Tidewater Glacier Surges Initiated at the Terminus

TerraSAR-X data were provided by DLR (project OCE1503), and funded by the Conoco Phillips-Lundin Northern Area Program through the CRIOS project (Calving Rates and Impact on Sea level). C.N. acknowledges funding from European Union/ERC (grant 320816) and ESA (project Glaciers CCI, 4000109873/14/I-NB).There have been numerous reports that surges of tidewater glaciers in Svalbard were initiated at the terminus and propagated up‐glacier, in contrast with downglacier‐propagating surges of land‐terminating glaciers. Most of these surges were poorly documented, and the cause of this behavior was unknown. We present detailed data on the recent surges of two tidewater glaciers, Aavatsmarkbreen and Wahlenbergbreen in Svalbard. High‐resolution time‐series of glacier velocities and evolution of crevasse patterns show that both surges propagated up‐glacier in abrupt steps. Prior to the surges, both glaciers underwent retreat and steepening, and in the case of Aavatsmarkbreen, we demonstrate that this was accompanied by a large increase in driving stress in the terminal zone. The surges developed in response to two distinct processes. 1) During the late quiescent phase, internal thermodynamic processes and/or retreat from a pinning point caused acceleration of the glacier front, leading to the development of terminal crevasse fields. 2) Crevasses allowed surface melt‐ and rain‐water to access the bed, causing flow acceleration and development of new crevasses up‐glacier. Upward migration of the surge coincided with stepwise expansion of the crevasse field. Geometric changes near the terminus of these glaciers appear to have led to greater strain heating, water production and storage at the glacier bed. Water routing via crevasses likely plays an important role in the evolution of surges. The distinction between internally triggered surges and externally triggered speed‐ups may not be straightforward. The behavior of these glaciers can be understood in terms of the enthalpy cycle model.Publisher PDFPeer reviewe

Dynamic vulnerability revealed in the collapse of an Arctic tidewater glacier

Abstract Glacier flow instabilities can rapidly increase sea level through enhanced ice discharge. Surge-type glacier accelerations often occur with a decadal to centennial cyclicity suggesting internal mechanisms responsible. Recently, many surging tidewater glaciers around the Arctic Barents Sea region question whether external forces such as climate can trigger dynamic instabilities. Here, we identify a mechanism in which climate change can instigate surges of Arctic tidewater glaciers. Using satellite and seismic remote sensing observations combined with three-dimensional thermo-mechanical modeling of the January 2009 collapse of the Nathorst Glacier System (NGS) in Svalbard, we show that an underlying condition for instability was basal freezing and associated friction increase under the glacier tongue. In contrast, continued basal sliding further upstream increased driving stresses until eventual and sudden till failure under the tongue. The instability propagated rapidly up-glacier, mobilizing the entire 450 km2 glacier basin over a few days as the till entered an unstable friction regime. Enhanced mass loss during and after the collapse (5–7 fold compared to pre-collapse mass losses) combined with regionally rising equilibrium line altitudes strongly limit mass replenishment of the glacier, suggesting irreversible consequences. Climate plays a paradoxical role as cold glacier thinning and retreat promote basal freezing which increases friction at the tongue by stabilizing an efficient basal drainage system. However, with some of the most intense atmospheric warming on Earth occurring in the Arctic, increased melt water can reduce till strength under tidewater glacier tongues to orchestrate a temporal clustering of surges at decadal timescales, such as those observed in Svalbard at the end of the Little Ice Age. Consequently, basal terminus freezing promotes a dynamic vulnerability to climate change that may be present in many Arctic tidewater glaciers

Observationally constrained surface mass balance of Larsen C IceShelf, Antarctica

Abstract. Combining several geophysical techniques, we reconstruct spatial and temporal patterns of surface mass balance (SMB) over Larsen C Ice Shelf (LCIS), Antarctic Peninsula. Continuous time series of snow height at five locations allow for multi-year estimates of seasonal and annual SMB over LCIS. There is high interannual variability, with an SMB of 395 ± 61 to 413 ± 42 mm w.e. y−1 in the north and a larger SMB of up to 496 ± 50 mm w.e. y−1 farther south. This difference between north and south is corroborated by winter snow accumulation derived from an airborne radar survey from 2009, which showed an average snow thickness of 0.95 m north of 76° S, and 1.12 m south of 78°. Analysis of ground-penetrating radar from several field campaigns allows for a longer-term perspective of spatial SMB: a particularly strong and coherent reflection horizon below 25–44 m w.e. of ice and firn is observed in radargrams collected across the shelf. We propose that this horizon was formed in a single melt season over the ice shelf. Combining ground and airborne radar with SMB output from a regional climate model confirms that SMB increases from north to south, overprinted by a gradient of increasing SMB to the west. Previous observations show a strong decrease in firn air content toward the west, which we attribute to spatial patterns of melt, refreezing, and densification, rather than SMB. </jats:p

The ice-free topography of Svalbard

We present a first version of the Svalbard ice-free topography (SVIFT1.0) using a mass-conserving approach for mapping glacier ice thickness. SVIFT1.0 is informed by more than 900’000 point-measurements of glacier thickness, totalling almost 8’300 km of thickness profiles. It is publicly available for download. Our estimate for the total ice volume is 6’253km3, equivalent to 1.6cm sea-level rise. The thickness map suggests that 13% of the glacierised area is grounded below sea-level. Thickness values are provided together with a map of error estimates that comprise uncertainties in the thickness surveys as well as in other input variables. Aggregated error estimates are used to define a likely ice-volume range of 5’200-7’400km3. The ice-front thickness of marine-terminating glaciers is a key quantity for ice-loss attribution because it controls the potential ice discharge by iceberg calving into the ocean. We find a mean ice-front thickness of 133m for the archipelago

Surge-type glaciers: controls, processes and distribution

Glacier surging is an internally triggered instability. Surge-type glaciers periodically alternate between long periods of slow flow (the quiescent phase) and short periods of fast flow (the surge phase). Surging yields down-glacier transport of mass and often results in large and sudden glacier advances.The surging phenomenon has always challenged the notion of normality in glacier flow dynamics. The mechanisms of surging remain poorly understood. Observation of different surge behaviors across the world has been used as evidence for the development of glacier type-specific surge models that lack transferability and representativeness. Although only about 1% of the entire glacier population has been observed to surge, the surge phenomenon questions the completeness of our understanding of glacier dynamics. This thesis uses different perspectives to gain a new understanding on the global, regional and local controls on surging and reconcile the mechanisms of surging under a single model. Through a geodatabase of surge-type glaciers, datasets of climate and glacier geometry variables and a global distribution model we explore the controls on the non-random distribution of surge-type glaciers on a global scale. The highest densities of surge-type glaciers are found in a well-defined climatic envelope bounded by temperature and precipitation thresholds, while glacier geometry exerts a second-order control on their distribution. We introduce the enthalpy cycle model which relates flow oscillations to imbalances between enthalpy gains and losses. Enthalpy balance is satisfied outside of the optimal surge envelope, in cold and dry or warm and wet regions. However, the intermediate conditions of the optimal surge envelope prevent enthalpy balance to be reached, yielding dynamics cycling of glacier flow. Thermal switch models have been used to explain surging of polythermal glaciers. We reconstruct the evolution of the thermal regime of six glaciers in Svalbard from existing and new data. The large and thick surge-type glaciers of our sample do not return to a cold-based conditions between surges, demonstrating that thermal switching cannot apply to surges of large glaciers in Svalbard. On the other hand, the thin and mostly cold glaciers display evidence of former warmbased thermal regimes, showing that switches in climate can make glaciers go in and out of surging. We demonstrate that the concept of enthalpy cycling can explain surge and surge-like behavior in Svalbard. Finally, we investigate the role played by local controls on the initiation and development of the surges of two large polythermal glaciers in Svalbard. First, passive seismics and DEM differencing enabled the reconstruction of the chronology of events that led to the catastrophic surge of the Nathorstbreen glacier system. Removal of backstress by the failure of the frozen glacier terminus triggered the catastrophic collapse of one of the tributaries of the glacier system, source of unusual seismic activity. Secondly, the upward propagating surge of Svalbard tidewater glacier Aavatsmarkbreen is understood in terms of changes in the force balance. Glacier retreat and thinning caused a rapid steepening of the glacier snout, which in turn increased the driving stresses substantially. Development of crevasse fields during the late quiescent and surge phases allowed transfer of surface meltwater to the bed, increasing basal water storage and causing ice acceleration. The increase in driving stress and surface-to-bed drainage both contributed to basal enthalpy production, and controlled the pattern of surge evolution