Kamb Ice Stream Research Articles

Origin and dynamic significance of longitudinal structures ("flow stripes") in the Antarctic Ice Sheet

Abstract. Longitudinal ice-surface structures in the Antarctic Ice Sheet can be traced continuously down-ice for distances of up to 1200 km. A map of the distribution of ~ 3600 of these features, compiled from satellite images, shows that they mirror the location of fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of metres per annum and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. They can be traced continuously through crevasse fields and through blue-ice areas, indicating that they represent the surface manifestation of a three-dimensional structure, interpreted as foliation. Flow lines are linear and undeformed for all major flow units described here in the Antarctic Ice Sheet except for the Kamb Ice Stream and the Institute and Möller Ice Stream areas, where areas of flow perturbation are evident. Parcels of ice along individual flow paths on the Lambert Glacier, Recovery Glacier, Byrd Glacier and Pine Island Glacier may reside in the glacier system for ~ 2500 to 18 500 years. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow configuration of the ice sheet may have remained largely unchanged for the last few hundred years, and possibly even longer. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

The U-Pb detrital zircon signature of West Antarctic ice stream tills in the Ross embayment, with implications for Last Glacial Maximum ice flow reconstructions

AbstractGlacial till samples collected from beneath the Bindschadler and Kamb ice streams have a distinct U-Pb detrital zircon signature that allows them to be identified in Ross Sea tills. These two sites contain a population of Cretaceous grains 100–110 Ma that have not been found in East Antarctic tills. Additionally, Bindschadler and Kamb ice streams have an abundance of Ordovician grains (450–475 Ma) and a cluster of ages 330–370 Ma, which are much less common in the remainder of the sample set. These tracers of a West Antarctic provenance are also found east of 180° longitude in eastern Ross Sea tills deposited during the last glacial maximum (LGM). Whillans Ice Stream (WIS), considered part of the West Antarctic Ice Sheet but partially originating in East Antarctica, lacks these distinctive signatures. Its U-Pb zircon age population is dominated by grains 500–550 Ma indicating derivation from Granite Harbour Intrusive rocks common along the Transantarctic Mountains, making it indistinguishable from East Antarctic tills. The U-Pb zircon age distribution found in WIS till is most similar to tills from the west-central Ross Sea. These data provide new specific targets for ice sheet models and can be applied to pre-LGM deposits in the Ross Sea.

Significant groundwater contribution to Antarctic ice streams hydrologic budget

Satellite observations have revealed active hydrologic systems beneath Antarctic ice streams, but sources and sinks of water within these systems are uncertain. Here we use numerical simulations of ice streams to estimate the generation, flux, and budget of water beneath five ice streams on the Siple Coast. We estimate that 47% of the total hydrologic input (0.98 km3 yr−1) to Whillans (WIS), Mercer (MIS), and Kamb (KIS) ice streams comes from the ice sheet interior and that only 8% forms by local basal melting. The remaining 45% comes from a groundwater reservoir, an overlooked source in which depletion significantly exceeds recharge. Of the total input to Bindschadler (BIS) and MacAyeal (MacIS) ice streams (0.56 km3 yr−1), 72% comes from the interior, 19% from groundwater, and 9% from local melting. This contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes.

Grounding-line dynamics and margin lakes

AbstractAt both corners of the now stagnant Kamb Ice Stream (KIS, West Antarctica) outlet, shear margins of different ages confine wedge-shaped areas with relatively flat, smooth surfaces that stagnated before the main trunk of the ice stream. We identify these features as lakes or past lakes, and consider scenarios for their development in a regional history of ongoing adjustment to grounding-line position. We focus here on the centuries leading up to the recent stagnation of KIS, a time when its grounding line appears to have advanced >100km from an earlier upstream location. Starting from stagnation of Crary Ice Rise and changes in the grounding zone of Whillans Ice Stream, we trace feedbacks associated with local thickening, ice grounding and thickness transients that both advance the grounding line and leave remnant lakes in their wake. These lakes in turn promote the development of secondary margins that may appear as ‘margin jumps’ in the ice record.

Recent changes in the flow of the Ross Ice Shelf, West Antarctica

Comparison of surface velocities measured during the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS, 1973 to 1978) and velocities measured via feature tracking between two Moderate-resolution Imaging Spectroradiometer (MODIS) mosaics (compiled from 2003/4 and 2008/9 images) reveals widespread slowing and minor areas of acceleration in the Ross Ice Shelf (RIS) over the approximately 30 year interval. The largest changes (−13 ma−2) occur near the Whillans and Mercer Ice Streams grounding line in the southernmost part of the ice shelf. Speed has increased over the interval (up to 5 ma−2) between the MacAyeal Ice Stream grounding line and the shelf front, and along the eastern shelf front. Changes in ice thickness computed using ICESat laser altimetry are used together with a well-tested model of the ice shelf to investigate underlying causes of change in the flow of the ice shelf over time. The observed transients represent a combination of recent forcings and ongoing response to ice stream discharge variations over the past millennium. While evidence of older events may be present, the modern signal is dominated by shorter time scale events, including the stagnation of Kamb Ice Stream about 160 years ago, recent changes in basal drag on the Whillans Ice Stream ice plain and, perhaps, iceberg calving. Details in embayment geometry, for example the shallow sea floor below Crary Ice Rise, modulate the spatial pattern of ice shelf response to boundary condition perturbations.

The influence of subglacial hydrology on the flow of Kamb Ice Stream, West Antarctica

AbstractIce streams on the Siple Coast, West Antarctica, have a complex history of flow because their basal motion is governed by time‐varying basal conditions. Although the mechanical interaction between ice and till is well established, very little is known about the potential effect of regionally scaled water transport in a basal water system, which has only recently become apparent. To investigate the combined effect of hydrological and mechanical processes, we developed the Hydrology, Ice and Till model, in which ice flow is coupled to a Coulomb‐plastic till layer and a basal water system consisting of discrete conduits. When the model is applied to Kamb Ice Stream (KIS), results confirm that it is capable of oscillating between fast and stagnant modes of flow. We show that when subglacial conduits are disregarded or do not extend to the grounding line, the oscillatory behavior of the ice stream is governed by the basal thermal regime. When conduits extend to the grounding line, the modelled ice stream oscillation period is increased, peak speeds are reduced, and oscillations may ultimately cease if the volume of water supplied is sufficiently high. Three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line. Modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years, although varying according to hydrological activity. Our results indicate that KIS could reactivate this century, given its hydrological setting and ~170 years of stagnation.

Kamb Ice Stream flow history and surge potential

AbstractA basal zone, tens of meters thick, of debris-laden ice was observed in Kamb Ice Stream, West Antarctica, using a video camera lowered into boreholes made by hot-water drilling. The debris content varies, sometimes abruptly, forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction. In most parts, the concentration of debris is low, a few percent by weight, with particles, often mud clots, dispersed in a matrix of clear ice. The nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of Kamb Ice Stream including processes leading up to the termination of its streaming behavior and possible reactivation.

Evidence of rapid subglacial water piracy under Whillans Ice Stream, West Antarctica

AbstractThe subglacial water system of lower Whillans Ice Stream on the Siple Coast, West Antarctica, contains numerous connected subglacial lakes in three hydrological basins (northern, central and southern). We use Ice, Cloud and land Elevation Satellite (ICESat) data to derive estimates of lake volume change and regional thickness changes. By combining these results with a water budget model, we show that a uniform, localized thickness increase perturbed the hydropotential, resulting in a change in course of a major flow path within the system in 2005. Water originating from upper Whillans and Kamb Ice Streams that previously supplied the southern basin became diverted toward Subglacial Lake Whillans (SLW). This diversion led to a tenfold filling rate increase of SLW. Our observation suggests that water piracy may be common in the Siple Coast region, where the gentle basal relief makes the basal hydropotential particularly sensitive to small changes in ice thickness. Given the previously inferred connections between water piracy and ice-stream slowdown elsewhere in the region, the subtle and complex nature of this system presents new challenges for numerical models.

Hydrologic forcing of ice stream flow promotes rapid transport of sediment in basal ice

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.

Variability in the mass flux of the Ross ice streams, West Antarctica, over the last millennium

AbstractWe synthesize previously published remote-sensing observations, radar data and model output to obtain a ~1000 year ice flow history for the Siple Coast ice-stream system in West Antarctica to investigate the timing and magnitude of changes in mass flux. The synthesized history shows significant short-term variability in ice-stream shear margin and grounding line position due to internal variability of the coupled system. The chronology highlights the interplay between adjacent ice streams, which implies that the behavior of any individual ice stream should not be examined in isolation. Furthermore, individual events cannot be fully interpreted without an understanding of the broad-scale, long-term variability in the ice sheet. In the context of this millennium-scale history, we interpret the relatively recent stagnation of Kamb Ice Stream (KIS) as just one stage in the thermodynamic cycle of an ice stream in this region. The changes in mass balance that result from the KIS stagnation may thus be viewed as century-scale 'noise' relative to the longer-term trend. Understanding and characterizing this noise is a necessary step before accurate model-based predictions of ice-sheet mass balance for the next century can be made.

The grounding zone of the Ross Ice Shelf, West Antarctica, from ice-penetrating radar

AbstractAs ice streams flow into the Ross Ice Shelf, West Antarctica, their bed coupling transitions from weak to transient to zero as the ice goes afloat. Here we explore the nature of the bed across these crucial grounding zones using ice-penetrating radar. We collected several ground-based 2 MHz radar transects across the grounding zones of Whillans and Kamb Ice Streams and inferred bed-reflectivity changes from in situ measurements of depth-averaged dielectric attenuation, made possible by the observation of both primary and multiple bed echoes. We find no significant change in the bed reflectivity across either grounding zone. Combined with reflectivity modeling, this observation suggests that a persistent layer of subglacial water (>∼0.2 m) is widespread several kilometers upstream of the grounding zone. Our results are consistent with previous inferences of gradual grounding zones across this sector of the Ross Ice Shelf from airborne radar and satellite altimetry. Separately, the only clear bed-reflectivity change that we observed occurs ∼40 km downstream of the Kamb Ice Stream grounding zone, which we attribute to the onset of marine ice accretion onto the base of the ice shelf. This onset is much nearer to the grounding zone than previously predicted.

Basal ice sequences in Antarctic ice stream: Exposure of past hydrologic conditions and a principal mode of sediment transfer

The brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in Kamb Ice Stream. The upper sequence (7.3 m) comprises clear ice and layers with dispersed and stratified debris. The lower sequence (8.2 m) consists of accretion ice with alternating layers of stratified and solid debris. The two sequences have volumetric debris contents of about 5% and 20%, respectively. We infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the Siple Coast plain where basal freezing currently dominates the basal thermal regime. The basal ice layer contains the equivalent of 2.1 ± 0.4 m of frozen sediment. The high volume of sediment is a result of debris‐rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on/off cycles. Fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the Ross Sea. Our results show that sediment wedges near grounding lines may form by melt out of basal ice debris. This is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till. Cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze‐on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override Coulomb‐plastic substrates.

Hydrochemistry of ice stream beds—evaporitic or microbial effects?

AbstractRecently, new data have been acquired on the chemical composition of waters within the glacial till beneath the Kamb Ice Stream (KIS, formerly Ice Stream C) and the Bindschadler Ice Stream (BIS, formerly Ice Stream D), two of the arteries of ice loss from the Antarctic Ice Sheet (AIS). We compare this water chemistry with basally stored subglacial waters from Arctic and Alpine glaciers and the only other measurements of sub‐ice sheet chemistry to date, that emerging from beneath Law Dome near Casey Station, SW Antarctica, during a jökulhlaup. We find that the chemistries pose significant questions about the hydrology and biogeochemistry of ice sheet beds. Copyright © 2010 John Wiley & Sons, Ltd.

Bacteria beneath the West Antarctic Ice Sheet

Subglacial environments, particularly those that lie beneath polar ice sheets, are beginning to be recognized as an important part of Earth's biosphere. However, except for indirect indications of microbial assemblages in subglacial Lake Vostok, Antarctica, no sub-ice sheet environments have been shown to support microbial ecosystems. Here we report 16S rRNA gene and isolate diversity in sediments collected from beneath the Kamb Ice Stream, West Antarctic Ice Sheet and stored for 15 months at 4 degrees C. This is the first report of microbes in samples from the sediment environment beneath the Antarctic Ice Sheet. The cells were abundant ( approximately 10(7) cells g(-1)) but displayed low diversity (only five phylotypes), likely as a result of enrichment during storage. Isolates were cold tolerant and the 16S rRNA gene diversity was a simplified version of that found in subglacial alpine and Arctic sediments and water. Although in situ cell abundance and the extent of wet sediments beneath the Antarctic ice sheet can only be roughly extrapolated on the basis of this sample, it is clear that the subglacial ecosystem contains a significant and previously unrecognized pool of microbial cells and associated organic carbon that could potentially have significant implications for global geochemical processes.

Spatial variation of radar-derived basal conditions on Kamb Ice Stream, West Antarctica

AbstractRadar profiles of bed echo intensity can survey conditions at the ice–bed interface and test for the presence or absence of water. However, extracting information about basal conditions from bed echo intensities requires an estimate of the attenuation loss through the ice. We used the relationship between bed echo intensities from constant-offset radar data and ice thickness to estimate depth-averaged attenuation rates at several locations on and near Kamb Ice Stream (KIS), West Antarctica. We found values varying from 29 dBkm–1 at Siple Dome to 15 dBkm–1 in the main trunk region of KIS, in agreement with a previous measurement and models. Using these attenuation-rate values, we calculated the relative bed reflectivity throughout our KIS surveys and found that most of the bed in the trunk has high basal reflectivities, similar to those obtained in the location of boreholes that found water at the bed. Areas of lower bed reflectivity are limited to the sticky spot, where a borehole found a dry bed, and along the margins of KIS. These results support previous hypotheses that the basal conditions at locations like the sticky spot on KIS control its stagnation and possible reactivation.

Radar signatures beneath a surface topographic lineation near the outlet of Kamb Ice Stream and Engelhardt Ice Ridge, West Antarctica

AbstractVisible and infrared satellite images reveal numerous lineations on the Siple Coast region of West Antarctica. We used 5 MHz ice-penetrating radar to probe the interior and the bed of the ice sheet beneath a lineation at the boundary between Engelhardt Ice Ridge and flat-ice terrain to the south of the Kamb Ice Stream (KIS) outlet. Results show curved reflectors that emerge from the bed beneath 600 m thick ice. The tops of the reflectors extend about 100m into the ice above the bed, where they become almost horizontal. Apparent reflectivity of the horizontal section is about 20 dB less than that of the bed. We conclude that the likely cause of such strong reflection is sea water that was accreted into basal crevasses when the flat-ice terrain was floating. Internal layers are warped downward just downslope from the basal reflectors. It is thought that the downwarping was caused by localized basal melting in the past. The spatial pattern of downwarping suggests that localized basal melting was stronger on the north side than on the south side of KIS; apparently ice/ocean interactions on the two sides of KIS were different.

Holocene accumulation and ice sheet dynamics in central West Antarctica

We derive depth‐age relationships across the ice divide between the Ross and Amundsen Seas by tracking radar‐detected layers from the Byrd ice core and a dated 105‐m core near the divide. The depth‐age relationships and an ice‐flow model are used to establish histories of accumulation and ice sheet dynamics over the past 8000 years. Results show that accumulation was approximately 30% higher than today from 5000 to 3000 years ago. Antarctic climate variability today is dominated by periodic fluctuations in strength of the circumpolar vortex, which raises the possibility that the vortex was systematically weaker during the period of high accumulation. Accumulation today decreases almost linearly across the divide. It is unlikely that this pattern has changed through the Holocene. The radar‐detected stratigraphy shows no evidence of the arched layers that are expected beneath a stable divide that is frozen to its bed, implying that the divide has also been migrating and/or the basal ice has been sliding through the Holocene. We cannot rule out the possibility of sliding because the basal ice is near its pressure melting point. Other evidence indicates that divide migration is likely. The Ross Sea sector is now near steady state, but it had a strong negative imbalance 200 years ago when Kamb Ice Stream was active. In contrast, recent speedups of Pine Island and Thwaites Glaciers have likely caused the mass balance of the Amundsen Sea sector to become negative. The divide is likely migrating toward the Ross Sea today.

The evolution of surface flow stripes and stratigraphic folds within Kamb Ice Stream: why don’t they match?

AbstractFlow stripes seen in satellite imagery of ice streams and ice shelves are caused by surface undulations with kilometer-scale spacing and meter-scale relief and generally indicate current or recent fast ice flow. On a similar scale, folding of internal ice stratigraphy depicted in cross-flow ice-penetrating radar profiles is also a common occurrence in ice streams, suggesting a possible relationship between the two sets of features. We have traced surface flow stripes in RADARSAT and MODIS imagery on Kamb Ice Stream, West Antarctica, from the onset of streaming flow into the near-stagnant trunk. We compare the morphology and evolution of the surface flow stripes to the folds seen in the internal stratigraphy in cross-ice-stream radar profiles. We find essentially no correspondence in the observed locations or spacings between the radar internal layer folds at depths greater than 100 m and the flow stripes on the surface. The gap in the radar data and the surface mappings in the top 100 m of firn prevents a precise depiction of how the flow stripes and fold patterns at depth diverge. We explore hypotheses about how flow stripes and internal stratigraphic folds can originate and evolve differently as ice flows downstream. We suggest that flow stripes are subject to surface processes that can modify their morphology independently of the internal stratigraphy, leading to changes in the pattern of flow stripes relative to the internal layers below.

Along-Track Focusing of Airborne Radar Sounding Data From West Antarctica for Improving Basal Reflection Analysis and Layer Detection

This paper presents focused synthetic aperture radar (SAR) processing of airborne radar sounding data acquired with the High-Capability Radar Sounder system at 60 MHz. The motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking. The processing and reflection analyses are applied to data from Kamb Ice Stream, West Antarctica. The SAR processor correlates the radar data with reference echoes from subsurface point targets. The references are 1-D responses limited by the pulse nadir footprint or 2-D responses that include echo tails. Unfocused SAR and incoherent integration are included for comparison. Echoes are accurately preserved from along-track slopes up to about 0.5deg for unfocused SAR, 3deg for 1-D correlations, and 10deg for 2-D correlations. The noise/clutter levels increase from unfocused SAR to 1-D and 2-D correlations, but additional gain compensates at the basal interface. The basal echo signal-to-noise ratio improvement is typically about 5 dB, and up to 10 dB for 2-D correlations in rough regions. The increased noise degrades the clarity of internal layers in the 2-D correlations, but detection of layers with slopes greater than 3deg is improved. Reflection coefficients are computed for basal water detection, and the results are compared for the different processing methods. There is a significant increase in the detected water from unfocused SAR to 1-D correlations, indicating that substantial basal water exists on moderately sloped interfaces. Very little additional water is detected from the 2-D correlations. The results from incoherent integration are close to the focused SAR results, but the noise/clutter levels are much greater.

Century‐scale discharge stagnation and reactivation of the Ross ice streams, West Antarctica

Flow features on the surface of the Ross Ice Shelf, West Antarctica, record two episodes of ice stream stagnation and reactivation within the last 1000 years. We document these events using maps of streaklines emerging from individual ice streams made using visible band imagery, together with numerical models of ice shelf flow. Forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration. According to our analysis, Whillans Ice Stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and MacAyeal Ice Stream either stopped or slowed significantly between 800 and 700 years ago, restarting about 150 years later. Until now, ice‐stream scenarios emphasized runaway retreat or stagnation on millennial timescales. Here we identify a new scenario: century‐scale stagnation and reactivation cycles, as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains. This introduces uncertainty into predictions for future sea‐level withdrawals by the West Antarctic Ice Sheet, which are based in part on recent slowing of Whillans Ice Stream and the stagnant condition of Kamb Ice Stream.