Southwestern Ross Sea Research Articles

Preservation efficiencies and accumulation rates for biogenic silica and organic C, N, and P in high‐latitude sediments: The Ross Sea

Rates of biogenic sediment accumulation (biogenic silica and organic C, N, and P) and pore water flux have been established for a variety of depositional environments in the Ross Sea. On the basis of 14C measurements in kasten cores, sediment accumulation rates ranged from 250 cm kyr−1 in the coastal basin of Granite Harbor to 1–2 cm kyr−1 in the shelf and slope environments of the northern and eastern Ross Sea. Burial of biogenic material was most rapid in the southwestern Ross Sea, where biogenic silica accumulation rates ranged from 2 to 31 g cm−2 kyr−1 and organic carbon accumulation rates ranged from 0.05 to 1.4 g cm−2 kyr−1. In the northern and eastern Ross Sea, biogenic silica and organic carbon accumulation rates typically equaled 0.02–0.03 g SiO2 cm−2 kyr−1 and 0.002–0.004 g C cm−2 kyr−1. Flux core measurements were used to estimate seabed regeneration rates for biogenic silica, organic carbon, and phosphorus. Pore water fluxes, in general, showed much less variability across the Ross Sea than did the biogenic accumulation rates. Pore water silicate fluxes in the study area ranged from 0.6 to 5.3 g SiO2 cm−2 kyr−1, whereas carbon fluxes ranged from 0.1 to 1.2 g C cm−2 kyr−1 and phosphate fluxes varied from −0.006 to 0.012 g P cm−2 kyr−1. Seabed preservation efficiencies were calculated for biogenic silica, organic carbon, and phosphorus by combining the solid‐phase and pore water data. The seabed preservation efficiencies for biogenic silica (1–86%) were greater than for organic carbon (1–71%) at all nine stations examined. The preferential preservation of biogenic silica relative to organic carbon also was apparent in Ross Sea sediments because the biogenic silica/organic carbon ratio in the material buried in the seabed generally was 2 times greater than the ratio in sediment particles arriving at the sediment‐water interface. Sediment accumulation rate correlated strongly with both the biogenic silica and the organic carbon preservation efficiency data. P preservation efficiencies remained relatively high (24–65%) even when the accumulation rates were low (1–2 cm kyr−1) because of the near‐zero phosphate fluxes out of the seabed. Of the total P in the seabed (0.04–0.09 wt. %), approximately 25% existed in the form of organic P. The total amount of biogenic silica accumulating on the Ross Sea shelf is ∼2.3 × 1012 g SiO2 yr−1, which is approximately an order of magnitude less than the Ledford‐Hoffman et al. [1986] estimate that was based on 210Pb chronologies. Biogenic silica accumulation rates in the southern, central, and western Ross Sea increased during the mid and late Holocene, reaching their maximum values during the past 500 to 1000 years.

Factors influencing the distribution of diatoms and other algae in the Ross Sea

Quantitative microscopic analyses of sediment trap samples collected in the Ross Sea between January 1990 and February 1992 reveal striking temporal and spatial differences in algal bloom composition and size. Trap samples from the southwestern Ross Sea (site A, 76°30′S, 167°30′E) were dominated by the diatom Fragilariopsis curia (maximum of 92%), a species associated with both sea ice and the retreating ice edge in the Ross Sea. This species was probably seeded by melting congelation ice. Highest flux of diatom valves to both upper and lower traps in the 1991–1992 season occurred as a distinct event in mid‐February 1991, after which flux decreased by 1 to 3 orders of magnitude. As overall flux decreased, an increase in relative abundance of Chaetoceros resting spores and the open water species Thalassiosira antarctica was observed in the lower trap, suggesting some localized lateral advection. In the south central Ross Sea (site B, 76°30′, S 175°W), algal diversity was much higher, with a greater contribution of nondiatom material. The prymnesiophyte Phaeocystis antarctica, dinoflagellates, and cysts of unknown affinity were much more common than at site A, as were the diatoms Fragilariopsis cylindrus, Fragilariopsis kerguelensis, and Thalassiosira gracilis. Highest diatom flux occurred later (February 20 to March 6,1991) at this site and was an order of magnitude lower as compared to maximum flux at site A. Distinct differences in bloom size and composition between sites A and B may be a function of upper water column structure, differences in the amount of sea ice melting, and the type of sea ice present at the time of ice breakout (congelation versus pack ice). Despite rich surface productivities, lower silica flux to the seafloor at site B results from the higher proportion of nonsiliceous algae. At site C (72°30′S, 172°30′E), in the northwestern Ross Sea, diatoms again dominated algal flux; however, assemblage composition differed from that observed at both sites A and B. Relative abundance of F. curta was lower, averaging 60–70%, while the remainder of the assemblage was made up of F. cylindrus, other more robust species of Fragilariopsis, and Chaetoceros resting spores. Algal flux at site C is intermediate in style between sites A and B. Significantly, spatial differences in diatom assemblages noted above appear to be reflected in seafloor surface sediments, suggesting that downcore diatom data provide an interpretable record of paleoproductivity in the Ross Sea.

Mesoscale Atmospheric Circulations over the Southwestern Ross Sea Sector, Antarctica

In this study the mesoscale atmospheric circulation over the southwestern Ross Sea sector during winter is examined. The hydrostatic meso-gamma-scale atmospheric model MAR (Modele Atmospherique Regional) is used. Polar night is assumed, and an idealized large-scale situation is prescribed, with zero geostrophic forcing. The impact of a partial sea-ice cover on the atmospheric circulation is assessed by prescribing lead fractions in the range of the observed values (i.e., between 0% and 30%). Simulations show that the propagation of katabatic airstreams over Terra Nova Bay is facilitated by the presence of leads because the identity of cold, dense, katabatic air is better marked in warmer environmental maritime conditions. Boundary layer fronts and mesocyclone activity are associated with the katabatic airstreams. They are enhanced by the presence of leads. In particular, when the lead fraction is prescribed to be between 20% and 30%, the model simulates mesocyclone intensities comparable to those observed. Taking into account that such a lead fraction is situated in the upper range of the observed values in the central Ross Sea during winter, these results suggest that winter Ross Sea mesocyclones could nor always result from a pure mesoscale forcing. In contrast to the summer situation, no snow precipitation occurs for the simulated winter case, probably because of the too-low absolute humidity content of the air. Such model behavior is in agreement with the observations, which reveal a summer precipitation maximum at McMurdo Station on Ross Island. It is also found that the position of the simulated mesocyclone over Terra Nova Bay is nor sensitive to the lead fraction. Furthermore, this meteorological situation favors the advection of relatively mild and moist maritime air over a long distance in the ice-sheet interior. This process, which is referred to as a moist-air intrusion, could affect the Antarctic ice-sheet mass balance.

A marine and terrestrial Sirius Group succession, middle Beardmore Glacier-Queen Alexandra Range, Transantarctic Mountains, Antarctica

A succession of Sirius Group glacigene sediments which crop out along the western margins of the Beardmore valley between Cherry Icefall and Hewson Glacier, below The Cloudmaker, is designated as the stratotype of the Cloudmaker Formation. This new formation overlies a multiply glaciated pavement (Dominion Erosion Surface) cut into the Precambrian Goldie Formation, and is disconformably overlain by the Meyer Desert Formation (Sirius Group). The Cloudmaker Formation comprises bedded and massive diamictons, bedded sands and silts, and laminated clays. Assemblages of foraminifera occur throughout the Cloudmaker Formation and indicate that these basal Sirius Group sediments were deposited in brackish glacial marine environments. The general absence of diatoms suggest these marine waters were ice-covered. Similar marine assemblages are also present in basal Sirius Group sediments at Oliver Bluffs, Dominion Range. Recycled marine diatom assemblages in the Sirius Group at the latter locality indicate that the host sediments have an age of < 3.8 Ma (Pliocene). The Cloudmaker Formation is placed in the Pliocene, although a latest Miocene age for the basal sediments cannot be ruled out. Stratigraphie, sedimentologic, and paleontologic evidence suggests that Beardmore valley was occupied by a fjord and tidewater glacier system that extended at least 165 km through the Transantarctic Mountains from the southwestern Ross Sea. The stratigraphy of The Cloudmaker Formation consists of a succession of members separated by disconformities. It is hypothesised that these strata were deposited by a dynamic valley glacier system that underwent a history of glacier advance and grounding alternating with glacier retreat and flotation over a marine water column. A combination of fjord basin sediment filling and sea-level oscillations may also have influenced the pattern of glacier ice advance and retreat within Beardmore Paleofjord. The marine Cloudmaker Formation is overlain by the terrestrial diamicton dominated Meyer Desert Formation. At Oliver Bluffs, the Meyer Desert Formation diamictons are interbedded with fluvial, and lacustrine sediments; successions that contain in situ vascular plant fossils (principally the Southern Beech Nothofagus), mosses, and beetle remains. A Magellanic-type flora and fauna occupied the coastal margins of the Beardmore Paleofjord. The vertical transition from the basal marine Cloudmaker Formation to terrestrial Meyer Desert Formation provides a sea-level datum that can be used to assess the extent of post-Sirius Group tectonic uplift. Uplift rates at the Cloudmaker section, 90 km inland from the Transantarctic Mountain front or rift shoulder margin in the Queen Alexandra Mountain block are determined to be ~ 429 or ~ 350 m/Myr. This assumes a total uplift of 1331 m for the uppermost marine sediments of the Cloudmaker Formation, and maximum diatom-based ages for the Sirius Group of < 3.1 Ma or < 3.8 Ma. Gross similarities in stratigraphy and interpreted paleoenvironments are apparent between The Cloudmaker succession (Beardmore Paleofjord) and the upper Miocene-Pliocene successions at the mouth of Taylor Paleofjord, 800 km to the north. Contrasting present day elevational settings for these two widely separated marine successions indicates the post-Sirius rate of tectonic uplift for the Transantarctic Mountains has been significantly greater in the Queen Alexandra block.

Simulation of the Mesocyclonic Activity in the Ross Sea, Antarctica

The mesocyclonic activity in the southwestern Ross Sea is examined, with emphasis on its forcing by katabatic winds. The three-dimensional version of the meso-gamma-scale primitive equation model Modele Atmospherique Regional is used in which a representation of cloud microphysical processes has been introduced. Idealized boundary conditions are prescribed. In particular, the ocean is assumed to be completely ice-free or partially ice-covered. The former case corresponds to a fall situation that coincides with the climatological maximum of estimated precipitation at McMurdo Station on Ross Island. Due to the propagation of katabatic winds over the ocean, boundary layer fronts form. Clouds are generated in the fronts. A surface pressure trough also forms and extends northeastward from Terra Nova Bay. Mesocyclones are simulated in the trough. When the ocean is ice-free, strong sensible heat fluxes toward the atmosphere amplify the strength of the trough up to typically observed values. Tn this case, snow precipitation is associated with the boundary layer fronts and occurs in particular over McMurdo Sound. It is also found that latent heat release due to precipitation formation in the fronts does not significantly affect the deepening of the trough.

Over-winter movements and diving behavior of female Weddell seals (Leptonychotes weddellii) in the southwestern Ross Sea, Antarctica

The movements and diving behavior of 18 adult female Weddell seals (Leptonychotes weddellii) were determined by satellite telemetry during the over-winter period in 1990 and 1991. Nine seals provided diving and movement data for 8 – 9 months. Seals that normally bred in the eastern part of McMurdo Sound spent most of the winter in the middle and northern parts of McMurdo Sound before the annual shore-fast ice had formed in those areas, or in the pack ice 0–50 km north of the sound and Ross Island. This is a greater use of pack ice, as opposed to shore-fast ice, in winter than was previously believed. Some long-distance movements (one over 1500 km in total) to the middle and northwestern parts of the Ross Sea also occurred. Although highly variable within and between individuals, dives indicative of foraging were primarily to mid-water regions (100 – 350 m) in both years, and were similar to those that have been observed in spring and summer, when Pleuragramma antarcticum is the primary prey of Weddell seals in McMurdo Sound.

Mesoscale cyclogenesis dynamics over the southwestern Ross Sea, Antarctica

Previous work has shown that frequent mesoscale cyclogenesis adjacent to Franklin Island is linked to the strong and persistent katabatic winds from East Antarctica which funnel into Terra Nova Bay and then blow out over the southwestern Ross Sea. Four mesoscale cyclones that formed near Terra Nova Bay between February 16 and 20, 1988 are examined to more clearly define the governing mechanisms. These events are investigated using all available observations, including automatic weather station data, high‐resolution satellite images, satellite soundings, and hemispheric synoptic analyses. The first two cyclones formed on low‐level baroclinic zones established by the synoptic scale advection of warm moist air toward the cold continental air blowing gently from East Antarctica. In the second case, baroclinic instability of this small‐scale cold front was apparently triggered by the enhanced upward vertical motion associated with the approach of a midtropospheric trough. The third mesocyclone formed shortly after on a baroclinic zone over the polar plateau; the second vortex completely disrupted the usual katabatic drainage over the plateau and forced warm moist air over the coastal slopes. All three cyclones moved to the north in the prevailing cyclonic flow, but the plateau vortex lasted for only 6 hours. The fourth mesoscale low formed in conjunction with an abrupt and intense surge of katabatic air from Terra Nova Bay which resharpened the coastal baroclinic zone. At the same time a transiting midtropospheric trough probably associated with lower tropospheric upward vertical motion apparently accelerated the katabatic winds and triggered the vortex formation. A similar katabatic wind‐forced mesocyclone formed near Byrd Glacier. The two vortices moved to the east‐southeast and northeast, respectively, apparently being steered by the generating katabatic airstreams, and merged just to the north of the Ross Ice Shelf. The combined vortex reintensified as another trough passed overhead and moved eastward to West Antarctica where it dissipated two days later.

Role of silicon as a limiting nutrient to Antarctic diatoms: evidence from kinetic studies in the Ross Sea ice-edge zone

During the austral summer of 1990 an intense, diatom-dominated, ice-edge phytoplankton bloom in the southwestern Ross Sea resulted in depletion of silicic acid, nitrate and phosphate to concentrations much lower than is typical for Antarctic surface waters. Silicic acid was depleted to <6 FM within the core of the meltwater field, where biogenic particulate silica concentrations exceeded 20 pm01 1-' Three Si uptake kinetic experiments were conducted on natural phytoplankton assemblages from the nutrient-depleted zone; 30Si-labeled Si(OH), was used to measure the uptake rate at concentrations ranging from 1 to 20 PM above ambient. Dependence of the spec~fic uptake rate ( V ) on silicic acid concentration conformed well to a Michaelis-Menten saturation model; maximum uptake rates (V,,,,,) ranged from 0.0022 to 0.0028 h' which corresponds to maximum growth rates of 0.08 to 0.10 doublings d-l. Half-saturation constants (km) ranged from 1.1 to 4.6 PM, a range similar to values found in other areas of the ocean and considerably lower than those previously reported for several Antarctic d~atom species in culture studies. Results indicate detectable, but weak, substrate limitation of s~llcic acid uptake rate by the naturally occurring diatom assemblage in the western Ross Sea. Significant Si limitation in other subsystems of the Southern Ocean would be possible only if their resident diatom assemblages had much lower affinity for silicic acid than we observed.

Cycling of biogenic silica within the upper water column of the Ross Sea

We determined the distributions of biogenic particulate silica and dissolved silicic acid in the upper 80 m over the continental shelf of the southern Ross Sea, Antarctica, during two occupations of an east-west transect at 76°30′S in mid-January and early February 1990. There was a persistent (at least 3½ weeks in duration) diatom bloom within a surface meltwater lens extending 100–150 km seaward from the edge of the receding pack ice, with biogenic silica concentrations frequently exceeding 20 μmol l −1 in the upper 15 m. There were also other significant maxima in biogenic silica, with concentrations greater than 7 μmol l −1 at distances of 250–500 km seaward of the ice edge. These maxima were apparently unrelated to meltwater effects. 30Si tracer experiments to measure the production and dissolution rates of biogenic silica as a function of depth within the upper 50 m at three stations within the ice-edge diatom bloom indicate that the specific production rate (i.e. the rate per unit of biogenic silica present) ranged from 0.05 to 0.12 day −1 ( 0.07–0.17 doublings day −1). The resulting vertically integrated silica production rates ranged from 27 to 50 mmol m −2 day −1, with a mean of 34. Vertically integrated rates of biogenic silica dissolution ranged from 16 to 28 mmol m −2 day −1, with a mean of 22, or 64% of the mean silica production rate. The mean resulting net rate of biogenic silica production within the bloom, 12 mmol m −2 day −1, leads to an estimated net annual silica production rate of 1.0 mol m −2 in the upper 50 m of the southwestern Ross Sea. A revised silica budget for the western Ross Sea, incorporating the most recent estimates of production, redissolution, accumulation in the sediments and efflux from the seabed, indicates that virtually all silica exported from the upper 50 m must reach the sea-floor unless advective transport from the east is significant.

Mesoscale Cyclogenesis over the Southwestern Ross Sea Linked to Strong Katabatic Winds

Abstract Two years of automatic weather station (AWS) observations and satellite images have been used to study mesoscale cyclogenesis along the Transantarctic Mountains. Twice-daily regional sea-level pressure analyses revealed the frequent formation of mesoscale cyclones adjacent to two regions where the discharge of cold boundary-layer air from east Antarctica is concentrated: near Terra Nova Bay/Franklin Island and Byrd Glacier. Between one and two new vortices on average formed each week in the former location with weak frequency maxima in December–March and August–September. There was a large difference between the cyclogenetic activity in the two years. The AWS array expanded in 1985 and resolved another cyclogenetic area near Byrd Glacier. This feature was half as active as the Franklin Island area and exhibited many of the same characteristics. About half of the Byrd Glacier cyclones developed simultaneously with vortices near Franklin Island. These developments are the result of a dry baroclinic...

Satellite Analyses of Antarctic Katabatic Wind Behavior

Abstract Prominent warm signatures of strong, negatively buoyant, katabatic airstreams are present at thermal infrared wavelengths as a result of intense vertical mixing and drift-snow transport within stable boundary layers. These tracers are used to illustrate several aspects of the behavior of katabatic winds in the Ross Sea sector of the Antarctic. The satellite features are compared with surface-based observations whenever possible. Converging surface-wind signatures upslope from Terra Nova Bay are shown to closely follow the observed time-averaged streamlines of drainage airflow. The satellite-observed core of the katabatic airstream descends to sea level via a direct route, but complex three-dimensional trajectories are manifested in marginal regions. Katabatic winds propagating horizontally for hundreds of kilometers over the southwestern Ross Sea do not exhibit the expected influence of the Coriolis force. Katabatic signatures are shown to be climatological features over the Ross Ice Shelf which ...

Biogenic sedimentation in McMurdo Sound, Antarctica

Surface sediments from eastern McMurdo Sound and fjord basins of the Victoria Land Coast are enriched in organic carbon and opal. At depths below 600 m, opal contents commonly exceed 30%; organic carbon contents average 1.5% and are as high as 3.5%. Opal and organic carbon are supplied by local production within and below sea ice and by advection from open water areas of the southwestern Ross Sea. The distribution of organic carbon and opal at the sea floor is consistent with cyclonic (clockwise) water circulation in McMurdo Sound. Advective transport from the Ross Sea supplies biogenic sediment to eastern and northern McMurdo Sound; the southwestern shelf is bathed by waters derived in part from beneath the Ross Ice Shelf which transport very little allochthonous carbon. The supply of biogenic debris in southwestern McMurdo Sound is further curtailed by sea-ice conditions, e.g., more prevalent multi-year sea ice which reduces photosynthesis, and the absence of summer basal melting and ice breakout which restricts the flux of sea ice and open-water production to the sea floor. Consistent with this hypothesis, organic carbon fluxes measured via sediment trapping beneath fast ice are one to two orders of magnitude higher in eastern versus western McMurdo Sound. Pronounced cross-sound gradients in shallow water benthic biomass and species diversity have previously been attributed to east-west variations in productivity. Our surface sediment data suggest that similar or even more dramatic contrasts in benthic community structure may exist in the deeper water areas of McMurdo Sound. Pervasive resuspension is documented by pronounced near-bottom flux increases measured by sediment trapping. Resuspension events, which may be more common during the austral winter, act to winnow and transport biogenic debris and fine terrigenous sediment from shallow regions (< 400 m) to protected shelf basins. Despite oxidative loss within the water column, the accumulation rate of organic carbon in the deep basins of McMurdo Sound averages 45 mg C m −2 day −1, more than an order of magnitude higher than the world average organic carbon flux to continental margins and equivalent to accumulation rates observed in many anoxic settings. Preservation efficiency of opal during transport and deposition appears anomalously high and the most opal-rich sediments of McMurdo Sound are depleted in organic carbon relative to mid- and low-latitude siliceous deposits. Despite this observation, if similar patterns of biogenic sedimentation exist on many other parts of the shelf, the Antarctic continental margin is an important sink for sedimentary organic carbon, as has been previously suggested for the silica system.

Recent diatom record of McMurdo Sound, Antarctica: Implications for history of sea ice extent

Analyses of diatom assemblages within surface sediments from McMurdo Sound, Antarctica, document the regional circulation patterns in this large embayment in the southwestern Ross Sea. Thalassiosira spp., indicative of water column primary productivity, is most common in eastern and portions of northwestern McMurdo Sound as a result of its advection into these regions from areas of open water primary productivity. Surface sediments from southwestern McMurdo Sound are composed mainly of Nitzschia curta, a small pennate form commonly found both as a member of the sea ice microbial community as well as a dominant component in ice edge blooms in the Ross Sea. The northward advection of oligotrophic water into the western Sound from beneath the McMurdo Ice Shelf results in the dominance of an autochthonous flora in the southwestern Sound. Sediments deposited during the past 500 years also have been dominated by Thalassiosira spp. and Nitzschia curta. Thalassiosira spp. concentrations reached a maximum between 1600 and 1875 A.D., corresponding to the time of the “Little Ice Age.” More persistent winds may have been responsible for more prevalent polynyas at that time, resulting in greater amounts of open water primary productivity and subsequent higher percentages of Thalassiosira spp. Increased areal extent and/or duration of coastal polynyas during the “Little Ice Age” suggests that within the southwestern Ross Sea, the production of High Salinity Shelf Water, and hence Antarctic Bottom Water, may have been greater at that time.

Biogenic-silica accumulation in the Ross Sea and the importance of Antarctic continental-shelf deposits in the marine silica budget

Thirty-five box cores were collected from the continental shelf in the Ross Sea during cruises in January and February, 1983. Pb-210 and Pu-239, 240 geochronologies coupled with biogenic-silica measurements were used to calculate accumulation rates of biogenic silica. Sediment in the southern Ross Sea accumulates at rates ranging from ≤0.6 to 2.7 mm/y, with the highest values occurring in the southwestern Ross Sea. Biogenic-silica content in surface sediments ranges from 2% (by weight) in Sulzberger Bay and the eastern Ross Sea to 41% in the southwestern Ross Sea. Biogenic-silica accumulation in the southwestern Ross Sea averages 2.7 × 10 −2 g/ cm 2/ y and is comparable to accumulation rates in high-productivity, upwelling environments from low-latitude continental margins ( e.g., Gulf of California, coast of Peru). The total rate of biogenic-silica accumulation in the southern Ross Sea is approximately 0.2 × 10 14 g/ y, with most of the accumulation occurring in basins (500–1000 m water depth). If biogenic-silica accumulation in the southern Ross Sea continental shelf is typical of other basins on the Antarctic continental shelf, as much as 1.2 × 10 14 g/ y of silica could be accumulating in these deposits. Biogenic-silica accumulation on the Antarctic continental shelf may account for as much as a fourth of the dissolved silica supplied to the world ocean by rivers and hydrothermal vents.

Observations of Sediment–laden Icebergs in Antarctic Waters: Implications to Glacial Erosion and Transport

AbstractSediment-laden icebergs are rarely sighted in Antarctic waters. However, during the recent Deep Freeze 79-USCGC Glacier expedition to the George V Coast and the south-western Ross Sea, nine sediment-laden icebergs and several pieces of pack ice with surficial sediment layers were observed. These observations include basal debris zones, debris slumped on to glaciers and floating ice, and englacial debris believed to have been incorporated along shear zones.Sediment samples collected from icebergs were texturally and mineralogically variable. Some were unsorted mixtures consisting of a wide variety of angular minerals and rock fragments; others consisted primarily of slate clasts, quartz sand, and rock flour.

Observations of Sediment–laden Icebergs in Antarctic Waters: Implications to Glacial Erosion and Transport

Abstract Sediment-laden icebergs are rarely sighted in Antarctic waters. However, during the recent Deep Freeze 79-USCGC Glacier expedition to the George V Coast and the south-western Ross Sea, nine sediment-laden icebergs and several pieces of pack ice with surficial sediment layers were observed. These observations include basal debris zones, debris slumped on to glaciers and floating ice, and englacial debris believed to have been incorporated along shear zones. Sediment samples collected from icebergs were texturally and mineralogically variable. Some were unsorted mixtures consisting of a wide variety of angular minerals and rock fragments; others consisted primarily of slate clasts, quartz sand, and rock flour.

Air census of two colonies of Adelie Penguins (Pygoscelis adeliae)in Ross Dependency, Antarctica

Air photographs taken on biological survey flights between 1962 and 1965 have yielded counts of Adelie Penguin breeding colonies at Beaufort Island in the southwestern Ross Sea and Inexpressible Island on the Victoria Land coast (Fig 1). These colonies are remote from expedition bases, seldom visited, and too large for accurate assessment from the ground. Earlier estimates of their size are summarised by Taylor (1964). Methods of estimating colony size are discussed by Caughley (1960) and Taylor (1962). This survey is based on serial black and white photographs taken at optimal times of the breeding cycle; counts are converted to estimates of breeding population size using data from a ground study at the Cape Royds (Fig 1) breeding colony and from known patterns of breeding at other colonies (Sladen, 1958; SapinJaloustre, 1960; Taylor. 1962; Stonehouse. 1963; Reid. 1964).

Food of Weddell Seals at McMurdo Sound, Antarctica

Stomach contents of 44 Weddell seals taken at McMurdo Sound, Antarctica, were examined. Fishes occurred in 97% of the stomachs that contained any food. Mysids, decapod crustaceans, amphipods and cephalopods were the in- vertebrate prey taken most frequently. All specimens examined were heavily in- fested with nematodes and cestodes, a condition related to the predominantly fish diet of this seal. INTRODUCrION During field studies of marine benthic invertebrates at McMurdo Sound, 1958 to 1961, the stomach contents of 44 adult Weddell seals, Leptonychotes weddelli (Lesson), were examined. This report describes the items of food taken by these seals and discusses these data in terms of published knowledge of the food habits of this species. Of the four truly Antarctic pinnipeds, the Weddell seal occurs farthest south. Its principal habitat is the fast sea ice or grounded ice along the coast. The species is circumpolar (Scheffer, 1958) and is found throughout the year near the United States McMurdo Station (77?51' S, 166?39' E) at McMurdo Sound in the southwestern Ross Sea. During the summer in this region seals use open leads and holes in the sea ice. In winter they live beneath the ice and utilize small blow holes or air in pockets under pressure ice. Although considerable literature relates to Antarctic seals there is much to learn of the ecology of individual species (Turbott, 1952, 1956; Gilmore, 1961). For information about the Weddell seal at McMurdo Sound, the best general source remains Wilson's (1907) account. To date, however, there has been little specific information available on the food habits of this species. ABSTRACT: Stomach contents of 44 Weddell seals taken at McMurdo Sound, Antarctica, were examined. Fishes occurred in 97% of the stomachs that contained any food. Mysids, decapod crustaceans, amphipods and cephalopods were the in- vertebrate prey taken most frequently. All specimens examined were heavily in- fested with nematodes and cestodes, a condition related to the predominantly fish diet of this seal. INTRODUCrION During field studies of marine benthic invertebrates at McMurdo Sound, 1958 to 1961, the stomach contents of 44 adult Weddell seals, Leptonychotes weddelli (Lesson), were examined. This report describes the items of food taken by these seals and discusses these data in terms of published knowledge of the food habits of this species. Of the four truly Antarctic pinnipeds, the Weddell seal occurs farthest south. Its principal habitat is the fast sea ice or grounded ice along the coast. The species is circumpolar (Scheffer, 1958) and is found throughout the year near the United States McMurdo Station (77?51' S, 166?39' E) at McMurdo Sound in the southwestern Ross Sea. During the summer in this region seals use open leads and holes in the sea ice. In winter they live beneath the ice and utilize small blow holes or air in pockets under pressure ice.

Influence of X-Rays on Limb Regeneration in Urodele Amphibians

Influence of X-Rays on Limb Regeneration in Urodele Amphibians