Western Ross Sea Research Articles

Isotopic constraints on the provenance of fine-grained sediment in LGM tills from the Ross Embayment, Antarctica

The major element, trace element and Nd, Sr, and Pb isotopic compositions were determined for the < 63 μm sediment fraction of fourteen onshore tills and twenty-one offshore tills in the Ross Sea and surrounding areas, Antarctica. The purpose was to determine the provenance of tills deposited on the continental shelf of the Ross Sea during the Last Glacial Maximum (LGM) and to assess the relative importance of the West and East Antarctic ice sheets in providing ice to the Ross Embayment during this time. Onshore tills exposed around the margins of the Ross Embayment associated with specific ice streams and outlet glaciers have Nd and Pb compositions that vary regularly with geographic position, reflecting changes in the age and isotopic compositions of basement rocks along the Ross Sea margin. Specifically, till ε Nd values range from − 5.6 to − 9.9 for the West Antarctic (Siple Coast) ice streams and the southern Transantarctic Mountains, while the ε Nd in the central portions of the mountains range from − 11.9 to − 14.9. Uranogenic Pb becomes less radiogenic in the same direction. The composition of Late Quaternary tills from the Ross Sea also varies geographically, with the < 63 μm fraction from both the western and eastern Ross Sea tills having higher ε Nd values (− 5.8 to − 7.5 and − 3.8 to − 6.9, respectively) than the till in the central Ross Sea (− 7.1 to − 12.5). Tills in the eastern Ross Sea (most proximal to West Antarctica) have isotopic and chemical compositions identical to onshore till from West Antarctica and the southern Transantarctic Mountains and were most likely derived from these areas. Tills in the western Ross Sea with the highest ε Nd values have low wt.% SiO 2 (46%), and high Ni, Nb, Ta and Sr contents requiring a large component of material eroded from extrusive igneous rocks from the adjacent Late Cenozoic McMurdo Volcanic Field. In contrast, tills in the central Ross Sea were most likely related to ice emanating from the central Transantarctic Mountains that flowed north across the Ross Embayment to the continental shelf margin. Overall, our data provide further direct evidence for the model described in a previously published companion paper [K.J. Licht, J.R. Lederer, J.R. Swope, Provenance of LGM glacial till (sand fraction) across the Ross Embayment, Antarctica, Quat. Sci. Rev. 24 (2005) 1499–1520.] that ice derived from the East Antarctic ice sheet was active and responsible for LGM till deposition throughout the western half of the Ross Sea (including the central Ross Sea). As a result, models for the past and future dynamics of the Antarctic ice sheets must account for a major East Antarctic contribution to the Ross ice sheet.

COMPETITION AMONG PENGUINS AND CETACEANS REVEALS TROPHIC CASCADES IN THE WESTERN ROSS SEA, ANTARCTICA

An apparent trophic cascade that appears during summer in the western Ross Sea, Antarctica, explains why the Antarctic silverfish (Pleuragramma antarcticum) there becomes cannibalistic; its principal prey, crystal krill (Euphausia crystallorophias) becomes scarce; and the diatom community is minimally grazed compared to adjacent areas. The krill is the major grazer of diatoms. On the basis of fieldwork at Ross Island, we suggest that the cascade results from foraging by unusually numerous Adélie Penguins (Pygoscelis adeliae), minke whales (Balaenoptera bonaerensis), and fish-eating killer whales (Orcinus orca). These species and other top predators apparently deplete the krill and silverfish. In drawing our conclusions, we were aided by two "natural experiments." In one "experiment," large, grounded icebergs altered the seasonal pattern of change in regional sea-ice cover, but not the seasonal change in penguin diet and foraging behavior that was also detected during the pre-iceberg era. In the other "experiment," a short-term polynya (opening in the ice) brought penguins and whales together in a confined area, this time altering both penguin diet and foraging behavior. We conclude that the foraging of penguins and whales, and not a formerly hypothesized seasonal decrease in sea-ice cover, explains (1) the annual switch in the penguins' prey from krill to silverfish, (2) the subsequent lengthening of penguin foraging trips, and (3) a marked decline of cetaceans in the area later in the season. Reduction in the middle-trophic-level prey is expressed in the relaxed grazing pressure on phytoplankton.

Effects of Flipper Bands on Foraging Behavior and Survival of Adélie Penguins (Pygoscelis Adeliae)

Abstract Since the 1950s, flipper bands have been used widely to mark penguins (Spheniscidae), but not without concerns regarding possible negative effects on survival and fitness. As part of a demographic study of Adélie Penguins (Pygoscelis adeliae) in the western Ross Sea, Antarctica, we investigated effects of flipper bands on foraging-trip duration and food loads, as well as apparent survival, during four breeding seasons (2000-2003), using mark-recapture and radio-frequency identification (RFID) technology. Foraging-trip durations were ∼8% (3.5 h) longer, on average, for banded compared with unbanded birds, but the effect varied among years. Food loads did not differ between banded and unbanded birds, but males carried heavier food loads than females. Flipper bands decreased apparent annual survival by 11–13% during 2000-2003, but over a longer time period (1996-2003) we observed high annual variability, including years of high survival for banded birds. Males had slightly higher survival than females in both banded and unbanded birds. Mechanisms resulting in band effects on foraging behavior and survival, the variable effect of bands by season, and the potential ameliorating effect of age or experience on the effects of bands need further investigation in Adélie and other penguin species. We recognize a need to understand and balance the negative consequences of flipper bands for penguins against the beneficial gains in information associated with their use. Effets du Baguage à l'Aileron sur le Comportement de Recherche Alimentaire et la Survie de Pygoscelis adeliae

Antarctic Bottom Water production and export by tides in the Ross Sea

Current measurements in the western Ross Sea show that Antarctic Bottom Water is produced at the outer edge of the Continental Shelf by tidal stirring of Antarctic Surface Water, Circumpolar Deep Water and Shelf Water. Current amplitudes during spring tides are greater than 1 m/s, sufficient to carry Circumpolar Deep Water onto the shelf from offshore and stir at least the lower portion of the water column to a nearly homogeneous mixture. The Antarctic Bottom Water produced every day during spring tides does not appear to have a seasonal signal. Its export rate is estimated to be 1.95 ± 1.85 × 106 m3/s.

Gametogenesis and reproductive strategies in some species of the Antarctic fish genus Trematomus (Nototheniidae) from Terra Nova Bay, Ross Sea

Species of the genus Trematomus are the most common nototheniids in the High-Antarctic Zone, and several live sympatrically on the Ross Sea continental shelf. Histological analysis of gonadal development of T. bernacchii, T. hansoni and T. newnesi was carried out on samples collected during the austral summer in the coastal waters of Terra Nova Bay, western Ross Sea. Gonads of both females and males of the three species were described based on macroscopic and histologic scales. Histologically, both males and females of T. bernacchii were in postspawning or in early maturing condition, thus indicating that they recently completed spawning. Conversely, most specimens of T. hansoni were in spawning condition, as evidenced by the histological appearance of gonads and by the high value of gonado-somatic index (GSI) of females between 20 and 36%. In the same period, females of T. newnesi had maturing ovaries, whereas males were at an early developmental stage of maturity. In all species, the testicular structure closely resembled the unrestricted spermatogonial type, although spermatogonia were more concentrated in the blind end of tubules. On the basis of the pattern of oocyte maturity observed within the ovarian lamellae, all Trematomus had a group synchronous ovary, as commonly reported in Antarctic fish. Furthermore, the presence of oocytes showing the commencement of yolk deposition in spent females confirms that vitellogenesis is a slow process, extending over at least 1 year. The reproductive strategies of Trematomus are discussed in terms of timing of the spawning season of each species as a function of the seasonal production cycle and of the interspecific relationships. The spawning activities of the three species investigated appeared to be mismatched with respect to time as well as location within the range of distribution of each.

Expansion and rapid retreat of the West Antarctic Ice Sheet in eastern Ross Sea: possible consequence of over-extended ice streams?

Marine geological data from the eastern Ross Sea provide compelling evidence that the West Antarctic Ice Sheet extended to the continental shelf break during the last glacial maximum. Swath bathymetry records show mega-scale glacial lineations within each of the four large glacial troughs on the shelf. These features extend virtually uninterrupted across the shelf and have sharp lateral boundaries. Sediment cores that sampled the lineations recovered diamicton that is interpreted as deformation till. The combined data indicate streaming ice. The great extent of the paleo-ice streams (up to 1000 km in length) is attributed to the fact that they flowed across unconsolidated sedimentary strata. Geomorphic features that are known to form at the retreating grounding line, such as recessional moraines and ridges, are restricted to the southeastern portion of the shelf. Iceberg furrows that occur at depths of up to 650 m overprint mega-scale lineations. These deep furrows are interpreted as having been formed by icebergs calved from the grounding line of the ice sheet, which indicates the absence of a fringing ice shelf. Sediment cores sampled a thin layer of glaciomarine sediments resting sharply on till. Sub-ice shelf deposits, similar to those sampled in the western Ross Sea, were not recovered in the eastern Ross Sea. These combined results imply rapid retreat of the ice sheet from the shelf. Crosscutting lineations with different orientations indicate changes in flow direction that probably occurred as the ice profile was lowered and the physiography of the sea floor across which the ice sheet was retreating changed. Grounding zone wedges vary in number and location within individual troughs, which indicates diachronous retreat of ice streams. Due to a lack of carbonate material for radiocarbon dating, all radiocarbon ages were derived from acid insoluble organic (AIO) fractions. These dates indicate early retreat of the ice sheet relative to the western Ross Sea and other areas of west Antarctica. However, the older dates must be viewed as suspect, due to a paucity of diatomaceous material and contamination from old carbon. While we were not successful at constraining the timing of ice sheet retreat from the continental shelf, the data do indicate that the ice sheet was grounded on the shelf during the LGM.

Broad-scale factors influencing the biodiversity of coastal benthic communities of the Ross Sea

Early ecological research in McMurdo Sound revealed local spatial gradients in community structure associated with variations in anchor ice disturbance, fast ice and snow cover, and the effects of predators. Research contrasting the east and west sides of McMurdo Sound has shown major differences in benthic communities, which have been attributed to oceanographic influences on the advection of water-column productivity and the frequency of fast ice break-out. Despite these regional and local differences, coastal benthic communities in McMurdo Sound show a high level of stability, and contain a variety of large and potentially very long-lived species. In Terra Nova Bay, about half way along the Victoria Land Coast of the western Ross Sea, the coastal benthic communities provide some insightful contrasts with those in McMurdo Sound. For example, the abundance and depth distribution of dominant species such as Sterechinus neumayeri and Adamussium colbecki are markedly different from McMurdo Sound. In both locations communities dominated by large sponges are most prolific in regions that are free from iceberg disturbance of the seabed. A recent assessment of northern Victoria Land coastal benthic communities, in conjunction with multibeam imagery of the seafloor, further highlights the importance of iceberg disturbance in structuring Antarctic benthic communities. A comparative synthesis of these coastal ecological studies enables us to generate hypotheses concerning the relative importance of different environmental drivers in structuring benthic communities. Overlain on the regular latitudinal shifts in physical factors such as light regime, are regional fluctuations that are controlled by atmospheric and oceanographic circulation patterns and coastal topography/bathymetry. Change in diversity along the western coast of the Ross Sea is predicted to be influenced by three main factors (1) ice disturbance (e.g., via anchor ice and advection of supercooled water or icebergs), (2) photosynthetically available radiation (affected by ice and snow cover and water clarity), (3) the locations of polynyas and advection of planktonic production and larvae. Interactions between these factors are expected to result in non-linear changes along the latitudinal gradient. While predictions generated from these hypotheses remain to be rigorously tested, they provide indications of how benthic communities may respond to changes in production, disturbance and the stability of coastal sea ice.

The Role of the Atmospheric Circulation in the Record Minimum Extent of Open Water in the Ross Sea in the 2003 Austral Summer

The contribution of the atmospheric circulation to a record minimum extent of open water in the polar Ross Sea (RS) region in the 2003 austral summer is examined. Two major findings are reached in this study. The first is that the origins of this anomaly are more complex than previously thought, with an anomalous atmospheric circulation contributing at least as much to the lack of open water as damming of sea ice by a large iceberg known as C‐19. Only in the western RS, where C‐19 lay, is damming found to restrict open water in the spring of 2002 (October‐December), but even here the coldest spring in the last 15 years extended the sea‐ice formation season. Elsewhere in the RS the divergent northward ice drift that normally occurs widely reversed to southward in early spring and was then followed by negligible ice motion. The most anomalous springtime ice drift occurred in the central and eastern RS rather than near C‐19 and was mirrored in the weakest southerly winds on record in central areas. The unusual southward ice drift in early spring 2002 caused widespread convergence and compaction of the normally thin and undeformed first‐year ice along and north of the central and eastern Ross Ice Shelf (RIS). Compacted ice with few leads would have been slow to melt in the warmest summer (January‐February) months. Direct observations also indicate that sea surface temperatures (SSTs) rapidly fell to freezing in the central and western RS in February 2003 supporting new ice formation. All the available data indicate these were due to the cold spring and the extensive, compact ice cover in the late spring of 2002 and in January 2003 preventing most of the incoming solar radiation from reaching the ocean. The second finding is that sea ice in the summer of 2003 would have been very extensive even if C‐19 had not occurred. This is based on comparisons with other years of extensive summer ice, notably one when no large iceberg occurred. Ice motion and atmospheric circulation patterns in this case resembled those in the spring of 2002 and the summer of 2003. Evidence that the anomalous atmospheric circulation in the RS in the spring of 2002 was typical of El Niño events is also discussed.

Opposing oceanic and atmospheric ENSO influences on the Ross Sea Region, Antarctica

Abstract. Here we discuss the cause and effect of opposing atmospheric and oceanic ENSO forcings in the Ross Sea, that lead to a net warming in the eastern Ross Sea and a net cooling in the western Ross Sea during El Niño years. During La Niña years the opposite is observed. The oceanic ENSO effect causes a ~1 K warming with a 3 month lag during El Niño years in comparison to La Niña time periods. During El Niño events, the atmospheric ENSO effect leads to a shift and weakening of the Amundsen Sea Low, causing enhanced import of colder West Antarctic air masses into the western Ross Sea. We find that this indirect ENSO effect is about one order of magnitude stronger (up to 15 K) in the western Ross Sea than the direct effect (~1 K), leading to a net cooling during El Niño and net warming during La Niña events.

EFFECTS OF FLIPPER BANDS ON FORAGING BEHAVIOR AND SURVIVAL OF ADÉLIE PENGUINS (PYGOSCELIS ADELIAE)

Abstract Since the 1950s, flipper bands have been used widely to mark penguins (Spheniscidae), but not without concerns regarding possible negative effects on survival and fitness. As part of a demographic study of Adelie Penguins (Pygoscelis adeliae) in the western Ross Sea, Antarctica, we investigated effects of flipper bands on foraging-trip duration and food loads, as well as apparent survival, during four breeding seasons (2000-2003), using mark-recapture and radio-frequency identification (RFID) technology. Foraging-trip durations were ∼8% (3.5 h) longer, on average, for banded compared with unbanded birds, but the effect varied among years. Food loads did not differ between banded and unbanded birds, but males carried heavier food loads than females. Flipper bands decreased apparent annual survival by 11–13% during 2000-2003, but over a longer time period (1996-2003) we observed high annual variability, including years of high survival for banded birds. Males had slightly higher survival than femal...

PALYNOLOGY OF THE NBP03-01A TRANSECT IN THE NORTHERN BASIN, WESTERN ROSS SEA, ANTARCTICA: A LATE PLIOCENE RECORD

Abstract Fifty-seven samples taken from ten piston cores collected along a transect off the continental margin of the Northern Basin, Ross Sea, Antarctica were analyzed for palynomorphs. Moderately diverse assemblages of marine microplankton and terrestrial palynomorphs were recovered. The palynomorph assemblages have been subdivided into two main groups: the in-situ flora (including acritarchs, dinoflagellate cysts, leiospheres and prasinophyte algae mainly composed of cymatiosphaerids), and the reworked flora (including dinoflagellate cysts, pollen and spores). The leiospheres are the most abundant palynomorphs. This prominence in the relative abundance of leiospheres has been reported as typical of assemblages found today at the limit between seasonal and pack ice in the Arctic. In-situ dinoflagellate cysts are sparse. They are mainly represented by Lejeunecysta, which, based on species similarities to those from Cape Roberts, are believed to be of Oligocene to Pliocene age. All other dinoflagellate cy...

Variability of dense water formation in the Ross Sea

This paper presents results from a model study of the interannual variability of high salinity shelf water (HSSW) properties in the Ross Sea. Salinity and potential temperature of HSSW formed in the western Ross Sea show oscillatory behaviour at periods of 5–6 and 9 years superimposed on long-term fluctuations. While the shorter oscillations are induced by wind variability, variability on the scale of decades appears to be related to air temperature fluctuations. At least part of the strong decrease of HSSW salinities deduced from observations for the period 1963–2000 is shown to be an aliasing artefact due to an undersampling of the periodic signal. While sea ice formation is responsible for the yearly salinity increase that triggers the formation of HSSW, interannual variability of net freezing rates hardly affects changes in the properties of the resulting water mass. Instead, results from model experiments indicate that the interannual variability of dense water characteristics is predominantly controlled by variations in the shelf inflow through a sub-surface salinity and a deep temperature signal. The origin of the variability of inflow characteristics to the Ross Sea continental shelf can be traced into the Amundsen and Bellingshausen Seas. The temperature anomalies are induced at the continental shelf break in the western Bellingshausen Sea by fluctuations of the meridional transport of circumpolar deep water with the eastern cell of the Ross Gyre. In the Amundsen Sea, upwelling due to a persistently cyclonic wind field carries the signal into the surface mixed layer, leading to fluctuations of the vertical heat flux, anomalies of brine release near the sea ice edge, and consequently to a sub-surface salinity anomaly. With the westward flowing coastal current, both the sub-surface salinity and deep temperature signals are advected onto the Ross Sea continental shelf. Convection carries the signal of salinity variability into the deep ocean, where it interacts with modified circumpolar deep water upwelled onto the continental shelf as the second source water mass of HSSW. Sea ice formation on the Ross Sea continental shelf thus drives the vertical propagation of the signal rather than determining the signal itself.

Trace metals distributions in coastal sea ice of Terra Nova Bay, Ross Sea, Antarctica

In an attempt to clarify the release of trace elements from the seasonal coastal sea ice, samples were periodically collected in a nearshore station inside the Gerlache Inlet (Terra Nova Bay, Western Ross Sea), during the summer 2000/01 and analysed for dissolved and particulate cadmium, copper, iron, manganese and lead, as well as salinity, suspended particulate matter, nutrients and phytoplankton pigments. In order to provide insight on the metal association with the particles included in the sea ice, the metal solid speciation was also investigated. Both vertical distributions within the ice cores and temporal variations at the seawater interface were studied, in an effort to fully characterize the system and correlation among the considered parameters. Concentrations and speciation patterns clearly indicate metal incorporation within the annual sea ice due to resuspension of sediments, followed by release of particulate metals during melting as a primary process affecting trace metal availability in the Antarctic coastal waters.

Provenance of LGM glacial till (sand fraction) across the Ross embayment, Antarctica

The provenance of Late Quaternary Ross Embayment till was investigated by comparing the coarse sand composition of East and West Antarctic source area tills with till samples from across the Ross Sea. The West Antarctic samples from beneath the Whillans (B) and Kamb (C) ice streams are petrologically distinct from samples of lateral moraines flanking several East Antarctic outlet glaciers. The characteristic assemblage of four West Antarctic samples includes felsic intrusive and detrital sedimentary lithic fragments, plagioclase and abundant quartz. In contrast, most of the ten East Antarctic till samples contains abundant mafic intrusive and detrital sedimentary lithic fragments as well as less abundant quartz. The distinctive composition of these source areas can be linked to 33 samples from 20 cores of Last Glacial Maximum (LGM) age till distributed across the Ross Sea. Western Ross Sea till samples exhibit mineralogic and lithological similarities to East Antarctic till samples, although these western Ross Sea tills contain higher percentages of felsic intrusive and extrusive lithic fragments. Eastern Ross Sea till samples are compositionally similar to West Antarctic till, particularly in their abundance of quartz and dearth of mafic and extrusive lithic components. Central Ross Sea till exhibits compositional similarities to both East and West Antarctic source terranes including a mafic lithic component, and marks the confluence of ice draining from East and West Antarctica during the LGM, thus West Antarctic-derived ice streams did not advance into the western Ross Sea. This indicates that even if pre-LGM equivalents of the present Siple Coast ice streams existed, they did not simply expand allowing West Antarctic-derived ice to dominate the LGM Ross Ice Sheet.

Seismic stratigraphy of the Plio-Pleistocene Ross Island flexural moat-fill: a prognosis for ANDRILL Program drilling beneath McMurdo-Ross Ice Shelf

Ross Island volcanic complex began forming with the emplacement of the basaltic shield volcanoes of Mt. Bird and Mt. Terror between ca. 4.6 and 1.3 Ma, though it has developed most significantly over the last 1 Ma during an eruptive phase resulting in the 3794-m-high composite vent of Mt. Erebus. Throughout this time, loading of the lithosphere at the southern end of the Terror Rift by the Ross Island volcanic pile has progressively depressed the crust, resulting in a subcircular flexural moat around the periphery of the island. Multichannel seismic reflection data collected from the McMurdo-Ross Ice Shelf (MRIS) reveal the stratigraphic architecture of the moat-fill on the southeastern side of Ross Island. The moat region has accommodated a well-stratified, regionally extensive sedimentary succession of at least 1.2 km below the seafloor in the deepest part of the depression. Three seismic stratigraphic units are identified that generally thicken and dip towards Ross Island and are bounded by angular (onlap) unconformities. We infer that the three units were deposited in accommodation space created during discrete phases of volcanic load-induced subsidence: 1. Unit III. Moderate to low-amplitude discontinuous reflectors are dislocated and tilted by normal faulting and interpreted to represent coarse-grained glacigenic and fine-grained marine sediments with likely intercalated volcanic ash. These strata may have started to accumulate during loading of the crust by Mt. Bird between ca. 4.6 and 3.8 Ma. 2. Unit II. Moderate to high-amplitude continuous reflectors that onlap Unit III and are interpreted to represent coarse-grained glacigenic and fine-grained marine sediments with likely intercalated volcanic ash. These strata are inferred to have accumulated in the crustal depression resulting from loading by Mt. Terror between ca. 1.8 and 1.3 Ma. 3. Unit I. Relatively continuous low-amplitude to seismically-opaque reflectors (Unit IB), onlap Unit II and grade upwards into moderate to high amplitude reflectors below the seafloor (Unit IA). Unit IB is interpreted to represent fine-grained pelagic marine sediments and volcanic ashes which may have accumulated during a phase of rapid subsidence at the initiation of Mt. Erebus loading between ca. 1.0 and 0.8 Ma. Unit IA is interpreted to represent progressive infilling of the flexural moat with more coarse-grained, probably glacigenic sediments during continuation of Mt. Erebus loading between ca. 0.8 and 0.2 Ma. These phases of load-induced subsidence may have periodically overdeepened the moat, making it likely that the sedimentary fill is relatively continuous and has largely escaped erosion by ice grounding during past glacial expansions of the West Antarctic Ice Sheet (WAIS) and the MRIS. The age relationships provided by radiometric dating of the various volcanic loads imply that the sediments here have the potential to provide a relatively continuous and high-resolution glacimarine record of the past behavior of the WAIS–MRIS and the climate history of the western Ross Sea region back to ∼5 Ma. This stratigraphic record is scheduled to be drilled by the ANDRILL Program in the austral summer of 2006. From the interpretation of seismic facies, we present a stratigraphic prognosis for the proposed drill site.

Magnetostratigraphic chronology of a late Eocene to early Miocene glacimarine succession from the Victoria Land Basin, Ross Sea, Antarctica

Drilling offshore from Cape Roberts, Antarctica, has enabled recovery of a 1472-m cumulative record of late Eocene–early Miocene history of sedimentary basin development and climate change in the Western Ross Sea. In this paper, we synthesize the results of palaeomagnetic analyses carried out on the CRP-1, CRP-2 and CRP-3 sediment cores, and present a chronology for the recovered Eocene–Miocene succession. Stepwise demagnetization data demonstrate that secondary overprints have been successfully removed and that characteristic remanent magnetizations (ChRMs) have been clearly identified in most of the samples. A close sampling interval has allowed a detailed magnetic polarity stratigraphy to be established for the composite succession. Correlation with the geomagnetic polarity time scale (GPTS) has been constrained by a number of 40Ar/ 39Ar and 87Sr/ 86Sr ages, as well as by a recently developed Antarctic siliceous microfossil zonation, and by calcareous nannoplankton biostratigraphy. The basal sediments of the Eocene–Miocene succession rest unconformably on Devonian sandstones of the Beacon Supergroup. A basal sandstone breccia, which probably represents the onset of rifting in the Victoria Land Basin (VLB), is overlain by a succession of sandstones that are interbedded with thin conglomerate beds. These sediments give way to more clearly glacially influenced mudstones and diamictite facies in the mid Oligocene, and, by the Oligocene–Miocene boundary, coincident with the Mi-1 glaciation, a permanent glacial dominance was imprinted on the sedimentary record. Average sediment accumulation rates were initially rapid in the late Eocene–early Oligocene (up to 60 cm/k.y.), but reduced to only a few cm/k.y. in the early Miocene as basin subsidence slowed.

Energetic plumes over the western Ross Sea continental slope

Rapid descent of dense Drygalski Trough (western Ross Sea, Antarctica) shelf water over the continental slope, within 100 to 250 m thick benthic plumes, is described. Speeds of up to 1.0 m/s are recorded flowing at an average angle of 35° to the isobaths, entraining ambient Lower Circumpolar Deep Water en route. This process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor Antarctic Bottom Water. Nonetheless, a 4‐hour duration pulse of undiluted shelf water was observed at depth (1407 m) directly north of the Drygalski Trough, moving at around 90 degrees to isobaths, and at a speed of 1.4 m/s. Thus the export of Ross Sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths, punctuated by rapid downhill cascades.

Effects of summer ice coverage on phytoplankton assemblages in the Ross Sea, Antarctica

An oceanographic cruise was conducted in the Ross Sea (Antarctica) during summer 2001 as part of the Italian National Program for Antarctic Research (PNRA). Extensive areas of pack ice occurred over the Ross Sea, atypical for summer when offshore waters are normally free of ice. The present study focuses on the effects of increased ice coverage on phytoplankton assemblages. Water samples collected at various depths at 72 hydrographical stations in offshore and coastal waters were used to determine size-fractionated phytoplankton biomass as chlorophyll a (chla) concentrations, and HPLC photosynthetic pigments. For the offshore waters, the average chla concentration was 57.8 mg m−2, approximately three times the values recorded under ice-free conditions during summer 1996. In coastal waters, the average chla concentrations were 102 and 206 mg m−2 during January and February, respectively, i.e., up to 2.5 times those of 1996. Micro- and nano-phytoplankton size fractions made up about 90% of the phytoplankton biomass over the entire study area and were composed primarily of diatoms with a pico-phytoplankton fraction dominated by prymnesiophyceans. The broken pack and melting ice was strongly coloured by an extensive algal biomass suggesting that the phytoplankton was a result of seeding from ice algal communities. The Ross Sea considered to be one of the most productive areas of the Southern Ocean, had primary production values about four-fold those of other areas. The lengthening of the ice season observed in the Western Ross Sea, associated with a considerable increase in phytoplankton biomass as observed in summer 2001, would have a major impact on the trophic structure of the entire ecosystem, and presumably, also on carbon export.

Effects of summer ice coverage on phytoplankton assemblages in the Ross Sea, Antarctica

An oceanographic cruise was conducted in the Ross Sea (Antarctica) during summer 2001 as part of the Italian National Program for Antarctic Research (PNRA). Extensive areas of pack ice occurred over the Ross Sea, atypical for summer when offshore waters are normally free of ice. The present study focuses on the effects of increased ice coverage on phytoplankton assemblages. Water samples collected at various depths at 72 hydrographical stations in offshore and coastal waters were used to determine size-fractionated phytoplankton biomass as chlorophyll a (chla) concentrations, and HPLC photosynthetic pigments. For the offshore waters, the average chla concentration was 57.8mgm−2, approximately three times the values recorded under ice-free conditions during summer 1996. In coastal waters, the average chla concentrations were 102 and 206mgm−2 during January and February, respectively, i.e., up to 2.5 times those of 1996. Micro- and nano-phytoplankton size fractions made up about 90% of the phytoplankton biomass over the entire study area and were composed primarily of diatoms with a pico-phytoplankton fraction dominated by prymnesiophyceans.The broken pack and melting ice was strongly coloured by an extensive algal biomass suggesting that the phytoplankton was a result of seeding from ice algal communities.The Ross Sea considered to be one of the most productive areas of the Southern Ocean, had primary production values about four-fold those of other areas. The lengthening of the ice season observed in the Western Ross Sea, associated with a considerable increase in phytoplankton biomass as observed in summer 2001, would have a major impact on the trophic structure of the entire ecosystem, and presumably, also on carbon export.

Early life stages in the life cycle of Antarctic silverfish, Pleuragramma antarcticum in Terra Nova Bay, Ross Sea

The nototheniid Pleuragramma antarcticum (Boulenger, 1902), is the dominant pelagic fish in waters of the continental shelf in High Antarctic regions. Larvae and juveniles of this species comprise the majority of ichthyoplankton at many locations around Antarctica including the Weddell Sea and the western Ross Sea, where it may amount to 98% of the ichthyoplankton. Its life cycle has been the subject of a number of studies but spawning and embryological development are still uncertain. Eggs with embryos and newly hatched larvae of P. antarcticum were collected in November 2002 near the Italian Antarctic station at Terra Nova Bay through holes drilled in the sea ice. Eggs and yolk-sac larvae were floating among the platelet ice below the solid cap of congelation ice. Eggs were 2.2–2.5 mm in diameter and contained embryos at an advanced stage of development. Hatching occurred from mid-November onwards, and newly hatched larvae averaged 9.3 mm SL. This paper provides the detailed description of embryos and newly hatched larvae in terms of pigmentation pattern and morphometric characteristics, thus allowing a significant advance in our understanding of the early life history of P. antarcticum in the Ross Sea, and extending the knowledge of the life cycle of this key Antarctic species.