Cosmonaut Sea Research Articles

Freshening of Antarctic Bottom Water Off Cape Darnley, East Antarctica

AbstractRecently, a source of Antarctic Bottom Water (AABW) was identified off Cape Darnley at the eastern end of the Weddell‐Enderby Basin. However, the behavior and long‐term variability of Cape Darnley Bottom Water (CDBW) are not clearly understood. Hydrographic observations from 1974 to 2016 were compared, and a decade‐long bottom temperature record was analyzed to clarify multidecadal changes in the CDBW in this region and its downstream influences. In the Cooperation Sea, CDBW spread northwestward with its deepest part reaching to approximately 4,900 dbar. CDBW freshening of 0.001–0.003 decade−1 was revealed. In the Cosmonaut Sea, long‐term AABW warming of approximately 0.01–0.03°C decade−1 was prominent in the deep basin, while freshening was detected on the upper continental slope. Spatial patterns suggest that an interbasin deep transport of excess freshwater is carried by CDBW and fed into the Weddell Gyre, which might act as an abyssal freshwater buffer.

Spatial distribution of micro- and meso-zooplankton in the seasonal ice zone of east Antarctica during 1983–1995

Historically, most studies about the geographic distribution of zooplankton in the Southern Ocean have been focused on the macro-sized zooplankton (2–20 mm), such as the Antarctic krill and larger-sized copepods. On the other hand, despite the high abundance and biomass, the distribution patterns of micro- (20–200 μm) and meso-sized (200 μm–2 mm) zooplankton communities are little understood. In this study, we investigated the distribution patterns of larger micro-zooplankton (100–200 μm) and meso-zooplankton communities in the seasonal ice zone in the Cosmonaut Sea near Syowa Station and examined the effects of environmental factors and water properties on these communities. The investigation was based on samples collected with 100 μm mesh nets, which are appropriate to estimate the quantitative abundance and community structure of micro- and meso-zooplankton species between 1983 and 1995. Cluster analysis of the samples revealed that the distribution of macro-zooplankton species was influenced by the temperature and salinity of ocean fronts. Among the meso-zooplankton, cyclopoid and small calanoid copepods tended to be ubiquitously distributed. However, among the micro-zooplankton, the distributions of foraminiferans and tintinnids were associated with sea ice extent. The distribution of micro- and meso-zooplankton communities could be used to estimate the impact of environmental changes on the marine ecosystem in the Southern Ocean.

New continental margin magnetic anomalies of East Antarctica

Over the past decade, Australian, Norwegian and Russian marine surveys have collected integrated seismic, gravity and magnetic data in the southern Indian Ocean. The more than 350,000 line-km of new airborne and marine magnetic observations for the East Antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns. This compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of East Antarctica. The eastern sector of the map from Bruce Rise in the west to the D'Urville Sea in the east is largely dominated by seafloor spreading magnetic anomalies. The ‘Adélie Rift Block’ of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric. Abrupt magnetic anomaly changes along the oceanic-continent transition in the Cooperation Sea including the Enderby Basin Anomaly extend for more than 1680km from the Kerguelen Plateau towards the Cosmonaut Sea. Three sectors of the East Antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation. Strong positive seafloor magnetic anomalies mark the southern margin of the Kerguelen Plateau, the Maud Rise and adjacent areas in the Riiser-Larsen Sea. The new compilation suggests that at least 300km of the Enderby Basin and Shackleton Basin may be part of the Cretaceous Kerguelen Volcanic Province and possibly maps an abandoned ‘fossil’ spreading center in the central Enderby Basin. The majority of the published age models for the Enderby Basin and “Australian sector” of the East Antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map.

Krill stocks in high latitudes of the Antarctic Lazarev Sea: seasonal and interannual variation in distribution, abundance and demography

The motivation of the LAKRIS project (Lazarev Krill Study) was to investigate the seasonal and interannual variability in distribution, demography, phenology and population dynamics of Antarctic krill Euphausia superba in poorly studied high-latitude areas. The core data set consists of four RMT-net-sampling surveys that were conducted in early summer (December 2005 and 2007), autumn (April 2004) and winter (July to August 2006). Supplementary data were analysed from the 1980s to view the results in a wider geographical context. The major observations are the following: (1) krill abundance and distribution showed geographical, seasonal and interannual fluctuations. Krill abundance was almost one order of magnitude lower compared to the Southwest Atlantic. (2) Krill density showed substantial differences between day and night for autumn and winter surveys. (3) A spatial gradient was observed for krill size groups, with larger sized (adult spawning) krill further south and medium-sized juveniles and immatures mainly between 60° and 67°S. (4) Data on maturity stage composition and larvae occurrence indicate an active and successful spawning in these high-latitude areas even under the conditions of sea-ice cover. At the end of the spawning season, females and males regressed in their external sexual characteristics (‘re-juvenation’) to overwintering immature stages. Major conclusions relate to a conceptual view of the spatial distribution of krill size classes and the occurrence of larvae in the context of major oceanographic features and current systems in the Lazarev Sea and adjacent ocean sectors. The Lazarev Sea obviously does not support a single self-maintaining population, but represents a complex transition zone of stocks with different origins from the Scotia Sea and the Cosmonaut Sea.

Mesozoic paleomagnetic anomalies south of Ceylon

Ceylon was one of the connecting continental links in the ensemble of the united Indian and Antarctic continents when Gondwana split up. However, the precise position of Ceylon is still undetermined. Some publications attribute its position to different regions near Enderby Land. The analysis of the linear magnetic anomalies in the Central Basin of the Indian Ocean south of Ceylon and near the Antarctic coast (the Cosmonaut Sea and the Riiser-Larsen Sea) yielded evidence for the position of Ceylon in the ensemble of the Indian and Antarctic continents: it was east of Gunnerus Ridge in the Cosmonaut Sea. The breaking away of Ceylon from Antarctica occurred in the period of chron M11r (136.44–136.90 Ma). Before breaking away, the eastern flank of the Gunnerus Ridge was joined to Ceylon and they formed a united continental block.

Polar baseline surface radiation measurements during the International Polar Year 2007–2009

Abstract. Downwelling and upwelling shortwave and longwave radiation components from six active polar sites, taking part of the Baseline Surface Radiation Network (BSRN), were selected for the period of the last International Polar Year (March 2007 to March 2009), and included in the BSRN-IPY dataset, along with metadata and supplementary data for some of the stations. Two sites, located at Svalbard archipelago (Ny Ålesund) and Alaska (Barrow), represent Arctic sea-level conditions. Four Antarctic stations represent both sea-level (Dronning Maud Land and Cosmonaut Sea) and high-elevation conditions (South Pole and East Antarctic Plateau). The BSRN-IPY dataset content and quality are discussed. The dataset is now available at doi:10.1594/PANGAEA.737668, and can be used for free after accepting the BSRN data release guidelines. The dataset has been summarized as monthly averages and subject to further evaluation according to strict criteria not previously applied.

Surface oceanography of BROKE-West, along the Antarctic margin of the south-west Indian Ocean (30–80∘E)

Abstract Hydrographic CTD and ADCP data were collected during the BROKE-West research voyage (January–March 2006) in the south-west Indian Ocean sector of the Antarctic margin. These data describe the large-scale circulation, water masses, fronts and summertime stratification in the surface layer over the continental shelf, slope and rise region between 30 and 80 ∘ E that forms CCAMLR Statistical Area 58.4.2. The surface circulation matched the full-depth circulation and consisted of the eastward flowing southern Antarctic Circumpolar Current front to the north, and the westward flowing Antarctic Slope Current associated with the Antarctic Slope Front along the continental slope to the south. Two sub-polar gyres were detected south of the Southern Boundary of the Antarctic Circumpolar Current: the eastern Weddell Gyre in the Cosmonaut Sea ( 30 – 50 ∘ E ) and the greater Prydz Bay Gyre in the Cooperation Sea ( 60 – 80 ∘ E ). In the eastern Weddell Gyre, the seasonal mixed layer depths were shallower, warmer and fresher relative to the regions to the east which were deeper, cooler and more saline. This spatial variability is found to be strongly correlated to the large-scale pattern of sea ice melt/retreat in the months preceding the voyage and the accumulated wind stress thereafter. Areas of upwelling warm deep waters into the surface layer are presented from positive anomalies of potential temperature and nutrient concentrations (nitrate and silicate). These anomalies were strongest in the eastern Weddell Gyre in the vicinity of the Cosmonaut Polynya/Embayment, north of Cape Anne and near the Southern Boundary of the Antarctic Circumpolar Current in the eastern sector of the survey. The summertime stratification (seasonal mixed layer, seasonal pycnocline and Tmin layer) are discussed relative to the distributions of chl a and acoustically determined Antarctic Krill (Euphausia superba) densities. Elevated chl a concentrations were found in the surface layer of the marginal ice zone and it is proposed that these are retained south of the fast, narrow jet of enhanced Antarctic Slope Current on the upper continental slope. There is qualitative evidence of these maxima being subducted and transported north in the seasonal pycnocline in response to Ekman convergence from the easterly winds in this region. The seasonal mixed layer within the sub-polar gyres had relatively low chl a concentrations with sub-surface maxima in the seasonal pycnocline and the top of the Tmin layer. Surface concentrations increased once again north of the Southern Boundary in the north-east of the survey. Krill and chl a concentrations were both co-located and decoupled at different locations across the survey. There was no clear oceanographic boundary influencing the distribution of the krill surveyed, though further work is necessary to properly synthesize this and other biological patterns with the oceanographic processes, given the varying time and length scales and intrinsic sampling limitations.

Distribution of squid and fish in the pelagic zone of the Cosmonaut Sea and Prydz Bay region during the BROKE-West campaign

The composition and distribution of squid and fish collected by Rectangular Midwater Trawls in the upper 200 m were investigated during the BROKE-West (Baseline Research on Oceanography, Krill and the Environment-West) survey (January-March 2006) in CCAMLR Subdivision 58.4.2 of the Southern Ocean. A total of 332 individuals were collected, with the most abundant fish species being Pleuragramma antarcticum (34%), Notolepis coatsi (27%) and Electrona antarctica (26%); and the most abundant squid being Galiteuthis glacialis (64%). Abundances of all species were among the lowest recorded using this type of gear. Cluster analysis revealed two distinct communities: a notothenioid-dominated coastal community and an oceanic community dominated by mesopelagic fish and squid. Environmental factors related to this segregation were explored using Redundancy Analysis (RDA). The notothenioid P. antarcticum was associated with shallow areas with high chlorophyll a concentrations. Larval stages of E. antarctica, N. coatsi and G. glacialis were found over deeper water and were positively correlated with higher temperatures and a deeper-reaching mixed layer. Postmetamorphic stages of E. antarctica were caught mostly after sunset and were negatively correlated with solar elevation. The observation of higher densities in the eastern part of the sampling area reflects a temporal rather than a geographical effect. Samples of the three most abundant fishes, E. antarctica, P. antarcticum and N. coatsi, were analysed for gut content. All species fed on a variety of mesozooplankton including copepods, amphipods and euphausiids, which is consistent with previous reports on similar life stages. Mean body energy density was highest for E. antarctica (27 kJ g-1), while it was similar for P. antarcticum and N. coatsi (22 kJ g-1). The high energy content emphasizing the importance as a food resource for top predators in the Southern Ocean.

Dimethylsulphide and dimethylsulphoniopropionate in the South-West Indian Ocean sector of East Antarctica from 30° to 80°E during BROKE-West

Abstract We investigated the spatial and vertical variations of dimethylsulphide (DMS), dissolved, particulate, and total dimethylsulphoniopropionate (DMSPd, DMSPp and DMSPt) in the south-west Indian Ocean sector of East Antarctica between 30°-80°E and south of 62°S from January to March 2006. Twenty seven sites were examined and mean concentrations in the upper 150 m of the water column were for DMS 8 nM (range nd-63 nM); DMSPd 5 nM (nd-36 nM); DMSPp 11 nM (nd-38 nM), and DMSPt 16 nM (nd-54 nM). The highest concentrations of the sulphur compounds occurred in the western sector of the study area on the Antarctic shelf and across the Antarctic Slope Front region (ASF) of the eastern Weddell Gyre (EWG) in the Cosmonaut Sea. Concentrations were lower north of the ASF region of the EWG and in the eastern sector of the Antarctic Circumpolar Current (ACC) and the greater Prydz Bay Gyre (PBG) circulation in the Cooperation Sea. The influence of melting pack ice on DMS and DMSPd concentrations was apparent in this study but was predominantly restricted to a few stations in the Cosmonaut Sea early in the voyage. DMS concentrations were extremely high at stations in the western sector of the Marginal Ice Zone (MIZ) in the early part of the study (mean 25 nM), reaching concentrations as high as 63 nM at station 42, decreasing to a mean concentration of 5.5 nM in the reduced MIZ to the east. In the Seasonal Mixed-Layer (SML) water (4-50 m) from the ACC and EWG, mean DMS concentrations were 5 nM, whilst very low concentrations of DMS occurred under forming sea ice (0.5 nM). Mean DMSPp concentrations in the top 50 m of the MIZ stations across the survey were closely similar (18-20 nM) and slightly higher than SML waters (13-14 nM), whilst mean DMSPd concentrations were 275 and 100% higher in samples from the MIZ stations (8-15 nM), compared with SML waters (4 nM). In the eastern sector, the coastal region across the shelf break and ASF region was largely free of ice and there was generally no elevation of DMSPd (mean 4 nM). This suggests that biological processes associated with the pack or fast ice in the ASF region of the western sector of the survey, in conjunction with the oceanography of the region, may have caused the high concentrations of DMS and DMSPd observed at latitudes >67°S. Whilst filtration affects did influence DMSPd concentrations at a few sites, it was clear that elevated DMS and DMSPd concentrations occurred in the top 50 m and Winter Water (WW) layers of MIZ stations, and in High-Salinity Shelf Water (HSSW) at specific locations on the Antarctic Shelf, and likely reflected melting sea ice. DMS and DMSPp were also detected below the euphotic zone in Modified Circumpolar Deep Water (MCDW) on the Antarctic slope, with DMSPp possibly transported there in sedimenting detritus and faecal material. The significant concentrations of DMS detected in lower MCDW (0.42-1.1 nM) and Antarctic Bottom Water (AABW, 2.7 nM) were likely to have been transported to these depths in wintertime as dense shelf-water overflows from the Cape Darnley and Prydz Bay source regions. The high concentrations of DMS and DMSPd observed in the surface waters of the MIZ further support the importance of the Antarctic region and the sea-ice zone to the global sulphur cycle.

The Cosmonaut Sea Wedge

A set of multi-channel seismic profiles (∼15,000 km) is used to study the depositional evolution of the Cosmonaut Sea margin of East Antarctica. We recognize a regional sediment wedge, below the upper parts of the continental rise, herein termed the Cosmonaut Sea Wedge. The wedge is situated stratigraphically below the inferred glaciomarine section and extends for at least 1,200 km along the continental margin with a width that ranges from 80 to about 250 km. The morphology of the wedge and its associated depositional features indicate a complex depositional history, where the deep marine depositional sites were influenced by both downslope and alongslope processes. This interaction resulted in the formation of several proximal depocentres, which at their distal northern end are flanked by elongated mounded drifts and contourite sheets. The internal stratification of the mounded drift deposits indicates that westward flowing bottom currents reworked the marginal deposits. The action of these currents together with sea-level changes is considered to have controlled the growth of the wedge. We interpret the Cosmonaut Sea Wedge as a composite feature comprising several bottom current reworked fan systems. The wide spectrum of depositional geometries in the stratigraphic column reflects dramatic variations in sediment supply from the continental margin as well as varying interaction between downslope and alongslope processes.

The macrozooplankton of the Cosmonaut Sea, east Antarctica (30°E–60°E), 1987–1990

The macrozooplankton of the Cosmonaut Sea, Indian sector of the Southern Ocean, were investigated during the austral summers (January/February) of 1987, 1988, 1989 and 1990. Macrozooplankton samples were collected with Bongo and MT nets in the top 200 m of the water column, along with integrated temperature and salinity. Multivariate analysis of macrozooplankton density data identified a consistent division between the assemblage structure of the neritic and oceanic zones, the latter being characterized by a number of species unique to the continental shelf including Euphausia crystallorophias, Pleuragramma antarcticum and Notothenia kempi. Samples collected in the oceanic zone were all located within the Coastal Current (CC), south of the Southern Boundary of the Antarctic Circumpolar Current (ACC). However, in both 1987 and 1988 water masses of ACC origin had been injected into the CC. These intrusions were an important but inter-annually variable source of mesoscale spatial variability, being associated with distinct macrozooplankton assemblage structure and densities. Inter-annually there was considerable variation in macrozooplankton densities and biomass. Densities, dominated by Eukrohnia hamata, Thysanoessa macrura and Euphausia superba (krill), averaged ∼250 ind.1000 m −3 from 1987 to 1989, but reached >750 ind.1000 m −3 in 1990, largely due to higher densities of E. hamata. Biomass was dominated by krill, and total levels reflected the decline in biomass of this species from 1986 to 1990. A number of significant correlations were identified between species densities and temperature and/or salinity. However, the wide biogeographic ranges of most species indicated that these correlations were largely spurious. Overall, the differences between macrozooplankton assemblages in ACC and CC water masses within years, and the differences in macrozooplankton densities between years, most likely reflected the different histories of spatially or temporally separated water masses that were structured by a combination of bottom-up and top-down processes. Comparison of the data from this study with other regions in the Antarctic demonstrated that there is considerable circumpolar consistency in the taxonomic composition of Antarctic macrozooplankton communities, although the densities and proportional contribution of species varies both spatially and temporally. The occurrence of a krill-dominated assemblage, characterized by low densities of other species, indicates that this species has a significant top-down impact on zooplankton communities. The spatial and temporal variation in krill biomass may therefore be a significant determinant of both mesoscale and circumpolar variation in the contribution of other taxonomic groups to Antarctic zooplankton assemblages. Locally, the decline in krill biomass in the Cosmonaut Sea between 1987 and 1990 indicated a weaker connection with the Cooperation Sea to the east, where the opposite trend was observed, than previously thought. Separation between these two seas is probably facilitated in part by the anti-cyclonic gyres observed in both regions.

Seismic morphology and distribution of inferred glaciomarine deposits along the East Antarctic continental margin, 20°E–60°E

Seismic-reflection profiles from the continental rise and abyssal plain along a nearly 2000-km long stretch of the East Antarctic margin have been studied, including the Cosmonaut Sea basin and adjacent parts of the Riiser-Larsen Sea to the west and the Cooperation Sea to the east. The recognition of glaciomarine deposits is based on their correlative basal reflector and acoustic characteristics. The lateral distribution of seismostratigraphic sediment thickness, internal architecture of reflectors and seafloor morphology reveal a number of deep-water depocentres with large paleo-channels. These main depocentres correspond to the coastal outlets of present-day ice streams and hence indicate longer-term stabilities in the ice-stream positions during the Neogene advance and retreat phases of the Antarctic Ice Sheet. Scattered paleo-channels and subordinate depocentres in the Cosmonaut Sea suggest that some of the ice streams might occasionally change their position or that also other ice streams were episodically involved during earlier glacial maxima. Several paleo-channels are > 30 km wide and > 500 m deep, the largest one being approximately 40 km wide and 1000 m deep. These channels show evidence of multiple incisions and have prominent, multi-storey levees. The giant conduits are considered to be formed through several extensive phases of mass wasting from the shelf edge and upper slope to the continental rise and adjoining abyssal plain. Downslope transport by sediment gravity flows predominates with a subordinate role of along-slope sediment drift by circumpolar contour current. The repetitive advances and retreats of the ice sheet and recharging of ice streams with terrigenous detritus allowed enormous amounts of glacigenic sediment to be transferred from the Antarctic interior to the shelf margin during glacial phases. It was probably a high rate of localized sediment supply by ice streams and a wide range of sediment grain sizes that promoted the long series of large and highly efficient turbidity currents. The study demonstrates that a seismostratigraphic mapping of deep-water depocentres and recognition of paleo-channels along a glaciated continental margin may provide important information on the long-term ice-sheet dynamics, particularly on the pattern of ice drainage represented by coastal ice streams.

A climatology of sea ice embayments in the Cosmonaut Sea, Antarctica

The Cosmonaut Sea off the coast of Antarctica is characterized by highly variable sea ice conditions. As the sea ice expands during the austral fall and winter, embayments, open bodies of water surrounded on three sides by sea ice, as well as polynyas often form. Using sea ice concentration data provided by satellite‐based passive microwave instruments from 1979 to 2004, we characterize these embayment events by their size and temporal evolution. The data suggests a striking periodicity in the occurrence of these embayments, with the exception of a “quiet” period between 1984 and 1987. The large embayment events seem to appear approximately every three years in the same location and persist for anywhere between mid‐June to mid‐August. These results indicate there is predictability in the occurrence of embayment events, adding important insight into the characterization of sea ice variability in this region.

A numerical study of the western Cosmonaut polynya in a coupled ocean–sea ice model

Employing results from a 0.4°, 40‐level fully global, coupled ocean–sea ice model, we investigated the role of physical processes emanating from atmosphere, ocean, and ice in the initiation, maintenance, and termination of a sensible heat polynya with a focus on the western Cosmonaut polynya that occurred during May–July 1999. The Cosmonaut polynya first appeared in early May 1999 in the form of an ice‐free embayment, transformed into an enclosed polynya on 5–9 July, and disappeared by late July, when the ice from the surrounding regions began to encircle the embayment. Except for the differences in ice concentrations, the time of appearance, size, and shape of the Cosmonaut polynya simulated by the model are in approximate agreement with the Special Sensor Microwave/Imager (SSM/I) observations. Between May and July 1999 the Cosmonaut Sea experienced two synoptic storms, both lasting ∼5 days. Followed by the passage of the first storm on 12–19 June, there was a remarkable growth in the size of the embayment by 21 × 103 km2. Associated with this, the sea surface temperature (SST) rose by 0.15°C, the upward heat flux jumped from 5 to 94 W m−2, and a net freshwater flux into the ocean increased by 2 cm d−1. By running the model simulation with a 20% wind speed increase, it is demonstrated that the twofold increase in SST and upward heat flux increased the embayment area by 15 × 103 km2 and decreased the ice concentration by approximately 10% from the control run. A similar, but somewhat weaker wind event that took place on 30 June to 10 July had less influence on the embayment area although the upward heat flux (65 W m−2) was comparable to the first event. By examining the vertical displacement of the −1.6°C isotherm depth prior to, during, and after these two storms, we demonstrate that the impetus provided by these storms was able to raise the −1.6°C isotherm depth by 30 m through wind‐driven mixing, making sufficient oceanic heat input from beneath the mixed layer available to prevent freezing and/or delay ice formation while ice in the adjacent regions continued to grow. A sudden shift in the ice drift direction from southwest to northeast (3 July) followed by the second storm, accompanied by large air‐sea temperature differences, caused the enclosure of the embayment, subsequent formation of the polynya, and its termination.

Interplay of turbidite and contourite deposition along the Cosmonaut Sea/Enderby Land margin, East Antarctica

Multichannel seismic reflection data from the Cosmonaut Sea margin of East Antarctica have been interpreted in terms of depositional processes on the continental rise. A major sediment lens is present below the upper continental rise along the entire Cosmonaut Sea margin. The lens probably consists of sediments supplied from the shelf and slope, being constantly reworked by westward flowing bottom currents redepositing the sediments into a large-scale plastered drift deposit prior to the main glacigenic input along the margin. High-relief elongated and sometimes semicircular depositional structures are found on the upper continental rise, stratigraphically above the regional sediment lens, and were mainly deposited by the action of closely spaced turbidity currents. On the lower continental rise, large-scale sediment bodies extend perpendicular to the continental margin and were deposited as a result of down-slope turbidity transport and westward flowing bottom currents. The elongated sediment mounds on the upper and lower continental rise were deposited after initiation of glacigenic input to the slope and rise.

Input of Glaciomarine Sediments along the East Antarctic Continental Margin; Depositional Processes on the Cosmonaut Sea Continental Slope and Rise and a Regional Acoustic Stratigraphic Correlation from 40° W to 80° E

Multichannel seismic reflection data from the Cosmonaut Sea margin of East Antarctica have been interpreted in terms of depositional processes in the continental slope and rise area. A major sediment lens is present below the upper continental rise along the entire Cosmonaut Sea margin. The lens probably consists of sediments supplied from the shelf and slope, being constantly reworked by westward flowing bottom currents, which redeposited the sediments into a large scale drift deposit prior to the main glaciogenic input along the margin. High-relief semicircular or elongated depositional structures are also found on the upper continental rise stratigraphically above the regional sediment lens, and were deposited by the combined influence of downslope and alongslope sediment transport. On the lower continental rise, large-scale sediment bodies extend perpendicular to the continental margin and were deposited as a result of downslope turbidity transport and westward flowing bottom currents after initiation of glacigenic input to the slope and rise. We compare the seismostratigraphic signatures along the continental margin segments of the adjacent Riiser Larsen Sea, the Weddell Sea and the Prydz Bay/Cooperation Sea, focussing on indications that may be interpreted as a preglacial-glaciomarine transition in the depositional environment. We suggest that earliest glaciogenic input to the continental slope and rise occurred in the Prydz Bay and possibly in the Weddell Sea. At a later stage, an intensification of the oceanic circulation pattern occurred, resulting in the deposition of the regional plastered drift deposit along the Cosmonaut Sea margin, as well as the initiation of large drift deposits in the Cooperation Sea. At an even later stage, possibly in the middle Miocene, glacial advances across the continental shelf were initiated along the Cosmonaut Sea and the Riiser Larsen Sea continental margins.

Relationship between synoptic forcing and polynya formation in the Cosmonaut Sea: 1. Polynya climatology

A polynya has been observed in satellite imagery to occur irregularly between the months of July and October in the Cosmonaut Sea. To investigate the processes of formation, maintenance, and decay of these polynyas, a 13‐year climatology of Antarctic atmospheric and surface conditions is assembled from numerical weather prediction model analyses and remotely sensed satellite observations. An analytical technique used previously to identify and track atmospheric low pressure systems is modified to identify polynyas from daily analyses of sea ice area. This information is used to prepare a set of monthly climatological conditions as well as their anomalies when a polynya is present. Previous studies have suggested a link between atmospheric forcing and polynya formation. Along with the drop in sea ice area, there is a drop in sea level pressure as well as an increase in atmospheric wind divergence when the Cosmonaut Sea polynya is present. In some months, the sea ice area decrease is preceded by the passage of an atmospheric low pressure system 1–2 days earlier, but due to the continuous presence of katabatic winds and the coarseness of the ECMWF data, there is not as definitive a relationship between wind divergence and sea level pressure or ice area. Errors associated with the processing of passive microwave remotely sensed data, as well as differences in spatial scale limit that can be determined about the ocean state from the data sets, suggest the need for improved surface classification techniques, as well as regional modeling studies.

Relationship between synoptic forcing and polynya formation in the Cosmonaut Sea: 2. Regional climate model simulations

In part I of this series [Arbetter et al., 2004], a relationship was found in the observations between synoptic atmospheric systems and polynya formation in the Cosmonaut Sea region of Antarctica. In this study, we use a regional coupled atmosphere‐sea ice model to investigate the role of atmospheric forcing of the polynyas in this area. The model successfully simulates both latent and sensible heat polynyas which are found in the region. In particular, a deep‐ocean polynya is formed coincident with the passage of an atmospheric synoptic scale system. The divergence in the sea ice is found to be highly correlated with the atmospheric divergence. We conclude that the initial formation of a polynya can be caused by the interaction of the atmospheric synoptic scale and the continental katabatic wind regimes. While we cannot directly evaluate the role of the ocean using the present model simulations, we performed experiments with different levels of prescribed basal heat flux. These experiments highlight a polynya which could be initiated by the winds and maintained by the oceanic heat. This leads to the concept of a wind‐driven sensible heat polynya, not typical of other deep‐ocean polynyas such as the Weddell polynya of the 1970s.

Demography and life cycle of Antarctic krill, Euphausia superba, in the Indian sector of the Southern Ocean: long-term comparison between coastal and open-ocean regions

Size/age composition and reproductive status of Antarctic krill, Euphausia superba, in the central part of the Indian sector of the Southern Ocean, e.g., the Cooperation Sea (Prydz Bay region) and the Cosmonaut Sea, during austral summers 1977-1990 were summarized to estimate growth rates, longevity, reproduction, recruitment, life span, and mortality rates. The life span of Antarctic krill exceeds 5 years in both the Cosmonaut and Cooperation seas. The age composition of the southern and northern groupings differs markedly, with substantial reduction in numbers of early age groups in the northern grouping. Long-term observations of spawning success, recruitment, and age composition suggest that a self-sustained grouping of krill persists in the Cooperation Sea south of the Antarctic Divergence. However, periodic gene flow via recruits from surrounding regions most probably accounts for the lack of spatial genetic differences between the Cooperation Sea and adjacent areas, thus preventing the establishment of an isolated subpopulation in the region investigated. The major factor responsible for the substantial interannual variability in krill dynamics appears to be macroscale oceanographic and atmospheric circulations, which determine a level of environmental isolation of the Cooperation Sea from adjacent waters.

Development of an Antarctic Regional Climate System Model. Part I: Sea Ice and Large-Scale Circulation

Abstract A coupled atmosphere–ice regional model previously used for simulations in the Arctic has been implemented in the Antarctic. Three 14-month simulations were performed for 1988–89, with different oceanic specifications. The year 1988 was interesting as there were several sensible heat polynya events in the Cosmonaut Sea region, the investigation of which is the goal of future finer-resolution simulations. Overall, the regional climate model produces reasonable simulations of the Antarctic at 100-km resolution. Root-mean-square errors range from 4 hPa in surface pressure, 4 K in near-surface air temperature, and 3 m s−1 in near-surface winds, to 1 K in air temperature and 2 m s−1 in winds at the 500-hPa level. Tests of the coupled system response to oceanic heat flux suggest that the sea-ice simulation is more sensitive than the atmospheric circulation, but it could be expected that the atmosphere would respond to these changes in sea ice over longer time periods than those of interest here. This s...