Floating Ice Shelves Research Articles

The case for an ice shelf in the pleistocene Arctic Ocean

During Pleistocene glacial maxima, including the last glacial maximum, a thick floating ice shelf covered all or most of the deep Arctic Ocean. This conclusion is based on a preponderance of results from field and modeling studies. Of special importance are recent discoveries in the Arctic Ocean: the deep ice plowmarks on the submarine Yermak Plateau and Chukchi Borderland, and the evidence for sediment erosion and overconsolidation on the top of Lomonosov Ridge. In our view, they all are clear signatures of a former ice shelf that became grounded at these sites. While floating in the Arctic Ocean, the ice shelf would have exerted backpressure against any marine ice sheets that may have been grounded on circum-Arctic continental shelves, in particular against grounded ice streams, checking their instability, damping their surges, and deflecting their flow. At the close of glacial time, ice-shelf disintegration permitted ice-stream surges, making it possible for erratic-laden icebergs to reach the central Arctic. These features are those of an Arctic Ice Sheet that acted as a unified dynamic system, as the Antarctic Ice Sheet acts today. The Arctic Ice Sheet consisted of marine ice domes grounded on the continental shelf in the western Arctic and an ice shelf that spread across the Arctic Ocean basin and grounded against the continental shelf in the eastern Arctic. This grounding would dam Eurasian rivers flowing toward the Arctic. If the ice-dammed lakes then became frozen to the bed, marine ice domes would eventually form on the continental shelf of the eastern Arctic. However, there is evidence that these Eurasian marine ice domes formed and collapsed repeatedly between 24 ka and 12 ka, suggesting that these domes were highly unstable.

Channelized bottom melting and stability of floating ice shelves

The floating ice shelf in front of Petermann Glacier, in northwest Greenland, experiences massive bottom melting that removes 80% of its ice before calving into the Arctic Ocean. Detailed surveys of the ice shelf reveal the presence of 1–2 km wide, 200–400 m deep, sub‐ice shelf channels, aligned with the flow direction and spaced by 5 km. We attribute their formation to the bottom melting of ice from warm ocean waters underneath. Drilling at the center of one of channel, only 8 m above sea level, confirms the presence of ice‐shelf melt water in the channel. These deep incisions in ice‐shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously.

Autonomous underwater vehicles (AUVs) and investigations of the ice–ocean interface in Antarctic and Arctic waters

AbstractLimitations of access have long restricted exploration and investigation of the cavities beneath ice shelves to a small number of drillholes. Studies of sea-ice underwater morphology are limited largely to scientific utilization of submarines. Remotely operated vehicles, tethered to a mother ship by umbilical cable, have been deployed to investigate tidewater-glacier and ice-shelf margins, but their range is often restricted. The development of free-flying autonomous underwater vehicles (AUVs) with ranges of tens to hundreds of kilometres enables extensive missions to take place beneath sea ice and floating ice shelves. Autosub2 is a 3600 kg, 6.7 m long AUV, with a 1600 m operating depth and range of 400 km, based on the earlier Autosub1 which had a 500 m depth limit. A single direct-drive d.c. motor and five-bladed propeller produce speeds of 1–2 m s−1. Rear-mounted rudder and stern-plane control yaw, pitch and depth. The vehicle has three sections. The front and rear sections are free-flooding, built around aluminium extrusion space-frames covered with glass-fibre reinforced plastic panels. The central section has a set of carbon-fibre reinforced plastic pressure vessels. Four tubes contain batteries powering the vehicle. The other three house vehicle-control systems and sensors. The rear section houses subsystems for navigation, control actuation and propulsion and scientific sensors (e.g. digital camera, upward-looking 300 kHz acoustic Doppler current profiler, 200 kHz multibeam receiver). The front section contains forward-looking collision sensor, emergency abort, the homing systems, Argos satellite data and location transmitters and flashing lights for relocation as well as science sensors (e.g. twin conductivity–temperature–depth instruments, multibeam transmitter, sub-bottom profiler, AquaLab water sampler). Payload restrictions mean that a subset of scientific instruments is actually in place on any given dive. The scientific instruments carried on Autosub are described and examples of observational data collected from each sensor in Arctic or Antarctic waters are given (e.g. of roughness at the underside of floating ice shelves and sea ice).

The geological evolution of southern McMurdo Sound - new evidence from a high-resolution aeromagnetic survey

Magnetic anomaly data are presented from a new helicopter-borne high-resolution aeromagnetic survey in southern McMurdo Sound. Anomaly data have been acquired at a common 305 m elevation above the McMurdo and southern McMurdo ice shelves and draped over the volcanic islands that pin them. The resulting anomaly patterns provide a significant advance in the understanding of the rift related geology beneath the floating ice shelves. More extensive Erebus Volcanic Province (McMurdo Volcanic Group) rocks are indicated along with a significant blanket of glaci-volcaniclastic sediment on the seafloor between the volcanic islands in southern McMurdo Sound. These glaci-volcaniclastic sediments are inferred to originate from former grounding of the southern McMurdo Ice Shelf as a marine ice sheet. A strong N-S fabric is also observed in the anomaly data suggesting that the rift structure observed in the Victoria Land basin persists to the south beneath the McMurdo and southern McMurdo ice shelves. W-N-W transfer faults identified within the Transantarctic Mountain rift flank to the west are not obvious in the aeromagnetic data set, implying that the 'Discovery Accommodation Zone' may be restricted to the region between a southward extension of the range bounding fault that marks the limit of the Victoria Land Basin and the right lateral offset in the Transantarctic Mountain front in southern Victoria Land.

Marine ice-sheet dynamics. Part 1. The case of rapid sliding

Marine ice sheets are continental ice masses resting on bedrock below sea level. Their dynamics are similar to those of land-based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line, where the ice reaches a critical flotation thickness. In order to predict the evolution of the grounding line as a free boundary, two boundary conditions are required for the diffusion equation describing the evolution of the grounded-ice thickness. By analogy with Stefan problems, one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat. The other condition must be determined by coupling the ice sheet to the surrounding ice shelves. Here we employ matched asymptotic expansions to study the transition from ice-sheet to ice-shelf flow for the case of rapidly sliding ice sheets. Our principal results are that the ice flux at the grounding line in a two-dimensional ice sheet is an increasing function of the depth of the sea floor there, and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland. These results indicate that marine ice sheets have a discrete set of steady surface profiles (if they have any at all) and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line.

A quantitative examination of modern sedimentary lithofacies formation on the glacially influenced Gulf of Alaska continental shelf

Continental-shelf lithofacies are described from a series of cores collected in the northern Gulf of Alaska, a high-energy paraglacial shelf experiencing rapid rates of sediment accumulation. Short-lived tracers ( 234Th and chlorophyll- a) indicate that during the annual peak in fluvial sediment input (summer), biologic sediment mixing coefficients in the surficial seabed are generally lower than other coastal environments (<20 cm 2 yr −1) and mixing extends downward <10 cm. 210Pb geochronology indicates that sediment accumulation rates (time scales of 10–100 yr) are 0.1–3 cm yr −1. The measured bioturbation and accumulation rates lead to predictions of moderate to bioturbated lithofacies, as observed. Primary depositional fabric is preferentially preserved where sediment accumulation rates >2 cm yr −1 and non-steady sediment deposition occurs. Depositional fabric is also observed in strata at 50–100 m water depths and is similar in appearance to beds that may form through deposition of wave-induced fluid-mud flows, which have been observed forming on other shelves with moderate to high wave energy. Five general lithofacies can be identified for the study area: inner-shelf sand facies, interbedded sandy mud facies, moderate-to-well-bioturbated mud facies, gravelly mud facies, and Tertiary bedrock facies. The moderate-to-well-bioturbated mud facies is areally dominant, representing over 50% of the shelf area, although roughly equal volumes (∼0.4 km 3) of strata with some preservation of primary fabric are annually accumulating. Lithofacies on this paraglacial shelf generally resemble mid- and low-latitude allochthonous shelf strata to a much greater degree than Holocene glacimarine strata formed on shelves dominated by icebergs and floating ice shelves. Paraglacial strata may be differentiated from non-glacial shelf strata by lower organic carbon concentrations, a relatively lower degree of bioturbation, and increased preservation of primary depositional fabric.

The environment and evolution of the West Antarctic ice sheet: setting the stage

The West Antarctic ice sheet is the last ice sheet of the type cradled in a warm, marine geologic basin. Its perimeter stretches into the surrounding seas allowing warmer ocean waters to reach the undersides of its floating ice shelves and its relatively low surface elevation permits snow-carrying storms to extend well into its interior. This special environment has given rise to theories of impending collapse and for the past quarter-century has challenged researchers who seek a quantitative prediction of its future behaviour and the corresponding effect on sea level. Observations confirm changes on a variety of time scales from the quaternary to less than a minute. The dynamics of the ice sheet involve the complex interaction of ice that is warm at its base and cold along the margins of ice streams; subglacial till that is composed of a combination of marine sediment and eroded sedimentary rocks; and water that moves primarily between the ice and bed, but whose flow direction can differ from the direction of ice motion. The pressure of the water system is often sufficient to float the ice sheet locally and small changes in the amount of water in the till can cause it to rapidly switch from very weak to very stiff.

"Back-arc" marine ice sheet in the Sea of Okhotsk

[1] Geomorphologic, marine geological, and paleoclimatic evidence, along with modeling results, indicate that, during the Late Pleistocene, the Sea of Okhotsk was glaciated by a “back-arc” marine ice-sheet, which was a continuous spillover of glaciation from the highlands of Northeast Siberia, buttressed by a submarine ridge of the Kurile Island Arc. That marine ice sheet also impounded and deflected the Lower Amur and Uda Rivers which resulted in reorganization of their drainage systems. The Okhotsk marine ice sheet was grounded on seafloor and reached to the outer (southern) edge of the submarine Academy Sill. From that limit, the ice sheet extended farther south, across the deep Kurile Basin, as a floating ice shelf. The Okhotsk Sea and Beringian Ice Sheets were the only “back-arc” Quaternary glaciers; they were two major sources of icebergs, meltwater, and ice-rafted debris supplied to the North Pacific Ocean during the Ice Ages.

Rheology of the Ronne Ice Shelf, Antarctica, inferred from satellite radar interferometry data using an inverse control method

The Antarctic Ice Sheet is surrounded by large floating ice shelves that spread under their own weight into the ocean. Ice shelf rigidity depends on ice temperature and fabrics, and is influenced by ice flow and the delicate balance between bottom and surface accumulation. Here, we use an inverse control method to infer the rigidity of the Ronne Ice Shelf that best matches observations of ice velocity from satellite radar interferometry. Ice rigidity, or flow law parameter B, is shown to vary between 300 and 900 kPa a1/3. Ice is softer along the side margins due to frictional heating, and harder along the outflow of large glaciers, which advect cold continental ice. Melting at the bottom surface of the ice shelf increases its rigidity, while freezing decreases it. Accurate numerical modelling of ice shelf flow must account for this spatial variability in mechanical characteristics.

Accelerated sea-level rise from West Antarctica.

Recent aircraft and satellite laser altimeter surveys of the Amundsen Sea sector of West Antarctica show that local glaciers are discharging about 250 cubic kilometers of ice per year to the ocean, almost 60% more than is accumulated within their catchment basins. This discharge is sufficient to raise sea level by more than 0.2 millimeters per year. Glacier thinning rates near the coast during 2002-2003 are much larger than those observed during the 1990s. Most of these glaciers flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is under way.

Seabed erosion on the Lomonosov Ridge, central Arctic Ocean: A tale of deep draft icebergs in the Eurasia Basin and the influence of Atlantic water inflow on iceberg motion?

The submarine Lomonosov Ridge, which bisects the Arctic Ocean, is covered by ∼450 m of hemipelagic sediments. High‐resolution chirp sonar and side‐scan data have documented erosion of a section of the ridge. Here we present multichannel seismic and chirp sonar data which show removal of unconsolidated sediments along the Eurasia Basin side of the Lomonosov Ridge, where the central ridge is shallower than 1000 m. The incomplete erosion of the hemipelagic drape and probable topographic control of ice motion suggest that the deepest draft glacier ice, which reached the central Arctic Ocean from the Eurasia Basin, was in the form of armadas of large icebergs embedded in sea ice rather than a single, continuous floating ice shelf. Massive, rapid discharge of glacier ice was probably required to produce the deepest draft (&gt;800 m) icebergs. Eastward and northward drift trajectories from a likely iceberg source area on the Kara‐Barents Sea margin would reflect the relative strength of Atlantic inflow through the Fram Strait. However, only icebergs with drafts exceeding bathymetric thresholds on the Lomonosov Ridge would leave an erosive imprint on the seabed. Advection of Atlantic water may have been relatively strong at times of discharge from the Saalian ice sheet. Icebergs exiting the Arctic Ocean caused massive erosion and redeposition of the hemipelagic sediments on top of Yermak Plateau.

Rigorous GPS data-processing strategies for glaciological applications

AbstractGlobal positioning system (GPS) data are now routinely used for many glaciological applications. In some common cases, systematic errors are unmodelled at the data-processing stage, although they are often presumed insignificant. In this paper, I investigate these assumptions for three different scenarios: (1) measurements on a moving glacier; (2) measurements on a floating ice shelf; and (3) precise height determination over large elevation ranges, such as for aircraft positioning in lidar/laser altimeter missions. In each case, systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the GPS processing software, under certain conditions. If these coordinate biases go undetected, short- and long-term measurements of horizontal ice velocity or rates of ice-thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques. More rigorous processing strategies are discussed that allow for bias-free parameter estimation.

Force-perturbation analysis of Pine Island Glacier, Antarctica, suggests cause for recent acceleration

AbstractPine Island Glacier, flowing into the Amundsen Sea from West Antarctica, thinned substantially during the 1990s, its grounding line receded by several km, and its velocity increased by &gt;10% to values approaching 3 km a–1. Here, we use these observations, together with estimates of ice thickness and surface strain rates, to estimate the perturbation in forces resisting ice flow compatible with the observations. The analysis assumes that such perturbations are transmitted far upstream from where they originate, and that creep response to the perturbations can be described by equations similar to those that govern ice-shelf creep. It indicates that observed acceleration between 1996 and 2000 could have been caused by progressive ungrounding within the most seaward 25 km ‘ice plain’ of the grounded glacier. Earlier retreat and thinning of the glacier’s floating ice shelf may have provided the conditions that initiated ungrounding of the ice plain. Our analysis indicates that continued ice-plain thinning at the current rate of about 2 ma–1 will result in a velocity increase by 1 km a–1 within the next 11 years as the ice plain becomes totally ungrounded.

Ice temperature and high geothermal flux at Siple Dome, West Antarctica, from borehole measurements

AbstractA vertical temperature profile through the West Antarctic ice sheet (WAIS) at the summit of Siple Dome reveals an elevated geothermal flux. This could be the root cause for the existence of a dynamic ice-stream system in the WAIS. Siple Dome is still frozen on its bed, but adjacent ice streams have bed temperatures at the pressure-melting point of ice. Although present-day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice, indirect effects like a rapid disintegration of the floating ice shelves or additional melt-water input at the surface could give rise to speed-up of the ice streams with an ensuing rise in sea level. Ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration, within a relatively short period of time, of at least part of the WAIS.

The Arctic Center of Quaternary ice and flood spreading: A deductive model

Early in the XIX century, the founders of glacial theory conceived of a ice cap centered on the North Pole and extending as far south as central Europe. However later this model was discarded. The deep Arctic Ocean, discovered in 1890s, was thought inconsistent with the model. Moreover, a belief took hold that the Arctic was not more severely glacierized than now, as polar snowfall seemed insucient to nourish much bigger ice masses. So the reigning concept of the XX century suggested that the past great ice sheets of Northern Hemisphere had mostly located on the mid-latitude continents. This concept was first challenged in 1970s, when Hughes et al. (1977) put forth the model of an Arctic Ice Sheet (AIS) that had formed largely in the Arctic Ocean. A core ice-shelf mechanism of Arctic Ice Sheet formation was proposed, which suggested: first, an inception of ice shelves in confined cold-water seas; second, turning the ice shelves into marine ice domes grounded on the polar continental shelves; and third, amalgamation of the Arctic terrestrial, marine-based, and floating ice components into a single Antarctic-style dynamic system. Now, a marine ice transgression hypothesis is proposed which suggests that the Arctic marine ice domes and thick floating ice shelf would push outwards and transgress onto adjacent lands. The Arctic Ice Sheet was an unstable, threshold-like system, prone to generate nonlinear responses to gradual change in forcing. The responses materialized in glacial surges, Heinrich events, and megafloods. As a result of the Earth's rotation, the system developed a west-to-east asymmetry.

Ice shelf tidal motion derived from ERS altimetry

Ocean tides beneath floating ice shelves can affect heat and water circulations within the subshelf cavity, which may influence patterns of basal melting. Accurate tidal predictions are essential to eliminate vertical tide motions when mapping the flow speeds and thickness changes of floating ice. Global ocean tide solutions are typically accurate to 2–3 cm in amplitude. However, their coarse resolution limits their utility in coastal regions. Moreover, the absence of satellite and extensive gauge station data in the polar regions has resulted in significant differences between recent tide model solutions around the Antarctic continent. We use 9 years of high‐resolution European Remote Sensing (ERS) satellite radar altimeter range measurements and a modified form of the orthotide method to generate tidal solutions for a series of ice shelves around the Antarctic Peninsula. We compare our tidal solution at George VI Ice Shelf on the western coast of the Antarctic Peninsula to direct measurements from British Antarctic Survey tide gauge records and contrast the results with solutions from the FES99 and Kantha2.0 (global) and the CATS01.02 (Antarctic) ocean tide models. The average root mean square vector difference between the tide gauge and tide model predictions of amplitude and phase was 10.9, 17.1, 4.2, and 15.4 cm for the ERS ice shelf, Kantha2.0, CATS01.02, and FES99 models, respectively. Tidal predictions derived from the ERS ice shelf model are closer to the tide gauge recordings than those of the global models, and the inclusion of ERS altimeter data may lead to improved global model performance.

Sediment core from beneath the Amery Ice Shelf, East Antarctica, suggests mid-Holocene ice-shelf retreat

The stability of floating ice shelves is an important indicator of ocean circulation and ice-shelf mass balance. A sub–ice-shelf sediment core collected during the Austral summer of 2000–2001 from site AM02 (69°42.8′S, 72°38.4′E) on the Amery Ice Shelf, East Antarctica, contains a full and continuous record of glacial retreat. The AM02 core site is ∼80 km south of the floating ice shelf edge and contains a 0.5-m-thick Holocene surface layer of siliceous mud and diatom ooze of marine origin. Core data are supportive of sub–ice-shelf circulation models that predict the landward flow of oceanic water, and prove that the landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ∼80 km. An increase in sea-ice–associated diatom deposition in the upper part of the Holocene suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum.

Holocene history of the Larsen-A Ice Shelf constrained by geomagnetic paleointensity dating

A sedimentary record collected from beneath the former Lar- sen-A Ice Shelf reveals the Holocene history of the Larsen-A re- gion. The record begins with the transition from grounded ice to a floating ice shelf, completed by 10.7 6 0.5 ka, and ends with the modern recession. The record contains several late Holocene dia- tomaceous ooze layers that suggest proximity to productive open- water events. Radiocarbon ages obtained from these sediments were complicated by the presence of detrital and reworked carbon. We have eliminated these complications and constructed a chro- nology for the Larsen-A Ice Shelf history via tuning of the geo- magnetic field paleointensity record with a reference curve. This approach provides chronological control to sediment sequences that lack appropriate material for radiocarbon dating. Geomag- netic paleointensity features with wavelengths of 2-3 k.y. can be recognized and interhemispherically correlated, illustrating the po- tential to use geomagnetic paleointensity variations as a global cor- relation and dating tool at sub-Milankovitch time scales.

Excess helium and neon in the southeast Pacific: Tracers for glacial meltwater

We analyze helium (He) and neon (Ne) isotope data sets from the southeast Pacific sector of the Southern Ocean collected in 1992 and 1994 and describe a new method to estimate glacial meltwater fluxes independent of previous approaches. The waters sampled during these cruises reveal large atmospheric He and Ne excess concentrations in the upper 500 m that are derived predominantly from the dissolution of air contained in glacial ice during melting underneath and at the front of floating ice shelves at elevated hydrostatic pressure. On the stations sampled, the meltwater fractions are largest near the northern and southern ice shelf fronts of George VI Ice Shelf (1.5–2.5%), consistent with prior inferences from δ18O. The inventories of 4He and Ne with local glacial meltwater origin south of 70°S between 210°E and 295°E are (2.5 ± 0.7)1011 cm3STP 4He and (7.9 ± 2.7)1011 cm3STP Ne, respectively. The associated meltwater volumes are 430 ± 120 Gt for 4He and 400 ± 140 Gt for Ne. For an estimated seawater residence time of 2 years on the continental shelf, the amounts of glacial meltwater translate into bulk melting rates of 215 ± 170 Gt yr−1 for 4He and 200 ± 170 Gt yr−1 for Ne. The values derived from 4He are expected to be larger than those obtained from Ne because of the terrigenic He component, which accounts for 10–15% of the He excess in this region. Our regional melting rate of 200–215 Gt yr−1 is comparable to a prior estimate but subject to uncertainties resulting from the station distribution, the air‐sea gas exchange, and the estimate of the net transport through the study region.

Physical structure of epishelf lakes of the southern Bunger Hills, East Antarctica

Epishelf lakes, positioned between ice-free areas and floating ice shelves or glaciers, are unusual tidal, but largely freshwater, environments found in both the Arctic and the Antarctic. The greatest concentration of these lakes is in the Bunger Hills, East Antarctica (66°S, 100°E). We present and discuss temperature and salinity profiles for five epishelf lakes from this region, most of which show unusual properties. White Smoke Lake is fresh and cold (&lt;0.1°C) throughout; Lake Pol’anskogo has two basins, one fresh and cold, the other saline and warm; ‘Southern’ Lake is cold and saline at depth; Transkriptsii Gulf has a deep, warm saline layer; and ‘Northern’ Lake is relatively warm throughout. The structures of these lakes can be explained in terms of a simple model in which the isolated saline water evident in three of the lakes entered the basins through the connection to the marine waters during periods of reduced freshwater input. By dating these marine incursions, periods of reduced melt, presumably due to colder temperatures, can be determined.