Southern Scotia Sea Research Articles

Geodynamic implications of the Cenozoic stress field on Seymour Island, West Antarctica

AbstractPalaeostress inferred from brittle mesostructures in Seymour (Marambio) Island indicates a Cenozoic to Recent origin for an extensional stress field, with only local compressional stress states. Minimum horizontal stress (σ3) orientations are scattered about two main NE–SW and NW–SE modes suggesting that two stress sources have been responsible for the dominant minimum horizontal stress directions in the north-western Weddell Sea. Extensional structures within a broad-scale compressional stress field can be linked to both the decrease in relative stress magnitudes from active margins to intraplate regions and the rifting processes that occurred in the northern Weddell Sea. Stress states with NW–SE trending σ3are compatible with back-arc extension along the eastern Antarctic Peninsula. We interpret this as due to the opening of the Larsen Basin during upper Cretaceous to Eocene and to the spreading, from Pliocene to present, of the Bransfield Basin (western Antarctic Peninsula), both due to former Phoenix Plate subduction under the Antarctic Plate. NE–SW σ3orientations could be expressions of continental fragmentation of the northern Antarctic Peninsula controlling eastwards drifting of the South Orkney microcontinent and other submerged continental blocks of the southern Scotia Sea.

Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage

We used the instantaneous growth rate method to determine the effects of food, temperature, krill length, sex, and maturity stage on in situ summer growth of krill across the southwest Atlantic sector of the Southern Ocean. The main aims were to examine the separate effects of each variable and to generate a predictive model of growth based on satellite-derivable environmental data. Both growth increments in length on moulting (GIs) and daily growth rates (DGRs, mm d 21 ) ranged greatly among the 59 swarms, from 0.58‐15% and 0.013‐0.32 mm d 21 . However, all swarms maintained positive mean growth, even those in the low chlorophyll a (Chl a) zone of the central Scotia Sea. Among a suite of indices of food quantity and quality, large-scale monthly Chl a values from SeaWiFS predicted krill growth the best. Across our study area, the great contrast between bloom and nonbloom regions was a major factor driving variation in growth rates, obscuring more subtle effects of food quality. GIs and DGRs decreased with increasing krill length and decreased above a temperature optimum of 0.58C. This probably reflects the onset of thermal stress at the northern limit of krill’s range. Thus, growth rates were fastest in the ice edge blooms of the southern Scotia Sea and not at South Georgia as previously suggested. This reflects both the smaller size of the krill and the colder water in the south being optimum for growth. Males tended to have higher GIs than females but longer intermoult periods, leading to similar DGRs between sexes. DGRs of equivalent-size krill tended to decrease with maturity stage, suggesting the progressive allocation of energy toward reproduction rather than somatic growth. Our maximum DGRs are higher than most literature values, equating to a 5.7% increase in mass per day. This value fits within a realistic energy budget, suggesting a maximum carbon ration of ;20% d 21 . Over the whole Scotia Sea/South Georgia area, the gross turnover of krill biomass was ;1% d 21

Small basins in the Scotia Sea: The Eocene Drake Passage gateway

After isostatic correction for their sedimentary fill, the depths of two small oceanic basins in the southern Scotia Sea suggest that both started to open in mid to late Eocene times. Models of the short magnetic anomaly profiles across the basins provide support for these determinations. The location of the basins, adjacent to the present-day Drake Passage marine gateway, and the timing of continental stretching leading up to their opening, during the Ypresian (early Eocene) onset of global cooling, mean that their importance is potentially far greater than their small size implies. Extension in the region of the two basins would have opened Drake Passage to shallow or intermediate depth oceanic circulation between the Pacific and Atlantic oceans for the first time. This coincided with a reorganisation of vertical mixing patterns in the global ocean, a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments, and the onset of a long decline in atmospheric carbon dioxide concentrations. Cenozoic global cooling may therefore have begun as a result of the shallow opening of Drake Passage.

Propagating rift during the opening of a small oceanic basin: The Protector Basin (Scotia Arc, Antarctica)

Abstract The opening of oceanic basins constitutes one of the key features of Plate Tectonics because it determines the rifting and displacement of the continental crustal blocks. Although the mechanisms of development of large oceans are well known, the opening and evolution of small and middle size oceanic basins have not been studied in detail. The Protector Basin, located in the southern Scotia Sea, is a good example of a small oceanic basin developed between two thinned continental blocks, the Pirie Bank and the Terror Rise, poorly studied up to now. A new set of multibeam bathymetry, multichannel seismic reflection, and gravity and magnetic anomaly profiles obtained on the SCAN 2001 cruise led us to determine that the Protector Basin probably opened during the period comprised between C5Dn (17.4 Ma) and C5ACn–C5ABr chrons (13.8 Ma), forming a N–S oriented spreading axis. The end of spreading is slightly younger to the north. The start of spreading is clearly diachronous, with the most complete set of chrons up to C5Dn in the southern profile, C5Cn in the middle section and only up to C5ADn in the northern part of the basin. The spreading axis propagated northwards during the basin development, producing the wedge shape of the basin. In addition, at the NE part of the basin, a reverse fault developed in the border of the Pirie Bank after basin opening accentuates the sharp northern end. Moreover, the northwestern part of the Pirie Bank margin is an extremely stretched continental crust with N–S elongated magnetic anomalies related to incipient oceanic southward propagating spreading axes. The Protector Basin shows the oldest evidence of E–W continental stretching and subsequent oceanic spreading during Middle Miocene, related with the eastward development of the Scotia Arc that continues up to Present. The relative rotation of continental blocks during the development of small sized oceanic basins by continental block drifting favoured the opening of wedge shape basins like the Protector Basin and conjugate propagating rifts.

The commercial harvest of krill in the southwest Atlantic before and during the CCAMLR 2000 Survey

Abstract A brief history of the commercial harvest and fishing patterns for krill (Euphausia superba) in the Atlantic sector (Area 48) of the Southern Ocean is presented, with an emphasis on the commercial activities of the krill fishery at the time of the CCAMLR 2000 Survey. During the period of the CCAMLR 2000 Survey, commercial krill fishing activities were centered at the South Shetland Islands in the southern Scotia Sea (Subarea 48.1). Fishing patterns, catches, catch rates, and biological information are also presented for the Japanese stern trawler Chiyo maru No. 5, which conducted krill fishing operations in the CCAMLR Subarea 48.1 from 31 January to 1 March, 2000. Information on length and maturity composition is summarized for five regions adjacent to the South Shetland Islands where the Chiyo maru No. 5 fished. The biological information collected from the commercial fishery agrees well with the findings of the CCAMLR 2000 Survey, as well as with a regional acoustic survey conducted by the US AMLR in Subarea 48.1 from February 22 to March 7, 2000. The relationship between the commercial and survey information emphasizes the value of collecting fine-scale biological information from commercial krill fisheries in the Southern Ocean.

The signature of river- and wind-borne materials exported from Patagonia to the southern latitudes: a view from REEs and implications for paleoclimatic interpretations

Riverine and wind-borne materials transferred from Patagonia to the SW Atlantic exhibit a homogeneous rare earth element (REE) signature. They match well with the REE composition of Recent tephra from the Hudson volcano, and hence this implies a dominance of material supplied by this source and other similar Andean volcanoes. Due to the trapping effect of proglacial and reservoir lakes, the larger Patagonian rivers deliver to the ocean a suspended load with a slightly modified Andean signature, that shows a REE composition depleted in heavy REEs. In this paper we redefine Patagonia as a source of sediments, which is in contrast with other sources located in southern South America. Quaternary sediments deposited in the northern and, to a lesser extent, in the southern Scotia Sea, and most of the dust in ice cores of east Antarctica have REE compositions very similar to the loess from Buenos Aires Province and to Patagonian eolian dust. However, we rule out Buenos Aires province as a Holocene major source of sediments. Similarly to Buenos Aires loess (a proximal facies), it is likely that the REE compositions of most sediment cores of the Scotia Sea and Antarctica reflect a distal transport of dust with an admixed composition from two main sources: a major contribution from Patagonia, and a minor proportion from source areas containing sediments with a clear upper crustal signature (e.g., western Argentina) or from Bolivia’s Altiplano. Evidence indicates that only during the Last Glacial Maximum, Patagonian materials were the predominant sediment source to the southern latitudes.

Provenance and transport of terrigenous sediment in the south Atlantic Ocean and their relations to glacial and interglacial cycles: Nd and Sr isotopic evidence

Sr and Nd isotopic compositions of Late Quaternary surface sediment and sediment cores from the south Atlantic and southeast Pacific sectors of the Southern Ocean are used to constrain the provenance and transport mechanisms of their terrigenous component. We report isotopic and mineralogical data for core samples from three localities, the Mid-Atlantic Ridge at 41°S and the northern and southern Scotia Sea. In addition, data for surface sediment samples from the south Atlantic and southeast Pacific sectors of the Southern Ocean are presented. The variations of Sr and Nd isotopic compositions of the bulk sediment samples in all cores were correlated with the magnetic susceptibility of the sediment and with the inferred glacial–interglacial stages. The isotopic data indicate that, during glacial periods, sediment was delivered from continental crust with a shorter residence time than that supplying material during interglacial periods. At the core site near the Mid-Atlantic Ridge, Nd isotopic, combined with mineralogical evidence indicates interglacial period deposition of a relatively high amount of kaolinite and silt with low ϵ Nd values < −8. The material was probably supplied by North Atlantic Deep Water from low latitudes. For glacial periods, a high contribution of silt and clay with ϵ Nd > −4.5, probably derived from southern South America, was indicated. The glacial–interglacial shift in sources may be due to either a decreasing influence of North Atlantic Deep Water during glacial times or by a larger contribution of glaciogenic detritus from southern South America. At the core site in the northern Scotia Sea, sediment of interglacial periods is dominated by smectite with ϵ Nd < − 6 and silt with ϵ Nd > −4. We suggest that smectite was derived from the Falkland shelf and silt was derived from the Argentinian shelf. During glacial periods, the Argentinian shelf was an important source for silt and chlorite with ϵ Nd > −4. The contribution from the Falkland shelf seems to have remained similar during glacial and interglacial periods. Hydrographic transport by bottom currents and turbidites could account for the high glacial detrital flux. An evaluation of the significance of an aeolian contribution to deep sea sediment suggests that it plays only a minor role. In the southern Scotia Sea, the Antarctic Peninsula is considered an important source for young material with ϵ Nd > −4, in particular during glacial periods. During interglacial periods, sediment supply from the Antarctic Peninsula was lower than during glacial times, resulting in a relatively high contribution of old material (ϵ Nd < −8) from East Antarctica. Deep water currents and icebergs could account for the transport of the old component to the southern Scotia Sea. The accumulation rates of material from the various source regions for glacial times are in agreement with an increase in the strength of the Antarctic Circumpolar Current. The production rate and the circulation pattern of bottom water in the Weddell Sea appear to have remained similar over most of the last 150 kyr.

Terrigenous sediment supply in the Scotia Sea (Southern Ocean): response to Late Quaternary ice dynamics in Patagonia and on the Antarctic Peninsula

Geochemical and mineralogical compositions of modern and Late Quaternary marine sediments from the Scotia Sea trace sources and transport paths of terrigenous sediment. We discuss downcore variations of compositional data of two sediment cores from the northern and southern Scotia Sea that correlate with fluctuations in magnetic susceptibility. Sediments were derived from very different sources at both localities, as revealed by contrasting clay-mineral assemblages. However, a common feature is the input of more basic and undifferentiated crustal material with the potential of high magnetic susceptibility during glacial periods, indicated by variable quartz/feldspar ratios and major, trace and rare earth elements. Terrigenous sediments mainly originate from nearby terrestrial sources or are introduced through interbasinal sediment transfer from adjacent seas. The observed temporal compositional variations have to be attributed to changes in the relative detrital contributions from the diverse source areas. Ice-mass extensions in southern Patagonia, on the Antarctic Peninsula and adjacent islands likely control the supply of glaciogenic detritus to the open ocean during times of glacial expansion, diluting the sediment input of interbasinal origin. Current transport is mainly responsible for sediment dispersal to the pelagic Scotia Sea and may amplify the glaciological source signals during glacial climate periods, because of a stronger wind forcing of the Antarctic Circumpolar Current.

The Osservatorio Geofisico Sperimentale marine magnetic surveys in the Antarctic Seas

About 40 000 km of marine magnetic and gradiometric data have been collected during eight geophysical surveys conducted since the Austral summer 1987/1988 in the circum-antarctic seas, by the research vessel OGS-Explora. For the most surveyed areas (Ross Sea, Southwestern Pacific Ocean, and Southern Scotia Sea), the analysis of the acquired data have contributed to clarify important aspects of their geological structure and tectonic evolution. The main scientific results, obtained combining other available geophysical data (multichannel seismic profiles and satellite-derived data), will be briefly illustrated.

Climatic Control on Variations of Terrigenous Particle Provenance and Transport in the Scotia Sea (Southern Ocean): Evidence from Nd, Pb, and Sr Isotopes

Terrigenous matter constitutes a considerable proportion of total sediment accumulation in Late Quaternary deposits of the Atlantic sector of the Southern Ocean. Input of lithogenic debris from land to sea, mainly to the shelf, is driven by glaciogenic supply from Antarctica, the river systems of South America, and wind transport through the southern westerlies. Deep and bottom water circulation most effectively control the distribution of terrigenous particles in the pelagic realm (Petschick et al., 1996). Variations of terrigenous sediment accumulation rates, grain size properties, and mineralogical composition, as inferred from sediment core studies, support the important role of ocean currents as major particle carriers throughout the late Quaternary (Diekmann et al., 1996; Diekmann and Kuhn, 1997). Clues exist for increased long-distance influxes of air-borne dust from Patagonia to both the Southern Ocean (Kumar et al., 1995) and Antarctica (Petit et al., 1990) during cold climatic stages. A Patagonian provenance of dust particles in ice cores from East Antarctica has been proposed from 143Nd/144Nd isotope data (Basile et al., 1997). This finding documents the importance of an atmospheric pathway during glacial periods, which has also been inferred from deep-sea sediments of the South Atlantic (Kumar et al., 1995). The authors postulated that the increase in primary productivity in the Southern Ocean during glacial periods has resulted from a relief of today's iron limitation by a greater supply of iron-rich aeolian dusts from dry areas of Patagonia. Nevertheless, their interpretation may be challenged because the aeolian dust component of the deep-sea sediments has not been isolated and analysed. Significant dust deposition in deep-sea sediments probably occurred in the southern Scotia Sea (sediment core location PS2319), indicated by down-core fluctuations of grain size parameters and magnetic susceptibility (Fig. 1). The latter may be correlated with dust records in Antarctic ice cores (Hofmann, in press). Maxima of magnetic susceptibility correspond to dust peaks in ice cores. Similar variations of magnetic susceptibility appear in sediment core PS2515 from the northern Scotia Sea (Fig. 1). However, little is known about provenance and proportions of primary aeolian dust components deposited directly on the sea floor. In order to investigate the hypothesis of an increased supply of aeolian dust into the Southern Ocean during glacial times, we have analysed the Nd, Pb, and Sr isotope compositions in bulk samples from sediment cores PS2515 and PS2319 of the Scotia Sea located at 53~ 45~ and 60~ 43~ respectively. Both sediment cores span the climate cycles of the last 140 ka, representing marine isotope stages 1 through 6 (Figs. 1). The isotope data of bulk sediments delineate systematic down-core variations in accordance with signatures of magnetic susceptibility (Fig. 1). In core intervals representing cold stages (marked by maxima of magnetic susceptibility) Nd and Sr isotopes exhibit a higher proportion of a mantle-

Trophic structure in open waters of the marginal ice zone in the Scotia-Weddell confluence region during spring (1983)

The structure of the food web was investigated in open waters adjacent to the marginal ice zone in the southern Scotia Sea in spring 1983. Diets were defined for dominant zooplankton, micronekton, and flying seabird species and then aggregated by cluster analysis into feeding groups. Most zooplankton were omnivorous, feeding on phytoplankton, protozoans, and in some cases, small metazoans (copepods). Only two species were found to be exclusively herbivorous:Calanoides acutus andRhincalanus gigas. Micronekton were carnivores with copepods being the dominant prey in all their diets. The midwater fishElectrona antarctica was the dominant food item in seven of the nine seabird species examined. Cephalopods, midwater decapod shrimps and carrion were also important in the diets of a few seabird species. Comparison (cluster analysis) of diets in spring with other seasons (winter, fall) indicated that over half the species examined (18 of 31) had similar diets in all seasons tested. The significant intraspecific shifts in diet that did occur were attributable to regional, seasonal, and interannual effects. A scheme is presented that describes the major energetic pathways through the open water ecosystem from phytoplankton to apex predators. At the base are phytoplankton and protozoans which are the principal food resource for the biomass copepods and krill. Krill and the biomass copepods are the principal forage of the midwater fishElectrona antarctica which, in turn, is the central diet component of flying seabirds as well as important food for the Antarctic fur seal and cephalopods. Krill are a major diet element for the fur seal and cephalopods, and the principal food of the minke whale.

Community structure and trophic ecology of zooplankton in the scotia sea marginal ice zone in winter (1988)

Zooplankton community structure and trophic ecology were investigated in the marginal ice zone of the Southern Scotia Sea during the austral winter of 1988 as part of the AMERIEZ program. In the study area, near the South Orkneys (34–49°W, 57–61.5°S), water emerging from the Weddell Sea mixed with Scotia Sea water to form a complex field of mesoscale eddies and meanders. Three primary zooplankton communities were identified: a shallow cold water assemblage typical of Weddell Sea water; a shallow/upper mesopelagic assemblage of subantarctic species introduced into the southern Scotia Sea with warm core eddies from the Polar Front; and a deep (>400 m) mesopelagic community with circumtantartic species. Zooplankton numerical dominants were cyclopoid copepods (mostly Oithona spp.); biomass dominants were four calanoid copepods, Calanoides acutus, Rhincalanus gigas, Calanus propinquus and Metridia gerlachei. Calanoides acutus and R. gigas were undergoing winter diapause in the mesopelagic zone (>400 m) whereas M. gerlachei and C. propinquus were distributed throughout the water column. In the subantarctic eddies M. gerlachei was replaced by M. lucens as a dominant and R. gigas was more abundant than in cold water to the south. Diet analysis of 35 dominant species revealed five feeding guilds among the zooplankton. One consisted primarily of small herbivorous copepods. Two closely related guilds consisting of copepods, krill and salps were omnivorous, feeding on phytoplankton, protozoans, metazoans and crustacean debris (molts). Two guilds were constituted by predatory copepods and chaetognaths. A sixth group, all copepods, which included important numerical and biomass dominants of the region, was trophically inactive. A comparison of gut fullness between winter 1988 and fall 1986 revealed that all but two of 19 small particle grazing species had significantly less food in their guts in winter. The exceptions were the biomass dominants C. propinquus and M. gerlachei, which were actively feeding in winter. Phytoplankton biomass was low in winter, averaging two-thirds that in fall. Several major trophic pathways from phytoplankton through apex predators (birds and mammals) are proposed for winter in the southern Scotia Sea with zooplankton playing a central role in each.

The Weddell-Scotia marginal ice zone: Physical oceanographic conditions, geographical and seasonal variability

Physical oceanographic conditions were measured in the Weddell and Scotia Sea marginal ice zones (MIZ's) during 1983, 1986 and 1988. The field work encompassed spring, autumn and mid-winter periods and included retreating, advancing and steady-state ice edges. Observed upper ocean structures, which typify MIZ's and reflect input of low salinity water from melting ice, included low salinity upper layers, lenses and fronts. An upper mixed layer was always present and was generally more fully developed in autumn and winter than at other times of year. Conditions in the deeper waters reflected regional oceanographic processes and significant differences were present between the Weddell and Scotia seas. Weddell Sea Water is a major source of water for the southern Scotia Sea, however, the upper Scotia Sea was dominated by warmer, less saline waters from Drake Passage. The colder, denser Weddell water appeared to have mixed isopycnally with deeper water in the Scotia Sea, present there at depths exceeding 500 m. The Scotia Sea was dominated by strong gradients and energetic mesoscale features, with currents exceeding 50 cm/s. The northwestern Weddell Sea had, in contrast, current speeds well below about 5 cm/s and small to negligible lateral water property gradients. Our observations suggest that the Weddell western boundary current was weaker than has been estimated in the past. In addition, we found scant evidence of deep winter convection in the Scotia Sea, a process which has been hypothesized in the past to contribute to deep water formation. No evidence was found during winter 1988 in the Scotia Sea of the modified water known as Weddell-Scotia Confluence water.

The stratification and water masses at Drake Passage

The waters at Drake Passage are known to be separated horizontally into four distinct zones of relatively small geostrophic shear by three fronts in which the shear and eastward flow are large. The two northern fronts, Subantarctic Front and Polar Front, that bound the Polar Frontal Zone comprise most of the transport of the Antarctic Circumpolar Current. Vertical sections of closely spaced (∼50 km) oceanographic stations made across Drake Passage in the summers of 1975 and 1976 produced quality measurements of temperature, salinity, oxygen, silicate, nitrate, and phosphate. Based on these data, a traditional description of characteristic distributions is presented. Good sampling resolution in the vertical allows maxima in hydrostatic stability to be traced laterally (approximately along isopycnals) between individual stations. Eleven strata of maximum stability are identified. Most vertically separate distinct water masses. The most pronounced stability stratum is associated with the seasonal pycnocline; north of the Polar Front, it separates the Subantarctic Surface Waters from Subantarctic Mode Water. To the south of the Polar Front the seasonal pycnocline is found slightly above the temperature minimum characteristic of the Antarctic Surface Waters in austral summer. This stability stratum extends northward near the base of the Subantarctic Mode Water. Another stability stratum extending across the entire passage forms the upper boundary for the Circumpolar Deep Water. South of the Polar Front, this stratum is at the base of the Antarctic Surface Water; to the north it is the lower boundary of the Antarctic Intermediate Water. Although Subantarctic Mode Water and Antarctic Intermediate Water are separated by a stability stratum north of the Polar Front, there is no such separation between Antarctic Intermediate Water and the temperature minimum of the Antarctic Surface Water south of the front. This suggests that Subantarctic Mode Water and Antarctic Intermediate Water are distinct and that the latter derives from the surface waters south of the Polar Front. At greater depths a horizontally discontinuous stratum is found between the Upper and Lower Circumpolar Deep Water. Two horizontally continuous stability strata are found near the base of the Circumpolar Deep Water, marking the transition to a high‐silicate variety of denser deep water from the Southeast Pacific Basin. The densest water mass found is Weddell Sea Deep Water, which flows through a gap in the South Scotia Ridge and along the southern Scotia Sea to fill the trough in the southern Drake Passage. It is overlain by a laterally continuous stratum of relative maximum stability.

Regional Relationships between Development and Growth in Larvae of Antarctic Krill, Euphausia Superba, From Field Samples

ABSTRACT Larvae of Euphausia superba were sampled during January-March 1981 in three Antarctic areas: the Scotia Sea (twice), waters near Elephant Island in the South Shetlands (twice), and Bransfield Strait. January-February larvae in the Scotia Sea averaged 250/m3 and 99% were calyptopes, dominated by stage 1. The oldest stages, Furcilia 3 and 4, had grown at slow rates, determined by comparisons of body lengths with larvae from other areas. Progressively younger stages reflected better growth, associated with a contemporary highly variable distribution of chlorophyll. Most rapid growth was inferred in a low density of larvae south of Elephant Island in March, where chlorophyll was at highest observed levels. Intermediate growth had occurred in larvae from north and east of Elephant Island, and Bransfield Strait, where chlorophyll concentrations were intermediate. During March, larvae averaged 3-6 stages older than during January-February. Abundances were 100–500/m3 along 250 km in the same area sampled in January. To the north and east of Elephant Island, principal modes in larval abundances were at younger stages (by 2-4 stages, respectively, = about 16-32 days of growth) than near the South Orkneys. An intermediate region showed an intermediate mode, implying intermediate timing of peak recruitment. Inferred development times of about 8 days per calyptopis and furcilia stage are shorter than previous estimates. In Scotia Sea areas having the highest densities of larvae in March, there were numerous developmental forms (variants) of furcilia stages 1 and 2, without clearly dominant forms. This evidence of indirect pathways during larval development and of slow growth was borne out by body-length measurements. Chlorophyll was near depletion in these areas of high densities of larvae. Data from earlier years indicate that absence of dominant forms in Furciliae 1 and 2 during March in the southern Scotia Sea may occur regularly, possibly contributing to the substantial range of body size of older year classes. Pathways of larval development observed in March were direct in coastal areas, compared with oceanic, in spite of variable ambient chlorophyll. Extremes in growth rate, population density, and food seem to interrelate, whereas intermediate states do not.

Antarctic Polar Front Zone in the Western Scotia Sea—Summer 1975

Abstract The component of the FDRAKE-75 data obtained by the R/V Conrad in the western Scotia Sea reveals a definite sequence of thermohaline stratification zones encountered on passing from Antarctic to Sub-antarctic waters. A Polar Front Zone, displaying multiple temperature minima, separates the Antarctic Zone, characterized by a single intense T-min above 200 m, from the Subantarctic Zone with its nearly isohaline layer from 100 m to over 400 m. The Antarctic Zones of the Weddell and the Scotia Seas aye separated by a cold, relatively homogeneous zone situated in the southern Scotia Sea called the Weddell-Scotia Confluence. The boundaries of the Polar Front Zone are highly meandered and isolated eddies of Subantarctic water may occur within the zone. The main axis of the Antarctic Circumpolar Current apparently lies close to the Subantarctic boundary of the, Polar Front Zone, while a secondary axis is associated with the southern limit of the Polar Front Zone. Inspection of the Islas Orcadas and Melvi...