Under-ice Surface Research Articles

Insights into the diet and feeding behavior of immature polar cod (Boreogadus saida) from the under-ice habitat of the central Arctic Ocean.

Polar cod (Boreogadus saida) is an endemic key species of the Arctic Ocean ecosystem. The ecology of this forage fish is well studied in Arctic shelf habitats where a large part of its population lives. However, knowledge about its ecology in the central Arctic Ocean (CAO), including its use of the sea-ice habitat, is hitherto very limited. To increase this knowledge, samples were collected at the under-ice surface during several expeditions to the CAO between 2012 and 2020, including the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The diet of immature B. saida and the taxonomic composition of their potential prey were analysed, showing that both sympagic and pelagic species were important prey items. Stomach contents included expected prey such as copepods and amphipods. Surprisingly, more rarely observed prey such as appendicularians, chaetognaths, and euphausiids were also found to be important. Comparisons of the fish stomach contents with prey distribution data suggests opportunistic feeding. However, relative prey density and catchability are important factors that determine which type of prey is ingested. Prey that ensures limited energy expenditure on hunting and feeding is often found in the stomach contents even though it is not the dominant species present in the environment. To investigate the importance of prey quality and quantity for the growth of B. saida in this area, we measured energy content of dominant prey species and used a bioenergetic model to quantify the effect of variations in diet on growth rate potential. The modeling results suggest that diet variability was largely explained by stomach fullness and, to a lesser degree, the energetic content of the prey. Our results suggest that under climate change, immature B. saida may be at least equally sensitive to a loss in the number of efficiently hunted prey than to a reduction in the prey's energy content. Consequences for the growth and survival of B. saida will not depend on prey presence alone, but also on prey catchability, digestibility, and energy content.

Diversity in Zooplankton and Sympagic Biota during a Period of Rapid Sea Ice Change in Terra Nova Bay, Ross Sea, Antarctica

Sea ice is a major driver of biological activity in the Southern Ocean. Its cycle of growth and decay determines life history traits; food web interactions; and populations of many small, ice-associated organisms. The regional ocean modelling system (ROMS) for sea ice in the western Ross Sea has highlighted two modes of sea ice duration: fast-melting years when water temperature warms quickly in early spring and sea ice melts out in mid-November, and slow-melting years when water temperature remains below 0 °C and sea ice persists through most of December. Ice-associated and pelagic biota in Terra Nova Bay, Ross Sea, were studied intensively over a 3-week period in November 1997 as part of the PIPEX (Pack-Ice Plankton Experiment) campaign. The sea ice environment in November 1997 exhibited features of a slow-melting year, and the ice cover measured 0.65 m in late November. Phytoplankton abundance and diversity increased in the second half of November, concomitant with warming air and water temperatures, melting sea ice and progressive deepening of a still weak pycnocline. Water column phytoplankton was dominated by planktonic species, both in abundance and diversity, although there was also some input from benthic species. Pelagic zooplankton were typical of a nearshore Antarctic system, with the cyclopoid copepod Oithona similis representing at least 90% of total abundance. There was an increase in numbers coinciding with the period of ice thinning. Conversely, ice-associated species such as the calanoid copepods Stephos longipes and Paralabidocera antarctica decreased over time and were found in low numbers once the water temperatures increased. Stratified sampling under the sea ice, to 20 m, revealed that P. antarctica was mainly found in close association with the under-ice surface, while S. longipes, O. similis, and the calanoid copepod Metridia gerlachei were dispersed more evenly.

Spatial Heterogeneity as a Key Variable Influencing Spring-Summer Progression in UVR and PAR Transmission Through Arctic Sea Ice

The transmission of ultraviolet (UVR) and photosynthetically available radiation (PAR) through sea ice is a key factor controlling under-ice phytoplankton growth in seasonally ice-covered waters. The increase towards sufficient light levels for positive net photosynthesis occurs concurrently with the sea ice melt progression in late spring when ice surface conditions shift from a relatively homogeneous high-albedo snow cover to a less reflective mosaic of bare ice and melt ponds. Here, we present a detailed dataset on the spatial and temporal progression of transmitted UVR and PAR in relation to changing quantities of snow, sea ice and melt ponds. Data were collected with a remotely operated vehicle (ROV) during the GreenEdge landfast sea ice campaign in June-July 2016 in southwestern Baffin Bay. Over the course of melt progression, there was a 10-fold increase in spatially averaged UVR and PAR transmission through the sea ice cover, reaching a maximum transmission of 31% for PAR, 7% for UVB and 26% for UVA radiation. The depth under the sea ice experiencing spatial variability in light levels due to the influence of surface heterogeneity in snow, white ice and melt pond distributions increased from 7 +/- 4 to 20 +/- 6 m over our study. Phytoplankton drifting in under-ice surface waters were thus exposed to variations in PAR availability of up to 43%, highlighting the importance to account for spatial heterogeneity in light transmission through melting sea ice. Consequently, we demonstrate that spatial averages of PAR transmission provided more representative light availability estimates to explain under-ice bloom progression relative to single point irradiance measurements during the sea ice melt season. Encouragingly, the strong dichotomy between white ice and melt pond PAR transmittance and surface albedo permitted a very good estimate of spatially averaged light transmission from drone imagery of the surface and point transmittance measurements beneath different ice surface types.

Chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic

We monitored the spatiotemporal progression of chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic between mid-March and early July 2008. CDOM abundance in bottom ice, as quantified by absorption coefficient at 325nm, aCDOM(325), showed a positive, linear relationship with the concentration of chlorophyll a, being low at the start of ice algal accumulation, highly enriched during the peak bloom and early post-bloom, and depleted again during sea ice melting. Vertical profiles of CDOM in early and late spring were typically characterized by slight to moderate elevations at both the surface and bottom and rather constancy within the interior ice. In the ice algae-thriving mid-spring, L-type profiles prevailed due to extremely high CDOM in the lowermost 10-cm layer. Bottom-layer CDOM in landfast ice (aCDOM(325): 15.8m−1) more than doubled that in drift ice (aCDOM(325): 6.6m−1). CDOM in the ice cover, except the bottom layer, was generally lower than or similar to that in the under-ice surface water. Salinity accounted for 58% of the CDOM variability in drift ice having minimal terrestrial and ice algal signatures. CDOM absorption spectra of algae-rich bottom ice samples exhibited ultraviolet (UV) absorption shoulders attributable to mycosporine-like amino acids (MAAs). These compounds were readily photodegradable by solar UV radiation. Our results suggest that 1) inclusion of organic solutes and in situ biological production are the dominant processes controlling the distribution of CDOM in sea ice in the study area, 2) biological production in bottom ice is a minor source of CDOM to the underlying surface water; 3) CDOM plays a critical role in shielding sympagic organisms in bottom ice against UV radiation; 4) the MAAs are effective photoprotectants only under low-UV conditions.

A model for under‐ice‐target acoustic scattering.

A model is described that has been developed to calculate the scatter of a high frequency acoustic pulse that originates from a stationary source and is scattered from a stationary target near the under-ice surface characteristic of the Arctic. Measured under-ice acoustic profile data are used to model the large scale under-ice surface roughness by an ensemble of ice keels. Additional measured data are used to model the small scale roughness of ice keels by an ensemble of ice blocks. The Helmholtz-Kirchhoff integral in the Kirchhoff approximation is used to calculate the scatter from an ice facet. The scatter from an ice keel is given by a coherent sum of the scatter from all the ice facets. A T-matrix is used to calculate the scatter from the stationary target. The first order terms in the multiple scattering process between the target and ice surface is also calculated, i.e., the scatter from the target of the free field scatter from ice facets and the scatter from ice facets of the free field scatter from the target. Numerical results show the extent to which the scatter from the target in the under-ice environment differs from its free field scatter.

10.1007/s11983-008-1007-0

For the first time, based on direct observations of carbon dioxide concentration at modern Russian drifting stations, the role of the Arctic sea ice in maximum seasonal changes in carbon dioxide distribution is considered. Carbon dioxide is generated during growth of ice mass at the sea ice undersurface and is absorbed at the surface of melting sea ice. The time of ice growth is three or four times greater than that of its melting in summer months, and, as result, the time of carbon dioxide generation is longer that that of its uptake. Consequently, the Arctic Ocean is a source of carbon dioxide on the mean annual scale (the effect of Arctic “breathing”). Experiments in a freezing chamber confirm these conclusions.

Early Spring Oceanic Heat Fluxes and Mixing Observed from Drift Stations North of Svalbard*

Abstract From several drifting ice stations north of Svalbard, Norway, observations were made in early spring of the ocean turbulent characteristics in the upper 150 m using a microstructure profiler and close to the under-ice surface using eddy correlation instrumentation. The dataset is used to obtain average heat fluxes at the ice–water interface, in the mixed layer, across the main pycnocline, as well over different water masses in the region. The results are contrasted with proximity to the branches of the warm and saline Atlantic water current, the West Spitsbergen Current (WSC), which is the main oceanic heat and salinity source both to the region and to the Arctic Ocean. Hydrographic properties show that the surface water mass modification is typically due to atmospheric cooling with relatively less influence of ice melting. Surface heat fluxes of O(100) W m−2 are found within the branches of the WSC and over shelf areas with elevated levels of mixing due to strong tides. Away from the shelves and WSC, however, ocean-to-ice turbulent heat fluxes are typical of the central Arctic. Deeper in the water column, entrainment from below together with equally important horizontal advection and diffusion increase the heat content of the mixed layer and contribute to the heat flux maximum in the upper layers. The results in this study emphasize the importance of mixing along the boundaries, over shelves, and topography for the cooling of the Atlantic water layer in the Arctic in general, and for the regional heat budget, hence the ice cover and cooling of the WSC north of Svalbard, in particular.

On the role of the Arctic sea ice in seasonal variability of carbon dioxide concentration in Northern latitudes

For the first time, based on direct observations of carbon dioxide concentration at modern Russian drifting stations, the role of the Arctic sea ice in maximum seasonal changes in carbon dioxide distribution is considered. Carbon dioxide is generated during growth of ice mass at the sea ice undersurface and is absorbed at the surface of melting sea ice. The time of ice growth is three or four times greater than that of its melting in summer months, and, as result, the time of carbon dioxide generation is longer that that of its uptake. Consequently, the Arctic Ocean is a source of carbon dioxide on the mean annual scale (the effect of Arctic “breathing”). Experiments in a freezing chamber confirm these conclusions.

The sub-ice algal community in the Chukchi sea: large- and small-scale patterns of abundance based on images from a remotely operated vehicle

We examined the sub-ice algal community in the Chukchi Sea during June 1998 using a remotely operated vehicle (ROV). Ice algae were observed on the under-ice surface at all ten stations (from 70°29′N to 72°26′N; 162°00′W to 153°56′W) and varied in abundance and distribution from small aggregations limited to depressions in the ice to nets, curtains and strands of Melosira. There was no relationship between percent cover of sub-ice algae and physical factors at the kilometer scale, but at the scale of individual ice floes the percent cover of sub-ice algae was positively correlated with distance from the floe edge and negatively correlated with snow depth. A significant positive relationship between the concentration of sediment pigments and percent cover of sub-ice could indicate a coupling between ice algal and benthic systems. Pieces of ice algae that appeared to be Melosira were observed on the seafloor to a depth of over 100 m and cells or spores of obligate ice algal taxa were collected from sediments from 44-m to 1,000-m deep. The large biomass of sub-ice algae observed at many stations in the Chukchi Sea and the presence of ice algae on the seafloor indicates that the distribution and abundance of sub-ice algae needs to be understood if we are to evaluate the role of ice algae in the Arctic marine ecosystem.

Sea Ice-Upper Ocean Ecosystems and Global Changes in the Arctic

In the Arctic, a clear temperature trend toward warming has been observed during the last two decades. The warming has led to a reduction in the area and thickness of the Arctic sea ice cover because of increased melting. Melt water is accumulated within the 0- to 30-m surface layer, markedly changing its hydrological and hydrochemical characteristics: this water is warmer and fresher, with lower nutrient concentrations. A stable thermocline at depths of 30–35 m weakens vertical mixing of water and hampers active exchange between nutrient-enriched water lying below the thermocline and nutrient-poor water under ice. This affects the qualitative and quantitative composition of upper-ocean phyto- and zooplankton. The changes in the climate and the ice environment cause marked alterations in the composition and structure of the biological communities of sea ice and under-ice surface water. Comparative analysis of materials collected in the anticyclonic Arctic Gyre at the ice stations Severnyi Polyus-22 (1975– 1981) and SHEBA (1997–1998) shows that over the last two decades the number of species of ice diatoms markedly decreased and the role of freshwater algae increased. The number of nematodes, copepods, amphipods, and turbellarians, which were the dominant groups in perennial ice, also declined sharply. The results suggest that the sea ice-upper ocean ecosystem is changing from typically marine to brackish-water. The main reason for these changes is probably global warming in the Arctic.

Turbulent stress at the ice/ocean interface and bottom surface hydraulic roughness during the SHEBA drift

Upper ocean turbulence and mean current data from the 1997–1998 Surface Heat Budget of the Arctic Ocean (SHEBA) drift in the western Arctic Ocean were used to estimate turbulent shear stress at the ice/ocean interface and hydraulic roughness of the ice undersurface. Techniques for determining interfacial stress from velocity covariance measurements in the ice/ocean boundary layer (IOBL) are complicated by buoyancy flux and the heterogeneity of the ice undersurface, where protrusions corresponding to relatively small upper surface features become major obstacles when compared to the scale of the IOBL. In addition, ice deformation forced relocation of the main SHEBA oceanographic program about midway through the experiment. Hence, there were two different measurement sites to consider. Three different methods were used to compute the undersurface roughness characterizing the undeformed, multiyear floe occupied by the SHEBA project. The first was straightforward (naive) application of the “law of the wall” to turbulent stress and mean velocity measurements from the turbulence instrument cluster nearest to the interface, assuming a logarithmic velocity profile and constant stress. For the second method, local turbulence closure was used to model the entire IOBL in order to estimate the impact of Coriolis stress attenuation and buoyancy forces on stress and shear near the boundary. A third method, proposed here for the first time, estimated mixing length and eddy viscosity from a scale inversely proportional to the wave number at the peak of the weighted vertical velocity spectrum. The last method appears most robust, in that it shows no overall difference in z0 between the two sites and only minor seasonal and directional differences. The best estimate for the undersurface roughness of undeformed, multiyear ice at the SHEBA site is 0.0048 m < z0 < 0.007, i.e., about 6 mm. Specification of a regional “aggregate” under‐ice roughness requires additional consideration of the added drag from isolated pressure keels and floe edges, along with reduced drag from smoother, newly frozen ice and open water.

Visual predators and the diel vertical migration of copepods under Arctic sea ice during the midnight sun

Despite the midnight sun, herbivore copepods Calanus hyperboreus, C. glacialis and Pseudocalanus acuspes displayed a normal diel vertical migration (NDVM) under the ice cover of Barrow Strait in spring, ascending into the chlorophyll-rich under-ice surface layer around maximum relative rate of change in irradiance (ΔI/I) at dusk but returning to depth a few hours later, well in advance of the dawn maximum ΔI/I. Nauplii prey being abundant above 50 m, the upward night-time incursions of the omnivore Metridia longa seldom reached beyond <25 m. In the absence of UV-B radiation or a temperature gradient, migration out of the euphotic layer was interpreted as a reaction to visual predators (e.g. Arctic cod Boreogadus saida and the hyperiid amphipod Themisto libellula). Swarms of T. libellula actively preying on copepods accumulated at the ice–water interface at dusk. Low vulnerability to visual predators and a more uniform vertical distribution of their food explained the limited DVM of the small omnivores Microcalanus pygmaeus, Oithona similis and Oncaea borealis. Once the feeding migrations developed, the daytime depth of the centre of mass of the distribution of a copepod was correlated to its size (r2 = 0.63). Our observations suggest that, under Arctic sea ice, interspecific differences in the pattern and extent of copepod DVM can be related to the vertical distribution of potential food and to vulnerability to visual predators.

Scale invariance of the intensity anomaly induced by the ice cover in the sound field of a tone source in an arctic waveguide

On the basis of the normal mode method and the theory of sound scattering by a stochastically rough surface of an elastic layer, the specific features of the frequency dependence of the intensity anomaly induced by the ice cover in the sound field generated by a tone source in an arctic waveguide are studied. The anomaly is determined by the difference between the level (in dB) of the coherent component of the sound intensity that occurs in the waveguide with the ice cover and the intensity level in a waveguide with an ice-free surface. Using the WKB approach, it is shown that the intensity anomaly is invariant for all values of the frequency, the mean ice thickness, and the rms amplitude and correlation radius characterizing the rough undersurface of ice on the condition that the ratios of all the aforementioned geometric parameters to the sound wavelength in water remain constant. This property is confirmed by precise calculations of the anomaly by the mode program and explained by the specific features of the frequency dependence of attenuation coefficients and their distribution in the numbers of normal modes. Possible practical applications of the revealed property of the intensity anomaly are suggested.

Parameterization of mixing in the ocean boundary layer

A method for estimating eddy viscosity and diffusivity based on a similarity theory for the vertical exchange scale of dominant turbulent eddies in a surface forced flow is used to predict flux profiles for momentum, heat, and salt in the ocean boundary layer (OBL). By neglecting time dependence in solving for the turbulence properties, the technique requires specifying only the mean density profile along with surface friction velocity and surface buoyancy flux. Momentum and scalar fluxes in stratified fluid below the well-mixed layer are determined by iteration. The method is illustrated in two ways. First, measurements of turbulent stress and heat flux from an intense storm during the 1994 Antarctic Zone Flux Experiment are simulated with reasonable accuracy. The model is forced by Reynolds stress measured near the surface, and by buoyancy flux estimated from the thermodynamic mass balance at the ice undersurface. It is then verified by comparing predicted friction velocity with measurements at other levels in the boundary layer, and by comparing predicted and measured turbulent heat flux. The latter requires accurate simulation of eddy diffusivity in the boundary layer. In the second demonstration, a prognostic model for the evolution of upper ocean temperature and salinity structure is formulated for an idealized storm scenario in which freezing or melting at the surface alternately induces destabilizing and stabilizing surface buoyancy flux. Since closure is local, prognostic equations for temperature and salinity only are carried. The model simulations are compared with a second-moment closure model (Mellor–Yamada level 2 1 2 ) forced in the same way. For surface heat flux (the main diagnostic), the two approaches agree quite well. In other respects, significant differences are noted, and it is suggested how data may be used to evaluate (and discriminate among) ocean turbulence models.

Hunting behavior of a marine mammal beneath the antarctic fast Ice

The hunting behavior of a marine mammal was studied beneath the Antarctic fast ice with an animal-borne video system and data recorder. Weddell seals stalked large Antarctic cod and the smaller subice fish Pagothenia borchgrevinki, often with the under-ice surface for backlighting, which implies that vision is important for hunting. They approached to within centimeters of cod without startling the fish. Seals flushed P. borchgrevinki by blowing air into subice crevices or pursued them into the platelet ice. These observations highlight the broad range of insights that are possible with simultaneous recordings of video, audio, three-dimensional dive paths, and locomotor effort.

Turbulent mixing in Barrow Strait

Data collected in Barrow Strait during April and May 1995 indicate that turbulence can occur throughout the water column, with the most energetic turbulence being associated with strong currents during spring tides. In the surface mixed layer (SML) under the first-year, land-fast ice, the average effective vertical diffusivity was proportional to current speed, with mean values ranging up to 0.02 m 2 s -1. Current speeds in the SML during tidal maxima sometimes exceeded 20 cm s -1 but turbulent stress levels remained low because the ice undersurface was hydrodynamically smooth. An asymmetry observed in under surface drag during the tidal cycle is associated with changes in eddy viscosity, possibly in response to the advection of a horizontal salinity gradient in the SML in this region. In the sharp halocline ( N≈15–20 cycles h -1) at the base of the SML, mixing was often too weak to transport heat, salt, and other scalars vertically. During such periods the halocline acts as a barrier to the transport of properties, such as nutrients, from the deeper water to the surface. Within the deeper, less-stratified, water ( N≈3–5 cycles h -1), energetic mixing frequently occurred, and was modulated by both the spring/neap tidal variability and within each tidal cycle. The water column was, on average, fresher than observed in previous spring measurements from this area, suggesting interannual variability in the Archipelago hydrography.

Use of the lateral line and tactile senses in feeding in four antarctic nototheniid fishes

Fishes of the family Nototheniidae (Pisces: Perciformes) dominate antarctic fish communities and have radiated to fill diverse niches. The most southern species must operate under an extended austral night and under thick sea ice, yet have eyes more typical of shallow coastal fishes. In winter, the eyes are probably useless, except for detecting bioluminescence. I compared the responses of four species to hydromechanical and tactile signals: two benthivores,Trematomus bernacchii andT. pennellii, a benthic planktivore,T. nicolai, andPagothenia borchgrevinki, which feeds near the ice undersurface and within ice cracks. The planktivores have dorsal mouths, with eyes oriented dorsally or laterally (Pagothenia); their lateral line canals and receptor organs are larger dorsally. The benthivores have more ventrally oriented mouths and eyes. All species responded to hydromechanical cues to the head, but only the two benthivores responded to trunk hydromechanical stimuli or tactile stimuli to the ventral trunk or pelvic fins. Possibly responses to plankton along the trunk are of little use if a reorientation washes pelagic prey away. In responding to trunk stimuli,T. bernacchii reorients its head to the target in two stages by slowly pivoting on its pelvic fins. In contrast,T. pennellii reorients in a single quick flip. It is argued that, becauseT. bernacchii has wider canals thanT. pennellii, it must move more slowly to reduce self-generated noise. It is likely that further studies of winter diet and prey behavior may reveal the relative advantages of the two repositioning styles.

Seasonal changes in phytoplankton and bacterioplankton distribution at the ice-water interface in the Antarctic neritic area

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 123:225-233 (1995) - doi:10.3354/meps123225 Seasonal changes in phytoplankton and bacterioplankton distribution at the ice-water interface in the Antarctic neritic area Delille D, Fiala M, Rosiers C To determine the relationship between phytoplankton and bacteria biomass near the ice-water interface in the Antartic, the seasonal distributions of phytoplankton and bacteria populations were investigated on the continental shelf of Terre Adélie during the ice coverage period. An under-ice surface station was sampled weekly from March 1991 to January 1992 for the bottom ice and for surface, 0.5 and 2 m depth seawater. Seawaterchlorophyll a values ranged from 0.9 mg m-3 in summer to 0.01 mg m-3 in winter. Values 50 times higher were recorded in the overlying ice. Bacterial abundance ranged from 0.5 x 1011 cells m-3 in July to 6.0 x 1011 cells m-3 after the ice break-up. Values reaching up to 2.5 x 1012 cells m-3 were recorded in sea ice. Bacterial biomass and chlorophyll a concentrations were significantly correlated in both sea ice and underlying seawater. Bacterial biomass represents between 1 and 10% of total microbialbiomass in sea ice and from 10% (summer) to up to 90% (winter) of the living community in the underlying seawater. Phytoplankton . Bacterioplankton . POC . Seasonal variations . Sea ice . Antarctica Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 123. Publication date: July 20, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

High-frequency forward scattering from the arctic canopy: Experiment and high-frequency modeling

A high-frequency forward scattering experiment carried out under multiyear ice is outlined. The experiment employed an omnidirectional transmitter emitting short tone bursts centered at either 25 or 45 kHz. The receiver was a four quadrant array, and the quadrants were recorded separately. Each quadrant had a broadbeam pattern, greater than 40 (70) deg at the 3-dB down points in the horizontal (vertical) direction at 45 kHz. A large number of transmitter/receiver geometries were obtained by varying both the transmitter depth and horizontal range. Horizontal transmitter/receiver separations ranged from 400 to 2800 m. Model/experiment comparisons are shown for horizontal ranges less than 500 m, since these ranges can be most readily compared with our high-frequency scattering models developed to date, which ignore refraction in the water column. Both individual realizations and ensemble experimental results are shown and compared with models. Specular point theory [D. E. Funk and K. L. Williams, J. Acoust. Soc. Am. 91, 2606 (1992)] is used as a starting point, and then catastrophe theory [P. L. Marston, in Physical Acoustics, Vol. XXI, edited by R. N. Thurston and A. D. Pierce (Academic, 1992); K. L. Williams et al., J. Acoust. Soc. Am. 94, 1890(A) (1993)] is used where inclusion of diffracted effects is essential. The models capture many of the attributes of the data. Much of the difference between models and experiment can be traced to limited knowledge of the ice canopy topography. Although the experiment was carried out along paths where the one-dimensional profile of the under-ice surface had been previously determined with upward looking sonar, and these profiles were used in the models, out of plane scattering could not be included. That would require full two-dimensional knowledge of the under-ice surface.

Extraction of the seabed reflectivity function using ice cracking noise as a signal source

A technique is described for measuring the reflection coefficient of the Arctic seabed with a single vertical array of hydrophones. Naturally occurring ice cracking noises were used as the acoustic sources. This method circumvents the difficulties and expense of introducing artificial sound sources through the thick Arctic pack ice. The measurements were made in April 1988, with a 22-element array suspended from the ice in 420 m of water. The range of the source is first determined using the direct arrival and multiple reflections from the seabed and underice surface. Then the source directivity is parametrized using the direct arrival path only, and extrapolated to reflected path angles. A plot of the reflection coefficient versus grazing angle clearly indicates the value of the critical angle. The sound speed of the sea bottom corresponding to this critical angle agrees well with that measured from a bottom grab sample taken during the field trip. Finally, an interesting phenomenon was an anomalous increase of the received pressure level around a grazing angle of 60°. This is associated with a leaky plate wave radiating at the ice–water interface.