Keith Makinson

Rapid erosion, drumlin formation, and changing hydrology beneath an Antarctic ice stream

What happens beneath a glacier affects the way it flows and the landforms left behind when it retreats. Direct observations from beneath glaciers are, however, rare and the subglacial environment remains poorly understood. We present new, repeat observations from West Antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record. We interpret erosion at a rate of 1 m a−1 beneath a fast-flowing ice stream, followed by cessation of erosion and the formation of a drumlin from mobilized sediment. We also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed. All these changes occurred on time scales of a few years or less. This variability suggests that an ice stream can reorganize its bed rapidly, and that present models of ice dynamics may not simulate all the relevant subglacial processes.

Modeling tidal currents beneath Filchner‐Ronne Ice Shelf and on the adjacent continental shelf: Their effect on mixing and transport

A depth-averaged tidal model has been applied to the southern Weddell Sea. The model domain covers the southern continental shelf, including the ocean cavity beneath Filchner-Ronne Ice Shelf. Reasonable agreement with the available current meter data has been achieved. Our results confirm that in areas with shallow water and large topographic gradients, tidal oscillations with peak velocities up to 1 m s−1 play a significant role in the vertical mixing and transport of water masses. The estimated energy dissipation beneath Filchner-Ronne Ice Shelf due to surface friction is 25 GW, approximately 1% of the worlds total tidal dissipation. Tidally induced Lagrangian residual currents converging at the ice front, an area of strong mixing, draw together water masses from the continental shelf and sub-ice shelf cavity. The model indicates that Lagrangian residual currents have fluxes of up to 250,000 m3 s−1, and speeds of over 5 cm s−1 along the ice front, with over 350,000 m3 s−1 being exchanged between the sub-ice shelf cavity and adjacent continental shelf. These currents are particularly efficient in ventilating the sub-ice shelf cavity within 150 km of Ronne Ice Front. Such strong tidal mixing will significantly modify the properties of water masses that flow through this region, particularly to the west of Berkner Island. The model predictions indicate that tidal processes strongly influence the oceanographic conditions in the vicinity of Ronne Ice Front. Shipborne observations along the ice front support many of the model predictions concerning the effect of tides on the hydrography.

Influence of tides on melting and freezing beneath Filchner-Ronne Ice Shelf, Antarctica

An isopycnic coordinate ocean circulation model is applied to the ocean cavity beneath Filchner-Ronne Ice Shelf, investigating the role of tides on sub-ice shelf circulation and ice shelf basal mass balance. Including tidal forcing causes a significant intensification in the sub-ice shelf circulation, with an increase in melting (3-fold) and refreezing (6-fold); the net melt rate and seawater flux through the cavity approximately doubles. With tidal forcing, the spatial pattern and magnitude of basal melting and freezing generally match observations. The 0.22 m a(-1) net melt rate is close to satellite-derived estimates and at the lower end of oceanographic values. The Ice Shelf Water outflow mixes with shelf waters, forming a cold (<-1.9 degrees C), dense overflow (0.83 Sv) that spills down the continental slope. These results demonstrate that tidal forcing is fundamental to both ice shelf-ocean interactions and deep-water formation in the southern Weddell Sea. Citation: Makinson, K., P. R. Holland, A. Jenkins, K. W. Nicholls, and D. M. Holland (2011), Influence of tides on melting and freezing beneath Filchner-Ronne Ice Shelf, Antarctica, Geophys. Res. Lett., 38, L06601, doi: 10.1029/2010GL046462.

Oceanographic conditions south of Berkner Island, beneath Filchner-Ronne Ice Shelf, Antarctica

We have made oceanographic measurements at two sites beneath the southern Filchner-Ronne Ice Shelf. Hot-water drilled access holes were made during January 1999, allowing conductivity-temperature-depth (CTD) profiling and the deployment of instrument moorings. The CTD profiles show that the entire water column is below the surface freezing point. We estimate the (summer) flux of water between the two sites to be 2×106 m3 s−1. The summer potential temperature-salinity properties of the water column suggest that this flow is part of a recirculation in the deepest part of the subice shelf cavity and the Filchner Depression. The recirculation is driven by a combination of the melting of deep basal ice and the freezing that results from the depressurization of the cold buoyant water as it ascends the ice shelf base. The source of the water was high-salinity shelf water (HSSW) produced in the Ronne Depression. This is the water that provides the external heat necessary for the strong melting at the deep grounding lines in the vicinity of Foundation Ice Stream. Instruments moored at the drill sites show that during the winter HSSW formed on the Berkner Shelf flows beneath the ice shelf and largely displaces the recirculating water from the two sites. This provides an externally driven through flow that is warmer (nearer the surface freezing point) and slower than the internal recirculation and which is low enough in density to escape the Filchner Depression.

Tidal influence on Rutford Ice Stream, West Antarctica : observations of surface flow and basal processes from closely spaced GPS and passive seismic stations

High-resolution surface velocity measurements and passive seismic observations from Rutford Ice Stream, West Antarctica, 40 km upstream from the grounding line are presented. These measurements indicate a complex relationship between the ocean tides and currents, basal conditions and ice-stream flow. Both the mean basal seismicity and the velocity of the ice stream are modulated by the tides. Seismic activity increases twice during each semi-diurnal tidal cycle. The tidal analysis shows the largest velocity variation is at the fortnightly period, with smaller variations superimposed at diurnal and semi-diurnal frequencies. The general pattern of the observed velocity is two velocity peaks during each semi-diurnal tidal cycle, but sometimes three peaks are observed. This pattern of two or three peaks is more regular during spring tides, when the largest-amplitude velocity variations are observed, than during neap tides. This is the first time that velocity and level of seismicity are shown to correlate and respond to tidal forcing as far as 40 km upstream from the grounding line of a large ice stream.

Effect of critical latitude and seasonal stratification on tidal current profiles along Ronne Ice Front, Antarctica

[1] The ice front region of Ronne Ice Shelf lies near the critical latitude of the semidiurnal M2 tide, the principal tidal constituent in the southern Weddell Sea. Here the Coriolis frequency almost equals the M2 tidal frequency, resulting in a strong dependence of the M2 tidal currents on depth and stratification and a boundary layer that can occupy the entire water column. Using data from four long-term moorings along Ronne Ice Front, we confirm the presence of strongly depth-dependent semidiurnal tidal currents and their sensitivity to changes in stratification. The time series show dramatic seasonal changes in tidal current profiles and significant interannual variability. During periods of stratification, the amplitude of the semidiurnal tides in the mid–water column shows a twofold increase and, despite being several kilometers offshore from the ice front, the tidal currents clearly show a second boundary layer originating from the adjacent ice shelf base. Together, these two boundary layers occupy most of the water column, up to 600 m deep, until intense sea ice formation and the production of High-Salinity Shelf Water erodes the vertical stratification. During winter when homogeneous conditions prevail, a single bottom boundary layer occupies the entire water column at some locations. This strong seasonality and sensitivity of the M2 tidal current to stratification highlights the difficulties of interpreting current data from short-term moorings while demonstrating that it is the best indicator for characterizing changes in stratification after direct observations of density variations.

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.

Modeling tidal current profiles and vertical mixing beneath Filchner-Ronne Ice Shelf, Antarctica

Abstract One of the warmest water masses beneath Filchner–Ronne Ice Shelf (FRIS) is dense, high salinity shelf water (HSSW) that flows into the sub-ice-shelf cavity from the ice front and occupies the lower portion of the water column. A one-dimensional turbulence closure ocean model has been applied to this high latitude sub-ice-shelf environment to demonstrate that tidal currents mix HSSW vertically through the water column and cause melting at the bottom of the ice shelf. Significantly FRIS lies near the critical latitude for the semidiurnal tide, where the Coriolis frequency equals the tidal frequency, resulting in a strongly depth-dependent tidal current and thick boundary layers. Using the model, the effect of the critical latitude, stratification, and the polarization of the tidal current ellipse on boundary layer structure and subsequent vertical mixing are examined. The model shows that stratification significantly affects how the shape of the tidal current ellipse varies with depth and that both...

Diurnal and semidiurnal tide‐induced lateral movement of Ronne Ice Shelf, Antarctica

[1] Recent GPS observations from a spatially extensive network across Ronne Ice Shelf show significant daily ice flow variations. At all sites, the almost-synchronous horizontal displacements occur at diurnal and semidiurnal tidal periods. During spring tides, displacements, velocities and strains near the ice front have superimposed oscillations that are 300% of their mean values and occur over a six-hour period, resulting in regular ice shelf flow reversals. Close to ice stream grounding lines, however, the horizontal diurnal and semidiurnal signals decay and almost vanish. From our analysis, we conclude that ice shelves respond primarily elastically to tidal tilting, thus accounting for the observed diurnal and semidiurnal flow variations, and their amplification toward the ice shelf front. Our findings suggest that detailed modeling of these data could provide improved ice shelf and ice stream models for correctly simulating ice shelf flow and predicting future ice sheet evolution. Citation: Makinson, K., M. A. King, K. W. Nicholls, and G. Hilmar Gudmundsson (2012), Diurnal and semidiurnal tide-induced lateral movement of Ronne Ice Shelf, Antarctica, Geophys. Res. Lett., 39, L10501, doi:10.1029/2012GL051636.

The BAS hot water drill: development and current design

Abstract 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.