast sea ice in McMurdo Sound can support a production rate of approximately 0.5 g C m−2 d−1 under optimal conditions, 76% of which is associated with the platelet layer where rates of nutrient exchange are relatively high. While adjustments in any biological coefficient will alter the magnitude of production in the model, the range of results permitted by uncertainty in their values is well within the bounds likely to result from normal variations in snow cover, or from the uncertainty in the rate of nutrient flux.

Changes in the sea-ice brine community during the spring-summer transition, McMurdo Sound, Antarctica. II. Phagotrophic protists

The land-fast sea-ice brine contains a diverse phagotrophic protist assemblage consisting of <5 pm heterotrophic flagellates, Cryothecomonas spp., heterotrophic dinoflagellates, and heterotrophic and rnixotrophic ciliates. Fine-scale horizontal spatial variability is a feature of this assemblage; samples taken within 1 m of each other can be dominated by different heterotrophic protists. Many of the larger heterotrophic protists found in the brine are also found in the water column. The photosynthetic ciliate Mesodinium rubrum is also common. In mid to late austral spring, the heterotrophic assemblage accounts for ca 10% of the total protist biomass in the brine and is dominated by Cryothecornonas spp. This flagellate can reach densities of over 106 cells I-' of brine. In the early austral summer, ciliates (primarily Strornbidium spp., Mesodiniurn rubrum and Didinium spp.) and heterotrophic dinoflagellates (primarily a small Gyrnnodinium sp. and Polykrikos sp.) increase in abundance in the brine. Ciliate densities of 2 3 X 103 1-' and heterotrophic dinoflagellate densities of 104 cells 1-' are common in the brine during early summer. By the end of January Cjust prior to ice decay and break-out), heterotrophic flagellates and ciliates can account for 50 % of the protist biomass.

Changes in the sea-ice brine community during the spring-summer transition, McMurdo Sound, Antarctica. I. Photosynthetic protists

Changes in the sea-ice brine community during the spring-summer transition, McMurdo Sound, Antarctica. I. Photosynthetic protists

The dielectric properties of brine in sea ice at microwave frequencies

The dc conductivity and complex dielectric constant at frequencies of 7.50, 9, 11, 30, and 40 GHz of 16 samples of sea water brine in equilibrium with sea ice with freezing temperatures ranging from - 2.8\deg C to -25.0\deg C have been measured. The data is analyzed to yield parameters occurring in a Debye relaxation equation so that the dielectric constant of brine may be calculated throughout the microwave region of the electromagnetic spectrum.