Ronne Ice Shelf Research Articles

Comparison of ice-shelf creep flow simulations with ice-front motion of Filchner-Ronne Ice Shelf, Antarctica, detected by SAR interferometry

Comparison between numerical model ice-shelf flow simulations and synthetic aperture radar (SAR) interferograms is used to study ice-flow dynamics at the Hemmen Ice Rise (HIR) and Lassiter Coast (LC) corners of the iceberg-calving front of the Filchnei—Ronne Ice Shelf, Antarctica. The interferograms are constructed from SAR images provided by the European Space Agency's remote-sensing satellites (ERS-1/2). Narrow bands of large shear strain rate are observed along the boundaries between fast-flowing ice-shelf ice and no-flow boundaries. Large rifts, opened where the ice shelf separates from the coast, appear to be filled with a melange of sea ice, ice-shelf fragments, and snow. Trial and error is used to find the best match between artificial interferograms, constructed from modelled ice flow, and the observed interferograms. We find that at both HIR and LC, ice with in the coastal boundary layers must be significantly softer than adjacent ice. At HIR the rift-filling ice melange transmits stress from one ice-shelf fragment to another; thus it must have mechanical competence and must moderate both separation of the ice shelf from the coast and the release of icebergs. However, the ice melange along the LC does not. The difference may be related to melange thickness, which could vary in the two locations due to differences in sub-ice-shelf oceanography or perhaps to regional atmospheric warming, currently under way along the Antarctic Peninsula. Future warming could weaken the melange ice around HIR as well, causing the ice shelf to lose contact with that shelf-front anchor.

Ice-shelf dynamics near the front of the Filchner-Ronne Ice Shelf, Antaretica, revealed by SAR interferometry: model/interferogram comparison

AbstractWe compare European remote-sensing satellite (ERS) synthetic aperture radar interferograms with artificial interferograms constructed using output of a finite-element ice-shelf flow model to study the dynamics of Filchner-Ronne Ice Shelf (FRIS), Antaretica, near Hemmen Ice Rise (HIR) where the iceberg-calving front intersects Berkner Island. We find that the model must account for rifts, mechanically competent sea ice which fills rifts, and ice softening in coastal boundary layers in order to agree with the ice-deformation pattern implied by observed interferograms. Analysis of the stress field in the model experiment that best matches the observed interferograms suggests that: (1) HIR introduces weakness into the ice shelf through the generation of large-scale rifts, and (2) the melange of sea ice and ice-shelf fragments that fills the rifts stabilizes the shelf front by providing mechanical coupling between the fractured shelf front and the adjacent coast. The rift-filling melange could melt more easily than the surrounding ice shelf and thus could represent a vulnerability of the FRIS to climate warming.

Ice-shelf dynamics near the front of the Filchner-Ronne Ice Shelf, Antaretica, revealed by SAR interferometry: model/interferogram comparison

AbstractWe compare European remote-sensing satellite (ERS) synthetic aperture radar interferograms with artificial interferograms constructed using output of a finite-element ice-shelf flow model to study the dynamics of Filchner-Ronne Ice Shelf (FRIS), Antaretica, near Hemmen Ice Rise (HIR) where the iceberg-calving front intersects Berkner Island. We find that the model must account for rifts, mechanically competent sea ice which fills rifts, and ice softening in coastal boundary layers in order to agree with the ice-deformation pattern implied by observed interferograms. Analysis of the stress field in the model experiment that best matches the observed interferograms suggests that: (1) HIR introduces weakness into the ice shelf through the generation of large-scale rifts, and (2) the melange of sea ice and ice-shelf fragments that fills the rifts stabilizes the shelf front by providing mechanical coupling between the fractured shelf front and the adjacent coast. The rift-filling melange could melt more easily than the surrounding ice shelf and thus could represent a vulnerability of the FRIS to climate warming.

Predicted reduction in basal melt rates of an Antarctic ice shelf in a warmer climate

Floating ice shelves are vulnerable to climate change at both their upper and lower surfaces. The extent to which the apparently air-temperature-related retreat of some northerly Antarctic Peninsula ice shelves1 presages the demise of their much larger, more southerly, counterparts is not known, but air-temperature effects are unlikely to be important in the near future. Oceanographic measurements from beneath the most massive of these southerly ice shelves—the Filchner–Ronne Ice Shelf2,3,4—have confirmed that dense sea water resulting from sea-ice formation north of the ice shelf flows into the sub-ice-shelf cavity. This relatively warm so-called High Salinity Shelf Water (HSSW) is responsible for the net melting at the ice shelf's base. Here I present temperature measurements, from the same sub-ice-shelf cavity, which show a strong seasonality in the inflow of HSSW. This seasonality results from intense wintertime production of sea ice, and I argue that the seasonal springtime warming can be used as an analogue for climate warming. For the present mode of oceanographic circulation, the implication is that warmer winters (a climate warming, leading to lower rates of sea-ice formation, would cause a reduction in the flux of HSSW beneath the ice shelf. The resultant cooling in the sub-ice cavity would lead, in turn, to a reduction in the total melting at the ice shelf's base. A moderate warming of the climate could thus lead to a basal thickening of the Filchner–Ronne Ice Shelf, perhaps increasing its longevity.

Seabed topography under the southern and western Ronne Ice Shelf, derived from seismic surveys

Seismic reflection measurements of ice thickness and water-column thickness have been made over the southern and western Ronne Ice Shelf, from which, the seabed elevation has been determined. A 2300 km traverse, covering an area over which little bathymetric data previously existed and at a station interval of 15 km, resulted in 152 new measurements of the seabed elevation. The seismic velocity in the ice shelf was determined using shallow refraction surveys for the snow and firn in the upper 100 m of the ice shelf, and by applying a model assuming a simple temperature profile for the deeper ice. Surface elevations were calculated assuming the ice to be in hydrostatic equilibrium. The estimated error in the seabed elevations is ±15 m. With the addition of the new data there is now complete coverage of the seabed elevation and water-column thickness for Ronne Ice Shelf at a spatial resolution of between 10–100 km. The seabed in the area to the south of Korff and Henry ice rises and the Doake Ice Rumples is about 100 m shallower than had previously been speculated, which will affect the validity of previous assumptions of water circulation and ice-ocean interaction in this area.

New oceanographic data from beneath Ronne Ice Shelf, Antarctica

Oceanographic data have been obtained via an access hole made through Ronne Ice Shelf. The site, which is the third in a series of similar studies, lies 17 km west of Korff Ice Rise where 825 m of ice overlies a 485‐m deep water column. Measurements included conductivity and temperature profiles, and an instrument mooring was deployed for long‐term measurements of currents, temperature and conductivity. At the sea floor there was a 150‐m layer of well‐mixed water with a potential temperature and salinity of −1.97°C and 34.72. The water cooled and freshened towards the ice‐shelf base, ultimately reaching −2.41°C and 34.51. The hydrographic and water current data imply a flow into the deepest part of the sub‐ice shelf cavity of about 200,000 m³ s−1 of the deeper, relatively warm water, which would be able to power an average basal melt rate of 0.2 m a−1 for the western portion of Ronne Ice Shelf.

Ice shelf basal melting at the grounding line, measured from seismic observations

Analysis of seismic data provides a new technique to measure basal melting of an ice shelf close to the grounding line. Internal reflections have been observed within the ice on a seismic reflection profile at the grounding line of Ronne Ice Shelf. The changes in ice thickness above and below these reflections have been used in a steady state model to calculate the basal mass flux of the ice shelf soon after the ice begins to float. Strain rates within the ice, calculated during the modeling, agree with values from nearby survey data. The calculated melt rates range between 0 and 7 m yr−1 with an estimated standard error of ±2.4 m yr−1, in reasonable agreement with earlier estimates based on surface glaciological observations. Limitations in the method include the difficulties in determining reflector geometry from a single seismic section and also the limitations imposed by the assumption of steady state. Additional seismic data would greatly reduce these limitations.

Geochemistry in an area recently uncovered from the Filchner ice shelf

Sediments were studied in the southern Weddell Sea (Antarctica) that had been both covered by the ice shelf prior to 1986 (ICS) and outside the ice shelf (IFS). The composition of particulate matter in these sediments shows that the sedimentation under the ice shelf is quantitatively similar to that outside the ice shelf, except that the former has a negligible content of easily degradable organic matter, indicating a very low biochemical activity. The 210Pb profile, from the ICS station, gave a sedimentation rate of 0.02 cm y −1 . This accumulation rate, combined with a particulate carbon content of approximately 1%, yields an annual net accumulation of 1.7 g C m −2. The profiles of nutrients in the interstitial water (under the ice shelf) indicate a very small flux to the overlaying water. Outside the ice shelf the flux is significantly higher, though still small.

A towed geophone system for use in snow-covered terrain

SUMMARY A new type of drag cable has been developed, which was successfully used to collect seismic reflection data on an Antarctic ice shelf. Standard geophone elements were encapsulated in polyurethane to form 25 m long, rectangular sections. Each of the 12 sections incorporated a group of 12 geophones. The resultant towed array resembles a 300m long flexible ski. The use of a towed cable provided substantial time and manpower savings over the use of planted geophones. In comparison with drag cables utilizing gimbal geophones, the towed array has a much lower coefficient of friction due to its smooth profile, and it is therefore possible to tow an array using snowmobiles. This provides a significant advantage because a system can be deployed by ski-equipped light aircraft to areas that are difficult or impossible to access using large vehicles. The simple construction of the towed array results in a lower cost than when gimbal cables are used. The main disadvantage is that the towed array has poorer wind-noise characteristics than gimbal cables or planted geophones. Use of the array enabled a team of four people to acquire 158 km of single-fold seismic reflection data on the Ronne Ice Shelf over two field seasons. The data are the first to show sub-seabed structure beneath this major ice shelf.

Clay mineral distribution in surface sediments of the South Atlantic: sources, transport, and relation to oceanography

Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.

Temperature variability beneath Ronne Ice Shelf, Antarctica, from thermistor cables

Thermistor cables have been deployed at two sites beneath Ronne Ice Shelf, Antarctica. One site is to the east of a submarine ridge that delineates the eastern boundary of the Ronne Depression, and the other is 100 km to the north, above the eastern slope of the depression. Long records from the cables (up to 22 months) indicate a large difference in the temperature variability at the two sites, being an order of magnitude greater in the Ronne Depression (site 2). Although the records appear otherwise similar, there is no significant correlation between them. The high variability in the site 2 record has allowed the construction of a simple descriptive model of the local oceanographic regime. Winter freezing in the open water north of the ice front generates Western Shelf Water (WSW), a type of High Salinity Shelf Water, which travels southwest beneath the ice shelf, appearing at site 2 as a slope‐trapped current at the bottom of the water column. Baroclinic instability in the flow manifests itself in the site 2 temperature record as oscillations on time scales of 5 to 15 days. The disturbances cause a periodic east‐west advection of water masses across the Ronne Depression. Site 2 is on the eastern slope of the depression, where the wave‐induced eastward motion forces Ice Shelf Water to rise, resulting in periodic ice‐platelet formation in the water column, as surmised from conductivity‐temperature‐depth measurements at the site. The depth of the WSW layer decreases by 40 to 60 m during a 100‐day period, starting some 4 months after the beginning of the summer. Assuming an absence of significant WSW production during the summer, the 4‐month delay implies a minimum average speed of WSW flow of about 0.02 m s−1. The WSW flux into the Ronne Depression is estimated at 3 × 105 m3s−1.

Aeromagnetic data from the southern Weddell Sea embayment and adjacent areas: synthesis and interpretation

A new regional aeromagnetic anomaly map covering the southern Weddell Sea and surrounding areas was produced by combining Russian and British magnetic anomaly data acquired between 1973 and 1989. The map is divided into four zones on the basis of characteristic anomaly wavelength: Antarctic Peninsula, Weddell Sea embayment, Haag Nunataks and the western coast of East Antarctica. Over the southern Weddell Sea and the Ronne and Filchner ice shelves, the magnetic field is relatively smooth indicating a thick sedimentary cover although there are some distinctive anomalies. A broad positive anomaly is clearly visible running W-E across the north of the area, which is interpreted as representing the continent — ocean boundary. The most prominent magnetic feature is the elongate Explora Anomaly which runs parallel to the coast of East Antarctica and continues over Berkner Island terminating at the Dufek Massif. We interpret the source of this anomaly as rift-related volcanic rocks probably emplaced around the same time as the Dufek intrusion. The rift itself is interpreted as running south between the Andenes escarpment and East Antarctica and terminating south of Berkner Island. Russian gravity profiles show that Berkner Island does not have an associated Bouguer anomaly, although a large positive anomaly (approx 80 mGal) is seen to the east of the island over the Filchner Ice Shelf, suggesting the presence of a second rift structure. Gravity modelling across these features shows thinned crust below the Ronne and Filchner ice shelves, indicating that the whole area has undergone significant extension. A gravity model across the flanks of Berkner Island is used to estimate the ratio of unstretched to stretched crustal thickness. The resulting value of approximately 2.7 indicates that enough extension has taken place to induce melting, supporting the idea that the source of the magnetic anomaly over Berkner Island is rift-related volcanic rocks.

Ocean tides under the Filchner-Ronne Ice Shelf, Antarctica

A depth-averaged finite-difference numerical model has been used to make a preliminary study of the tides under the Filchner–Ronne Ice Shelf. Open boundary conditions were specified using the global ocean model of Schwiderski. Tidal constituents for the two principal semi-diurnal constituents M2 and S2, and the two principal diurnal constituents Ο1 and K1 were extracted from computed sea-surface elevations by harmonic analysis. Measured values near to the grounding line could only be reproduced satisfactorily by increasing the bottom friction coefficient under the ice to 50 times the open-ocean value. This destroys any agreement near the ice front or at pelagic sites. It is thought that a friction coefficient which varies with distance under the ice would be able to reproduce better all the available measurements. More tidal measurements are required to validate any model of the region with model experiments being used to help pinpoint possible sites for instrument deployment.

Ocean tides under the Filchner-Ronne Ice Shelf, Antarctica

A depth-averaged finite-difference numerical model has been used to make a preliminary study of the tides under the Filchner–Ronne Ice Shelf. Open boundary conditions were specified using the global ocean model of Schwiderski. Tidal constituents for the two principal semi-diurnal constituents M2 and S2, and the two principal diurnal constituents Ο1 and K1 were extracted from computed sea-surface elevations by harmonic analysis. Measured values near to the grounding line could only be reproduced satisfactorily by increasing the bottom friction coefficient under the ice to 50 times the open-ocean value. This destroys any agreement near the ice front or at pelagic sites. It is thought that a friction coefficient which varies with distance under the ice would be able to reproduce better all the available measurements. More tidal measurements are required to validate any model of the region with model experiments being used to help pinpoint possible sites for instrument deployment.

Experimental fracture and mechanical properties of Antarctic ice: preliminary results

An experimental study of the fracture mechanics and rheology of ice from the Ronne Ice Shelf, Antarctica, is currently being undertaken. The apparent critical stress-intensity factor (or apparent fracture toughness, KQ) for crack propagation has been measured using a three-point bend method for inducing crack growth perpendicular to the axis of cylindrical ice-core specimens. Tensile crack nucleation under applied uniaxial compressive stress has also been evaluated. Both methods have allowed a profile of ice elastic and fracture properties with depth through the ice shelf to be constructed. Preliminary results indicate that the measured elastic modulus increases with depth through the firn and upper meteoric ice before reaching a constant value in the deeper, dense meteoric and basal marine ice. The resistance to fracture, as measured by changes in apparent fracture toughness and crack-nucleation stress, increases with depth right through the firn and meteoric ice layers. A simple fracture mechanics model applied to the Ronne Ice Shelf indicates that crevasses form from small surface cracks, less than 40 cm deep, which quickly grow to depths of 40–60m and then remain stable.

Experimental fracture and mechanical properties of Antarctic ice: preliminary results

An experimental study of the fracture mechanics and rheology of ice from the Ronne Ice Shelf, Antarctica, is currently being undertaken. The apparent critical stress-intensity factor (or apparent fracture toughness, K Q) for crack propagation has been measured using a three-point bend method for inducing crack growth perpendicular to the axis of cylindrical ice-core specimens. Tensile crack nucleation under applied uniaxial compressive stress has also been evaluated. Both methods have allowed a profile of ice elastic and fracture properties with depth through the ice shelf to be constructed. Preliminary results indicate that the measured elastic modulus increases with depth through the firn and upper meteoric ice before reaching a constant value in the deeper, dense meteoric and basal marine ice. The resistance to fracture, as measured by changes in apparent fracture toughness and crack-nucleation stress, increases with depth right through the firn and meteoric ice layers. A simple fracture mechanics model applied to the Ronne Ice Shelf indicates that crevasses form from small surface cracks, less than 40 cm deep, which quickly grow to depths of 40–60m and then remain stable.

Weddell Sea tectonics and Gondwana break-up: an introduction

The Weddell Sea, part of the circumpolar Southern Ocean, is probably the most remote, least known and least accessible sea in the world. Difficult ice conditions have limited the acquisition of ship data, although this has been partly offset in recent years by access to satellite radar altimetry data. The Weddell Sea was originally defined by the Admiralty Hydrographic Department in 1932 and redefined by the Antarctic Place Names Committee in 1976 (Hattersley-Smith 1991). It is bounded on the western side by the east coast of the Antarctic Peninsula, on the southern side by the Ronne and Filchner ice fronts, and on the southeastern side by the Dronning Maud Land and Coats Land coasts of East Antarctica (Fig. 1). The South Scotia Ridge separates the Weddell Sea from the Scotia Sea to the north and a line joining Southern Thule in the South Sandwich Islands and Kapp Norvegia in Dronning Maud Land, separates it from the South Atlantic Ocean to the NE. Within this volume, papers relate to the Weddell Sea as defined above, together with part of the adjoining South Atlantic Ocean up to 50°E, and to the geology of the once neighbouring continents of Gondwana. The term Weddell Sea embayment is also used informally throughout this volume to include the embayment area to the south of the Weddell Sea now covered by the Ronne and Filchner ice shelves, including Berkner Island, and the continental shelf north of the Ronne and Filchner ice fronts (Figs 1 & 2).

New seismic data from the Ronne Ice Shelf, Antarctica

We present the first images of sub-seabed structure to be obtained from beneath the Ronne Ice Shelf. Single-fold profiles totalling 158 km were recorded using a new form drag cable with explosives as source. The survey was conducted in an area lying 100 km SE of the Orville Coast, Antarctic Peninsula. The ice shelf at this location is about 350 m thick and the seabed undulates gently between 650 and 675 m below sealevel. In the western part of the area, acoustic basement outcrops at the seabed. In the eastern part, a set of open folds were observed with wavelengths of 5–6 km, amplitudes of 200–300 m and a NNE-SSW strike direction. These features match those observed on previous marine data obtained in the narrow shore lead at the front of the ice shelf and allow the strike of the structures to be traced for 70 km to the SW of the original observations. Existing models of the plate tectonic development of West Antarctica have anticipated major dextral strike-slip motion between the Antarctic Peninsula and the Weddell Sea but the new data shows no evidence for this, indeed the structures can be interpreted as resulting from an episode of sinistral motion.

Modeling the formation and deposition of frazil ice beneath Filchner‐Ronne Ice Shelf

Large areas of the Filchner‐Ronne Ice Shelf are underlain by layers of marine ice, which form when supercooled seawater circulating beneath the ice shelf freezes. The freezing process initially produces a suspension of disc‐shaped frazil ice crystals, and these are subs quently deposited onto the ice shelf base in areas where the flow of water is slack enough. This has been modeled assuming that the freezing takes place within buoyant plumes of Ice Shelf Water ascending the ice shelf base from source regions near the grounding lines of the major inlet glaciers. The deposition of the majority of the suspended frazil ice is found to occur in spatially discrete bursts, where peak rates of accumulation at the ice shelf base exceed 1 m yr−1 of solid ice. There is a good correlation between the location of the zones of crystal deposition and the position of the upstream limits of the marine ice layers. The high rates of localized accumulation account for the rapid buildup observed in the layer thickness, which then gradually declines as the marine ice is carried downstream with the flow of the ice shelf. Model results also suggest an origin for the ice platelets observed at depth in the water column near the Filchner Ice Shelf.

Tidal flexure at ice shelf margins

Hinge zones accommodate the differential tidal movement between floating ice shelves, which move with the sea tide, and grounded ice sheets, which do not. Observations on Rutford Ice Stream and Ronne Ice Shelf, Antarctica, using the kinematic method of Global Positioning System, have yielded the first continuous tidal displacement profiles from an ice sheet hinge zone. The form of these tidal displacement profiles indicates that the flexure can be fit to an elastic beam model using a parameter space optimization technique. The model matches the observed tidal deflection profile to within 5 cm which is similar to the observational uncertainty. The same model and parameter fitting technique is then successfully applied to various other published and unpublished tidal displacement data from Doake Ice Rumples, Bach Ice Shelf, and Ekström Ice Shelf, Antarctica, and Jakobshavns Glacier, Greenland. I conclude that the elastic beam model with a single value of the elastic modulus (0.88±0.35 GPa) adequately describes almost all the data and so can usefully be applied elsewhere. Earlier studies of tidal dissipation in ice shelf hinge zones, based on a viscous or transient creep rheology, showed that perhaps 30% of global tidal energy dissipation could be occurring in ice shelf hinge zones. This study suggests, however, that an elastic rheology may be equally appropriate, in which case this estimation of the tidal energy dissipation is excessively high.