Sub-ice Shelf Research Articles

A new depositional model for ice shelves, based upon sediment cores from the Ross Sea and the MaC. Robertson shelf, Antarctica

We document the similarity of depositional facies occurring in gravity cores recovered from two disjunct regions of the Antarctic continental shelf: the Ross Sea and the MaC. Robertson shelf. The facies sequence model is represented in two cores, one collected during the 1995-1 cruise of the R/VNathaniel R. Palmer(core NBP95 TC-18) and the other collected by the RSVAurora Australisduring cruise 149 in 1995 (core 149 39GC38). Both cores show a succession of facies indicative of ice-shelf retreat during the late-Pleistocene to Holocene transition. Distinct lithofacies range in thickness from a few tens of cm to 1 m and consist of (from bottom to top) a coarse, granulated sandy mud; laminated silt and clay; structureless silly clay; poorly sorted sandy siliceous mud; and siliceous mud and ooze. These facies represent the passage of distinct depositional regimes across the core sites, including sub-ice shelf beneath a basal debris zone; sub-ice shell distal to a debris zone; calving-line transition; and open marine. This facies model represents an advance in our understanding of Glacial marine stratigraphy for the Antarctic continental shelf and will provide the basis for more realistic palaeoglacial reconstructions.

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.

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.

Late quaternary glacial history of the South Orkney Plateau, Antarctica

Piston cores from the South Orkney Plateau penetrated overcompacted diamictons in water depths of up to 250 m. Detailed textural and petrological analyses of these diamictons indicate that they are basal tills. Seismic records from the plateau show a widespread surface of glacial erosion and provide additional evidence of an ice cap grounded to a depth of 250 m. Piston cores from the slope of the plateau penetrated diatomaceous muds resting directly on poorly sorted muds with very little to no biogenic material. The ice-rafted debris in these glacial-marine sediments is composed almost exclusively of material derived from the South Orkney Islands. This implies deposition beneath an ice shelf as opposed to iceberg rafting. In contrast, diatomaceous muds contain relatively abundant exotic iceberg-rafted material and reflect a glacial-maritime setting similar to that of today. The sharp contact separating diatomaceous surface sediments from basal tills and sub-ice shelf deposits indicates that the ice cap and ice shelf retreated from the plateau rapidly. Radiocarbon dates for diatomaceous muds from a glacial trough on the plateau indicate that the ice cap and ice shelf retreated from the plateau prior to 6000 to 7000 years ago. The homogeneity of surficial diatomaceous sediments suggests that sea ice conditions over the plateau have not changed radically since that time.

A two-dimensional model for the thermohaline circulation under an ice shelf

The production of Antarctic Bottom Water is influenced by Ice Shelf Water which is formed due to the modification of shelf water masses under huge ice shelves. The coupling of inflow conditions, thermohaline processes at the ice shelf base and the sub-ice shelf circulation is studied with a two-dimensional thermohaline circulation model which has been developed for a section perpendicular to the ice shelf edge. Different boundary conditions appropriate to the Filchner Ice Shelf regime are considered. The model results indicate that, in general, shelf water is transported toward the grounding line, where at the ice shelf base melting occurs with a maximum rate of 1.5 my−1. Accumulation of ice takes place at the end of the melting zone close to the ice shelf edge with a rate on the order of 0.1 my−1. The location of this accumulation zone determines whether or not the density increase by salt rejection causes an upper circulation cell and the separation of the modified water mass from the ice shelf base at mid-range depth. At the ice shelf edge the simulated temperature, salinity, helium and δ18O values for the temperature minimum layer are typical for Ice Shelf Water. However the sub-ice shelf circulation is highly variable as well as sensitive to changes in boundary conditions. Moderate changes in the characteristics of the inflowing water or in sea-floor topography may double the intensity of the circulation. Non-linear processes in the accumulation zone cause variabilities which can be described by an ice shelf edge oscillator influencing the entire circulation regime.

Sedimentology of the lower huronian supergroup (early proterozoic), Elliot lake area, Ontario, Canada

The Early Proterozoic Huronian Supergroup is a slightly metamorphosed sedimentary succession locally > 10 km thick, exposed to the north of Lake Huron, Ontario, Canada. The lower part of the supergroup, comprising the Matinenda, McKim, Ramsay Lake and Pecors formations, was studied in the Quirke Syncline area, Elliot Lake, and regions immediately to the west and south. Thirteen lithofacies are present: trough and planar cross-stratified sandstone, quartz pebble conglomerate, clay-rich sandstone, lenticular-bedded sandstone-siltstone-mudstone, laminated sandstone-siltstone-mudstone, massive sandstone- massive pebbly sandstone, massive diamictite, stratified matrix-supported diamictite, stratified clast-supported diamictite, graded diamictite, and massive siltstone. The lithofacies may be grouped into associations which correspond approximately to the mapped formations. The crossbedded sandstones and conglomerates comprise a fluvial association (Matinenda Formation), which was formed mainly in shallow braided channels. Clay-rich sandstone and the lenticular and laminated lithofacies, constituting portions of the McKim and Pecors Formations, were deposited on the strand and offshore areas of braid deltas, interfingering laterally and vertically with the Matinenda fluvial deposits. Four glacigenic subassociations are present in the Ramsay Lake Formation. An ice-proximal subassociation (I) consists of massive pebbly sandstone and diamictite which progressively truncates the Matinenda and McKim Formations to the north. The massive sandstone may be an ice-override deposit formed by erosion and reworking of the underlying Matinenda Formation, or a sub-ice shelf rain-out accumulation of locally eroded glacial debris. Subassociations II and III consist mainly of subaqueous sediment gravity flow and ice rain-out deposits. Subassociation IV is interpreted as an ice-proximal fluvial outwash deposit containing resedimented blocks of moraine material. Stratigraphic and paleocurrent data indicate a south-dipping paleoslope that tilted southeast during the deposition of the fluvial and deltaic units. A trough of relatively deep water extended NW-SE across the region, transverse to the E-W divergent continental margin hinge line that is assumed to have existed to the south of the project area. Analogies are suggested to Mesozoic-Recent basins on the Atlantic margin of Canada, where transverse paleogeographic trends reflect either landward extension of active oceanic fracture zones, or reactivation of earlier structural lineaments.