Ross Sea Area Research Articles

Cytogenetic diversity of notothenioid fish from the Ross sea: historical overview and updates

Cytogenetics provides a unique platform to study in situ structural, functional, and evolutionary aspects of the genome. As such it holds powerful promise in decoding mechanisms and processes of genome architectural changes and their role in organism’s diversification and evolution. Since the early 80s, such an approach has been applied to the study of the Antarctic notothenioid fishes. In almost three decades, the cytogenetic information has expanded to cover half of the known species inhabiting the high Antarctic waters. Although started 10 years later, cytogenetic studies of species from the Ross sea region have provided valuable contributions to this bulk of knowledge. Here, we synthesize the currently available cytogenetic information on Antarctic notothenioid fishes from the Ross Sea Region, inclusive of both conventional karyotyping and gene mapping. In addition, new karyotypic data on four species (Lepidonotothen squamifrons, Trematomus scotti, T. loennbergii, and T. lepidorhinus) are provided. In discussing these data, specific focus is made on the patterns and subtleties of cytogenetic diversity at inter- and intra-specific levels aiming at contributing to the refinement of the knowledge of fish diversity in a region, the Ross Sea area, whose primary ecological value is widely recognized.

Anomalous South Pacific lithosphere dynamics derived from new total sediment thickness estimates off the West Antarctic margin

Paleotopographic models of the West Antarctic margin, which are essential for robust simulations of paleoclimate scenarios, lack information on sediment thickness and geodynamic conditions, resulting in large uncertainties. A new total sediment thickness grid spanning the Ross Sea–Amundsen Sea–Bellingshausen Sea basins is presented and is based on all the available seismic reflection, borehole, and gravity modeling data offshore West Antarctica. This grid was combined with NGDC's global 5 arc minute grid of ocean sediment thickness (Whittaker et al., 2013) and extends the NGDC grid further to the south. Sediment thickness along the West Antarctic margin tends to be 3–4km larger than previously assumed. The sediment volume in the Bellingshausen, Amundsen, and Ross Sea basins amounts to 3.61, 3.58, and 2.78millionkm3, respectively. The residual basement topography of the South Pacific has been revised and the new data show an asymmetric trend over the Pacific–Antarctic Ridge. Values are anomalously high south of the spreading ridge and in the Ross Sea area, where the topography seems to be affected by persistent mantle processes. In contrast, the basement topography offshore Marie Byrd Land cannot be attributed to dynamic topography, but rather to crustal thickening due to intraplate volcanism. Present-day dynamic topography models disagree with the presented revised basement topography of the South Pacific, rendering paleotopographic reconstructions with such a limited dataset still fairly uncertain.

High-resolution mineral dust and sea ice proxy records from the Talos Dome ice core

Abstract. In this study we report on new non-sea salt calcium (nssCa2+, mineral dust proxy) and sea salt sodium (ssNa+, sea ice proxy) records along the East Antarctic Talos Dome deep ice core in centennial resolution reaching back 150 thousand years (ka) before present. During glacial conditions nssCa2+ fluxes in Talos Dome are strongly related to temperature as has been observed before in other deep Antarctic ice core records, and has been associated with synchronous changes in the main source region (southern South America) during climate variations in the last glacial. However, during warmer climate conditions Talos Dome mineral dust input is clearly elevated compared to other records mainly due to the contribution of additional local dust sources in the Ross Sea area. Based on a simple transport model, we compare nssCa2+ fluxes of different East Antarctic ice cores. From this multi-site comparison we conclude that changes in transport efficiency or atmospheric lifetime of dust particles do have a minor effect compared to source strength changes on the large-scale concentration changes observed in Antarctic ice cores during climate variations of the past 150 ka. Our transport model applied on ice core data is further validated by climate model data. The availability of multiple East Antarctic nssCa2+ records also allows for a revision of a former estimate on the atmospheric CO2 sensitivity to reduced dust induced iron fertilisation in the Southern Ocean during the transition from the Last Glacial Maximum to the Holocene (T1). While a former estimate based on the EPICA Dome C (EDC) record only suggested 20 ppm, we find that reduced dust induced iron fertilisation in the Southern Ocean may be responsible for up to 40 ppm of the total atmospheric CO2 increase during T1. During the last interglacial, ssNa+ levels of EDC and EPICA Dronning Maud Land (EDML) are only half of the Holocene levels, in line with higher temperatures during that period, indicating much reduced sea ice extent in the Atlantic as well as the Indian Ocean sector of the Southern Ocean. In contrast, Holocene ssNa+ flux in Talos Dome is about the same as during the last interglacial, indicating that there was similar ice cover present in the Ross Sea area during MIS 5.5 as during the Holocene.

Ross Sea deep-ocean and epipelagic microzooplankton during the summer-autumn transition period

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 67:123-137 (2012) - DOI: https://doi.org/10.3354/ame01588 Ross Sea deep-ocean and epipelagic microzooplankton during the summer-autumn transition period Karl A. Safi1,*, Karen V. Robinson2, Julie A. Hall3, Jill Schwarz3, Elizabeth W. Maas3 National Institute of Water and Atmospheric Research, 1PO Box 11-115, Hamilton, 2PO Box 8602, Christchurch, and 3Private Bag 14901, Wellington, New Zealand *Email: k.safi@niwa.co.nz ABSTRACT: Microzooplankton populations are key participants in the transfer, recycling and export of carbon in epipelagic waters, but little is known about their role, abundance and diversity at depth, especially in the Antarctic region. We surveyed microzooplankton populations and their potential prey in the Ross Sea area from 66° S to 77° S during the New Zealand IPY-CAML survey, 12 February to 11 March 2008. Samples were taken throughout the available water column at depths between ~5 and 3400 m, allowing us to compare deeper, largely unknown, mesopelagic and bathypelagic waters with the traditionally studied epipelagic zone. Microzooplankton diversity and abundance were highest in the high chlorophyll a, diatom-dominated waters of the epipelagic zone, but occasionally, peaks did occur in the upper mesopelagic before populations declined with depth. Ciliates and dinoflagellates declined more rapidly than heterotrophic nanoflagellates, which led to the latter dominating at depth. Ciliate populations in epipelagic waters were correlated with bacteria, picophytoplankton and chlorophyll a, but these potential prey did not correlate well with heterotrophic nanoflagellates in deeper or low-biomass waters. Despite their rapid decline with depth, due to the large volume of deeper waters, the total integrated microzooplankton biomass exceeded that found in the epipelagic zone. This indicates that the deeper waters of the Ross Sea, at this time, contained a significant pool of microzooplankton biomass. These deeper populations are likely to aid in the recycling and remineralisation of sinking phytoplankton biomass traditionally thought to be exported to the ocean floor. KEY WORDS: Microzooplankton · Protists · Mesopelagic · Bathypelagic · Epipelagic · Ross Sea Full text in pdf format PreviousNextCite this article as: Safi KA, Robinson KV, Hall JA, Schwarz J, Maas EW (2012) Ross Sea deep-ocean and epipelagic microzooplankton during the summer-autumn transition period. Aquat Microb Ecol 67:123-137. https://doi.org/10.3354/ame01588 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 67, No. 2. Online publication date: October 02, 2012 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2012 Inter-Research.

A double bottom simulating reflector in the western Ross Sea, Antarctica

[1] We present seismic evidence of the presence of gas hydrates and free gas in Victoria Land Basin (western Ross Sea, Antarctica). The occurrence of gas hydrates is inferred from a bottom simulating reflector (BSR), the first identified in the Ross Sea area. The gas presence was analyzed through targeted reprocessing of the multichannel seismic reflection data (3000 m streamer, 120 channels, 60-fold) acquired in 1990 by the Italian R/V OGS-Explora. The seismic data were reprocessed in order to increase the signal-to-noise ratio by adopting a true-amplitude approach, allowing a subsequent amplitude variations with offset (AVO) analysis to extract information on both P and S wave reflectivity. Along a profile from the deeper basin to an intrabasin structural high, the BSR evolves into crosscutting reflectors (CCRs) and enhanced amplitude reflectors (EARs). Locally, another seismic event is interpreted to be a second BSR (BSR0), identified about 150 m below the first BSR and is possibly related to the presence of a mixture of gases comprising methane and hydrocarbons with heavier molecular weight and therefore associated with a deeper hydrate stability zone. The presence of faults controls the upward gas migration, connecting the free gas zone below the BSR and the mud volcanoes and pockmarks on the seafloor. The modeling of the hydrate equilibrium zone shows that the observed BSR/BSR0 could be stable at the present pressure-temperature conditions.

On a new Antarctic species of Symplectoscyphus Marktanner-Turneretscher, 1890 (Cnidaria, Hydrozoa, Sertulariidae), with an annotated checklist of the Antarctic species of the genus

A new Antarctic species of the genus Symplectoscyphus Marktanner-Turneretscher is described and figured, and its position amongst the allied Antarctic species of the genus is discussed. The material was collected from the Ross Sea area (Antarctica) during the BioRoss survey to the western Ross Sea and Balleny Islands in 2004. Symplectoscyphus frondosus, sp. nov., is unique amongst the Antarctic known species of the genus in having large, erect, rigid, muchbranched, bottlebrush-shaped stems. An annotated checklist of the Antarctic species of the genus is also included. Sixteen of the 19 known Antarctic species are endemic to the Antarctic Region (84%), two are also present in subAntarctic waters and only one has a wider distribution.

Atmospheric chemistry of an Antarctic volcanic plume

We report measurements of the atmospheric plume emitted by Erebus volcano, Antarctica, renowned for its persistent lava lake. The observations were made in December 2005 both at source, with an infrared spectrometer sited on the crater rim, and up to 56 km downwind, using a Twin Otter aircraft; with the two different measurement platforms, plume ages were sampled ranging from <1 min to as long as 9 h. Three species (CO, carbonyl sulfide (OCS), and SO2) were measured from both air and ground. While CO and OCS were conserved in the plume, consistent with their long atmospheric lifetimes, the downwind measurements indicate a SO2/CO ratio about 20% of that observed at the crater rim, suggesting rapid chemical conversion of SO2. The aircraft measurements also identify volcanogenic H2SO4, HNO3 and, recognized for the first time in a volcanic plume, HO2NO2. We did not find NOx in the downwind plume despite previous detection of NO2 above the crater. This suggests that near‐source NOx was quickly oxidized to HNO3 and HO2NO2, and probably NO32−(aq), possibly in tandem with the conversion of SO2 to sulfate. These fast processes may have been facilitated by “cloud processing” in the dense plume immediately downwind from the crater. A further striking observation was O3 depletion of up to ∼35% in parts of the downwind plume. This is likely to be due to the presence of reactive halogens (BrO and ClO) formed through heterogeneous processes in the young plume. Our analysis adds to the growing evidence for the tropospheric reactivity of volcanic plumes and shows that Erebus volcano has a significant impact on Antarctic atmospheric chemistry, at least locally in the Southern Ross Sea area.

Quasi-parabolic reflecting bottom surfaces of the Drygalski Antarctic floating ice tongue

Very high frequency deep radio sounding systems for ice thickness measurements are practically the only useful apparatuses for large scale radar flight surveys in polar regions. The morphology of the bottom surface of an Antarctic floating ice tongue, in the Ross Sea area, East Antarctica, was studied using the arrival times of signal echoes of the radio sounding system. The amplitude variations of radar signals from the reflecting surface were analyzed to determine the gain or the loss of the reflectors. Such surfaces show quasi-parabolic geometrical shapes at the ice/water interface with both concave and convex faces towards the sounding system. Electromagnetic analysis performed on radar echoes indicates that amplitude variations detected by the antenna are focusing or defocusing effects only due to the reflector's shape. A factor in the radar equation that represents the surface shape when coherent reflectors are involved is introduced. This factor allows us to determine more precisely the morphology and electromagnetic characteristics of the interface between the media investigated by means of radio echo sounding.

Methanesulphonic acid (MSA) stratigraphy from a Talos Dome ice core as a tool in depicting sea ice changes and southern atmospheric circulation over the previous 140 years

Firn core methanesulphonic acid (MSA) stratigraphy from Talos Dome (East Antarctica) was compared with anomalies of the satellite-measured sea ice extent (1973–1995) in the Ross Sea and Wilkes Land oceanic sector. In spite of the sparseness of sea ice data, the MSA maxima fit with many positive sea ice anomalies in the Ross Sea. This evidence suggests that marine biogenic activity enhanced by large sea ice cover is an important, but not exclusive, factor in controlling MSA concentration in snow precipitation at Talos Dome. Other than source intensity, differences in regional atmospheric transport mechanisms affect the arrival of MSA-rich aerosol at Talos Dome. To clarify the role of transport processes in bringing biogenic aerosol to Talos Dome, a spectral analysis was applied to the MSA, SOI (South Oscillation Index), and SAM (Southern Annular Mode) record. Synchronicity or phase shift between the chemical signature and atmospheric circulation modes were tested. The variations in the MSA profile have a periodicity of 6.9, 4.9, 3.5, and 2.9 years. The 6.9 and 2.9 year periodicities show a strong positive correlation and are synchronous with corresponding SOI periodicity. This variability could be related to an increase in MSA source intensity (by dimethylsulphide from phytoplanktonic activity) linked to the sea ice extent in the Ross Sea area, but also to an increased strength in transport processes. Both of these factors are correlated with La Niña events (SOI positive values). Furthermore, SAM positive values are related to an increased sea ice extent in the Ross Sea sector and show two main periodicities 3.3 and 3.8 years. These periodicities determine the MSA variability at 3.5 years. However, the effect of intensification of the polar vortex and the consequent reduction in transport process intensity, which reduce the delivery of air masses enriched in MSA from oceanic areas to Talos Dome, make the effect of the SAM on the MSA concentration at Talos Dome less active than the SOI. In this way, snow deposition at the Talos Dome records larger MSA concentration by the combined effects of increased source emissions and more efficient transport processes. The MSA record from Talos Dome can therefore be considered a reliable proxy of sea ice extent when the effect of changes in transport processes in this region of Antarctica is considered. Over the previous 140 years, these conditions occur with a periodicity of 6.9 years.

Horizontal and vertical distribution and demography of euphausiids in the Ross Sea and its adjacent waters in 2004/2005

The horizontal and vertical distribution and population structure of euphausiids in the Ross Sea and its adjacent waters were investigated during the summers of 2004/2005 using stratified towed samples. Nine species of euphausiids occurred in the survey area. Among them, Euphausia triacantha was dominant in biomass north of the southern boundary of the Antarctic circumpolar current (SB). Thysanoessa spp. was widely distributed north of the continental slope, while E. superba was distributed from the SB to the slope, where it showed the highest biomass. Juvenile E. superba was distributed offshore near the SB and remained at the surface, but gravid females were dominant in the slope and mainly occurred in the middle layers (400–600 m). Adult and juvenile E. crystallorophias were found at 200–300 m in the colder water of the continental shelf. In general, the peak biomass of euphausiids was found in the mid layers of the Ross Sea area. The life span and the number of spawns for major species are also discussed.

Evidence of calcium carbonates in coastal (Talos Dome and Ross Sea area) East Antarctica snow and firn: Environmental and climatic implications

Micrometre-sized aeolian dust particles stored in Antarctic firn and ice layers are a useful tool for reconstructing climate and environmental changes in the past. The mineral content, particle concentration and chemical composition of modern dust in firn cores from the peripherycal dome (Talos Dome) and coastal area of East Antarctica (Ross Sea sector) were investigated. During analyses there was a considerable decrease in microparticle concentrations within a few hours of ice sample melting, accompanied by a systematic increase in the concentration of calcium ions (Ca 2+) in solution. Based on mineralogical phase analyses, which reveal the presence of anhydrous and hydrous calcium carbonates such as calcite (CaCO 3), monohydrocalcite (CaCO 3·H 2O) and ikaite (CaCO 3·6H 2O, hexahydrate calcium carbonate), the observed variations in concentrations are ascribed to the partial dissolution of the carbonate content of samples. Soluble carbonate compounds are thus primary aerosols included into the samples along with insoluble aluminosilicate minerals. We hypothesize hydrous carbonates may derive from the sea ice surface, where ikaite typically forms at the early stages of sea ice formation. Back trajectory calculations show that favourable events for air mass advection from the sea ice surface to Talos Dome are rare but likely to occur.

Breaking molds: Oswaldella laertesi, sp. nov., a unique Antarctic species of Oswaldella Stechow, 1919 (Cnidaria: Hydrozoa: Kirchenpaueriidae)

A new species of the Antarctic genus Oswaldella Stechow is described and figured, and its position amongst the remaining species of the genus is discussed. The material was collected from the Ross Sea area (Antarctica) during the BioRoss survey of the western Ross Sea and Balleny Islands in 2004. Oswaldella laertesi, sp. nov., is unique amongst known species of the genus in having hydrocladia arranged in three longitudinal rows.

Variation among females in egg lipid content and developmental success of echinoderms from McMurdo Sound, Antarctica

Egg lipid and protein content of different females of Antarctic echinoderms in McMurdo Sound, Antarctica, were measured to assess variation among females and developmental success. Egg triacylglycerol content of the Antarctic sea urchin Sterechinus neumayeri, when less than 70 ng, correlated with embryos that failed to develop past the 4-day-old blastula stage. In contrast asteroids (Odontaster meridionalis, O. validus, Acodontaster hodgsoni) all produced eggs that developed normally, even with variable egg lipid content. This difference might be related to dietary sources for more herbivorous sea urchins compared to more omnivorous and predatory asteroids. Low egg lipid content, with resulting poor embryonic survivorship, suggests that herbivorous sea urchins may be under unusual levels of nutritional stress in McMurdo Sound during the time frame studied (2004–2005). This nutritional stress might be related to the presence of large icebergs, known to have reduced primary production in the Ross Sea area.

A Description of Seven Antarctic Marine Gymnamoebae Including a New Subspecies, Two New Species and a New Genus: Neoparamoeba aestuarina antarctica n. subsp., Platyamoeba oblongata n. sp., Platyamoeba contorta n. sp. and Vermistella antarctica n. gen. n. sp.

Seven marine gymnamoebae were isolated from different environments of seawater, slush (pack ice meltwater), and sediment in the Ross Sea area of Antarctica. All amoebae were isolated and maintained at temperatures below 4 degrees C. Growth, rate of locomotion, and general morphology were observed at an environmentally appropriate temperature (1 degrees C) and at room temperature (approximately 25 degrees C). Molecular (srDNA sequences) and microscopical techniques were used to identify the gymnamoebae and establish their phylogenetic affinities. Three isolates (S-131-2, SL-200, and W4-3) were assigned to a psychrophilic subspecies of Neoparamoeba aestuarina, N. aestuarina antarctica n. subsp., one isolate (S-205) was assigned to a new species of Platyamoeba, P. oblongata n. sp., two isolates (W51C#4 & W51C#5) were also assigned to a new species of Platyamoeba, P. contorta n. sp., and one isolate (S-241) was a novel psychrophilic gymnamoeba Vermistella antarctica n. gen. n. sp. Molecular and morphological results revealed that V. antarctica was not related to any described family of gymnamoebae. Strains S-205, W51C#4, and W51C#5 were capable of locomotion at room temperature, while strains SL-200, S-131-2, W4-3, and S-241 exhibited locomotion only below approximately 10 degrees C. Our results imply that the Antarctic environment is host both to cosmopolitan gymnamoebae that have acquired adaptations for existence at low environmental temperature and to apparently novel psychrophilic amoebae described here for the first time.

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.

Clay mineral fluctuations and surface textural analysis of quartz grains in Pliocene–Quaternary marine sediments from Wilkes Land continental rise (East-Antarctica): Paleoenvironmental significance

Pliocene–Quaternary marine sediments from Wilkes Land continental rise were investigated using clay mineral analysis and quartz grain surface texture observations. The studied piston cores were collected during a geophysical and geological survey in the framework of the Italian and Australian WEGA Project. The cores were collected along two transects crossing the channel-mound system that characterizes the Wilkes Land continental rise. Massive and laminated facies were recognized in these sedimentary sequences. The clay mineral assemblages are dominated by illite. Smectite and chlorite occur in intermediate amounts, whereas kaolinite is scarce. Illite derives from the hinterland of the Mertz and Ninnis glaciers. Smectite and kaolinite derive from the Ross Sea area and they are transported by westward marine bottom currents. Chlorite is derived both from the adjacent continent and from the Victoria Land. Different concentrations of individual clay minerals in the massive and laminated facies are controlled by the interplay between different source areas and the main sedimentary processes. Glacial and eolian quartz grains prevail in the massive units indicating short transportation and rapid deposition from melting ices, with no further significant transport action by marine currents. In the laminated facies the high amount of quartz grains affected by high-energy subaqueous transport features are related both to turbiditic processes and to bottom current transport.

Ross Sea Ice Motion, Area Flux, and Deformation

Abstract The sea ice motion, area export, and deformation of the Ross Sea ice cover are examined with satellite passive microwave and RADARSAT observations. The record of high-resolution synthetic aperture radar (SAR) data, from 1998 and 2000, allows the estimation of the variability of ice deformation at the small scale (∼10 km) and to assess the quality of the longer record of passive microwave ice motion. Daily and subdaily deformation fields and RADARSAT imagery highlight the variability of motion and deformation in the Ross Sea. With the passive microwave ice motion, the area export at a flux gate positioned between Cape Adare and Land Bay is estimated. Between 1992 and 2003, a positive trend can be seen in the winter (March–November) ice area flux that has a mean of 990 × 103 km2 and ranges from a low of 600 × 103 km2 in 1992 to a peak of 1600 × 103 km2 in 2001. In the mean, the southern Ross Sea produces almost twice its own area of sea ice during the winter. Cross-gate sea level pressure (SLP) gradients explain ∼60% of the variance in the ice area flux. A positive trend in this gradient, from reanalysis products, suggests a “spinup” of the Ross Sea Gyre over the past 12 yr. In both the NCEP–NCAR and ERA-40 surface pressure fields, longer-term trends in this gradient and mean SLP between 1979 and 2002 are explored along with positive anomalies in the monthly cross-gate SLP gradient associated with the positive phase of the Southern Hemisphere annular mode and the extrapolar Southern Oscillation.

226Ra and Ba concentrations in the Ross Sea measured with multicollector ICP mass spectrometry

Analysis of 226Ra concentrations in natural samples has been limited by the low efficiency of counting techniques, or by the time-consuming nature of thermal ionization mass spectrometry (TIMS). In this study, we have developed and tested the use of multiple ion-counting plasma source mass spectrometry to provide a fast and effective analytical technique for 226Ra. We use a Nu Instruments mass spectrometer to collect 226Ra and the spike isotope, 228Ra, synchronously, thereby maximising ion yields. This approach requires assessment of the gain of the ion counters and of mass discrimination, both of which are achieved using a U standard before and after analysis. Molecular interferences in the Ra spectrum must also be assessed. It is shown that, when the torch region is contaminated with Ba, significant interferences (∼100 cps) at +225.8 and 227.8 amu are observed. When clean of this Ba interference, however, the Ra spectrum has a very low background so that rapid and accurate measurement of 226Ra concentration is possible. We use this new technique to measure 226Ra concentrations in three water column profiles from the Ross Sea area for which Ba concentrations were also measured. 226Ra concentrations range from 0.36 to 0.42 fmol/kg, and Ba concentrations from 78.2 to 90.3 nmol/kg. The 226Ra/Ba ratio is constant at 4.7×10 -9 mol/mol, within error of surface water values measured by TIMS. The constancy of the 226Ra/Ba in the Ross Sea, at a value typical for the open ocean, argues against addition of significant Ra or Ba by meltwater or groundwater from the Antarctic continent. This result makes the dating of carbonates from the area using 226Ra/Ba possible. The lack of surface depletion in either Ba or 226Ra differs from the pattern observed in the open Southern Ocean where concentrations increase with depth, suggesting that either the calcite-dominated productivity in the Ross Sea area does not remove 226Ra and Ba as effectively as opal-dominated productivity in the Southern Ocean, or that water column mixing in the Ross Sea is effective at the time scale of removal.

Sensitivity analysis of a robust diagnostic general circulation model of the Ross Sea

The work aims to explore the sensitivity response of a robust-diagnostic model of the Ross Sea area to different forcings and boundary conditions. Numerical experiments using different open boundary velocities, wind stress fields, constraints for sea surface elevation, objectively analyzed or constant ad hoc thermohaline fields with different relaxation coefficients are proposed. Results show that realistic patterns and velocities in the basin are induced mainly by the wind forcing, while its variability can explain part of the modulation of the current field observed but does not affect significantly the Ross island recirculation, mainly controlled by the thermohaline field. The in–out flow below the Ross Ice Shelf (RIS) is influenced by the assumptions adopted for the Sea Surface Elevation along the shelf itself, while the wind variability can explain the exchange between the RIS cavity and the sea. Even if the model has to be improved, especially for what concerns the sea–ice interactions that are not present in this version, comparisons between current meters moorings data averages and reconstructed fields show that features can be explained by wind stress variability, while circulation patterns are in good agreement with experimental observations and other theoretical studies.

Life history stages of Pyramimonas tychotreta (Prasinophyceae, Chlorophyta), a marine flagellate from the Ross Sea, Antarctica

SUMMARY During a summer cruise to the Ross Sea (Antarctica) areas of snow-covered sea ice were red-coloured due to high concentrations of the recently described Pyramimonas tychotret aDaugbjerg. Light microscopy of living material revealed that the population was comprised of quadriflagellate motile cells and thick-walled cysts. The red colour was due to large numbers of secondary carotenoid-containing granules, positioned in the periphery of motile cells and cysts. Mature cysts also contained numerous starch grains and lipid droplets. Cells from a red-coloured field sample turned green overnight as the secondary carotenoids disappeared when cells were placed in low light conditions. The sample then exhibited the typical grass-green colour of motile cells observed in water samples from the area. Under reduced light motile cells showed strong positive phototaxis. The encystment process involved the asexual transformation of quadriflagellate cells into cysts. A single type of square cyst scale, with perforated floors and walls, replaced the body scales of motile cells. A marked extension, often ending in a hook was at each corner of the cyst scales. Germinating cysts produced four motile cells. Electron microscopy showed the cyst wall to be tri-layered, with a thin, electrondense inner layer, a thick middle layer and a thin outer layer. Sea ice samples with dense populations of motile cells and cyst stages also contained elongate uniflagellate cells. These cells were covered with box scales, foot-print scales, an underlayer of pentagonal scales, limuloid scales and flagellar hair scales identical to those present on the quadriflagellate stage. We tentatively suggest that the uniflagellate stage represents a gamete and its presence implies the occurrence of sexual reproduction. Although, fusion of gametes was not observed, a biflagellate cell with a larger volume was seen which may have been a zygote. How this stage fits into of the life history remains to be explained.