Weddell Sea Sector Research Articles

Late Holocene ice-flow reconfiguration in the Weddell Sea sector of West Antarctica

Here we report Late Holocene ice sheet and grounding-line changes to the Weddell Sea sector of West Antarctica. Internal radio-echo layering within the Bungenstock Ice Rise, which comprises very slow-flowing ice separating the fast-flowing Institute and Möller ice streams, reveals ice deformed by former enhanced flow, overlain by un-deformed ice. The ice-rise surface is traversed by surface lineations explicable as diffuse ice-flow generated stripes, which thus capture the direction of flow immediately prior to the creation of the ice rise. The arrangement of internal layers can be explained by adjustment to the flow path of the Institute Ice Stream, during either a phase of ice sheet retreat not longer than ∼4000 years ago or by wholesale expansion of the grounding-line from an already retreated situation not sooner than ∼400 years ago. Some combination of these events, involving uplift of the ice rise bed during ice stream retreat and reorganisation, is also possible. Whichever the case, the implication is that the ice sheet upstream of the Bungenstock Ice Rise, which currently grounds over a >1.5 km deep basin has been, and therefore may be, susceptible to significant change.

Reconstruction of changes in the Weddell Sea sector of the Antarctic Ice Sheet since the Last Glacial Maximum

The Weddell Sea sector is one of the main formation sites for Antarctic Bottom Water and an outlet for about one fifth of Antarctica's continental ice volume. Over the last few decades, studies on glacial–geological records in this sector have provided conflicting reconstructions of changes in ice-sheet extent and ice-sheet thickness since the Last Glacial Maximum (LGM at ca 23–19 calibrated kiloyears before present, cal ka BP). Terrestrial geomorphological records and exposure ages obtained from rocks in the hinterland of the Weddell Sea, ice-sheet thickness constraints from ice cores and some radiocarbon dates on offshore sediments were interpreted to indicate no significant ice thickening and locally restricted grounding-line advance at the LGM. Other marine geological and geophysical studies concluded that subglacial bedforms mapped on the Weddell Sea continental shelf, subglacial deposits and sediments over-compacted by overriding ice recovered in cores, and the few available radiocarbon ages from marine sediments are consistent with major ice-sheet advance at the LGM. Reflecting the geological interpretations, different ice-sheet models have reconstructed conflicting LGM ice-sheet configurations for the Weddell Sea sector. Consequently, the estimated contributions of ice-sheet build-up in the Weddell Sea sector to the LGM sea-level low-stand of ∼130 m vary considerably.In this paper, we summarise and review the geological records of past ice-sheet margins and past ice-sheet elevations in the Weddell Sea sector. We compile marine and terrestrial chronological data constraining former ice-sheet size, thereby highlighting different levels of certainty, and present two alternative scenarios of the LGM ice-sheet configuration, including time-slice reconstructions for post-LGM grounding-line retreat. Moreover, we discuss consistencies and possible reasons for inconsistencies between the various reconstructions and propose objectives for future research. The aim of our study is to provide two alternative interpretations of glacial–geological datasets on Antarctic Ice-Sheet History for the Weddell Sea sector, which can be utilised to test and improve numerical ice-sheet models.

Micromorphological and microchemical indicators of pedogenesis in Ornithogenic Cryosols (Gelisols) of Hope Bay, Antarctic Peninsula

Micromorphological investigations of Antarctic soils are comparatively scarce, however, they could help understand the genesis of cryogenic soils under extreme polar conditions and different biotic factors. In most areas of Maritime Antarctica, the soil structure is apparently influenced by the local lithology, cryoturbation, guano deposition and reaction of guano solutions. The present study was carried out in Hope Bay, in the northern part of the Antarctic Peninsula, possibly one of the oldest sites of penguin occupation in the region. We describe and analyze the most important micro and sub-microscopic features of selected Ornithogenic Cryosols (Gelisols) from this part of Antarctic Peninsula, representing a transitional climatic zone between the wetter South Shetlands and the much drier Weddell Sea sector. Nine pedons representing the main ornithogenic soils found in ice-free areas of Hope Bay were selected for the micromorphological study. Undisturbed soil blocks were sampled at different depths, ranging between 0 and 30cm. The microstructure and sub-microstructure were further investigated using a JEOL 8200 and a Zeiss scanning electron microscopes, both coupled with an microprobe (SEM/WDS). These pedons have a very limited surface accumulation of organic matter formed by mosses and lichens, changing abruptly to a mineral phosphatic horizon of bleached colours, and usually hardcemented by ice. A small to medium-sized granular structure is generally observed, with ovoidal, subrounded forms, including several well-defined ornithogenic materials, such as P-rich organic remains, nodular phosphates forms and minute fragments of bone apatite. The chemical composition of ornithogenic materials and phosphatic coatings indicates the presence of discrete forms of taranakite, minyulite, leucophosphite, struvite and fluorapatite, typical of phosphatization process in these soils. Phosphatization and enhanced chemical alteration of the substrate and is one of the main soil-forming process in ornithogenic soils, resulting in P-rich ovoidal aggregates formation. P-rich solutions penetrate desiccation fractures and cleavage planes in large clasts and react preferably with plagioclases. P reacts with Al and Fe to form various amorphous and crystalline P phases. Present day permafrost cementation of deeper phosphate layers indicate that warmer/wetter conditions occurred in the past, during which P was leached downwards and reacted with the rock substrates, developing stable ovoidal aggregates, now incorporated in ice-cemented subsurface horizon.

Steep reverse bed slope at the grounding line of the Weddell Sea sector in West Antarctica

The bed of the West Antarctic Ice Sheet is, in places, more than 1.5 km below sea level1, 2. It has been suggested that a positive ice-loss feedback may occur when an ice sheet’s grounding line retreats across a deepening bed1, 2, 3. Applied to the West Antarctic Ice Sheet, this process could potentially raise global sea level4 by more than 3 m. Hitherto, attention has focussed on changes at the Siple Coast5, 6, 7 and Amundsen Sea embayment8, 9, 10 sectors of West Antarctica. Here, we present radio-echo sounding information from the ice sheet’s third sector, the Weddell Sea embayment, that reveals a large subglacial basin immediately upstream of the grounding line. The reverse bed slope is steep, with about 400 m of decline over 40 km. The basin floor is smooth and flat, with little small-scale topography that would delay retreat, indicating that it has been covered with marine sediment5, 11 and was previously deglaciated. Upstream of the basin, well-defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin. Evidence so far suggests that the Weddell Sea sector of the West Antarctic Ice Sheet has been stable, but in the light of our data we propose that the region could be near a physical threshold of substantial change.

Marine geological constraints for the grounding-line position of the Antarctic Ice Sheet on the southern Weddell Sea shelf at the Last Glacial Maximum

The history of grounded ice-sheet extent on the southern Weddell Sea shelf during the Last Glacial Maximum (LGM) and the timing of post-LGM ice-sheet retreat are poorly constrained. Several glaciological models reconstructed widespread grounding and major thickening of the Antarctic Ice Sheet in the Weddell Sea sector at the LGM. In contrast, recently published onshore data and modelling results concluded only very limited LGM-thickening of glaciers and ice streams feeding into the modern Filchner and Ronne ice shelves. These studies concluded that during the LGM ice shelves rather than grounded ice covered the Filchner and Ronne troughs, two deep palaeo-ice stream troughs eroded into the southern Weddell Sea shelf. Here we review previously published and unpublished marine geophysical and geological data from the southern Weddell Sea shelf. The stratigraphy and geometry of reflectors in acoustic sub-bottom profiles are similar to those from other West Antarctic palaeo-ice stream troughs, where grounded ice had advanced to the shelf break at the LGM. Numerous cores from the southern Weddell Sea shelf recovered sequences with properties typical for subglacially deposited tills or subglacially compacted sediments. These data sets give evidence that grounded ice had advanced across the shelf during the past, thereby grounding in even the deepest parts of the Filchner and Ronne troughs. Radiocarbon dates from glaciomarine sediments overlying the subglacial deposits are limited, but indicate that the ice grounding occurred at the LGM and that ice retreat started before ∼15.1 corrected 14C kyrs before present (BP) on the outer shelf and before ∼7.7 corrected 14C kyrs BP on the inner shelf, which is broadly synchronous with ice retreat in other Antarctic sectors. The apparent mismatch between the ice-sheet reconstructions from marine and terrestrial data can be attributed to ice streams with very low surface profiles (similar to those of “ice plains”) that had advanced through Filchner Trough and Ronne Trough at the LGM. Considering the global sea-level low-stand of ∼130 m below present, a low surface slope of the expanded LGM-ice sheet in the southern Weddell Sea can reconcile grounding-line advance to the shelf break with limited thickening of glaciers and ice streams in the hinterland. This scenario implies that ice-sheet growth in the Weddell Sea sector during the LGM and ice-sheet drawdown throughout the last deglaciation could only have made minor contributions to the major global sea-level fluctuations during these times.

Inter-annual variations observed in spring and summer Antarctic sea ice extent in recent decade

The growth and decay of sea ice are complex processes and have important feedback onto the oceanic and atmospheric circulation. In the Antarctic, sea ice variability significantly affects the primary productivity in the Southern Ocean and thereby negatively influences the performance and survival of species in polar ecosystem. In present days, the awareness on the sea ice variability in the Antarctic is not as matured as it is for the Arctic region. The present paper focuses on the inter-annual trends (1999-2009) observed in the monthly fractional sea ice cover in the Antarctic at 1 × 1 degree level, for the November and February months, derived from QuikSCAT scatterometer data. OSCAT scatterometer data from India’s Oceansat-2 satellite were used to asses the sea ice extent (SIE) observed in the month of November 2009 and February 2010 and its deviation from climatic maximum (1979-2002) sea ice extent (CMSIE). Large differences were observed between SIE and CMSIE, however, trend results show that it is due to the high inter-annual variability in sea ice cover. Spatial distribution of trends show the existence of positive and negative trends in the parts of Western Pacific Ocean, Ross Sea, Amundsen and Bellingshausen Seas (ABS), Weddell Sea and Indian ocean sector of southern ocean. Sea ice trends are compared with long-term SST trends (1982-2009) observed in the austral summer month of February. Large-scale cooling trend observed around Ross Sea and warming trend in ABS sector are the distinct outcome of the study.

Antarctic topography at the Eocene–Oligocene boundary

We present a reconstruction of the Antarctic topography at the Eocene–Oligocene (ca. 34Ma) climate transition. This provides a realistic key boundary condition for modeling the first big Antarctic ice sheets at this time instead of using the present day bedrock topography, which has changed significantly from millions of years of tectonism and erosion. We reconstruct topography using a set of tools including ice sheet-erosion models, models of thermal subsidence and plate movement. Erosion estimates are constrained with offshore sediment volumes estimated from seismic stratigraphy. Maximum and minimum topographic reconstructions are presented as indicators of the range of uncertainty. Our results point to a significant upland area in the Ross Sea/Marie Byrd Land and Weddell Sea sectors. In addition, East Antarctic coastal troughs are much shallower than today due to the restoration of material that has been selectively eroded by the evolving ice sheets. Parts of East Antarctica have not changed since the E–O boundary because they were protected under non-erosive cold-based ice. The reconstructions provide a better-defined boundary condition for modeling that seeks to understand interaction between the Antarctic ice sheet and climate, along with more robust estimates of past ice volumes under a range of orbital settings and greenhouse gas concentrations.

Glacial/interglacial ice-stream stability in the Weddell Sea embayment, Antarctica

The resilience of the Antarctic Ice Sheet and its effect on global sea level depends on the dynamics of ice streams. Antarctic ice streams are known to be responsive to changes at the ocean interface and, as expected, most have thinned in response to ocean warming and sea-level rise since the Last Glacial Maximum (LGM). Here we provide direct and unexpected evidence that points toward the glacial/interglacial stability of the Slessor and Recovery glaciers, ice streams of the East Antarctic Ice Sheet (EAIS) which merge with the West Antarctic Ice Sheet (WAIS) to form the Filchner Ice Shelf in the Weddell Sea embayment. Cosmogenic-nuclide measurements in the Shackleton Range suggest that the Slessor and Recovery ice streams were not significantly thicker than today during the LGM. We hypothesise that the glaciers did not thicken because the grounding line was not able to migrate seaward beyond the deep Thiel/Crary Trough beneath the Filchner Ice Shelf immediately offshore. This discovery reveals how a topographic threshold can affect the dynamics of ice streams. It also reduces uncertainties on the thickness, extent and volume of the Antarctic Ice Sheet in a large but unknown sector of the Antarctic Ice Sheet; it constrains the potential sea-level rise from Antarctica; it helps explain observed anomalies in glacio-isostatic adjustment; above all it suggests that the behaviour of the Atlantic-facing Weddell Sea sector of the WAIS contrasts with that of the Pacific-facing Ross and Amundsen Sea sectors.

A network of autonomous surface ozone monitors in Antarctica: technical description and first results

Abstract. A suite of 10 autonomous ozone monitoring units, each powered using renewable energy, was developed and built to study surface ozone in Antarctica during the International Polar Year (2007–2009). The monitoring systems were deployed in a network around the Weddell Sea sector of coastal Antarctica with a transect up onto the Antarctic Plateau. The aim was to measure for a full year, thus gaining a much-improved broader view of boundary layer ozone seasonality at different locations as well as of factors affecting the budget of surface ozone in Antarctica. Ozone mixing ratios were measured based on UV photometry using a modified version of the commercial 2B Technologies Inc. Model 202 instrument. All but one of the autonomous units measured successfully within its predefined duty cycle throughout the year, with some differences in performance dependent on power availability and ambient temperature. Mean data recovery after removal of outliers was on average 70% (range 44–83%) and precision varied between 1.5 and 8 ppbv, thus was sufficiently good to resolve year-round the main ozone features of scientific interest. We conclude that, with adequate power, and noting a minor communication problem, our units would be able to operate successfully at ambient temperatures down to −60 °C. Systems such as the one described in this paper, or derivatives of it, could therefore be deployed either as local or regional networks elsewhere in the Arctic or Antarctic. Here we present technical information and first results from the experiment.

Gravity anomalies of the Antarctic lithosphere

Anomalous free-air gravity signals in and around the Antarctic continent have been reported for some decades. Recent definition of the Antarctic gravity field from field-based oversnow traverses and supporting data from Earth-orbiting satellites reveal discrete regions of both negative and positive free-air gravity anomalies. The data from these observations have enabled us to construct a free-air gravity anomaly map of Antarctica. Negative free-air gravity anomalies are found to occur mainly on the Antarctic continent, in particular, in the Wilkes Land, Ross Sea, central continental, and Weddell Sea sectors. Positive free-air gravity anomalies are found to occur mainly in the offshore circum-continental sectors. While each of these regions of anomalies provides excellent opportunities for further investigation, including identification of the causes of the negative and positive free-air gravity anomalies, special attention is given to the negative free-air gravity anomaly sites of the continent proper. Three potential sources of the negative free-air gravity anomalies are identified: the mantle, lithosphere, and crust. Examination of thermally induced density variations in the mantle based upon seismic tomography, and analysis of mantle-related gravity anomaly wavelengths favor a gravity anomaly source other than the mantle. Examination of the subcrustal lithosphere based on upper-mantle thermal structure, the origin of the lithosphere, and crustal influences on the underlying lithosphere, including radiogenic heat, implies that the source of the negative free-air gravity anomalies is less likely to be the subcrustal lithosphere and more likely to be located in the Antarctic crust. Examination of possible crustal features that might account for these anomalies leads to a consideration of subglacial topography and specific locales of anomalously low rock density.

Antarctic sea ice extent during the Holocene reconstructed from inland ice core evidence

Interannual fluctuations in the Antarctic sea ice extent (SIE) affect the global albedo and heat balance, and they may affect global CO2 levels. We found that the annual trend of sodium ion fluxes in the snowpack in the Dome Fuji region of East Antarctica can serve as a proxy for the present‐day SIE in the southern Indian Ocean and Weddell Sea sectors. We then used sodium ion profiles obtained from the Dome Fuji ice core to reconstruct the SIE during the Holocene. We observed that (1) the beginnings of both the climatic optimum (12.0 ka BP) and neoglaciation (5.4 ka BP) in the Southern Ocean, indicated by siliceous microfossil records, are consistent with decreases (11.8 ka BP) and increases (5.4 ka BP) in the sodium ion fluxes in the Dome Fuji ice core, and (2) the interannual fluctuation in the SIE during some periods of the neoglaciation was greater than the interannual fluctuation during the climatic optimum.

Antarctic ice sheet and sea ice regional albedo and temperature change, 1981–2000, from AVHRR Polar Pathfinder data

Spring–summer (November, December, January) ice sheet and sea ice regional surface albedo, surface temperature, sea ice concentration and sea ice extent averages and trends from 1981 to 2000 have been calculated for the Antarctic area. In this research the AVHRR Polar Pathfinder 5-km EASE-Grid Composites and the combined SMMR and SSMI data sets from the National Snow and Ice Data Center (NSIDC), Boulder, Colorado have been employed. A regional analysis has been made for five longitudinal sectors around Antarctica: the Weddell Sea (WS), the Indian Ocean (IO), the Pacific Ocean (PO), the Ross Sea (RS) and the Bellingshausen–Amundsen Sea (BS). The IO and PO sectors show ice sheet albedos of 0.85 and temperatures of − 25 °C. The corresponding values in the RS and BS sectors are 0.80 and − 16 °C respectively. The sea ice albedo is about 0.60 in the RS, BS and WS sectors and 0.55 in the IO and PO sectors. The average sea ice temperature varies around − 12 °C. All the sectors show slight increasing spring–summer albedo trends and decreasing spring–summer temperature trends and similar interannual variability in albedo and surface temperature. The steepest ice sheet albedo trend of 0.0019 ± 0.0009/yr is found in the RS sector. The steepest sea ice albedo trend of 0.0044 ± 0.0017 /yr occurs in the PO sector. The steepest temperature trends for both the ice sheet and sea ice occur in the BS sector, having values of − 0.075 ± 0.040 °C/yr and − 0.107 ± 0.027 °C/yr respectively. The sea ice concentration shows slight increasing trends, the highest being in the PO sector (0.3 ± 0.12%/yr), whereas the sea ice extent trends are near zero with the exception of the RS sector (14,700 ±8600 km 2/yr) and the BS sector (− 13,000 ± 6400 km 2/yr).

On the influence of adequate Weddell Sea characteristics in a large-scale global ocean circulation model

Global ocean circulation models usually lack an adequate consideration of high-latitude processes due to a limited model domain or insufficient resolution. Without the processes in key areas of the global thermohaline circulation, the characteristics and flow of deep and bottom waters cannot be modeled realistically. In this study, a high-resolution (~20 km) ocean model focused on the Weddell Sea sector of the Southern Ocean is combined with a low-resolution (2° × 2°) global ocean model applying the state estimation technique. Temperature, salinity, and velocity data on two Weddell Sea sections from the regional model are used as constraints for the large-scale model in addition to satellite altimetry and sea-surface temperatures. The differences between the model with additional constraints and without document that the Weddell Sea circulation exerts significant influence on the course of the Antarctic Circumpolar Current with consequences for Southern Ocean water mass characteristics and the spreading of deep and bottom waters in the South Atlantic. Furthermore, a warming trend in the period 1993–2001 was found in the Weddell Sea and adjacent basins in agreement with float measurements in the upper Southern Ocean. Teleconnections to the North Atlantic are suggested but need further studies to demonstrate their statistical significance.

Increased exposure of Southern Ocean phytoplankton to ultraviolet radiation

[1] Satellite remote sensing of both surface solar ultraviolet radiation (UVR) and chlorophyll over two decades shows that biologically significant ultraviolet radiation increases began to occur over the Southern Ocean three years before the ozone “hole” was discovered. Beginning in October 1983, the most frequent occurrences of enhanced UVR over phytoplankton-rich waters occurred in the Weddell Sea and Indian Ocean sectors of the Southern Ocean, impacting 60% of the surface biomass by the late 1990s. These results suggest two reasons why more serious impacts to the base of the marine food web may not have been detected by field experiments: (1) the onset of UVR increases several years before dedicated field work began may have impacted the most sensitive organisms long before such damage could be detected, and (2) most biological field work has so far not taken place in Antarctic waters most extensively subjected to enhanced UVR.

On the secular trends in sea ice extent over the antarctic region based on OCEANSAT-1 MSMR observations

Space-based passive microwave radiometers have been used to monitor southern ocean sea ice characteristics over the Antarctic region for the last 25 years. Decade-long observations from SMMR onboard Nimbus-7 during 1978-1987 indicated significant long-term trends in sea ice cover over different regions of the southern polar ocean, in addition to the strong seasonal and inter-annual variability in various sea ice characteristics. In the post-SMMR era, passive microwave monitoring of the polar region has continued with the availability of SSM/I onboard DMSP series of satellites. With the launch of MSMR onboard India's OCEANSAT-1 in May 1999, an independent set of multi channel passive microwave observations of the Antarctic and Arctic sea ice have become available. In this paper, we present the analysis of the first two years of MSMR observed brightness temperatures to infer sea ice extent over the Antarctic region. The analysis brings out the characteristic seasonal variability of sea ice extents over different sectors of the Southern Polar Ocean. The annual average sea ice extent values esimated from MSMR for 1999-2001 for important regions, e.g. the Weddell Sea and Ross Sea sectors, are found to be significantly different compared to the average values observed through SMMR. The MSMR estimates suggest a continuation of the secular trends observed during the SMMR period. The trend in the sea ice extent over the Antarctic region as a whole was observed to be slightly positive (0.043 million km 2 per year). The MSMR estimates of the sea ice extent over the entire Southern Ocean region, of 12.634 million km 2 during 1999-2001, support the continuation of the secular increase observed during the SMMR as well as SSM/I periods. The observed increasing trend in sea ice extent derived from SMMR and MSMR data is discussed along with some more recent estimates derived from SSM/I measurements, with possible indications of an acceleration in time. In the current greenhouse induced global warming scenario, with amplified warming over the polar region, this represents an intriguing result requiring continued investigations.

A new magnetic map of the Weddell Sea and the Antarctic Peninsula

As part of the Antarctic Digital Magnetic Mapping Project (ADMAP) workers from VNIIOkeangeologia (Russia), the British Antarctic Survey (UK) and the Naval Research Laboratory (USA) have brought together almost all of the available magnetic data in the area 0–120°W, 60–90°S. The final map covers the whole Weddell Sea and adjacent land areas, the Antarctic Peninsula and the seas to the west, an area comparable in size with that of the USA. This paper describes the methods used during the compilation of the map and reviews briefly some of the main features shown on it. Distinct magnetic provinces are associated with Precambrian rocks of the East Antarctic craton, highly extended continental crust in the Weddell Sea embayment, the arc batholith of the Antarctic Peninsula, and oceanic crust of the northern Weddell Sea, which was created as a direct consequence of South America–Antarctica plate motion and oceanic crust generated at the Pacific–Antarctic ridge. The magnetic anomaly map thus provides an overview of the fragmentation of south-western Gondwana and the tectonic development of the Weddell Sea sector of Antarctica.

Whistler observations of the plasmasphere/plasmapause from stations of the British Antarctic Survey

VLF whistler observations from Eights Station, Antarctica were crucial to the discovery and exploration of the plasmapause (“Carpenter's knee”) by Don Carpenter in the mid-1960s. The Weddell Sea sector of Antarctica is particularly well suited to such work because of the high whistler rate (conjugate to thunderstorm regions), proximity to the ground footprint of the average plasmapause, low electromagnetic noise levels (far from power lines, etc.), low ionospheric absorption (in winter), and wave amplification due to the South Atlantic Geomagnetic Anomaly. VLF recordings have been made at Halley, Antarctica, (76°S,27°W, L∼4.3) since 1967; the station is located on a similar L-shell to Eights and its successor Siple, but eastward in longitude by about 2 h in magnetic local time. In this paper, we review some of the research on the structure and dynamics of the plasmasphere/plasmapause which has been based on whistler data from Halley. In particular, the use of Halley and Siple as a station pair corotating with the Earth through the Sun–Earth frame has enabled the complex dynamics of the duskside bulge region to be better understood. For example, features consistent with narrow dense sunward-pointing plasma tails, have been delineated. Whistler data from Halley have also provided much information on fine structure within the plasmasphere. The paper discusses some important results of inner plasmaspheric probing using fixed-frequency (∼20 kHz) whistler data from the lower latitude Faraday station (65° S, 64° W,L∼2.3) , including annual density variations and magnetic storm effects, and concludes by indicating some directions for the future.

The crustal structure and sedimentation of the Weddell Sea embayment: implications for Gondwana reconstructions

The Weddell Sea embayment is central to reconstructions of the West Antarctic region of Gondwana. Some reconstructions represent this area as a pre-break-up component, the Filchner block, defined by the present 2000 m isobath and coastline but take no account of crustal extension and sedimentary progradation of the continental margin since break-up. Seismic refraction shows a wide, deep sedimentary basin beneath the central part of the embayment, indicating extensive rifting and stretching of continental crust during the break-up of Gondwana. This may have increased the width of the block by about 300 km. Potential field data suggest extensive progradation of sediments on the Weddell Sea continental margin. This may have moved the margin seaward between 200 and 400 km. Taken together, these modifications in the dimensions of the pre-break-up Filchner block represent between one half and one third the area of the present Weddell Sea embayment. The presence of extensive rifting is indicative that hot, thin crust was present during break-up providing further evidence that several closely spaced mantle plumes (or a single ‘megaplume’) impinged on the Weddell Sea sector of Gondwana in the early Jurassic. The lithospheric stretching in the embayment provided a mechanism for the rotation of the Ellsworth Mountains region during the early stages of Gondwana break-up commensurate with the paleomagnetic data.

Middle Cambrian rift-related volcanism in the Ellsworth Mountains, Antarctica: tectonic implications for the palaeo-Pacific margin of Gondwana

The Ellsworth Mountains of West Antarctica represent part of a displaced terrane once situated along the palaeo-Pacific margin of Gondwana, prior to supercontinent break-up, adjacent to South Africa and the Weddell Sea coast of East Antarctica. Middle Cambrian sedimentary rocks of the southern Ellsworth Mountains host locally thick volcanic and subvolcanic rocks forming five igneous centres. Geochemically, most of the igneous samples are mafic, with a subordinate suite of evolved types. The mafic suite is geochemically varied, ranging from MORB (mid-ocean ridge basalt)-like compositions to shoshonitic and lamprophyric (e.g. La N/Yb N = 0.95 to 15.2), with εNd i values ranging from +5.2 to −2.0, correlating with Ti/Y. They are interpreted as representing melts derived from more than one mantle source, with the MORB-like rocks being derived from a depleted mantle source, and the more enriched compositions representing partial melting of lithospheric mantle. Silicic rocks contain melt contributions from Late Proterozoic crust, which is inferred to form the basement of the Ellsworth Mountains. We interpret these igneous rocks as having been formed in a continental rift environment, with MORB-like basalts erupted near the rift axis, and melts from lithospheric mantle emplaced on the rift shoulder. Such an interpretation is consistent with the sedimentary host-rock palaeogeography and contemporaneous structures. This Middle Cambrian rift event is correlated spatially and temporally with rift-related sedimentary rocks in South Africa. It is currently unclear what rifted off the southern African–Weddell Sea sector of the Gondwana palaeo-Pacific margin at that time.

Mesoscale cyclone activity over Antarctica during 1991: 2. Near the Antarctic peninsula

A survey of mesoscale cyclogenesis near the Antarctic peninsula has been carried out for 1991 based on digital satellite imagery collected at Palmer Station. Slightly higher cyclonic activity was found over the Bellingshausen Sea sector (BSS) than in the Weddell Sea sector (WSS). Both sides were characterized by a pronounced formation maximum in summer and a minimum in winter. Comma clouds were the dominant vortex type, and their average diameter was around 370 km. Trajectories show that many vortices in the BSS move northeastward toward the Drake Passage, with some of the most well developed moving north‐northeastward to the southern tip of South America, bringing severe weather conditions to that area; little systematic motion was noted over the WSS. The fraction of mesoscale cyclones with middle and/or high cloud on the west side of the Antarctic peninsula was much higher than on the east side. These numerous and deep mesoscale cyclones are a consequence of the frequent outbreaks of cold air associated with synoptic‐scale cyclones that move cold air northward over the relatively warm Bellingshausen Sea, with convective instability resulting. This happens much less often in the WSS, where low‐level baroclinic instability and vortex stretching are the dominant formation mechanisms and air‐sea interaction is usually much less vigorous due to the stabilizing influence of the extended sea‐ice cover.