Ronne Ice Shelf Research Articles

Temperature and salinity beneath Ronne Ice Shelf, Antarctica

Salinity and temperature measurements have been made beneath Ronne Ice Shelf, Antarctica. Access to the seawater was gained by hot‐water drilling through 562 m of ice at a site 300 km from the ice front. The ice column is composed of 516 m of meteoric‐origin ice, underlain by 31 m of consolidated saline ice, with a further 15 m of unconsolidated slush at the base. The 360‐m‐deep water column beneath the ice shelf consists broadly of a 210‐m layer of Ice Shelf Water (ISW) at a potential temperature of −2.30°C and salinity 34.53, overlying a 100‐m‐thick layer of modified Western Shelf Water (−2.03°C, 34.64). The layers are separated by a weak 50‐m‐thick pycnocline. Isotopic analyses of water samples support this interpretation of the water types. Comparison of the data with published oceanographic observations from the shore lead indicate that the source of the water is either the western Berkner shelf or north of the ice front in the Ronne Depression. The measurements from the drill site broadly match predictions from a simple plume model of the sub‐ice‐shelf water circulation. In that model the basal slope of the ice shelf plays a central role in determining the properties of the ISW plume. With the exception of the upper few tens of meters the temperature and salinity of the entire water column vary by about 0.04°C and 0.03 on tidal timescales, and there is evidence for a longer‐term drift, presumably connected with shifts in the larger‐scale circulation. There is also much variability from internal wave activity. The salinity and deuterium budgets indicate that the meltwater fraction in the ISW observed at the drill site was between 5.3 and 6.5‰, in reasonable agreement with values previously found for ISW in the open ocean.

The BAS hot water drill: development and current design

Over the past decade the British Antarctic Survey has developed a hot water drilling system that uses components easily carried by Twin Otter aircraft. The system has recently been upgraded and was successfully used in the 1990/91 and 91/92 field seasons to penetrate ice up to 562 m thick on Ronne Ice Shelf, Antarctica. Holes of at least 0.13 m in diameter were created and maintained. The hot water drill incorporates 300 kW of heating power with a water re-circulation system, removing the need for continuous snow melting while drilling. The drill nozzles have been designed to combat the problems of the rapid re-freezing of the water-filled hole. In the event of borehole closure above the nozzle, a less powerful but still effective drilling action is available upwards. A compressible mechanical valve system is incorporated which increases the back pressure when the operator is drilling too fast. This system ensures the nozzle is always freely suspended, resulting in a vertical hole. The work has allowed oceanographic measurements to be made in the sea-water underlying George VI Ice Shelf and Ronne Ice Shelf, Antarctica. The access holes have also been used for the installation of sensors in the ice and the ocean for long term temperature monitoring.

Determination of ice-shelf bottom melting by time-domain reflectometry

AbstractFor our work in the Filchner-Ronne Ice Shelf Programme (FRISP), we have developed a new technique for measuring the bottom-melting rate with high reliability. The method is based on time-domain reflectometry (TDR) measurements of transmission lines inserted into melt holes. The TDR-data are digitally recorded on magnetic tape. System resolution has been estimated at 0.2 m. Hence, re-measuring after 1 year gives an accuracy of 10% for melting rates of 2m a-1. Two transmission lines for TDR measurements were installed during the German FRIS Expedition 1989–90. This paper describes the design of the system. Examples of recorded wave forms are given.

Determination of ice-shelf bottom melting by time-domain reflectometry

AbstractFor our work in the Filchner-Ronne Ice Shelf Programme (FRISP), we have developed a new technique for measuring the bottom-melting rate with high reliability. The method is based on time-domain reflectometry (TDR) measurements of transmission lines inserted into melt holes. The TDR-data are digitally recorded on magnetic tape. System resolution has been estimated at 0.2 m. Hence, re-measuring after 1 year gives an accuracy of 10% for melting rates of 2m a-1. Two transmission lines for TDR measurements were installed during the German FRIS Expedition 1989–90. This paper describes the design of the system. Examples of recorded wave forms are given.

The origin of green icebergs in Antarctica

A comparison of samples from a translucent green iceberg with a core from the Ronne Ice Shelf revealed an excellent agreement in isotopic composition, crystal structure, and incorporated sediment particles. Marine shelf ice which constitutes the basal portion of some ice shelves is considered to be the source of green icebergs. It most likely results from “ice pump” processes which produce large amounts of ice platelets in the water column beneath ice shelves. These subsequently accumulate and become compacted into bubble‐free, desalinated ice. Iceberg and drift‐buoy trajectories indicate that green icebergs observed in the Weddell Sea originate from the Amery Ice Shelf rather than from the Ronne Ice Shelf, although this ice shelf is also a potential source.

Evidence for basal marine ice in the Filchner–Ronne ice shelf

THE Filchner–Ronne ice shelf, which drains most of the marine-based portions of the West Antarctic ice sheet, is the largest ice shelf on Earth by volume. The origin and properties of the ice that constitutes this shelf are poorly understood, because a strong reflecting interface within the ice and the diffuse nature of the ice–ocean interface make seismic and radio echo sounding data difficult to interpret1,2. Ice in the upper part of the shelf is of meteoric origin, but it has been proposed2,5 that a basal layer of saline ice accumulates from below. Here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the Ronne ice shelf. We observe a change in ice properties at about 150 m depth, which we ascribe to a change from meteoric ice to basal marine ice. The basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf.

On the thermohaline circulation beneath the Filchner-Ronne Ice Shelves

In the Weddell Sea oceanographic data and numerical models demonstrate that Ice Shelf Water, one ingredient in the production of Weddell Sea Bottom Water, is formed by thermohaline interaction of High Salinity Shelf Water with the base of the Filchner-Ronne ice shelves. South of Berkner Island a passage with a water column thickness of about 300 m linking the Filchner and the Ronne regimes is important for the ventilation of the sub-ice shelf cavities. To simulate the flow we tested a two-dimensional thermohaline circulation model on several sections which approximate different geometries of a sub-ice shelf channel bounded by the ocean bottom and the ice shelf base. Temperature and salinity profiles measured in front of the Filchner-Ronne ice shelves are used to force the model. The results indicate that the circulation is sensitive to both salinity (density) forcing and depth of the shelf bottom prescribed at the open boundary representing the Ronne Ice Shelf edge. Where the shelf is shallow, 400 m deep, a closed circulation cell within the Ronne cavity acts like an ice pump with accumulation rates of marine ice at the ice shelf base up to 1.5 m y−1. The total outflow at the Ronne Ice Shelf edge is supported by an inflow from the Filchner regime. Where the shelf is deeper, a flow from the Ronne into the Filchner cavity develops if the bottom salinity at the Ronne Ice Shelf edge exceeds a critical value of 34.67. Seasonal variability imposed at both edges modifies the circulation pattern at the Filchner Ice Shelf edge such that the depth and magnitude of Ice Shelf Water outflow correspond with observations in the Filchner Depression.

A one‐dimensional model of ice shelf‐ocean interaction

Large‐scale oceanic circulation beneath Antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice‐ocean interface. Dense, saline waters are drawn underneath the ice shelves and emerge, cooled and diluted, as plumes of Ice Shelf Water. A simple, one‐dimensional model of this process has been developed, in which the Ice Shelf Water plume is treated as a turbulent gravity current, initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice. Subsequent evolution of the plume, as it ascends along an ice shelf base of specified geometry, can be simulated. The model has been applied to a flow line on Ronne Ice Shelf, Antarctica, to explain the observed distribution and rate of basal melting and freezing. Calculations indicate that the present mean melt rate of 0.6 m yr−1 would increase to 2.6 m yr−1 if the underlying water were to warm by 0.6°C. This would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf.

Organic carbon flux and oxygen penetration into sediments of the Weddell Sea: indicators for regional differences in export production

The flux of reactive organic carbon (C org) into sediments of the southern and eastern Weddell Sea was estimated by modelling measured oxygen and nitrate pore-water profiles. Highest flux of reactive organic carbon into the sediment was calculated for the shelf region (500 and 600 mmol C m −2 year −1), whereas for pelagic and continental slope sediments C org fluxes of less than 60 mmol C m −2 year −1 and 100–200 mmol Cm −2 year −1 respectively were calculated. The oxygen penetration depth (OPD) ranged from less than 2 cm in shelf sediments to much greater than 40 cm in pelagic sediments. For the first time, sediments covered by the Filchner Ice Shelf (probably cut off from a source of primary production for a few decades) were sampled. In this area a restricted vertical flux of reactive organic carbon was expected. However, the C org content of these sediments was as high as that of Antarctic shelf sediments, which suggests lateral transport of organic matter. In contrast, pore-water profiles and calculated reactive organic carbon fluxes off Filchner Ice Shelf are similar to those of much deeper depositional environments (3000–4000 m water depth).

Ocean circulation beneath the Ronne ice shelf

THE intimate thermal contact between the base of Antarctic ice shelves and the underlying ocean enables changes in climate to have a rapid impact on the outflow of ice from the interior of Antarctica1,2. Furthermore, water modified by passage under ice shelves, particularly in the Weddell Sea, is believed to be an important constituent of Antarctic Bottom Water3—a water mass that can be observed as far north as 50° N in the deep oceans4. Antarctic Bottom Water is both cold and oxygen-rich, and plays an important part in the cooling and ventilation of the world's oceans. Because of the difficulty in gaining access, the oceano-graphic regime beneath ice shelves is very poorly sampled5. By successfully drilling through the ice, however, we were able to obtain oceanographic data from beneath the largest Antarctic ice shelf, the Ronne–Filchner ice shelf in the southern Weddell Sea. We find that our data agree well with the predictions of a relatively simple oceanographic plume model of sub-ice-shelf circulation6. This model can therefore be used with some confidence to investigate the links between climate changes, ice-shelf melting and bottom-water production.

‘NO’ as a conservative tracer in the Weddell Sea

The salinity maximum of the Warm Deep Water advecting into the Weddell Sea lies about 200 m below the temperature maximum and an NO minimum. The NO minimum is horizontally as well as vertically resolved on two sections towards the eastern and southern coast of the Weddell Sea, one towards Cap Norvegia, the other towards the Filchner Ice Shelf, whereas the temperature and salinity maxima are horizontally resolved on the former section only. Thus NO is a valuable complementary tool in studying the boundary current along the coast of the Weddell Sea. The NO signal indicates a non-continuity within the boundary current, as the minimum in the downstream section (less than 480 μmol kg −1 ) is deeper than the upstream one ( more than 490 μmol kg −1). The temperature maximum as well as the NO minimum descend from a depth of about 400 m in the Cap Norvegia section to about 600 m in the Filchner Ice Shelf section, the salinity maximum being correspondingly lowered. A plot of NO vs. salinity shows a continuous mixing line between Warm Deep Water and the freshest part of the Winter Water interval, thus essentially displaying Winter Water as itself lying on a mixing line. This indicates that Warm Deep Water is being advected well into the winter surface layer.

Iron in the water column of the Weddell Sea

Total, dissolved and suspended iron has been determined in the water column of the Weddell Sea area during the austral summer 1988–1989. Some data are also presented for dissolved manganese, suspended manganese and suspended aluminium. The average value for dissolved iron was found to be 1.2 nM, with somewhat higher values at the Filchner Ice Shelf. The total iron was found to be considerably higher, with a range between 1 and 6 nM in the central Weddell Sea and 1 and 25 nM at the shelves. Transport of iron from the shelves into the Weddell Sea basin is demonstrated. This is observed both for total and dissolved iron.

Numerical modelling of ice shelf dynamics

By considering the basic stress equations for a unit volume of ice, a set of differential equations describing ice shelf flow is derived. In view of the lack of basal shear stresses at the bottom of ice shelf a model simulation which is restricted to the horizontal dimensions will not imply substantial errors. The model is applied to the Filchner-Ronne Ice Shelf, Antarctica, and model equations are solved in terms of finite differences on a 10 × 10 km grid. Present ice thickness data and boundary conditions, i.e. the balance velocities at the grounding line and strain rates at the ice front are entered as input. Using a non-linear Glen-type flow law (n=3) and a constant depth-averaged flow law parameter, representing an ice temperature of −17°C, a convincing velocity field is derived as a solution of the model equations. The model takes into account restrained flow across ice rumples where sufficient field data are available. A diagnostic run reproducing present velocity magnitudes is followed by two prognostic runs, each representing 2000 years of simulation. Transient ice thickness changes are obtained from solving the mass conservation equation. Two different assumptions concerning basal melting rates demonstrate its importance to ice shelf dynamics. Assumptions are: a) no basal melting, b) basal melting rates (−2m a−1 to +3m a−1) as derived from model results and geophysical field data.

Rapid disintegration of the Wordie Ice Shelf in response to atmospheric warming

THE breaking up of ice shelves around the Antarctic Peninsula has been cited1 as a “sign that a dangerous warming is beginning in Antarctica”. Here we present satellite images showing the disintegration of the Wordie Ice Shelf, which lies off the west coast of the Antarctic Peninsula (Fig. 1). Fracture, either in the form of surface crevasses or rifts extending to the bottom of the ice shelf, has been responsible for iceberg calving and weakening the central region of the ice shelf. These fracture processes, which led to retreat of the ice front, were apparently enhanced by the presence of increased amounts of melt water, resulting from a warming trend recorded in mean annual air temperatures in Marguerite Bay. If this warming trend continues, other nearby ice shelves on the Antarctic Peninsula may be at risk. But substantial additional warming would be required before similar processes could initiate breakup of the Ross and Filchner–Ronne ice shelves, which help stabilize the West Antarctic ice sheet.

Ice‐ocean interaction on Ronne Ice Shelf, Antarctica

Detailed glaciological studies have been completed at 28 sites lying on an approximate flow line, extending 760 km across Ronne Ice Shelf. Parameters measured at each location include ice velocity, thickness, principal strain rates, surface elevation, temperature, and accumulation rate. The data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site, from the principles of mass and energy conservation. These calculations indicate basal melting in excess of 1 m yr−1 over the first 100 km of the flow line downstream of the grounding line, where the ice shelf is between 1200 m and 1600 m thick. The maximum melt rates in this region occur near the inland margin and exceed 4 m yr−1. Melting continues at a lesser rate over the next 200 km before freezing commences. Freezing then dominates up to the final 100 km before the ice front, resulting in the accumulation of a layer of basal sea ice up to 50 m thick. This is rapidly removed as melt rates increase to over 6 m yr−1 at the ice front. The cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85% of the mass discharged across the grounding line before it reaches the ice front. Supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections, observed during radio echo sounding of ice thickness. In the region where mass balance calculations suggest an accretion of basal sea ice, reflection coefficients are consistently low, ranging from −6 dB to −38 dB. Most of these weak reflections are believed to originate from the true base of the ice shelf, the additional energy loss being the result of increased attenuation of the radar signal within the saline layer. The derived pattern of basal melting and freezing is consistent with a simple model of sub‐ice‐shelf oceanic circulation, involving a deep thermohaline convection cell. Dense, saline water, which is formed over the continental shelf during winter when the sea surface freezes, drains into the deepest parts of the sub‐ice‐shelf cavity. At the inland margin, where this water mass comes into contact with the ice shelf, its temperature is 1°C above the local pressure freezing point. Melting of ice results, producing a buoyant outflow of cold, relatively fresh water, along the ice shelf base. Basal freezing occurs towards the ice front, where the ascending water becomes supercooled. This circulation has important implications for the production of Antarctic Bottom Water and for the response of the ice shelf to driving stresses, through the temperature dependent viscosity of ice.

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

AbstractNew tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

The use of tiltmeters to study the dynamics of Antarctic ice-shelf grounding lines

Abstract New tiltmeter data are presented from Doake Ice Rumples on Ronne Ice Shelf, Antarctica. Five sites which showed a tidal ice-shelf flexure have been analysed using an elastic beam model to investigate the variation of flexure amplitude with distance from the grounding line. An earlier study on Rutford Ice Stream which also used an elastic model required an ice thickness much less than that observed. Reworking the Rutford Ice Stream data suggests that this greatly reduced ice thickness is not required, given the current sparse data coverage. The elastic model is used to improve the estimated grounding-line position on Rutford Ice Stream. Some of the difficulties in modelling ice-shelf flexure and locating grounding lines are discussed.

The 520-Year Temperature Record of a 100 M Core from the Ronne Ice Shelf, Antarctica

Evidence for climatic changes during the last 520 years was inferred from 18O content of a 100 m ice core from the Ronne Ice Shelf, Antarctica. The core was stratigraphically dated using seasonal variations of l8O content. The mean 18O content of the annual layers calculated on the basis of this dating decreases with depth z according to δ18O (‰) = −27.3 - 0.049z (m) and reflects first of all the decrease of the 18O content of the near-surface layers in the catchment area of the core from the drilling site 250 km to the south. This effect was corrected by assuming a linear decrease of the 18O content of the near-surface layers with increasing distance from the drilling site. Corrected δ18O values show a large scatter from year-to-year due to the local variability. The smoothed isotopic record displays variations in different time scales, which are caused most probably by climatological induced temperature variations. The gradient of 18O content with the 10 m firn temperature of 1.15‰/K found in the middle part of the Filchner-Ronne Ice Shelf was used to transfer the 18O series to a temperature record.

The 520-Year Temperature Record of a 100 M Core from the Ronne Ice Shelf, Antarctica

Evidence for climatic changes during the last 520 years was inferred from 18O content of a 100 m ice core from the Ronne Ice Shelf, Antarctica. The core was stratigraphically dated using seasonal variations of l8O content. The mean 18O content of the annual layers calculated on the basis of this dating decreases with depth z according to δ18O (‰) = −27.3 - 0.049z (m) and reflects first of all the decrease of the 18O content of the near-surface layers in the catchment area of the core from the drilling site 250 km to the south. This effect was corrected by assuming a linear decrease of the 18O content of the near-surface layers with increasing distance from the drilling site. Corrected δ18O values show a large scatter from year-to-year due to the local variability. The smoothed isotopic record displays variations in different time scales, which are caused most probably by climatological induced temperature variations. The gradient of 18O content with the 10 m firn temperature of 1.15‰/K found in the middle part of the Filchner-Ronne Ice Shelf was used to transfer the 18O series to a temperature record.

Evaluation Of Landsat, Spot, and Sojuzkarta Data Of Antarctica For Ice and Climate Research

Satellite data have proved suitable and cost-effective for ice and climate research in Antarctica. They have been used in numerous research efforts, including: monitoring coastal change, determining velocities of outlet glaciers, defining blue ice areas, tracking the movement of icebergs, and as a base for overlaying radar and other data. This paper reviews the data acquired since 1972 by the Landsat, SPOT and Sojuzkarta space systems and illustrates where good-quality (±10% cloud cover) data are available.The Landsat 1, 2, and 3 satellites acquired approximately 10 000 multispectral scanner (MSS) (80 m resolution) and return-beam vidicon (RBV) (30 m resolution) photographic and digital images of Antarctica between 1972 and 1983. The quality of these images has been evaluated by USGS to determine the feasibility of using them for glaciological, climatological, and geological research. Results have been plotted on a base map of Antarctica and show about 45% of the nominal scene centers, or about 70% of the geographic area of the continent, to be covered by good-quality imagery. Landsat 4 started acquiring data in 1982 and Landsat 5 in 1984. Together the two satellites acquired more than 4 500 thematic mapper (TM) (30 m resolution) and MSS images of Antarctica through the 1988–89 austral summer season, which may be the last season of Landsat 4 and 5 acquisition. The majority of the scenes have been acquired by two major projects: the West German Institut für Angewandte Geodäsie project to acquire data from the Ronne Ice Shelf area to the Greenwich meridian, and the USGS project to acquire data of coastal Antarctica and the inland boundary of the ice shelves, for use by the Scientific Committee on Antarctic Research (SCAR) community. Landsat 4 and 5 have different orbital parameters from Landsat 1, 2, and 3. As a result Landsat 4 and 5 imagery have different areal coverage. The data also have been archived differently. While the Landsat 1, 2, and 3 data exist in film format, many Landsat 4 and 5 TM images exist only as high-density digital tapes, and, as a result, the scenes cannot be examined and evaluated before purchase. Computer evaluation of the Landsat 4 and 5 data indicate that about 1350 or about 30% of the images have 10% cloud cover or less. However, it is difficult for automatic systems or those unfamiliar with the area to discriminate between clouds and snow. Based on a sampling of the data, it is more likely that 10–15%, or about 500 scenes, are of good quality. Those scenes which have been examined and evaluated as part of the SCAR cooperative acquisition project and are known to be of good quality are plotted and compared with the earlier Landsat data. The combined plot gives accurate and reliable information on the location of good quality Landsat 1–5 data of Antarctica.More than 7 000 Sojuzkarta scenes have been collected over Antarctica since 1976. The most useful data consist of photography from the KATE-200 system (20 m resolution) and the KFA-1000 system (6 m resolution). The data have been plotted by the U.S. Geological Survey and will soon be available as an open-file report. Those scenes which have been evaluated as having 10% cloud cover or less are plotted separately and overlaid on the Landsat plots. Scenes which have been examined are excellent, but an insufficient number of scenes have been inspected to evaluate the accuracy of the cloud cover estimation.The Systeme Probatoire d'Observation de la Terre (SPOT) was launched in 1986 and has acquired more than 1200 scenes of Antarctica. SPOT data consist of multi-spectral (20 m resolution) and panchromatic (10 m resolution) photographic or digital imagery. SPOT also possesses a mechanical plane mirror which permits off-nadir imaging and the generation of stereoscopic images. The locations of these scenes are now being plotted by the U.S. Geological Survey. Scenes with 10% cloud cover or less will be combined with Landsat and Sojuzkarta data to provide an accurate summary of the availability and coverage of high-quality satellite data of Antarctica.