Search

Icequakes radiating from an ice-stream base provide insights into otherwise difficult to observe sub-kilometer-scale basal heterogeneity. We detect basal icequakes beneath an ∼3-km-wide seismic sensor network installed on the Whillans Ice Plain (WIP) in West Antarctica, and we use S-wave back-projection to detect and locate thousands of basal icequakes occurring over 14 and 21 days in January 2014 and 2015, respectively. We find flow-parallel streaks of basal icequakes beneath the WIP, which we conjecture are related to the presence of mega-scale glacial lineations (MSGLs) indicated by ice-penetrating radar, with at least one streak originating in a local trough adjacent to a MSGL. Patterned basal seismicity can be caused by systematic spatial variation in basal pore pressure, bed-material frictional properties, or both. We interpret these flow-parallel icequake streaks as being due to frictionally heterogeneous bed materials in the presence of a streamlined ice-stream bed: bedform ridges correspond to aseismic, high-porosity deforming till, and some troughs to ephemeral exposures of deeper, seismogenic material such as lodged till or older sediments or rocks. Our results are consistent with MSGL formation by either erosion in troughs to expose deeper seismogenic material, or deposition of aseismic high-porosity till in bedform highs. Our results also suggest that evolving subglacial geomorphology can impact basal traction by reorganizing the spatial distribution of basal materials with varying mechanical properties.

Ice flow model, pairing, calculation of total recoverable mass flux, latitudinal model, and fireball statistical analysis

Marine sediments preserve archives of glacier behavior from many proxies, with lithofacies analysis providing direct evidence of glacial extent and dynamics. Many of these lithofacies have corresponding physical and geochemical properties that may be identified through quantitative, nondestructive logging properties. This study applies supervised and unsupervised classification to downcore logging data to attempt to model temperate glacimarine facies, which are independently identified via visual lithofacies analysis based on core photographs, digital X-radiography, and computed tomography scans. We test the limits of these methods by modeling both broad glacial and interglacial and small-scale variations in Late Pleistocene (

It has been suggested that the grounding line of the Last Glacial Maximum (LGM) ice sheet in the Ross Sea, Antarctica, receded in an approximately north-to-south pattern during the Holocene. An implication of this hypothesis is that geological evidence from the southwestern Ross Sea has been used widely to interpret retreat histories of the West Antarctic Ice Sheet (WAIS) across the wider Ross Sea embayment. Accurately constraining the timing and pattern of marine-based ice sheet retreat in this embayment is critical to understanding the drivers that may have triggered this event, and its contribution to rapid sea-level rise events. Here, we present new multibeam swath bathymetry data that identifies well-preserved glacial features indicating that thick (>700 m) marine-based ice derived from the East Antarctic Ice Sheet coastal outlet glaciers dominated the ice sheet input into the southwestern Ross Sea during the last phases of glaciation. Subglacial geomorphic features indicate that ice derived from present outlet glacier valleys in South Victoria Land flowed southeastward. This is more consistent with flowlines from model-based interpretations of an earlier retreat of the WAIS in the central Ross Sea than with previous land-based geological reconstructions. This implies that coastal records of deglaciation along the Transantarctic Mountains front record only the final phases of glacial retreat in the Ross Sea. Therefore, chronological data from the central embayment are required to accurately constrain the timing of large-scale glacial retreat in the Ross Sea and to identify the mechanisms that drove it.

Recent observations indicate that ice-ocean interaction drives much of the recent increase in mass loss from the Greenland Ice Sheet; however, the role of ocean forcing in driving past glacier change is poorly understood. To extend the observational record and our understanding of the ocean-cryosphere link, we used a multi-proxy approach that combines new data from proglacial lake sediments, 14 C-dated in situ moss that recently emerged from beneath cold-based ice caps, and 10 Be ages to reconstruct centennial-scale records of mountain glacier activity for the past ∼10 k.y. in West Greenland. Proglacial lake sediment records and 14 C dating of moss indicate the onset of Neoglaciation in West Greenland at ca. 5 ka with substantial snowline lowering and glacier expansion at ca. 3.7 ka followed by additional ice expansion phases at ca. 2.9, ca. 1.7, and ca. 1.4 ka and during the Little Ice Age. We find that widespread glacier growth at ca. 3.7 ka in West Greenland coincides with marked cooling and reduced strength of the West Greenland Current in Disko Bugt. The transition to cooler ocean conditions at ca. 3.7 ka identified in Disko Bugt is registered by marine proxy data farther afield in East Greenland and on the northwestern Icelandic shelf, implying large-scale paleoceanographic changes across the North Atlantic during this interval. The similarity between glacier change on West Greenland and multiple marine and terrestrial records across the North Atlantic suggests that glaciers are strongly influenced by changes in ocean circulation and consequently implies that the ocean-cryosphere teleconnection is a persistent feature of the Arctic system.

Source at https://doi.org/10.1130/G38195.1. During the last glacial period large parts of the Arctic, including the Barents Sea, north of Norway and Russia, were covered by ice sheets. Despite several studies indicating that melting occurred beneath much of the Barents Sea ice sheet, very few meltwater-related landforms have been identified. We document ∼200 seafloor valleys in the central Barents Sea and interpret them to be tunnel valleys formed by meltwater erosion beneath an ice sheet. This is the first account of widespread networks of tunnel valleys in the Barents Sea, and confirms previous predictions that large parts of the ice sheet were warm based. The tunnel valleys are interpreted to be formed through a combination of steady-state drainage and outburst floods close to the ice margin, as a result of increased melting within a period of rapid climate warming during late deglaciation. This is the first study documenting widespread tunnel valley formation at the northern reaches of a Northern Hemisphere paleo–ice sheet, during advanced deglaciation and beneath a much reduced ice sheet. This indicates that suitable conditions for tunnel valley formation may have occurred more widely than previously reported, and emphasizes the need to properly incorporate hydrological processes in current efforts to model ice sheet response to climate warming. This study provides valuable empirical data, to which modeling results can be compared.

The evolution of the Antarctic ice sheet during the mid-Pliocene warm period (MPWP) remains uncertain and has important implications for our understanding of ice sheet response to modern global warming. The extent to which marine-based sectors of the East Antarctic Ice Sheet (EAIS) retreated during the MPWP is particularly contentious, with geological observations and geochemical analyses being cited to argue for either a relatively minor or a significant ice sheet retreat in response to mid-Pliocene warming. The stability of marine-based ice sheets is intimately linked to bedrock elevation at their grounding lines, and previous ice sheet modeling assumed that Antarctic bedrock elevation during the MPWP was the same as today with the exception of a correction for the crustal response to ice loading. However, various processes may have perturbed bedrock elevation over the past 3 m.y., most notably vertical deflections of the crust driven by mantle convective flow, or dynamic topography. Here we present simulations of mantle convective flow that are consistent with a wide range of present-day observables and use them to predict changes in dynamic topography and reconstruct bedrock elevations during the MPWP. We incorporate these elevations into a simulation of the Antarctic ice sheet during the MPWP and find that the correction for dynamic topography change has a significant effect on the stability of the EAIS within the marine-based Wilkes Basin, with the ice margin in that sector retreating considerably further inland (200–560 km) relative to simulations that do not include this correction for bedrock elevation.

The mid-Pliocene warm period (MPWP, 3.3–2.9 Ma), with reconstructed atmospheric p CO2 of 350–450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 ± 10 m above modern, requiring total loss of the Greenland and marine West Antarctic Ice Sheets and a substantial loss of the East Antarctic Ice Sheet, with only a concurrent 2–3 °C rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (δ18Ob) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the δ18O of Antarctic ice (δ18Oi) was the same in the Pliocene as it is today, and while the sensitivity of δ18Ob to changing meltwater δ18O has been previously considered, these analyses neglect conservation of 18O/16O in the ocean-ice system. Using well-calibrated δ18O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the δ18Oi of the Pliocene Antarctic ice sheet was at minimum 1‰–4‰ higher than present. Assuming conservation of 18O/16O in the ocean-ice system, this requires lower Pliocene seawater δ18O without a corresponding change in ice sheet mass. This effect alone accounts for 5%–20% of the δ18Ob difference between the MPWP interglacials and the modern. With this amended isotope budget, we present a new Pliocene GMSL estimate of 9–13.5 m above modern, which suggests that the East Antarctic Ice Sheet is less sensitive to radiative forcing than previously inferred from the geologic record.

A recent focus of major international exploration in East Antarctica has been aimed at revealing its bedrock topography and imaging its tectonic architecture and evolution. Here we present the tectonic interpretation of regional-scale lineaments revealed by the Radarsat mosaic of Antarctica on the ice sheet surface in the Vostok–Dome C– Adventure Basin region. These lineaments appear in the radar backscatter textures as alignments of marked tonal variations with lengths of tens to hundreds of kilometers and were identifi ed using an automated methodology. We explore the origin scenarios for the ice sheet surface lineaments by comparing their azimuthal trends and spatial distribution with the main morphotectonic features of the bedrock. Azimuthal analysis reveals that lineaments cluster around two preferential directions interpreted as structural or tectonic domains. These show strong correlations with azimuths of tectonic fabrics in the bedrock. The main lineament domain parallels the morphotectonic features of the study area, namely the Adventure Basin and the Concordia and Aurora Trenches. The second lineament set corresponds to the mean orientation of the Lake Vostok depression. The spatial analyses of the two lineament domains strengthen our fi ndings and interpretations. Comparisons with wind and ice fl ow directions exclude their infl uence on the identifi ed lineament pattern. Results reveal the tectonic origin of the lineament domains, and demonstrate the method’s usefulness as a tool for tectonic studies of regions characterized by thick covers. These regions include other areas of the East Antarctic craton such as the Gamburtsev Subglacial Mountains, as well as deserts or surfaces of other planets.

The geological nature of glacier beds plays a key role in ice sheet dynamics. Whereas little is known about Greenland’s subglacial geology, the presence of basal sediments is a necessary condition for fast-flowing Antarctic ice streams. Such sediments sustain subglacial till layers, which if water saturated and under high pore-water pressure provide little resistance to ice flow. Using receiver function modeling of teleseismic P-waves, we report a thick (at least tens of meters) sediment layer beneath a site in Greenland’s ablation zone, ∼15 km away from the western ice sheet margin. Although we do not discuss the origin or detailed nature of these subglacial sediments, we suggest that they are capable of sustaining a till layer. Due to the prevalence of an inefficient, pressurized subglacial drainage system, this till layer would typically be under high pore pressures and fail at the shear stresses transmitted from the overlaying ice. Ice flow resistance is thus focused on regions where till is consolidated or absent, or where form drag over obstacles takes place. In contrast to Antarctica, Greenland’s surface melt now affects practically all latitudes, and efficient hydraulic connections between ice sheet surface and bed are pervasive throughout the ablation zone. This implies that rapid, melt-induced changes in subglacial water pressure are possible. The time required for basal till strength to react to these changes depends on the till’s properties. We estimate that formation and destruction of flow resistance can occur on time scales of less than a few years. This could lead to changes in ice flow that are currently difficult to predict.

To contrast continental and alpine subglacial weathering regimes and thereby assess the role of large ice masses in chemical weathering, borehole and outlet water samples were collected from multiple locations on a major, land-terminating outlet of the Greenland Ice Sheet. Boreholes, reaching ice depths to 824 m, were drilled to the bed with hot-water methods in four areas of the ice sheet ablation zone along a 45 km transect extending inland from the outlet terminus. The bulk chemical composition of these samples shows substantially less influence of sulfides and carbonates than found in alpine glaciers, suggesting that the sediment under this region of the ice sheet has become depleted of accessory minerals. The waters show wide variability in chemical composition over both large and small temporal-spatial scales, suggesting large ranges in length of subglacial water storage and in rates of abrasion and comminution of subglacial earth materials. The dissolved solids concentrations found in the Greenland Ice Sheet are comparable to and in some cases exceed those of alpine glaciers, suggesting that large ice masses are capable of generating substantial dissolved loads through silicate weathering mechanisms.

Reconstructions of ice sheet fluctuations during the Holocene, which encompassed cooler and warmer conditions than those that are captured in the historic record, help to elucidate ice margin sensitivity to climate change. We used amino acid geochronology to constrain the history of the western Greenland Ice Sheet margin during intervals of relative warmth in the middle Holocene. We measured the extent of amino acid racemization in 251 ice sheet–reworked marine bivalve shells from three locations spanning western Greenland. A significant relationship between shell age and the ratio of aspartic acid (Asp) isomers (Asp D/L) was revealed using Bayesian model fitting on 20 radiocarbon-dated shell fragments. The range of Asp-inferred bivalve ages at each site corresponds well with independent records of early Holocene ice retreat and late Holocene ice advance. Furthermore, the frequency of Asp-inferred bivalve ages from the three widely separated locations is nearly identical, with most ages between 5 and 3 ka, coinciding with optimum oceanic conditions. Because ice margin changes in western Greenland are tightly linked with oceanographic conditions, the distribution of reworked bivalve ages provides important information about relative ice margin position during smaller-than-present ice sheet configurations. This approach adds a new chronometer to our toolkit for constraining smaller-than-present ice sheet configurations and may have wide applicability around Greenland.

Limited data on the past dynamics of the Greenland Ice Sheet, exceeding the ice-core records, have led to partly contradictory reconstructions. Whereas the Scoresby Sund sector of the Greenland Ice Sheet has been suggested to be stable and not much larger than at present during the peak Pleistocene glaciations, the southeastern sector of the ice sheet has been inferred to be much more dynamic. Here we present seismic data showing that glacigenic debris-flow deposits dominate the younger than ca. 2.58 Ma succession of the Scoresby Sund trough mouth fan on the East Greenland continental margin, suggesting much more frequent expansions of the Greenland Ice Sheet to the shelf break than found previously. From our new observations of the Pleistocene stratigraphy, we revise previous findings and conclude that the Scoresby Sund sector of the Greenland Ice Sheet during the Pleistocene (1) developed in line with other parts of the East Greenland Ice Sheet, and (2) was more dynamic and sensitive to past climatic changes than hitherto realized. This rapid response of the glacier to climate forcing indicates how dynamic is the glacier ice front and what one might expect of the glacier as the influences of climate warming become more pronounced.

Antarctic subglacial highlands are where the Antarctic ice sheets first developed and the “pinning points” where retreat phases of the marine-based sectors of the ice sheet are impeded. Due to low ice velocities and limited present-day change in the ice-sheet interior, West Antarctic subglacial highlands have been overlooked for detailed study. These regions have considerable potential, however, for establishing the locations from which the West Antarctic Ice Sheet originated and grew, and its likely response to warming climates. Here, we characterize the subglacial morphology of the Ellsworth Subglacial Highlands, West Antarctica, from ground-based and aerogeophysical radio-echo sounding (RES) surveys and the Moderate-Resolution Imaging Spectroradiometer (MODIS) Mosaic of Antarctica. We document well-preserved classic landforms associated with restricted, dynamic, marine-proximal alpine glaciation, with hanging tributary valleys feeding a significant overdeepened trough (the Ellsworth Trough) cut by valley (tidewater) glaciers. Fjord-mouth threshold bars down-ice of two overdeepenings define both the northwest and southeast termini of paleo-outlet glaciers, which cut and occupied the Ellsworth Trough. Satellite imagery reveals numerous other glaciated valleys, terminating at the edge of deep former marine basins (e.g., Bentley Subglacial Trench), throughout the Ellsworth Subglacial Highlands. These geomorphic data can be used to reconstruct the glaciology of the ice masses that formed the proto–West Antarctic Ice Sheet. The landscape predates the present ice sheet and was formed by a small dynamic ice field(s), similar to those of the present-day Antarctic Peninsula, at times when the marine sections of the West Antarctic Ice Sheet were absent. The Ellsworth Subglacial Highlands represent a major seeding center of the paleo–West Antarctic Ice Sheet, and its margins represent the pinning point at which future retreat of the marine-based West Antarctic Ice Sheet would be arrested.

Interactions between subglacial hydrology and the ocean make the existence of estuaries at the grounding zones of ice sheets likely. Here we present geophysical observations of an estuary at the downstream end of the hydrologic system that links the active subglacial lakes beneath Whillans Ice Stream to the ocean beneath the Ross Ice Shelf, Antarctica. This subglacial estuary consists of a hydropotential low upstream of the grounding zone, which is linked to the ocean by a hydropotential trough and a large subglacial channel. This subglacial channel, which is imaged using active source seismic methods, has an apparent width of 1 km and a maximum depth of 7 m. The hydropotential trough continues upstream of the grounding zone and results from an along-flow depression in surface elevations. Pressure differences along the trough axis are within a range that can be overcome by tidally induced processes, making the interaction of subglacial and ocean water likely.

Ice sheet models suggest that once formed, the large, high-\ud altitude East Antarctic Ice Sheet was relatively self-stabilizing, due to its cold upper surface. The ice sheet hysteresis problem results from an inability to reconcile this expectation with geological evidence for episodes of ice sheet retreat. A classic example of this problem is manifested in benthic foraminiferal oxygen isotope records across the Oligocene-Miocene boundary (ca. 23 Ma), which display a transient ~1‰ excursion to higher values. The inferred increase and subsequent decrease in ice volume has been linked to advance and retreat of the Antarctic ice sheet across the continental shelf. However, oxygen isotope records alone do not provide unambiguous records of temperature and ice volume, hindering assessment of the driving mechanism for these variations. Here we present new benthic foraminiferal Mg/Ca, Li/Ca, and U/Ca records across the Oligocene- Miocene boundary from Ocean Drilling Program Sites 926 and 929. Our records demonstrate that Atlantic bottom-water temperatures varied cyclically, with the main cooling and warming steps followed by ice growth and decay respectively. We suggest that enhanced organic carbon burial acted as a positive feedback as climate cooled. Several lines of evidence suggest that the deglaciation was associated with an input of carbon to the ocean-atmosphere system, culminating in a previously unidentified seafloor dissolution event. We suggest that one of the initial sources of carbon was organic matter oxidation in ocean sediments. This study demonstrates that carbon cycle feedbacks should be considered when evaluating the stability of ancient ice sheets.

We document the age and extent of late Quaternary glaciofluvial processes in Garwood Valley, McMurdo Dry Valleys, Antarctica, using mapping, stratigraphy, geochronology, and geochemical analysis of sedimentary and ice deposits. Geomorphic and stratigraphic evidence indicates damming of the valley at its Ross Sea outlet by the expanded Ross Sea ice sheet during the Last Glacial Maximum. Damming resulted in development of a proglacial lake in Garwood Valley that persisted from late Pleistocene to mid-Holocene time, and in the formation of a multilevel delta complex that overlies intact, supraglacial till and buried glacier ice detached from the Ross Sea ice sheet. Radiocarbon dating of delta deposits and inferred relationships between paleolake level and Ross Sea ice sheet grounding line positions indicate that the Ross Sea ice sheet advanced north of Garwood Valley at ca. 21.5 ka and retreated south of the valley between 7.3 and 5.5 ka. Buried ice remaining in Garwood Valley has a similar geochemical fingerprint to grounded Ross Sea ice sheet material elsewhere in the southern Dry Valleys. The sedimentary sequence in Garwood Valley preserves evidence of glaciofluvial interactions and climate-driven hydrological activity from the end of the Pleistocene through the mid-Holocene, making it an unusually complete record of climate activity and paleoenvironmental conditions from the terrestrial Antarctic.

The AND-2A drill hole (ANDRILL [Antarctic Geological Drilling Program] Southern McMurdo Sound Project), ∼10 km from the East Antarctica coastline, records nearly 6 m.y. of sedimentation across the Miocene climatic optimum at a high-latitude site. Sedimentological studies of bedforms and particle size distributions indicate that the paleoenvironment was strongly affected by waves and currents, consistent with deposition in a glacially influenced neritic environment. We document abrupt shifts in mud percent within glacial-interglacial cycles ca. 17.8 Ma and between ca. 16.7 and 15.7 Ma that we attribute to the hydrodynamic effects of wave stirring tied to episodes of ice growth and decay. Although wave climate and geodynamic forcing of the paleobathymetry simultaneously affect wave stirring on a high-latitude shelf, both are ultimately controlled by the size of the ice sheet. The mud percent record displays cyclicity at short-eccentricity time scales (94–99 k.y.) and, unexpectedly, ice retreat phases interpreted from the particle size record coincide with eccentricity minima. We attribute the eccentricity-paced ice retreat phases during the late Early Miocene polythermal glacial conditions and the cool orbital parameters to marine ice sheet instability in response to changes in ocean circulation and heat transport. The particle size record of the AND-2A core provides unique near-field evidence for orbitally paced changes in high-latitude climate and ice volume during the Miocene climatic optimum and important insights into the mechanisms of ice sheet growth and decay in a period of global warmth.

The seismic stratigraphy and architecture of the Beaufort Sea shelf and slope are investigated using a comprehensive grid of high-resolution two-dimensional seismic reflection data. Three cross-shelf troughs, representing locations of former ice streams draining a 1000-km-long section of the Laurentide Ice Sheet (LIS) are examined: the Mackenzie, Amundsen Gulf, and M’Clure Strait systems. Dynamics of these paleo-ice streams influenced ice-sheet configuration and may have forced abrupt climatic change through delivery of ice and freshwater to the Arctic Ocean. A comprehensive understanding of their geometry and dynamics is crucial for constraining numerical models of the LIS. Evidence for two Quaternary ice advances to the shelf break is interpreted from the Mackenzie Trough. By contrast, seismic stratigraphy of the Amundsen Gulf Trough, 400 km east of the Mackenzie Trough, records at least nine Quaternary ice advances. Here, the outer shelf consists of stacked till sheets, extending to the shelf break and forming a trough-mouth fan. The contrasting glacial histories of these neighboring ice streams are explained by their positions within the LIS; the Mackenzie Trough ice stream was situated at the extreme northwest ice-sheet margin, whereas the Amundsen Gulf ice stream had a more central location and larger drainage basin, supplying significant quantities of ice and sediment to the Arctic Ocean through much of the Quaternary. The M’Clure Strait Trough probably possesses a similar architecture to the Amundsen Gulf Trough, and an even larger trough-mouth fan.

Ice loss from the marine-based, potentially unstable West Antarctic Ice Sheet (WAIS) contributes to current sea-level rise and may raise sea level by ≤3.3 m or even ≤5 m in the future. Over the past few decades, glaciers draining the WAIS into the Amundsen Sea Embayment (ASE) have shown accelerated ice flow, rapid thinning, and fast retreat of the grounding line (GL). However, the long-term context of this ice loss is poorly constrained, limiting our ability to accurately predict future WAIS behavior. Here we present a new chronology for WAIS retreat from the inner continental shelf of the eastern ASE, based on radiocarbon dates from three marine sediment cores. The ages document a retreat of the GL to within ∼100 km of its modern position before ca. 10,000 calibrated (cal.) yr B.P. This early deglaciation is consistent with ages for GL retreat from the western ASE. Our new data demonstrate that, in contrast to the Ross Sea, WAIS retreat from the ASE shelf was largely complete by the start of the Holocene. Our results further suggest either slow GL retreat from the inner ASE shelf throughout the Holocene, or that any episodes of fast GL retreat must have been short-lived. Thus, today’s rapid retreat may be exceptional during the Holocene and may originate in recent changes in regional climate, ocean circulation, or ice-sheet dynamics.

The geologic record from high-latitude continental margins shows that soft-bedded ice streams are capable of eroding, transporting, and depositing large volumes of sediment. Interpretation of these records relies on correct understanding of how sediments were transferred. We use a three-dimensional numerical ice-flow model, with physically based processes taking place in a Coulomb-plastic basal till layer, to test a new hypothesis related to sedimentary processes occurring beneath ice streams. Based on recent observations of a 15-m-thick debris-bearing basal ice layer (BIL) in Kamb Ice Stream, Antarctica, we propose that sediment entrainment by freeze-on, followed by englacial transport, and eventually meltout, represent efficient mechanisms whereby ice streams erode their bed and redistribute sediments. Our experimental setup produces results where ice stream flow is characterized by oscillations between fast and stagnant modes of flow. We show that there is a strong coupling between the amplitude of the ice stream oscillations and the amount of sediment eroded and transferred out of the modeled system due exclusively to the formation and advection of a BIL. We also show that increased incorporation of water from a basal water system amplifies the oscillations and thus the growth of the BIL. The patterns of ice stream flow, and associated sediment fluxes and transport seen in our model, are consistent with modern Antarctic ice streams as well as the seemingly erratic behavior of paleo–ice streams during the last deglaciation.

The long-term behavior of the East and West Antarctic Ice Sheets, and their respective responses to forcing provide essential context for assessment of modern dynamic changes in ice-flow regimes and ice-sheet and shelf margins. The western Ross Sea discharges ice from both the East and West Antarctic Ice Sheets, and the paleoglacial record from this region is therefore valuable in unraveling their long-term behavior. New, high-resolution multibeam bathymetric data reveal snapshots of well-preserved glacial landforms on the seafloor around Ross Island and McMurdo Sound. Glacial lineations, grounding zone wedges, draped recessional moraines, and meltwater channels record a series of different ice-flow events in the region, contradictions between which require major phases of ice-flow reorganization. From the glacial geomorphology, we reconstruct a four-stage model of ice-flow evolution for the last glacial cycle, consisting of: (1) northeastward flow into the Ross Sea from McMurdo Sound; (2) westward flow from the Ross Sea, around Ross Island, and onto the Victoria Land coast and coastal seafloor trough; (3) a deglacial phase of ice-sheet thinning, minor shifts in flow, and grounding line retreat into McMurdo Sound; and (4) grounding line pinning on Ross Island during regional retreat, uncoupling of a remnant Ross Island ice cap, and local oscillation of Victoria Land outlet glaciers. We find that East Antarctic Ice Sheet ice discharge had a strong influence on ice-flow geometry in this part of the Ross Sea during the last glacial stage, but that it was not necessarily in phase with the behavior of the West Antarctic Ice Sheet. It is similarly evident that the ice streams that drained the Ross Sea over the continental shelf at the Last Glacial Maximum did not all operate synchronously, and exerted different drawdown power at different times. Finally, we conclude that Ross Island acts as an important pinning point in the Ross Sea ice-sheet-shelf system, stabilizing grounding line retreat and encouraging lasting ice-shelf development.

The Pleistocene ice sheets left a clear signature of erosion, but the rate at which ice sheets erode is difficult to determine from either paleolandscapes or observations of contemporary processes. Here we use two years of sediment flux data, derived from meltwaters emerging from an outlet glacier in west Greenland, to calculate an average rate of subglacial erosion across a catchment extending >50 km inland from the ice margin. Erosion in this zone occurs at 4.8 ± 2.6 mm a −1 , a rate 1–2 orders of magnitude greater than previous estimates of erosion rate beneath the Greenland Ice Sheet. Our results suggest that where surface meltwaters are able to access the bed, the rate of erosion by ice sheets is in keeping with the rapid erosion observed at temperate alpine glaciers. During deglacial phases, when meltwater was abundant, ice sheet margins should therefore have acted as highly efficient agents of erosion.

Recent changes in speed, thinning, and retreat rates of marine-terminating outlet glaciers have raised concerns about the future stability of the Greenland Ice Sheet. Establishing a longer term record of outlet glacier retreat rates is essential to provide a context for present-day observations and to improve and constrain numerical models of outlet glacier behavior. New exposure dating (10Be) of streamlined bedrock surfaces and glacial erratic boulders of Sermilik Fjord, southeast Greenland, the present-day drainage route of Helheim Glacier, documents rapid retreat (∼80 m a−1) of this major marine-terminating outlet glacier at the close of the last glaciation. The glacier front retreated ∼80 km to within 20 km of the present-day (2010) position of Helheim Glacier in

Ice streams dominate the discharge of continental ice sheets. Recent observations and reconstructions have revealed that large-scale reorganizations in their flow trajectory (flow switching) can occur over relatively short time scales. However, the underlying causes of such behavior, and the extent to which they are predictable, are poorly known. This paper documents a major episode of ice-stream flow switching during the late Weichselian deglaciation of the southwestern Barents Sea and explores various hypotheses for its causation. Regional bathymetric data show that two ice streams that had similar, adjoining, topographically constrained source areas had very different trajectories and dynamics on the outer shelf. At the late Weichselian maximum, the Hakjerringdjupet ice stream flowed westward along the cross-shelf trough of Hakjerringdjupet, while the Soroya Trough ice stream flowed northward into Ingoydjupet, forming a tributary of the Bjornoyrenna ice stream. Initial retreat of the Hakjerringdjupet ice stream was rapid but with episodic periods of grounding. As it retreated onto the higher, rougher topography of the inner shelf, we infer a reduction in ice velocity and a dramatic decrease in the pace of retreat, as recorded by nested sequences of recessional moraines. Following (and probably in response to) this, we suggest that there was a short-lived surge/readvance of an adjacent lobe onto Fugloybanken. In contrast, the adjacent Soroya Trough ice stream remained active throughout deglaciation, before retreating rapidly, with no stillstands or readvances. We argue that the different retreat histories of the ice streams were determined by variations in bed topography/bathymetry, which modulated the grounding line response to sea-level variation. Such a mechanism is likely to be an important control on the long-term behavior of marine-based ice streams and outlet glaciers in Antarctica and Greenland and suggests that gathering data on their subglacial topography should be a priority.

We analyze Holocene ice flow using global positioning system and radio-echo sounding data acquired near the Amundsen-Weddell ice divide, between Pine Island Glacier and the Institute Ice Stream, West Antarctica. The data show that this part of the West Antarctic Ice Sheet (WAIS) has maintained a stable ice flow regime and ice divide position for similar to 7000 yr. Independent glacial geological data, in support of this assertion, suggest that the interior of the WAIS west of the Ellsworth Mountains has thinned by only a few hundred meters since the Last Glacial Maximum (LGM) and that the position of the ice divide may be recurrent over longer (more than 20 k.y., perhaps pre-Quaternary) time scales. We suggest that basal topography constrains ice flow in this sector of the WAIS, making the ice divide relatively insensitive to perturbations around the margins of the ice sheet compared with other ice divides. Large-scale forcing measured elsewhere in the WAIS as major reorganizations of the ice sheet surface and flowline pattern may be manifested only in simple vertical changes of the Amundsen-Weddell interior.

The catastrophic break-ups of the floating Larsen A and B ice shelves (Antarctica) in 1995 and 2002 and associated acceleration of glaciers that flowed into these ice shelves were among the most dramatic glaciological events observed in historical time. This raises a question about the larger West Antarctic ice shelves. Do these shelves, with their much greater glacial discharge, have a history of collapse? Here we describe features from the seafloor in Pine Island Bay, West Antarctica, which we interpret as having been formed during a massive ice shelf break-up and associated grounding line retreat. This evidence exists in the form of seafloor landforms that we argue were produced daily as a consequence of tidally influenced motion of mega-icebergs maintained upright in an iceberg armada produced from the disintegrating ice shelf and retreating grounding line. The break-up occurred prior to ca. 12 ka and was likely a response to rapid sea-level rise or ocean warming at that time.

The timing, nature, and causes of grounded ice-sheet retreat following the Last Glacial Maximum (LGM) in Marguerite Trough, west Antarctic Peninsula, and subsequent early Holocene ice-shelf decay, are presented in this paper. We use sedimentological, foraminiferal, geotechnical, and accelerator mass spectrometer (AMS) radiocarbon data from marine cores from the mid-continental shelf, together with previously published AMS dates, to establish a sedimentological and chronological model. Initial ice-sheet retreat through the outer- and mid-shelf sectors of Marguerite Trough was under way by ca. 14 ka B.P., was rapid, and coincided with the sea-level rise of meltwater pulse 1a. An ice shelf formed during this retreat, and fine-grained, laminated muds reflecting meltwater-derived suspension settling and/or tidal pumping were deposited. During this time the ice sheet remained grounded on the inner shelf. Ice-shelf breakup and retreat of the calving front, from ca. 13.2 to 12.5 ka B.P., was slow (∼100 m a –1 ) across the outer- and mid-shelf, with calving bay conditions remaining for at least 3.5 ka. We interpret this ice-shelf decay to have been driven by an incursion of Weddell Sea Transitional Water onto the shelf. In contrast, grounding-line and ice-shelf retreat in the inner bay occurred from ca. 9.3 ka B.P. and was driven by Circumpolar Warm Deep Water encroaching onto the continental shelf. At this time the mid-shelf was an open-marine environment characterized by hemipelagic deposition. These findings highlight the importance of oceanographic controls in the breakup of Antarctic Peninsula ice shelves during the Holocene.

Glacial sediments from the Prince Charles Mountains, East Antarctica, record late Pleistocene ice thickness variability in the Lambert Glacier–Amery Ice Shelf system, one of the world9s largest ice drainages. A former glacial limit, demarcated by minimally weathered deposits, follows a concave longitudinal profile, indicating a zone of strong ice streaming through the northernmost 500 km of the Lambert Graben. In situ 10 Be and 26 Al exposure ages from these relatively unweathered deposits indicate that the most recent phase of ice lowering occurred between ca. 18 and 8 ka, preceding by as many as 6 k.y. the deglaciation of adjacent coastal regions. Earlier onset of deglaciation in an area of strong ice streaming suggests a heightened sensitivity of the East Antarctic Ice Sheet to climate and sea-level changes following the Last Glacial Maximum than previously recognized.

We used a 26 Al- 10 Be burial isochron method to date the glacial stratigraphic section in Missouri, USA, that records the largest advances of the Laurentide Ice Sheet. This permits an improved comparison of terrestrial and marine records of glaciation. The first recorded advance of the Laurentide Ice Sheet reached 39°N, near the extreme southern limit of North American glaciation, 2.4 Ma. The next advance to this latitude took place near the beginning of the mid-Pleistocene transition, 1.3 Ma, and three more took place from 0.75 to 0.2 Ma. There is no evidence that the Laurentide Ice Sheet advanced south of ∼45°–47°N between 2.4 and 1.3 Ma. This chronology: (1) shows that North American continental glaciation postdated Cordilleran alpine glaciation; (2) is consistent with the hypothesis that both of these events were threshold responses to tropical cooling; (3) is consistent with the hypothesis that the first advance of the Laurentide Ice Sheet was glaciologically anomalous due to the presence of deformable preglacial regolith; (4) is not consistent with the hypothesis that this deformable regolith persisted until the mid-Pleistocene transition; and (5) indicates that the increase in global ice volume at the mid-Pleistocene transition was at least in part the result of a more extensive Laurentide Ice Sheet.

Current models describing past configurations of the East Antarctic ice sheet are poorly constrained by observations. Exposure dating of bedrock surfaces using in situ–produced cosmogenic nuclides provides an ideal tool for directly constraining former changes in ice sheet elevation. We present cosmogenic radionuclide 10 Be and 26 Al measurements in bedrock surfaces and glacially transported cobbles in the Grove Mountains, a group of nunataks within the slow-flowing interior ice sheet dome, hundreds of kilometers from the coastal ice margin and from ice streams. Samples were collected in vertical transects over 500 m of relief. Minimum bedrock and erratic exposure ages show a trend of increasing age with height above the ice sheet, spanning a period from 0.3 to 4.0 Ma and 50–900 ka, respectively. No evidence was found for thicker ice at the Last Glacial Maximum compared to modern ice thickness. The older bedrock exposure ages of 2.5–4.0 Ma require steady-state erosion rates of −1 . The measured two-isotope bedrock exposure ages are successfully modeled when changes in surface elevation of the ice sheet are described by a combination of linear long-term ice surface lowering and shorter term high-frequency glacial-interglacial oscillations. The best-fit model requires a long-term thinning rate of 50 m m.y. −1 and an elevation change of 100 m over a 100 k.y. glacial cycle.

Approximately 34 m.y. ago, Earth's climate transitioned from a relatively warm, ice-free world to one characterized by cooler climates and a large, permanent Antarctic Ice Sheet. Understanding this major climate transition is important, but determining its causes has been complicated by uncertainties in the basic patterns of global temperature and ice volume change. Here we use an unusually well exposed coastal incised river-valley complex in the Western Desert of Egypt to show that eustatic sea level fell and then rose by ∼40 m 2 m.y. prior to establishment of a permanent Antarctic Ice Sheet. This fall in sea level is associated with a positive oxygen isotope excursion that has been interpreted to reflect global cooling, but instead records buildup of an Antarctic Ice Sheet with a volume ∼70% of the present-day East Antarctic Ice Sheet. Both the sea-level fall and subsequent rise were coincident with a transient oscillation in atmospheric CO2 concentration down to ∼750 ppm, which climate models indicate may be a threshold for Southern Hemisphere glaciation. Because many of the carbon emission scenarios for the coming century predict that atmospheric CO2 will rise above this same 750 ppm threshold, our results suggest that global climate could transition to a state like the Late Eocene, when a large permanent Antarctic Ice Sheet was not sustainable.

The retreat history of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum is important for understanding the process of rapid deglaciation, constraining models that seek to predict the future trajectory of the ice sheet, and for estimating rates of sea-level change. Here we report new glacial geologic data from the southwestern Weddell Sea embayment that demonstrate that this part of the WAIS was thinner than previously suggested, and that there was progressive thinning of the ice sheet by 230–480 m since ca. 15 ka. We use geomorphological data and a numerical ice sheet model to reconstruct the ice sheet in the Weddell Sea at the Last Glacial Maximum. The volume of this ice would have added between 1.4 and 2.0 m to postglacial sea-level rise and would not have been sufficient to contribute significantly to meltwater pulse 1A, a rapid rise in sea level ∼14,200 yr ago.

The pattern of late Weichselian (ca. 20 ka) ice flow in the northern Barents Sea is not well known, due mainly to a lack of marine data east of Svalbard. Several years with little summer sea ice have allowed acquisition of swath-bathymetric imagery of well-preserved subglacial landforms characterizing late Weichselian ice-flow directions over ∼150,000 km2 of the northwestern Barents Sea. We show that a major ice dome was located on easternmost Spitsbergen or southern Hinlopen Strait, at least 500 km west of its previously inferred position in the northern Barents Sea. This dome controlled the regional flow pattern; ice flowed eastward around Kong Karls Land into Franz Victoria Trough and north through Hinlopen Strait. An ice dome west of Kong Karls Land is required to explain the observed ice-flow pattern, but does not preclude an additional ice dome to the southeast. Discrepancies with earlier ice-sheet reconstructions reflect the lack of previous seafloor observations, with evidence limited mainly to past ice loading and postglacial rebound. The new pattern of ice-flow directions shows predominantly eastward rather than northward flow, with Franz Victoria Trough a major drainage pathway with a full-glacial balance flux of >40 km3 yr−1.

Sediment delivery rates through an entire ice age are investigated using seismic records of glacial erosion products from the Fennoscandian Ice Sheet deposited offshore of mid-Norway in the 2.7 m.y. old Naust Formation (volume 100,000 km 3 ). The mean sedimentation rate over the ice age is ∼0.24 m k.y. −1 , with bedrock lowering of ∼520 m in the ice-sheet catchment. The mean sediment delivery is 2–3 times higher for the most recent 600 k.y. than for earlier Naust sequences. The hypothesis that glacial erosion is most rapid early in an ice age, when there was presumably much weathered bedrock and preglacial sediment available, is not accepted for our study area. More important are changing ice-sheet dimensions and dynamics, varying intensity of individual glacial cycles, and complex ice-sheet dynamics in single deglaciations. The mean rate of sediment delivery from ice sheets is an order of magnitude higher than from fluvial activity in the huge Amazon and Mississippi systems, implying that ice is a very effective agent of long-term denudation during cold periods of Earth history.