Large Ice Shelves Research Articles

Modelling of rift propagation on Ronne Ice Shelf, Antarctica, and sensitivity to climate change

The calving of icebergs from large Antarctic ice shelves is controlled mainly by the formation and propagation of rifts originating from the side margins of the ice shelf and local areas of grounding. Using InSAR, we observe the evolution of rifts along Hemmen Ice Rise, on Ronne Ice Shelf, Antarctica prior to the large calving event of October 1998. We couple these observations with a computer model combining the viscous flow of an ice shelf with a linear elastic fracture mechanics description of the propagation of rifts. The model reveals that the ice melange trapped in between the rifts exerts a major control on the propagation of rifts, and in turn on ice shelf stability. Melting of the ice melange from oceanic or atmospheric warming would significantly increase the propagation rate of rifts and threaten the ice‐shelf stability.

The Error of Judgment: Struggling for Neutrality in Science and Journalism

The Error of Judgment: Struggling for Neutrality in Science and Journalism

Spatial and temporal variability of net snow accumulation over the Antarctic from ECMWF re-analysis project data

Forecasts from the ECMWF re-analysis project (ERA) covering the period 1979–1993 are used to examine the spatial and temporal distribution of net snow accumulation (precipitation minus evaporation) over the Antarctic continent. There is generally good agreement between the spatial distribution of net accumulation in the model data, when the 15 year mean annual accumulation is considered, and the equivalent maps produced in earlier studies from in situ data. One of the major differences is the westerly displacement of the accumulation maximum on the western side of the Antarctic Peninsula as a result of the model orography having a high degree of smoothing in the east–west direction. The mean annual net accumulation in the ERA data for the whole of the continent is 151 mm year−1, equivalent to a total accumulation of 2106×1012 kg year−1. The mean accumulation value is in reasonable agreement with the best estimates from glaciological data, which recent studies have suggested is in the range 150–170 mm year−1. The lower value from ERA is partly a result of overestimation of evaporation/sublimation from the large ice shelves during the summer and spring, and an underestimation of the precipitation in the interior of the continent. The accumulation from the ERA data is the same as that computed from the operational ECMWF forecasts. During the 15 year data period, the mean accumulation varied from 129.1 mm (1987) to 171.8 mm (1984). Considering the continent as a whole, net accumulation was at a minimum in the summer season, although in coastal parts of West Antarctica, the minimum occurs in spring. The ERA accumulation data for the interior of the continent show no annual cycle and are significantly smaller than the available in situ measurements. At certain coastal sites in West Antarctica there is a clear relationship between annual precipitation and cyclone activity, although in East Antarctica such a relationship is only apparent in monthly data. Copyright © 1999 Royal Meteorological Society

The semi-annual oscillation and Antarctic climate. Part 1: influence on near surface temperatures (1957–79)

We studied the influence of the semi-annual oscillation (SAO) on near-surface temperatures in Antarctica, using observations of 27 stations that were operational during (part of) the period 1957–79. For the annual cycle of surface pressure, the second harmonic explains 17–36% of the total variance on the Antarctic Plateau, 36–68% along the East Antarctic coast and almost 80% on the west coast of the Peninsula, and decreases further to the north. As a result of the amplification of the wave-3 structure of the circulation around Antarctica, a significant modification of the seasonal cooling is observed at many stations. The magnitude of this modification is largely determined by the strength of the temperature inversion at the surface: the percentage of the variance explained by the second harmonic of the annual temperature cycle is then largest on the Antarctic Plateau (11–18%), followed by the large ice shelves and coastal East Antarctica (6–12%) and stations at or close to the Peninsula (0–5%). A significant coupling between the half-yearly wave in surface pressure and that in surface temperature is found for coastal East Antarctica, which can be directly explained by the changes in meridional circulation brought about by the SAO. We show that the coupling of Antarctic temperatures to the meridional circulation is not only valid on the seasonal time scale of the SAO, but probably also on daily and interannual time scales. This has important implications for the interpretation of time series of Antarctic temperatures, a problem that will be addressed in part 2 of this paper.

Spatial and temporal variation of sublimation on Antarctica: Results of a high‐resolution general circulation model

In this paper we use output of a high‐resolution general circulation model (ECHAM‐3 T106, resolution 1.1°×1.1°) to study the spatial and temporal variation of sublimation on Antarctica. First, we compare model results with available observations of sublimation rates. The yearly cycle, with small latent heat fluxes during the winter, is well reproduced, and the agreement with sparsely available spot observations is fair. The model results suggest that a significant 10–15% of the annual precipitation over Antarctica is lost through sublimation and that sublimation plays an important role in the formation of blue ice areas. A preliminary analysis of the atmospheric boundary layer moisture budget shows that the spatial variation of sublimation in the coastal zone of East Antarctica can be explained by variations of horizontal advection of dry air. Dry air advection, and thus surface sublimation, is enhanced in areas where katabatic winds are strong and have a large downslope component and where the Antarctic topography drops suddenly from the plateau to the coastal zone. In areas where horizontal advection is small, like the plateau and the large ice shelves, special conditions must be met to make significant sublimation at the surface possible.

Relating the occurrence of crevasses to surface strain rates

AbstractThe presence of crevasses on the surface of ice masses indicates that a fracture criterion has been met. Understanding how crevasses form will provide information about the stress and strain-rate fields in the ice. This study derives a relationship between measurements of strain rate and observations of crevassing on the surface of ice masses. A literature search yielded 17 polar and alpine locations where strain rates had been measured and crevassing recorded. By plotting strain rates (converted to stresses using a creep law) using axes representing the surface-parallel principal stresses, failure envelopes were derived by enclosing measurements where surface crevassing was absent. The derived failure envelopes were found to conform well to theoretical ones predicted by the Coulomb and the maximum octahedral shear stress (von Mises) theories of failure. The derived failure envelopes were scaled by the tensile strength, which was found to vary from 90 to 320 kPa. There was no systematic variation of tensile strength with either temperature at 10 m depth or the method used to locate the crevasses. The observed variation in tensile strength could result from variations in ice properties (e.g. crystal size, impurity content or density) or could be related to uncertainty in the constitutive relation. Creep flow and fracture share a very similar temperature dependence, suggesting similar crystal-scale processes are responsible for both. The observed relationship will provide a supplementary tool with which to verify and test models of ice dynamics against remotely sensed imagery. The study also indicates that a temperature rise of a few degrees throughout the ice column will not result directly in any increase in calving rates from the large Antarctic ice shelves such as the Filchner–Ronne or Ross Ice Shelves.

Relating the occurrence of crevasses to surface strain rates

AbstractThe presence of crevasses on the surface of ice masses indicates that a fracture criterion has been met. Understanding how crevasses form will provide information about the stress and strain-rate fields in the ice. This study derives a relationship between measurements of strain rate and observations of crevassing on the surface of ice masses. A literature search yielded 17 polar and alpine locations where strain rates had been measured and crevassing recorded. By plotting strain rates (converted to stresses using a creep law) using axes representing the surface-parallel principal stresses, failure envelopes were derived by enclosing measurements where surface crevassing was absent. The derived failure envelopes were found to conform well to theoretical ones predicted by the Coulomb and the maximum octahedral shear stress (von Mises) theories of failure. The derived failure envelopes were scaled by the tensile strength, which was found to vary from 90 to 320 kPa. There was no systematic variation of tensile strength with either temperature at 10 m depth or the method used to locate the crevasses. The observed variation in tensile strength could result from variations in ice properties (e.g. crystal size, impurity content or density) or could be related to uncertainty in the constitutive relation. Creep flow and fracture share a very similar temperature dependence, suggesting similar crystal-scale processes are responsible for both. The observed relationship will provide a supplementary tool with which to verify and test models of ice dynamics against remotely sensed imagery. The study also indicates that a temperature rise of a few degrees throughout the ice column will not result directly in any increase in calving rates from the large Antarctic ice shelves such as the Filchner–Ronne or Ross Ice Shelves.

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.

Fluctuations in ice shelves and outlet glaciers in Antarctica

Abstract Interpretation of maps and satellite imagery reveals that during the last 90–100 years the large ice shelves and outlet glaciers of Antarctica have undergone pronounced changes, especially abrupt fluctuations in their frontal positions and dimensions. During the period under consideration phases of advance and disintegration resembling surges were manifested by many Antarctic glaciers. During the advance phases of 1947–1960 and 1965–1975 the Larsen, Filchner, and Amery ice shelves and the Shirase outlet glacier displayed pronounced surges. Analysis of fluctuations in the major glaciers of West and East Antarctica revealed asynchronous and out‐of‐phase elements in their behavior. This is apparently related to the differences in the dynamics and mass balances of the glaciers which manifested themselves in differing responses to the same climatic fluctuations.

Antarctic cryogenic sediments: Biotic and inorganic facies of ice shelf and marine-based ice sheet environments

Cryogenic sediments, defined as marine deposits that form in association with ice (icebergs, sea ice, ice shelves or marine-based ice sheets), are widespread in high latitude regions. Existing models for cryogenic depositional processes are of limited utility because of (1) lack of direct observational data, (2) reliance on older land-exposed sections from which inferential conclusions have been drawn, (3) employment of inadequate glaciologic and oceanographic concepts, and (4) failure to incorporate adequately information on the biotic content of these deposits. As a first step toward developing a synthetic model for cryogenic deposition, we relate glaciologic, sedimentologic, and biotic processes of the modern ice shelf and marine-based ice sheet environments using data from Antarctic continental shelves. The principal deposit beneath grounded marine-based ice sheets is lodgement till, which often contains reworked and fragmented fossil material. This till is identical in all important sedimentologic respects with terrestrial lodgement deposits. Marine lodgement tills commonly form geographically extensive sheets which are restricted to continental shelf sites, and which may be differentiated from other cryogenic deposits by their high degree of compaction and reworked fossils. Waterlain till may form beneath grounded ice in areas where debris melting out of the glacier sole falls through a thin water layer (up to a few m) between the ice and its bed. Waterlain till usually lacks fossil material, unless the source material for the subglacial debris is fossiliferous, and is less compact than lodgement till. Deposition beneath large polar ice shelves (e.g., Ross, Filchner-Ronne) is largely restricted to a zone (probably <100 km in width) near the grounding line. In this zone, a complex of primary and secondary cryogenic deposits (including waterlain till, ice shelf rafted detritus (ISRD), flow till, slumps, etc.) is likely to interfinger. Processes controlling this deposition include: local basal conditions of the ice sheet and ice shelf (freezing or melting); presence or absence of subglacial outflow, marine currents, and tidal action; sediment supply; long- and short-term grounding line movement; sea-floor topography; and water depth. Deposition of ISRD is limited elsewhere beneath these large ice shelves because high surface accumulation rates cause downward particle paths for ice and debris. Thus most debris is released soon after passing the grounding line. Biotic activity beneath ice shelves declines rapidly, in diversity and abundance, between the calving margin and grounding line. Deposition rates seaward of the calving margin are much higher than beneath the large ice shelves. Here, icebergs derived from outlet glaciers and ice streams that terminate directly in the sea, may carry significant loads of subglacial, englacial, or superglacial debris, which is released by a variety of mechanisms as the icebergs melt. This ice-rafted detritus mixes with biotic material from planktonic organisms as it falls through the water column to form compound IBRD. If currents are present in the water column or at the seafloor, sorting may occur to form residual IBRD. On shallow banks, grounded icebergs carrying subglacial debris may melt in situ forming iceberg till, and iceberg ploughing of sediment may form iceberg turbate. Both these deposits may be difficult to distinguish from lodgement till but both are restricted to areas with modern water depths less than ≈ 450 m .

The Antarctic ice sheet, its history and response to sea level and climatic changes over the past 100 million years

The development, surface form, flow and erosive activity of the Antarctic ice sheet over the past 100 million years is related to geological evidence that indicates long-term trends of global climate. The glaciological description of the evolution of the ice sheet covers its interactions with the atmosphere and with the ocean. Discussion focuses on changes of the equilibrium line altitude (ELA) with time, of the accumulation rate with global temperature and basal melting of ice shelves by ocean waters. The tendency of global climate to change rapidly or to drift slowly between two near-equilibrium states involves two types of changes. The former that has been widely recognized is reversible. A second type involving ice sheets or ice shelves appears to be almost irreversible since they remain in their changed state unless a much larger reversal of factors driving climate occurs. We refer to any rapid but reversible changes as climatic jumps and to the latter as climate-ice switches. Variability of climate in response to changing radiation with the earth's orbital parameters and other causes is included in our model. Extensive glaciation of East Antarctica commenced after 40 m.y. B.P. with wide fluctuations until shortly after 20 m.y. B.P. A climate-ice switch then prevented any major deglaciation. A further climate-ice switch associated with the ELA falling below sea level occurred soon after 10 m.y. B.P. This led to rapid formation of ice shelves and the marine ice sheet of West Antarctica which advanced as grounded ice to the edge of the continental shelf. After 5 m.y. B.P. the grounded ice started to retreat, extensive ice shelves remained and most glacial debris was then deposited beneath the inner regions of ice shelves. Response to Northern Hemisphere glaciation since 2.4 m.y. B.P. during glacial periods involved extension of the grounded ice area, a large decrease of mass input and decreased melting of outer parts of large ice shelves while the melting rate beneath inner regions was probably similar to the present. Marine core evidence suggests that retreat caused by global cooling has been dominant since 5 m.y. B.P. and that changes in extent due to Northern Hemisphere glaciation since 2.4 m.y. B.P. have had different effects on the major and on smaller northerly ice shelves.

Glacial Geodetic Contributions to the Mass Balance and Dynamics of Ice Shelves

The glacial geodetic contribution to the mass balance and dynamics of ice shelves includes repeated determinations of the absolute position (ϕ,λ,Η) of selected points (using satellite methods), the establishment of relative positions (y,x,Δh) in deformation figures, and height measurements. The results are used to establish ice-flow velocities and directions, strain and rotation rates, and changes in height. Modelling of deformation parameters at a few points over a large ice shelf is made possible by the collocation method. Results of these observations and analysis of Ekström Ice Shelf for the period 1979–87 are reported.

Glacial Geodetic Contributions to the Mass Balance and Dynamics of Ice Shelves

The glacial geodetic contribution to the mass balance and dynamics of ice shelves includes repeated determinations of the absolute position (ϕ,λ,Η) of selected points (using satellite methods), the establishment of relative positions (y,x,Δh) in deformation figures, and height measurements. The results are used to establish ice-flow velocities and directions, strain and rotation rates, and changes in height. Modelling of deformation parameters at a few points over a large ice shelf is made possible by the collocation method. Results of these observations and analysis of Ekström Ice Shelf for the period 1979–87 are reported.

A Time-Dependent Simulation of the Ross Ice Shelf Flow (Abstract)

The finite-element model discussed by MacAyeal and Thomas (1982) has been improved to include solution of the heat equation within each element, and to accelerate convergence to solution of the momentum-balance equations in terms of ice-shelf spreading rates. The model is now sufficiently rapid to permit both snapshot and time-marching simulations of a large ice shelf at high spatial and temporal resolution (grid size 10 km; time step 0.1 a). Here, we describe three model simulations of Ross Ice Shelf behavior.

A Time-Dependent Simulation of the Ross Ice Shelf Flow (Abstract)

The finite-element model discussed by MacAyeal and Thomas (1982) has been improved to include solution of the heat equation within each element, and to accelerate convergence to solution of the momentum-balance equations in terms of ice-shelf spreading rates. The model is now sufficiently rapid to permit both snapshot and time-marching simulations of a large ice shelf at high spatial and temporal resolution (grid size 10 km; time step 0.1 a). Here, we describe three model simulations of Ross Ice Shelf behavior.

Ice-rafted detritus and paleotemperature: Late Cenozoic relationships in the Ross Sea Region

Four “Eltanin” piston cores spanning 66°S to 71°S along a meridian which passes through the Ross Sea have been used to compare paleotemperature records based on radiolarian faunas with ice-rafted detritus (IRD) accumulation rates. Biostratigraphic/magnetostratigraphic core-bottom ages range from 0.7 m.y.B.P. to 3.2 m.y.B.P. Paleotemperature variations are clearly correlatable from core to core, while fluctuations in IRD accumulation rates at these sites show slight correspondence. Paleotemperature and IRD values in each core were compared to determine the succession of intervals of obviously positive, negative and nil correlation. This information was compared with a two-part model of IRD deposition derived from a consideration of the important principles of glaciology, iceberg dynamics, oceanography, paleoclimatology, paleo-oceanography and glacial history which have a probable bearing on IRD production and distribution. Case I of the model includes the present-day environment. It assumes the presence of a large ice shelf in the Ross Sea and unimportant winter/summer differences in the location of IRD accumulation zones. Regions of maximum interglacial IRD accumulation occurred beneath the ice shelf and near the Antarctic Convergence. During glacial expansion, the zone beneath the ice shelf diminished in importance and large amounts of IRD were deposited seaward of the ice-shelf margin as well as in the vicinity of a more northerly, glacial Antarctic Convergence. Case II reflects conditions prior to ice-shelf development with a significant seasonal variation in the distribution of IRD. The warmer episodes were probably marked by uniformly high rates of IRD accumulation between the continent and a region of high winter sea-surface temperature gradient. Cool periods saw two separate zones develop, one close to the continent and another close to the region of strong thermal contrast. Model comparison of IRD-paleotemperature correlations established for cores in this study reveals a gradual but irregular migration of the zones of greatest average glacial and interglacial IRD accumulation northward over about 5° of latitude from 3.2. m.y.B.P. to approximately 2.4 m.y.B.P. followed by a southward retreat to about their present position by 1.6 m.y.B.P. This was succeeded by a lengthy northward migration which culminated by approximately 0.3 m.y.B.P. and a final recovery to the current glacial-interglacial configuration. Case II “ice-shelf-free” conditions best explain core data in the interval between 3.2 and 2.6 m.y.B.P., suggesting an episode of significantly reduced Antarctic glaciation. Extensive perennial sea ice events are indicated at about 2.3, 2.1, 1.7, 1.1, and 0.3 m.y.B.P.

The climatic effects of large-scale surges of ice sheets

Surges in ice masses of glacier size are now well accepted in glaciology. There seems no reason why a similar phenomenon should not occur in bodies of ice as large as continental ice sheets.If a continental ice sheet surged into the sea it would have a considerable effect on world sea-level. This is proposed as the mechanism of past sea-level fluctuations (cyclothems) of the Carboniferous and Tertiary.The effect of a surge of the Antarctic Ice Sheet on world climate is considered, with particular reference to the origin of ice ages.The requirements of an ice-age mechanism are discussed and it is concluded that a periodic surge of the Antarctic Ice Sheet, perhaps induced by a decrease in insolation to the south polar region, has all the requirements of an ice-age inducing mechanism. In particular, any oscillating system must have capacitance (storage) and impedance (resistance). It is not easy to find a system in nature with a sufficiently long period of oscillation. However, the build up of ice on Antarctica would provide a sufficiently slow charging of storage, and the ice sheet itself would provide the storage to yield a system of long enough period.It is proposed that when the Antarctic Ice Sheet surges, a large ice shelf is produced which increases the albedo of the Earth. The resulting cooling leads to the formation of secondary ice sheets in the Northern Hemisphere, which in turn leads to a further increase in albedo and further cooling. The break up of the ice shelf and its replacement by ocean would lead to a large decrease in the Earth's albedo. The resulting warming would lead to the rapid melting of the subsiduary ice sheets and the ending of the ice age.

Origin of Ice Ages: an Ice Shelf Theory for Pleistocene Glaciation

A RECENT paper by Wilson1 suggests that ‘thermal’ surges of the Antarctic ice sheet led to the growth of large ice shelves in the Southern Ocean, and that these shelves cooled the Earth sufficiently to cause glaciations in the northern hemisphere. That the southern hemisphere may dominate the northern in this way is the antithesis of a suggestion in an earlier paper2 by me. However, both that paper2 and a later one3 do discuss the possibility of large thermal surges in Antarctica (see especially ref. 2, 185; ref. 3, 312), and since these papers were published I have been examining the further possibility that the surges act as ‘triggers’ for northern glaciation. The theoretical results of this examination, which will be published elsewhere, are inconclusive: the ice shelf theory for glaciation involves so many glaciological, oceancgraphical and meteorological assumptions that it may be impossible to justify all of them. It is the purpose of this communication to suggest how field evidence may be used to test the theory. Really convincing tests will probably require contributions from field workers in every part of the world.

THE FILCHNER ICE SHELF

(1959). THE FILCHNER ICE SHELF. Annals of the Association of American Geographers: Vol. 49, No. 2, pp. 110-119.