East Antarctic Outlet Glaciers Research Articles

Mid-Holocene thinning of David Glacier, Antarctica: chronology and controls

Abstract. Quantitative satellite observations only provide an assessment of ice sheet mass loss over the last four decades. To assess long-term drivers of ice sheet change, geological records are needed. Here we present the first millennial-scale reconstruction of David Glacier, the largest East Antarctic outlet glacier in Victoria Land. To reconstruct changes in ice thickness, we use surface exposure ages of glacial erratics deposited on nunataks adjacent to fast-flowing sections of David Glacier. We then use numerical modelling experiments to determine the drivers of glacial thinning. Thinning profiles derived from 45 10Be and 3He surface exposure ages show David Glacier experienced rapid thinning of up to 2 m/yr during the mid-Holocene (∼ 6.5 ka). Thinning slowed at 6 ka, suggesting the initial formation of the Drygalski Ice Tongue at this time. Our work, along with ice thinning records from adjacent glaciers, shows simultaneous glacier thinning in this sector of the Transantarctic Mountains occurred 4–7 kyr after the peak period of ice thinning indicated in a suite of published ice sheet models. The timing and rapidity of the reconstructed thinning at David Glacier is similar to reconstructions in the Amundsen and Weddell embayments. To identify the drivers of glacier thinning along the David Glacier, we use a glacier flowline model designed for calving glaciers and compare modelled results against our geological data. We show that glacier thinning and marine-based grounding-line retreat are controlled by either enhanced sub-ice-shelf melting, reduced lateral buttressing or a combination of the two, leading to marine ice sheet instability. Such rapid glacier thinning events during the mid-Holocene are not fully captured in continental- or catchment-scale numerical modelling reconstructions. Together, our chronology and modelling identify and constrain the drivers of a ∼ 2000-year period of dynamic glacier thinning in the recent geological past.

Delayed maximum and recession of an East Antarctic outlet glacier

Abstract During the last glaciation, East Antarctic outlet glaciers contributed to a grounded ice sheet in the Ross Embayment. The timing of maximum ice extent, as well as of subsequent deglaciation of these outlets, has implications for the behavior of the Antarctic Ice Sheet (AIS) and its impact on global sea level. We present 45 radiocarbon ages of lacustrine cyanobacteria from the Lake Wellman region alongside Hatherton Glacier, which are the first terrestrial data to both record advance of an Antarctic glacier to its maximum position as well as document a high-resolution chronology of subsequent retreat. Seventeen new exposure ages are widely scattered, but the youngest four are in broad agreement with the radiocarbon data. Hatherton Glacier slowly thickened from 13,000 to 9500 yr B.P. and then thinned steadily until at least ca. 2800 yr B.P. Our work affords evidence of both a delayed maximum and recession of an East Antarctic outlet glacier compared to the global Last Glacial Maximum (LGM) and supports growing evidence of a time-transgressive local LGM within the Ross Sea sector of the ice sheet. Both observations are consistent with the idea that the timing of outlet glacier expansion and timing of recession are controlled by the balance between dynamic thinning from ocean forcing and increased accumulation due to atmospheric warming.

Englacial drainage structures in an East Antarctic outlet glacier

AbstractGround-penetrating radar data acquired in the 2016/17 austral summer on Sørsdal Glacier, East Antarctica, provide evidence for meltwater lenses within porous surface ice that are conceptually similar to firn aquifers observed on the Greenland Ice Sheet and the Arctic and Alpine glaciers. These englacial water bodies are associated with a dry relict surface basin and consistent with perennial drainage into an interconnected englacial drainage system, which may explain a large englacial outburst flood observed in satellite imagery in the early 2016/17 melt season. Our observations indicate the rarely-documented presence of an englacial hydrological system in Antarctica, with implications for the storage and routing of surface meltwater. Future work should ascertain the spatial prevalence of such systems around the Antarctic coastline, and identify the degree of surface runoff redistribution and storage in the near surface, to quantify their impact on surface mass balance.

Ice flow variations at Polar Record Glacier, East Antarctica

ABSTRACTRelatively little is known about the physical mechanisms that drive the dynamics of the East Antarctic outlet glaciers. Here we conduct a remote-sensing investigation of the Polar Record Glacier (PRG), East Antarctica to analyze its ice flow acceleration, ice front variations and ice surface melting. Ice flow speeds at PRG increased by up to 15% from 2005 to 2015, with substantial interannual fluctuations. The ice velocities also showed seasonal variations, accelerating by up to 9% between September and January. Multiple mechanisms contribute to the observed seasonal variations: the initial acceleration may result from the lost back-stress provided by the sea ice in the austral spring and the later speedup relate to the surface meltwater that leads to weakened ice shelf and shear margins. The sensitivity of the PRG to oceanic forcing is confirmed by comparing the secular ice velocity increases with ocean temperatures. These measurements suggest that the dynamics of East Antarctic ice shelves are sensitive to melt at both the surface and base, at a range of timescales.

Cosmogenic nuclides constrain surface fluctuations of an East Antarctic outlet glacier since the Pliocene

Understanding past changes in the Antarctic ice sheets provides insight into how they might respond to future climate warming. During the Pliocene and Pleistocene, geological data show that the East Antarctic Ice Sheet responded to glacial and interglacial cycles by remaining relatively stable in its interior, but oscillating at its marine-based margin. It is currently not clear how outlet glaciers, which connect the ice sheet interior to its margin, responded to these orbitally-paced climate cycles. Here we report new ice surface constraints from Skelton Glacier, an outlet of the East Antarctic ice sheet, which drains into the Ross Ice Shelf. Our multiple-isotope (10Be and 26Al) cosmogenic nuclide data indicate that currently ice-free areas adjacent to the glacier underwent substantial periods of exposure and ice cover in the past. We use an exposure-burial model driven by orbitally-paced glacial–interglacial cycles to determine the probable ice surface history implied by our data. This analysis shows that: 1) the glacier surface has likely fluctuated since at least the Pliocene; 2) the ice surface was >200 m higher than today during glacial periods, and the glacier has been thicker than present for ∼75–90% of each glacial–interglacial cycle; and 3) ice cover at higher elevations possibly occurred for a relatively shorter time per Pliocene cycle than Pleistocene cycle. Our multiple-nuclide approach demonstrates the magnitude of ice surface fluctuations during the Pliocene and Pleistocene that are linked to marine-based ice margin variability.

Instability of the Antarctic Ross Sea Embayment as climate warms

AbstractCollapse of the Antarctic Ice Sheet since the Last Glacial Maximum 18,000 years ago is most pronounced in the Ross Sea Embayment, which is partly ice‐free during Antarctic summers, thereby breaching the O‐ring of ice shelves and sea ice surrounding Antarctica that stabilizes the ice sheet. The O‐ring may have vanished during Early Holocene (5000 to 3000 B.C.), Roman (1 to 400 A.D.), and Medieval (900 to 1300 A.D.) warm periods and reappeared during the Little Ice Age (1300 to 1900 A.D.). We postulate further collapse in the embayment during the post‐1900 warming may be forestalled because East Antarctic outlet glaciers “nail” the Ross Ice Shelf to the Transantarctic Mountains so it can resist the push from West Antarctic ice streams. Our hypothesis is examined for Byrd Glacier and a static ice shelf using three modeling experiments having plastic, viscous, and viscoplastic solutions as more data and improved modeling became available. Observed crevasse patterns were not reproduced. A new research study is needed to model a dynamic Ross Ice Shelf with all its feeder ice streams, outlet glaciers, and ice calving dynamics in three dimensions over time to fully test our hypothesis. The required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf. Calving must then successively pull the outlet glacier “nails” so collapse of the marine West Antarctic Ice Sheet proceeds to completion.

Rapid Holocene thinning of an East Antarctic outlet glacier driven by marine ice sheet instability.

Outlet glaciers grounded on a bed that deepens inland and extends below sea level are potentially vulnerable to ‘marine ice sheet instability'. This instability, which may lead to runaway ice loss, has been simulated in models, but its consequences have not been directly observed in geological records. Here we provide new surface-exposure ages from an outlet of the East Antarctic Ice Sheet that reveal rapid glacier thinning occurred approximately 7,000 years ago, in the absence of large environmental changes. Glacier thinning persisted for more than two and a half centuries, resulting in hundreds of metres of ice loss. Numerical simulations indicate that ice surface drawdown accelerated when the otherwise steadily retreating glacier encountered a bedrock trough. Together, the geological reconstruction and numerical simulations suggest that centennial-scale glacier thinning arose from unstable grounding line retreat. Capturing these instability processes in ice sheet models is important for predicting Antarctica's future contribution to sea level change.

Antarctic marine ice-sheet retreat in the Ross Sea during the early Holocene

Geological constraints on the timing of retreat of the Last Glacial Maximum (LGM) Antarctic Ice Sheets provide critical insights into the processes controlling marine-based ice-sheet retreat. The over-deepened, landward-sloping bathymetry of Antarctica's continental shelves is an ideal configuration for marine ice-sheet instability, with the potential for past and future ice-sheet collapse and accelerated sea-level rise. However, the chronology of retreat of the LGM ice sheet in the Ross Sea is largely constrained by imprecise radiocarbon chronology of bulk marine sediments or by coastal records that offer more reliable dating techniques but which may be influenced by local piedmont glaciers derived from East Antarctic outlet glaciers. Consequently, these coastal records may be ambiguous in the broader context of retreat in the central regions of the Ross Sea. Here, we present a sedimentary facies succession and foraminifera-based radiocarbon chronology from within the Ross Sea embayment that indicates glacial retreat and open-marine conditions to the east of Ross Island before 8.6 cal. (calibrated) kyr B.P., at least 1 k.y. earlier than indicated by terrestrial records in McMurdo Sound. Comparing these data to new modeling experiments, we hypothesize that marine-based ice-sheet retreat was triggered by oceanic forcings along most of the Pacific Ocean coastline of Antarctica, but continued Holocene retreat into the inner shelf region of the Ross Sea occurred primarily as a consequence of bathymetric controls on marine ice-sheet instability.

Erosion and transport by Byrd Glacier, Antarctica during the Last Glacial Maximum

Glacial till samples from seven, modern-Holocene Byrd Glacier moraines were characterized using particle size analysis, sand petrography and detrital zircon geochronology and compared to Ross Sea tills deposited during the Last Glacial Maximum. The goal was to identify the paleoflow path for Byrd Glacier and assess the use of multiple provenance techniques. The coarse sand fraction of Byrd Glacier tills is dominated by lithic fragments of adjacent bedrock outcrops, except samples from the Lonewolf Nunataks, which have a higher proportion of mineral to lithic fragments, as well as a recognizable exotic component. Cluster analysis shows that Byrd Glacier tills, with the exception of the two Lonewolf Nunataks sites, do not cluster strongly with Ross Sea samples because they have a higher proportion of lithic fragments. This indicates that comminution must be an active subglacial process beneath East Antarctic outlet glaciers. Byrd Glacier tills are also typically coarser grained that Ross Sea tills and their maturity is a reflection of both glacial processes and rock type. Measured U/Pb ages of detrital zircons from Byrd Glacier tills range from Triassic to Archean (240–3540 Ma) with a dominance of grains 530–600 Ma. Ross Sea till samples show spatial variability in U/Pb age distributions, with the core sites west of the 180° longitude line showing similarity to most Byrd Glacier tills, whereas core NBP9407-39, east of 180° long., is dominated by ∼100 Ma grains. Ross Sea tills also contain a recognizable detrital zircon fraction eroded inland of the Transantarctic Mountains. Both provenance methods indicate that the ice flow line for Byrd Glacier during the LGM was to the east of Ross Island and extended on either side of Ross Bank, with the majority of ice flowing to the Ross Sea's Central Basin. Our analysis shows that sand petrography and detrital zircon U/Pb age spectra provide complementary datasets that produce similar ice flow reconstructions and reveal valuable information about glacial processes and ice covered bedrock.

Elevated East Antarctic outlet glaciers during warmer-than-present climates in southern Victoria Land

We document Plio-Pleistocene changes in the level of Taylor Glacier, an outlet glacier in southern Victoria Land that drains Taylor Dome on the periphery of the East Antarctic Ice Sheet (EAIS). Chronologic control comes from 3He cosmogenic-nuclide analyses of 27 boulders sampled from drifts and moraines in Kennar Valley, a small hanging valley that opens onto a peripheral lobe of Taylor Glacier in the Quartermain Mountains. Assuming a constant boulder-erosion rate of 10 cm Myr −1, our preferred age model spans the last 3.1 Myr and calls for stepped ice recession from a local highstand ~ 200 m above the present base of Taylor Glacier at the mouth of Kennar Valley. The texture and sedimentology of all mapped moraines and drifts indicate deposition from cold-based ice, analogous with the modern Taylor Glacier at the mouth of Kennar Valley. The Kennar Valley glacial record shows an uncharacteristic relationship with average global temperatures, exhibiting higher-than-present ice levels during globally warm periods, including the Pliocene climatic optimum (~ 3.1 Ma) and Marine Oxygen Isotope Stage (MIS) 31 (~ 1.07 Ma). The Kennar Valley record also suggests that the rate of ice-surface lowering accelerated after the mid-Pleistocene transition at ~ 0.9 Ma. Correlation of our moraine record with published reports for fluctuations of Taylor Glacier elsewhere in the Quartermain Mountains, and with a dated moraine record from Ferrar Glacier (a second outlet for Taylor Dome), reveals similar ice-surface changes, highlighting minor, but widespread ice recession in southern Victoria Land since the mid- to late-Pliocene. Our record for minimal variability in the East Antarctic Ice Sheet contrasts with recent data from nearby marine cores that call for dynamic fluctuations in the volume of grounded ice in the Ross Embayment, and significant reduction of the West Antarctic Ice Sheet (WAIS) during warmer-than-present intervals. Taken together, these records from the Ross Embayment call for considerable variation in the response of marine-based West Antarctic ice and terrestrial East Antarctic outlet glaciers during Plio-Pleistocene time.

Dynamics and mass balance of four large East Antarctic outlet glaciers

AbstractThe East Antarctic ice sheet (EAIS) is Earth’s largest reservoir of fresh water and has the potential to raise sea level by ~50 m. A significant amount of the ice sheet’s mass is discharged by outlet glaciers draining through the Transantarctic Mountains, the balance characteristics of which are largely unknown. Here the mass balance is estimated for four glaciers draining ice from the EAIS through the Transantarctic Mountains into the Ross Sea embayment: David, Mulock, Byrd and Nimrod glaciers. Remote-sensing observations are used to map changes in ice flow and surface elevation, and ultimately to compute the mass balance of each glacier using the input–output method and three separate estimates for accumulation rate. Results computed using this method indicate small positive balances for David (2.41±1.31 Gt a–1), Mulock (1.91±0.84 Gt a–1) and Nimrod (0.88±0.39 Gt a–1) glaciers, and a large positive imbalance for Byrd Glacier (21.67±4.04 Gt a–1). This large imbalance for Byrd Glacier is inconsistent with other observations, and is likely due to an overestimation of accumulation rates across large regions of the interior catchment.

Increased flow speed on a large East Antarctic outlet glacier caused by subglacial floods

Large ice streams and outlet glaciers drain Greenland and Antarctica. An observed acceleration of ice velocity in one of these outlet glaciers, Byrd Glacier, East Antarctica coincides with a large water discharge from two subglacial lakes, allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice.

Provenance of LGM glacial till (sand fraction) across the Ross embayment, Antarctica

The provenance of Late Quaternary Ross Embayment till was investigated by comparing the coarse sand composition of East and West Antarctic source area tills with till samples from across the Ross Sea. The West Antarctic samples from beneath the Whillans (B) and Kamb (C) ice streams are petrologically distinct from samples of lateral moraines flanking several East Antarctic outlet glaciers. The characteristic assemblage of four West Antarctic samples includes felsic intrusive and detrital sedimentary lithic fragments, plagioclase and abundant quartz. In contrast, most of the ten East Antarctic till samples contains abundant mafic intrusive and detrital sedimentary lithic fragments as well as less abundant quartz. The distinctive composition of these source areas can be linked to 33 samples from 20 cores of Last Glacial Maximum (LGM) age till distributed across the Ross Sea. Western Ross Sea till samples exhibit mineralogic and lithological similarities to East Antarctic till samples, although these western Ross Sea tills contain higher percentages of felsic intrusive and extrusive lithic fragments. Eastern Ross Sea till samples are compositionally similar to West Antarctic till, particularly in their abundance of quartz and dearth of mafic and extrusive lithic components. Central Ross Sea till exhibits compositional similarities to both East and West Antarctic source terranes including a mafic lithic component, and marks the confluence of ice draining from East and West Antarctica during the LGM, thus West Antarctic-derived ice streams did not advance into the western Ross Sea. This indicates that even if pre-LGM equivalents of the present Siple Coast ice streams existed, they did not simply expand allowing West Antarctic-derived ice to dominate the LGM Ross Ice Sheet.

A new velocity map for Byrd Glacier, East Antarctica, from sequential ASTER satellite imagery

AbstractNew ice-velocity measurements are obtained for the main trunk of Byrd Glacier, East Antarctica, using recently acquired Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery. The velocities are derived from the application of a cross-correlation technique to sequential images acquired in 2000 and 2001. Images were co-registered and ortho-rectified with the aid of a digital elevation model (DEM) generated from ASTER stereo imagery. This paper outlines the process of DEM generation, image co-registration and correction, and the application of the cross-correlation technique to obtain ice velocities. Comparison of the new velocity map with earlier measurements of velocity from 1978 indicates that the glacier has undergone a substantial deceleration between observations. Portions of the glacier flowing at speeds of ~850ma–1 in 1978/79 were flowing at ~650ma–1 in 2000/01. The cause of this change in ice dynamics is not known, but the observation shows that East Antarctic outlet glaciers can undergo substantial changes on relatively short timescales.

An isotopic model for basal freeze‐on associated with subglacial upward flow of pore water

Subglacial freezing in polar glaciers can have a significant dynamical effect. Recent studies have shown that freezing of pore water flowing upward through subglacial fine‐grained sediments at the freezing interface and progression of this freezing front downward are responsible for fast ice flow stoppage in ice streams. The upward pore water flow leads to the formation of debris‐bearing basal ice layers. A model for stable isotope composition, both in δD and δ18O, is developed for predicting the isotopic composition of the ice segregated by such a mechanism. The development of this isotopic model for water films present along the grains of the subglacial sediment predicts the absence of apparent fractionation for the ice formed. This prediction is tested against two East Antarctic outlet glaciers by studying the δD‐δ18O relationships in the basal ice layers of these glaciers.

Observing the Antarctic Ice Sheet Using the RADARSAT-1 Synthetic Aperture Radar

This paper discusses the RADARSAT-1 Antarctic Mapping Project (RAMP). RAMP is a collaboration between NASA and the Canadian Space Agency (CSA) to map Antarctica using the RADARSAT -1 synthetic aperture radar. The project was conducted in two parts. The first part, which had the data acquisition phase in 1997, resulted in the first high-resolution radar map of Antarctica. The second part, which occurred in 2000, remapped the continent below 80°S Latitude and is now using interferometric repeat-pass observations to compute glacier surface velocities. Project goals and objectives are reviewed here along with several science highlights. These highlights include observations of ice sheet margin change using both RAMP and historical data sets and the derivation of surface velocities on an East Antarctic outlet glacier using interferometric data collected in 2000.

Ice-dynamic conditions of Shirase Glacier, Antarctica, inferred from ERS SAR interferometry

AbstractThe surface velocity field of Shirase Glacier, a fast-flowing East Antarctic outlet glacier, is determined from ERS synthetic aperture radar (SAR) images by means of speckle tracking using phase correlation, a technique which matches small image kernels of two complex SAR images by maximization of the local coherence. Velocity estimates are used to calculate surface strain rates, which are then used to calculate the large-scale, vertically integrated force balance and to determine the major stress components resisting the driving stress. For the whole glacier system, the driving stress is largely balanced by the basal drag, but with contributions from lateral drag up to 15% of the driving stress at the grounding line. Longitudinal stress gradients have only local importance to the balance of forces, limited to an area of a few square kilometers near the grounding line, where they resist the driving stress. In the grounded part of the glacier, >90% of the total ice velocity is due to basal sliding. Comparison with a balance-flux distribution of the Antarctic ice sheet suggests that the glacier in the downstream part of the Shirase drainage basin is close to equilibrium, showing a slight negative imbalance.

Late quaternary lakes in the mcmurdo sound region of antarctica

Lake levels within the enclosed drainage basins of the Dry Valleys adjacent to McMurdo Sound have fluctuated widely during the Late Quaternary due to (a) local climate change and the consequent variation in the evaporation–precipitation regime, and (b) glacial fluctuations, resulting in changes in the catchment and meltwater drainage areas of the glaciers and, in some cases, in the volumes of the available lake basins. Three types of lakes can be distinguished on the basis of their water source: (1) lakes receiving the bulk of their water from melting of local alpine glaciers; (2) proglacial lakes associated with outlet glaciers from the East Antarctic Ice Sheet; (3) proglacial lakes associated with the marine oxygen‐isotope stage 2 Ross Sea ice sheet and its precursors. The Dry Valleys contain an exceptionally long lacustrine record, extending back at least 300,000 years. Lacustrinesedimentation is cyclical, occurring over periods of about 100,000 years. During the last such cycle, relatively small lakes, both adjacent to East Antarctic outlet glaciers and fed by meltwater from alpine glaciers, existed during stage 5. However, these local lakes gave way to large proglacial lakes adjacent to the Ross Sea ice sheet in stage 2. The same relationship apparently occurred during the previous 100,000‐year cycle. Dating of lacustrine sediments suggests that lakes proglacial to the Ross Sea ice sheet have existed during episodes of sea‐level lowering during global glaciations. Lakes proglacial to outlet glaciers from the East Antarctic Ice Sheet have formed coincident with episodes of high eustatic sea level during interglacial periods.

Ice-dynamic conditions across the grounding zone, Ekströmisen, East Antarctica

AbstractA detailed investigation was performed of ice-dynamic conditions across the southern grounding zone of Ekströmisen, at the mouth of a regular East Antarctic outlet glacier not characterized by ice-streaming. Accurate field measurements along a profile 20 km long served as input in a two-dimensional numerical ice-flow model in order to calculate the variation of stress, strain rate and velocity with depth. The model results point to a sharp transition between the mechanics of grounded and floating ice, with a transition zone of only a few km between the two. Vertical shear, most of it near the base, was found to be the dominant flow mechanism in grounded ice. This part is also characterized by a distinct succession of surface undulations which are in turn controlled by variations in resistive stresses at the bottom. The associated phase shift between driving stress and basal drag was found to be accommodated by differential longitudinal pushes and pulls at the base. The flow in the upper half of the profile, on the other hand, is extensive everywhere. The adjacent ice shelf is characterized by small stress and velocity gradients in both the vertical and horizontal directions of the vertical section, and therefore has little deformation. A derived longitudinal deviatoric normal stress of only one-tenth of the value required for freely floating ice shelves reflects a large back-stress originating from friction along the side-walls of the narrow embayment.

Ice-dynamic conditions across the grounding zone, Ekströmisen, East Antarctica

AbstractA detailed investigation was performed of ice-dynamic conditions across the southern grounding zone of Ekströmisen, at the mouth of a regular East Antarctic outlet glacier not characterized by ice-streaming. Accurate field measurements along a profile 20 km long served as input in a two-dimensional numerical ice-flow model in order to calculate the variation of stress, strain rate and velocity with depth. The model results point to a sharp transition between the mechanics of grounded and floating ice, with a transition zone of only a few km between the two. Vertical shear, most of it near the base, was found to be the dominant flow mechanism in grounded ice. This part is also characterized by a distinct succession of surface undulations which are in turn controlled by variations in resistive stresses at the bottom. The associated phase shift between driving stress and basal drag was found to be accommodated by differential longitudinal pushes and pulls at the base. The flow in the upper half of the profile, on the other hand, is extensive everywhere. The adjacent ice shelf is characterized by small stress and velocity gradients in both the vertical and horizontal directions of the vertical section, and therefore has little deformation. A derived longitudinal deviatoric normal stress of only one-tenth of the value required for freely floating ice shelves reflects a large back-stress originating from friction along the side-walls of the narrow embayment.