Sasha P. Carter

Using radar-sounding data to identify the distribution and sources of subglacial water: application to Dome C, East Antarctica

Basal radar reflectivity is the most important measurement for the detection of subglacial water. However, dielectric loss in the overlying ice column complicates the determination of basal reflectivity. Dielectric attenuation is a function of ice temperature and impurity concentration. Temperature distribution is a function of climate history, basal heat flow and vertical strain rate, all of which can be partially inferred from the structure of dated internal layers. Using 11 dated layers, isotope records from Dome C, East Antarctica, and a model of the spatial variation of geothermal flux, we calculate the vertical strain rate and accumulation-rate history, allowing identification of areas where the basal melt rate exceeds 1.5 mm a -1 . The accumulation-rate history and vertical strain rates are then used as inputs for a transient temperature model. The model outputs for the present-day temperature distribution are then combined with depth-dependent ionic concentrations to model dielectric loss and infer basal reflectivity. The resulting reflection coefficients are consistent (� -5 dB) across a variety of subglacial water bodies. We also identify a high reflectivity >-15 dB in Concordia Trench and along suspected subglacial water-flow routes in Vincennes Basin. Highland areas tend to have highly variable reflection coefficients near -30 dB, consistent with an ice-bedrock interface. This combined model also identifies three areas of enhanced basal melting along Concordia Ridge, Concordia Subglacial Lake and Vincennes Basin. Melt at Concordia Subglacial Lake exceeds 5 mm a -1 . The inferred basal melt at these locations is not possible without enhanced geothermal flux. We demonstrate how radar-sounding data can provide both input and verification for a self-consistent model of vertical strain, vertical temperature distribution and meltwater distribution.

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

The dynamic response of the Greenland Ice Sheet (GrIS) depends on feedbacks between surface meltwater delivery to the subglacial environment and ice flow. Recent work has highlighted an important role of hydrological processes in regulating the ice flow, but models have so far overlooked the mechanical effect of soft basal sediment. Here we use a three-dimensional model to investigate hydrological controls on a GrIS soft-bedded region. Our results demonstrate that weakening and strengthening of subglacial sediment, associated with the seasonal delivery of surface meltwater to the bed, modulates ice flow consistent with observations. We propose that sedimentary control on ice flow is a viable alternative to existing models of evolving hydrological systems, and find a strong link between the annual flow stability, and the frequency of high meltwater discharge events. Consequently, the observed GrIS resilience to enhanced melt could be compromised if runoff variability increases further with future climate warming.

Synthesizing multiple remote-sensing techniques for subglacial hydrologic mapping: application to a lake system beneath MacAyeal Ice Stream, West Antarctica

We present an analysis of the active hydrologic system of MacAyeal Ice Stream (MacIS), West Antarctica, from a synthesis of multiple remote-sensing techniques: satellite laser altimetry; satellite image differencing; and hydrologic potential mapping (using a satellite-derived DEM and a bedrock DEM from airborne radio-echo sounding). Combining these techniques augments the information provided by each one individually, and allows us to develop a protocol for studying subglacial hydrologic systems in a holistic manner. Our study reveals five large active subglacial lakes under MacIS, the largest of which undergoes volume changes of at least 1.0 km 3 . We discuss the hydrologic properties of this system and present evidence for links between the lakes. At least three of the lakes are co-located with sticky spots, i.e. regions of high local basal shear stress. We also find evidence for surface elevation changes due to ice-dynamic effects (not just water movement) caused by changes in basal resistance. Lastly, we show that satellite radar altimetry is of limited use for monitoring lake activity on fast-flowing ice streams with surfaces that undulate on � 10 km length scales.

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.

Modeling 5 years of subglacial lake activity in the MacAyeal Ice Stream (Antarctica) catchment through assimilation of ICESat laser altimetry

Subglacial lakes beneath Antarcticas fast-moving ice streams are known to undergo ∼1 km 3 volume changes on annual timescales. Focusing on the MacAyeal Ice Stream (MacIS) lake system, we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from Ice, Cloud and land Elevation Satellite (ICESat) laser altimetry. We construct a steady-state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the ICESat-derived filling or draining rates. The modeled volume change rates of five large subglacial lakes in the downstream portion of MacIS are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered. However, the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream. Overall, we show that, for this fast-flowing ice stream, most subglacial lakes receive >90% of their water from distant distributed sources throughout the catchment, and we confirm that water is transported from regions of net basal melt to regions of net basal freezing. Our study provides a geophysically based means of validating subglacial water models in Antarctica and is a potential way to parameterize subglacial lake discharge events in large-scale ice-sheet models where adequate data are available.

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 5 mm this century. Furthermore, better constraints on future sea level change from this region will require improved estimates of geothermal heat flux and subglacial water transport.

A decade of progress in observing and modelling Antarctic subglacial water systems.

In the decade since the discovery of active Antarctic subglacial water systems by detection of subtle surface displacements, much progress has been made in our understanding of these dynamic systems. Here, we present some of the key results of observations derived from ICESat laser altimetry, CryoSat-2 radar altimetry, Operation IceBridge airborne laser altimetry, satellite image differencing and ground-based continuous Global Positioning System (GPS) experiments deployed in hydrologically active regions. These observations provide us with an increased understanding of various lake systems in Antarctica: Whillans/Mercer Ice Streams, Crane Glacier, Recovery Ice Stream, Byrd Glacier and eastern Wilkes Land. In several cases, subglacial water systems are shown to control ice flux through the glacier system. For some lake systems, we have been able to construct more than a decade of continuous lake activity, revealing internal variability on time scales ranging from days to years. This variability indicates that continuous, accurate time series of altimetry data are critical to understanding these systems. On Whillans Ice Stream, our results from a 5-year continuous GPS record demonstrate that subglacial lake flood events significantly change the regional ice dynamics. We also show how models for subglacial water flow have evolved since the availability of observations of lake volume change, from regional-scale models of water routeing to process models of channels carved into the subglacial sediment instead of the overlying ice. We show that progress in understanding the processes governing lake drainage now allows us to create simulated lake volume time series that reproduce time series from satellite observations. This transformational decade in Antarctic subglacial water research has moved us significantly closer to understanding the processes of water transfer sufficiently for inclusion in continental-scale ice-sheet models.