Basal mechanics of Ice Stream B, west Antarctica: 2. Undrained plastic bed model

Based on the results of our studies of the physical conditions beneath Ice Stream B, we formulate a new analytical ice stream model, the undrained plastic bed model (henceforth the UPB model). Mathematically, the UPB model is represented by a non-linear system of four coupled equations which express the relationships among ice sliding velocity, till strength, water storage in till, and basal melt rate. We examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant (less than hundreds of years). Temporal variability is introduced into the UPB model only by the direct dependence of till void ratio changes (ė = ∂e/∂t) on the basal melting rate m_r. Since till strength τ_b{e} and ice stream velocity U_b{τ_b} change as long as till void ratio varies, the first condition for ice stream stability is that of constant till water storage ė = 0. The second condition for ice stream stability arises from the feedback between ice stream velocity, till strength, and the basal melting rate which depends on shear heating m_r{ U_b τ_b}. This is the “weak till” condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress τ_b < (n + 1)^(−1) τ_d. The salient feature of the UPB model is its ability to produce two thermo mechanically controlled equilibrium states, one with a strong bed and slow ice velocities (“ice sheet” mode) and one with a weak bed and fast ice velocities (“ice-stream” mode). This bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in West Antarctica. Basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes. The UPB model demonstrates that ice streams may be prone to thermally triggered instabilities, during which small perturbations in the basal thermal energy balance grow, leading to generation or elimination of the basal conditions which cause ice streaming

Sedimentary processes at the base of a West Antarctic ice stream: Constraints from textural and compositional properties of subglacial debris

Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clay-rich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, sub-glacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams

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.This work was carried out with support from the Isaac Newton Trust, Cecil H. and Ida M. Green Foundation and Natural Environment Research Council (grant NE/E005950/1 and NE/J005800/1).This is the final version of the article. It was originally published in Geophysical Research Letters and is also available from the Wiley website at http://onlinelibrary.wiley.com/doi/10.1002/2014GL059250/abstract. © American Geophysical Union 201

Reactivation of Kamb Ice Stream tributaries triggers century-scale reorganization of Siple Coast ice flow in West Antarctica

Ongoing, centennial-scale flow variability within the Ross ice streams of West Antarctica suggests that the present-day positive mass balance in this region may reverse in the future. Here, we use a three-dimensional ice-sheet model to simulate ice flow in this region over 250 years. The flow responds to changing basal properties, as a subglacial till layer interacts with water transported in an active subglacial hydrological system. We show that a persistent weak bed beneath the tributaries of the dormant Kamb Ice Stream is a source of internal ice-flow instability, which reorganizes all ice streams in this region, leading to a reduced (positive) mass balance within decades and a net loss of ice within two centuries. This hitherto unaccounted for flow variability could raise sea-level by 5mm this century. Better constraints on future sea-level change from this region will require improved estimates of geothermal heat flux and subglacial water transport.This work was carried out with support from the Isaac Newton trust, Cecil H. and Ida M. Green Foundation and Natural Environment Research Council (grants NE/E005950/1 and NE/J005800/1). SFP was supported by the U.S. Department of Energy Office of Science, Biological and Environmental Research program. ST acknowledges support from National Science Foundation (grant #0338295). SPC was supported by funding from the Cryospheric Sciences program of NASA and HAF was supported by funding from NSF (grant ANT-0838885 (Fricker)). The source code for the results presented can be obtained by contacting the corresponding author directlyThis is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/2015GL06578

The shallow shelf approximation as a "sliding law" in a thermomechanically coupled ice sheet model

The shallow shelf approximation is a better ``sliding law'' for ice sheet modeling than those sliding laws in which basal velocity is a function of driving stress. The shallow shelf approximation as formulated by \emph{Schoof} [2006a] is well-suited to this use. Our new thermomechanically coupled sliding scheme is based on a plasticity assumption about the strength of the saturated till underlying the ice sheet in which the till yield stress is given by a Mohr-Coulomb formula using a modeled pore water pressure. Using this scheme, our prognostic whole ice sheet model has convincing ice streams. Driving stress is balanced in part by membrane stresses, the model is computable at high spatial resolution in parallel, it is stable with respect to parameter changes, and it produces surface velocities seen in actual ice streams.Comment: 12 pages of text; 4 tables; 27 figures; submitted to JGR Earth Surfac

Crater Lakes on Mars: Development of Quantitative Thermal and Geomorphic Models

Impact craters on Mars have served as catchments for channel-eroding surface fluids, and hundreds of examples of candidate paleolakes are documented [1,2] (see Figure 1). Because these features show similarity to terrestrial shorelines, wave action has been hypothesized as the geomorphic agent responsible for the generation of these features [3]. Recent efforts have examined the potential for shoreline formation by wind-driven waves, in order to turn an important but controversial idea into a quantitative, falsifiable hypothesis. These studies have concluded that significant wave-action shorelines are unlikely to have formed commonly within craters on Mars, barring Earth-like weather for approx.1000 years [4,5,6]

A new methodology to simulate subglacial deformation of water-saturated granular material

The dynamics of glaciers are to a large degree governed by processes operating at the ice–bed interface, and one of the primary mechanisms of glacier flow over soft unconsolidated sediments is subglacial deformation. However, it has proven difficult to constrain the mechanical response of subglacial sediment to the shear stress of an overriding glacier. In this study, we present a new methodology designed to simulate subglacial deformation using a coupled numerical model for computational experiments on grain-fluid mixtures. The granular phase is simulated on a per-grain basis by the discrete element method. The pore water is modeled as a compressible Newtonian fluid without inertia. The numerical approach allows close monitoring of the internal behavior under a range of conditions. <br><br> Our computational experiments support the findings of previous studies where the rheology of a slowly deforming water-saturated granular bed in the steady state generally conforms to the rate-independent plastic rheology. Before this so-called critical state, deformation is in many cases accompanied by volumetric changes as grain rearrangement in active shear zones changes the local porosity. For previously consolidated beds porosity increases can cause local pore-pressure decline, dependent on till permeability and shear rate. We observe that the pore-water pressure reduction strengthens inter-granular contacts, which results in increased shear strength of the granular material. In contrast, weakening takes place when shear deformation causes consolidation of dilated sediments or during rapid fabric development. Both processes of strengthening and weakening depend inversely on the sediment permeability and are transient phenomena tied to the porosity changes during the early stages of shear. <br><br> We find that the transient strengthening and weakening in turn influences the distribution of shear strain in the granular bed. Dilatant strengthening has the ability to distribute strain during early deformation to large depths, if sediment dilatancy causes the water pressure at the ice–bed interface to decline. Oppositely, if the ice–bed interface is hydrologically stable the strengthening process is minimal and instead causes shallow deformation. The depth of deformation in subglacial beds thus seems to be governed by not only local grain and pore-water feedbacks but also larger-scale hydrological properties at the ice base

Inferring ice fabric from birefringence loss in airborne radargrams : application to the eastern shear margin of Thwaites Glacier, West Antarctica

This work is ITGC Contribution No. ITGC-036 and is an output from the Thwaites Interdisciplinary Margin Evolution (TIME) project as part of the International Thwaites Glacier Collaboration (ITGC), supported by Natural Environment Research Council (NERC) research grant #NE/S006788/1 supporting T. J. Young and P. Christoffersen, and National Science Foundation (NSF) research grant #1739027 supporting S. M. Tulaczyk and D. M. Schroeder. Logistics for this project were provided by the NSF-U.S. Antarctic Program and NERC-British Antarctic Survey. R. Culberg is supported by a USA Department of Defense NDSEG Fellowship and N. L. Bienert is supported by an NSF Graduate Research Fellowship.In airborne radargrams, undulating periodic patterns in amplitude that overprint traditional radiostratigraphic layering are occasionally observed, however, they have yet to be analyzed from a geophysical or glaciological perspective. We present evidence supported by theory that these depth-periodic patterns are consistent with a modulation of the received radar power due to the birefringence of polar ice, and therefore indicate the presence of bulk fabric anisotropy. Here, we investigate the periodic component of birefringence-induced radar power recorded in airborne radar data at the eastern shear margin of Thwaites Glacier and quantify the lateral variation in azimuthal fabric strength across this margin. We find the depth variability of birefringence periodicity crossing the shear margin to be a visual expression of its shear state and its development, which appears consistent with present-day ice deformation. The morphology of the birefringent patterns is centered at the location of maximum shear and observed in all cross-margin profiles, consistent with predictions of ice fabric when subjected to simple shear. The englacial fabric appears stronger inside the ice stream than outward of the shear margin. The detection of birefringent periodicity from non-polarimetric radargrams presents a novel use of subsurface radar to constrain lateral variations in fabric strength, locate present and past shear margins, and characterize the deformation history of polar ice sheets.Publisher PDFPeer reviewe