Effect of topography on subglacial discharge and submarine melting during tidewater glacier retreat.

-We explored secular variations in subglacial discharge and submarine melting with an idealized model -Subglacial discharge increases as tidewater glaciers retreat along retrograde beds -Submarine melting depends on subglacial discharge and therefore promotes unstable retreat on retrograde bedsTo first order, subglacial discharge depends on climate, which determines precipitation fluxes and glacier mass balance, and the rate of glacier volume change. For tidewater glaciers, large and rapid changes in glacier volume can occur independent of climate change due to strong glacier dynamic feedbacks. Using an idealized tidewater glacier model, we show that these feedbacks produce secular variations in subglacial discharge that are influenced by subglacial topography. Retreat along retrograde bed slopes (into deep water) results in rapid surface lowering and coincident increases in subglacial discharge. Consequently, submarine melting of glacier termini, which depends on subglacial discharge and ocean thermal forcing, also increases during retreat into deep water. Both subglacial discharge and submarine melting subsequently decrease as glacier termini retreat out of deep water and approach new steady state equilibria. In our simulations, subglacial discharge reached peaks that were 6–17% higher than preretreat values, with the highest values occurring during retreat from narrow sills, and submarine melting increased by 14% for unstratified fjords and 51% for highly stratified fjords. Our results therefore indicate that submarine melting acts in concert with iceberg calving to cause tidewater glacier termini to be unstable on retrograde beds. The full impact of submarine melting on tidewater glacier stability remains uncertain, however, due to poor understanding of the coupling between submarine melting and iceberg calving.Funding was provided by the National Oceanic and Atmospheric Association (NA13OAR4310098) and the U.S. National Science Foundation (PLR-1504288 and PLR-1504521).Ye

Modeling Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields

Sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed, and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization, geometry, and sedimentary structures. Ultimately, eskers are a record of past glacio‐fluvial sediment transport, but there is currently no physical model for this process. We develop a 1‐D model of morphodynamics in semicircular bedrock‐floored subglacial channels. We adapt a sediment conservation law developed for mixed alluvial‐bedrock conditions to subglacial channels. Channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice, to which we add the closure or enlargement due to sediment deposition or removal, respectively. We apply the model to an idealized land‐terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus, or the formation of an incipient esker, is inherent to the dynamics of the channelized water flow. The alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation: We show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply‐ or transport‐limited system, as has been hypothesized for proglacial sediment yields. We also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply, but perhaps more importantly, ice‐surface slope and the temporal pattern of water delivery to the bed

Temporal variability of meltwater and sediment transfer dynamics at an Arctic glacier, Storglaciären, northern Sweden

In glacierised regions, suspended sediment fluxes are highly responsive to climate‐driven environmental change and can provide important information regarding the relationships between glacier variations, climate and geomorphic change. As a result, understanding patterns of suspended sediment transport and their relationship with meltwater delivery is of critical importance. However, studies of glacial suspended sediment transport are often limited by interpreting patterns of suspended sediment transfer based on whole-season data, allowing precise patterns to become masked. This thesis aims to contribute to the understanding of suspended sediment transfer in glacierised basins through the investigation of patterns of suspended sediment delivery to the proglacial area of Storglaciären, a small polythermal valley glacier located in the Tarfala valley, Arctic Sweden. High temporal resolution discharge and suspended sediment concentration data were collected during two summer field campaigns at Storglaciären. Interpretations of suspended sediment transport data were made using diurnal hysteresis and sediment availability data, combined with suspended sediment shape and magnitude data classified by applying principal component and hierarchical cluster analyses. Analysis of the dominant discharge generating processes at Storglaciären was also conducted using principal component analysis, allowing patterns of discharge to be better understood. This was complemented by analysis of the structure and evolution of the glacier drainage system by linear reservoir modelling and flow recession analysis. The results suggest that patterns of discharge and suspended sediment transport at Storglaciären are complex, with distinct processes and magnitudes of transport evident at both proglacial outlet streams, Nordjåkk and Sydjåkk. These processes are intrinsically linked to meteorological variables, with both ablation-driven and precipitation-driven discharge exerting influence over patterns of suspended sediment transport in the proglacial area of Storglaciären

The effect of buttressing on grounding line dynamics

Determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice-sheet models. Here, we investigate the role of lateral shear and ice-shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets. We derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non-local ice-shelf properties. Application of these results to a laterally confined version of the MISMIP 1a experiment shows that the boundary condition at the ice-shelf front (i.e. the calving law) is a major control on the location and stability of the grounding line in the presence of buttressing, allowing for both stable and unstable grounding line positions on downwards sloping beds. These results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice-shelf properties, in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed. Consequently, the marine ice-sheet instability hypothesis may not apply to buttressed marine ice sheets

Response of Marine‐Terminating Glaciers to Forcing: Time Scales, Sensitivities, Instabilities, and Stochastic Dynamics

Recent observations indicate that many marine‐terminating glaciers in Greenland and Antarctica are currently retreating and thinning, potentially due to long‐term trends in climate forcing. In this study, we describe a simple two‐stage model that accurately emulates the response to external forcing of marine‐terminating glaciers simulated in a spatially extended model. The simplicity of the model permits derivation of analytical expressions describing the marine‐terminating glacier response to forcing. We find that there are two time scales that characterize the stable glacier response to external forcing, a fast time scale of decades to centuries, and a slow time scale of millennia. These two time scales become unstable at different thresholds of bed slope, indicating that there are distinct slow and fast forms of the marine ice sheet instability. We derive simple expressions for the approximate magnitude and transient evolution of the stable glacier response to external forcing, which depend on the equilibrium glacier state and the strength of nonlinearity in forcing processes. The slow response rate of marine‐terminating glaciers indicates that current changes at some glaciers are set to continue and accelerate in coming centuries in response to past climate forcing and that the current extent of change at these glaciers is likely a small fraction of the future committed change caused by past climate forcing. Finally, we find that changing the amplitude of natural fluctuations in some nonlinear forcing processes, such as ice shelf calving, changes the equilibrium glacier state

Conditions for Growth and Retreat of the Laurentide Ice Sheet

Results of three dimensional numerical modelling of the North American ice sheets in response to the Earth's orbital radiation variations are reviewed in relation to the conditions for formation and retreat of the ice sheets. The last interglacial develops as a clear result of the preceding high summer radiation levels and is not very dependent on the climatic paramaterisation. The magnitude and timing of the last glacial maximum provides a means of fine tuning the climatic parameterisation. In between these two periods the extent of ice sheet advances and retreat is strongly sensitive to the magnitude of the ice sheet albedo feedback parameter. The time changes of the radiation, climate, ice sheet cover and bedrock depression are out of phase and as a result equilibrium is not attained. The distribution of land surface elevation plays a key role in the pattern of seeding of the ice sheet growth and the subsequent advances, coalescence and retreat. The dispersal pattern of bedrock in till can be expected to reflect the growth and advance phases of the ice sheet development rather than the maximum configuration. Finally, the cycles of ice ages over the last 500,000 years from the modelling follows the occurrence of extreme summer radiation levels over a wide latitude band of 40-80°N due to coincidence of obliquity and perihelion features superimposed on the hysteresis effects of the ice cover.Les résultats obtenus à partir de la modélisation numérique tri-dimensionnelle des calottes glaciaires de l'Amérique du Nord selon les variations de l'insolation des latitudes sont examinés en fonction des conditions à l'origine de la formation et du retrait des inlandsis. L'existence du dernier interglaciaire est nettement le résultat des hauts niveaux antérieurs d'insolation estivale et très peu celui des paramètres climatiques. L'ampleur et la durée du dernier maximum glaciaire permettent de préciser les paramètres climatiques en cause. Entre ces deux périodes, l'importance de la progression des glaciers et de leur retrait subséquent est grandement influencée par l'albédo de la calotte glaciaire. Les changements temporels de l'insolation, du climat, de la couverture de glace et de l'enfoncement du substratum sont décalés les uns par rapport aux autres, résultant en un déséquilibre. La répartition des altitudes de la surface terrestre joue un rôle clé dans le processus de formation de la calotte glaciaire, ainsi que des récurrences, de la coalescence et du retrait subséquents. On peut s'attendre à ce que le mode de dispersion des dépôts glaciaire expriment davantage les phases de croissance et de récurrence de la calotte glaciaire que sa configuration maximale. En dernier lieu selon le modèle, les cycles de glaciation au cours des 500 000 dernières années suivent l'apparition de niveaux extrêmes d'insolation le long d'une large bande de 40° à 80° de latitude, en raison de l'obliquité de l'écliptique et des caractéristiques du périhélie surimposés à l'effet d'hystéréris causé par la couverture de glace.Die Ergebnisse eines dreidimensionalen numerischen Modells der nordamerikanischen Eisdecken entsprechend den Variationen der Sonneneinstrahlung in verschiedenen Breiten werden in Bezug auf die Bedingungen fur die Bildung und den Riickzug der Eisdecken untersucht. Die letzte Interglazialzeit ist eine klare Folge der vorhergehenden hohen sommerlichen Sonneneinstrahlungen und ist nicht sehr abhângig von den klimatischen Parametern. Der Umfang und die Dauer des letzten glazialen Maximums erlauben die klimatischen Parameter genau zu bestimmen. Zwischen diesen beiden Perioden war der Umfang des Vorrückens und Rückzugs der Eisdecke stark beeinflupt vom Umfang der Albedo-Parameter der Eisdecke. Die zeitlichen Wechsel der Sonneneinstrahlung, des Klimas, der Eisdecke und der Senkung des Landes sind nicht phasengleich, und so wird kein Gleichgewicht erreicht. Die Verteilung der Erdoberflächenerhebungen nimmt eine Schlüsselrolle ein in dër Verteilung des Wachsens der Eisdecke und den darauf folgenden Rückphasen, dem Zusammenwachsen und dem Rückzug. Es ist anzunehmen, dap die Art der Verteilung des Landes in der Grundmoräne eher das Anwachsen und die VorstoBphasen der Eisdecke spiegelt als ihre maximale Gestalt. Zuletzt entsprechen dem Modell nach die Zyklen der Vereisung während der letzten 500,000 Jahre dem Vorkommen extremer sommerlicher Sonneneinstrahlung entlang eines breiten Gùrtels von 40-80o nördlicher Breite, infolge der Übereinstimmung von Neigungswinkel und Charakteristika der Sonnennähe, überlagert durch die Hysteresis-Wirkungen der Eisdecke

Hydrometeorology, Suspended Sediment and Conductivity in a Large Glacierized Basin, Slims River, Yukon Territory, Canada (1993-94)

The Slims River was monitored for global solar radiation, air temperature, discharge, suspended sediment, and dissolved load in 1993 and 1994. Peak seasonal discharge occurred late in the summer and reflects a typical glacierized basin hydrograph, with increased bare ice surfaces contributing strongly to discharge in July and August. Air temperature, rather than global solar radiation, was most strongly correlated with discharge in both years, but during sustained ablation, air temperature becomes a poor index of meltwater production. Precipitation was infrequent and of low magnitude. The variance in suspended sediment concentration could be explained only in part by discharge; frequent clockwise hysteresis and seasonal sediment concentration peaks unrelated to discharge variations also contributed to this variance. High concentrations of Ca2+ and Mg2+ in meltwaters reflect the lithological influence of carboniferous sedimentary rocks in the basin. Conductivity and individual cation concentrations decreased during both seasons and were inversely related to discharge. Diurnal conductivity amplitude was greatest during glacier melt, and frequent clockwise hysteresis was observed in both years.En 1993 et 1994, on a installé des appareils sur la rivière Slims pour mesurer le rayonnement solaire global, la température de l'air, le débit, les sédiments en suspension et la charge dissoute. Le débit saisonnier de pointe se produisait tard durant l'été et reflète l'hydrographe typique d'un bassin englacé, où les surfaces de glace vive plus étendues en juillet et en août contribuent fortement au débit. Au cours de ces deux années, la température de l'air plutôt que le rayonnement solaire global était très fortement corrélée au débit, mais la température de l'air devient un index médiocre de la production d'eau de fonte durant une période d'ablation intense. Les précipitations étaient rares et de faible intensité. La variance dans la concentration des sédiments en suspension pourrait s'expliquer en partie seulement par le débit; une hystérésis dextrorse fréquente et des pointes non reliées à la variation du débit dans la concentration saisonnière de sédiments contribuaient également à cette variance. De fortes concentrations de Ca2+ et de Mg2+ dans l'eau de fonte reflètent l'influence lithologique des roches sédimentaires carbonifères dans le bassin. La conductivité et la concentration en cations individuels diminuaient durant les deux saisons et étaient inversement reliées au débit. L'amplitude de conductivité diurne était la plus grande durant la fonte glaciaire, et on a observé une fréquente hystérésis dextrorse au cours des deux années