Nicholas R. J. Hulton

The Glimmer community ice sheet model

[1] We present a detailed description of the Glimmer ice sheet model, comprising the physics represented in the model and the numerical techniques used. Established methods are combined with good software design to yield an adaptable and widely applicable model. A flexible framework for coupling Glimmer to global climate forcing is also described. Testing and benchmarking is of crucial importance if the outputs of numerical models are to be regarded as credible; we demonstrate that Glimmer performs very well against the well-known EISMINT benchmarks and against other analytical solutions for ice flow. Glimmer therefore represents a well-founded and flexible framework for the open-source development of ice sheet modeling.

Late-glacial glacier events in southernmost South America : A blend of 'northern' and 'southern' hemispheric climatic signals?

ABSTRACT. This paper examines new geomorphological, chronological and modelling data on glacier fluctuations in southernmost South America in latitudes 46–55°S during the last glacial–interglacial transition. Establishing leads and lags between the northern and southern hemispheres and between southern mid‐latitudes and Antarctica is key to an appreciation of the mechanisms and resilience of global climate. This is particularly important in the southern hemisphere where there is a paucity of empirical data. The overall structure of the last glacial cycle in Patagonia has a northern hemisphere signal. Glaciers reached or approached their Last Glacial Maxima on two or more occasions at 25–23 ka (calendar) and there was a third less extensive advance at 17.5 ka. Deglaciation occurred in two steps at 17.5 ka and at 11.4 ka. This structure is the same as that recognized in the northern hemisphere and taking place in spite of glacier advances occurring at a time of high southern hemisphere summer insolation and deglaciation at a time of decreasing summer insolation. The implication is that at orbital time scales the‘northern’ signal dominates any southern hemisphere signal. During deglaciation, at a millennial scale, the glacier fluctuations mirror an antiphase southern’ climatic signal as revealed in Antarctic ice cores. There is a glacier advance coincident with the Antarctic Cold Reversal at 15.3–12.2 ka. Furthermore, deglaciation begins in the middle of the Younger Dryas. The implication is that, during the last glacial–interglacial transition, southernmost South America was under the influence of sea surface temperatures, sea ice and southern westerlies responding to conditions in the southern’ Antarctic domain. Such asynchrony may reflect a situation whereby, during deglaciation, the world is more sensitized to fluctuations in the oceanic thermohaline circulation, perhaps related to the bipolar seesaw, than at orbital timescales.

'Calving laws', 'sliding laws' and the stability of tidewater glaciers

Abstract A new calving criterion is introduced, which predicts calving where the depth of surface crevasses equals ice height above sea level. Crevasse depth is calculated from strain rates, and terminus position and calving rate are therefore functions of ice velocity, strain rate, ice thickness and water depth. We couple the calving criterion with three ‘sliding laws’, in which velocity is controlled by (1) basal drag, (2) lateral drag and (3) a combination of the two. In model 1, velocities and strain rates are dependent on effective pressure, and hence ice thickness relative to water depth. Imposed thinning can lead to acceleration and terminus retreat, and ice shelves cannot form. In model 2, ice velocity is independent of changes in ice thickness unless accompanied by changes in surface gradient. Velocities are strongly dependent on channel width, and calving margins tend to stabilize at flow-unit widenings. Model 3 exhibits the combined characteristics of the other two models, and suggests that calving glaciers are sensitive to imposed thickness changes if basal drag provides most resistance to flow, but stable if most resistance is from lateral drag. Ice shelves can form if reduction of basal drag occurs over a sufficiently long spatial scale. In combination, the new calving criterion and the basal–lateral drag sliding function (model 3) can be used to simulate much of the observed spectrum of behaviour of calving glaciers, and present new opportunities to model ice-sheet response to climate change.

A modelling reconstruction of the last glacial maximum ice sheet and its deglaciation in the vicinity of the Northern Patagonian Icefield, South America

ABSTRACT. A time‐dependent model is used to investigate the interaction between climate, extent and fluctuations of Patagonian ice sheet between 45° and 48°S during the last glacial maximum (LGM) and its subsequent deglaciation. The model is applied at 2 km resolution and enables ice thickness, lithospheric response and ice deformation and sliding to interact freely and is perturbed from present day by relative changes in sea level and equilibrium line altitude (ELA). Experiments implemented to identify an LGM configuration compatible with the available empirical record, indicate that a stepped ELA lowering of 750 to 950 m is required over 15000 years to bracket the Fenix I‐V suite of moraines at Lago Buenos Aires. However, 900 m of ELA lowering yields an ice sheet which best matches the Fenix V moraine (c. 23000 a BP) and Caldenius’ reconstructed LGM limit for the entire modelled area. This optimum LGM experiment yields a highly dynamic, low aspect ice sheet, with a mean ice thickness of c. 1130 m drained by numerous large ice streams to the western, seaward margin and two large, fast‐flowing outlet lobes to the east. Forcing this scenario into deglaciation using a re‐scaled Vostok ice core record results in an ice sheet that slowly shrinks by 25% to c. 14500 a bp, after which it experiences a rapid collapse, loosing some 85% of its volume in c. 800 years. Its margins stabilize during the Antarctic Cold Reversal after which it shrinks to near present‐day limits by 11 000 a bp.

THE LANDFORMS AND PATTERN OF DEGLACIATION IN THE STRAIT OF MAGELLAN AND BAHÍA INÚTIL, SOUTHERNMOST SOUTH AMERICA

ABSTRACT. We report the results of glacial geomorphological mapping of the Strait of Magellan and Bahía Inútil, southernmost South America. Our aims are to determine the pattern and process of deglaciation during the last glacial–interglacial transition, and to provide a firm geomorphological basis for the interpretation of radiocarbon, cosmogenic isotope and amino acid dates for the timing of deglaciation. The area is important because it lies in a southerly location, providing a link between Antarctica and southern mid‐latitudes, and also lies in the zone of the southern westerlies which are a key element in regional climate change. Our mapping shows that it is possible to make a clear weathering and morphological distinction between last glaciation and older landforms and sediments. Within the last glacial deposits we have identified a number of former glacier limits. The key to delineating many of these limits is continuous meltwater channels that run for several kilometres along the outer edge of discontinuous moraine belts. There are four distinct belts of moraines within the deposits of the last glaciation in the central part of the Strait of Magellan. There are two closely spaced major limits (Band C) at the north end of the Strait that reach Punta Arenas airport on the west side, and Península Juan Mazia on the east side. A third limit (D) terminates south of Punta Arenas on the west side, and passes close to Porvenir on the east. In Bahía Inútil there is a more complex pattern with a prominent outer limit (C) and a series of four equally prominent limits (D1 to D4) on both sides of the bay. South of these limits, there is a fourth group of moraine limits (E) on both coasts of the northern end of Isla Dawson, reflecting the last fluctuations of the Magellan glacier before final deglaciation of the southern end of the Strait. There are a number of drumlins within the outer moraine limits. The drumlins are draped by small, younger moraines showing that they have been overridden by subsequent advance(s). The coastlines of the study area are cut by a number of shorelines which record the presence of ice‐dammed lakes in the Strait of Magellan and Bahía Inútil during the later stages of deglaciation. We conclude that there are four main readvances or stillstands that marked the transition from the Last Glacial Maximum to the onset of the Holocene.

Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations

The relict moraines in Argentine Patagonia archive major expansions of the Patagonian Ice Sheet throughout the Quaternary, and are one of the few terrestrial climate proxies in the middle latitudes of the Southern Hemisphere that extends beyond the last glacial cycle. Determining their individual ages has proved challenging but has important implications for our understanding of terrestrial climate change in southern South America over the duration of the Quaternary. Here, for the first time, we demonstrate that sediment on outwash terraces can be directly dated to determine the timing of early–middle Quaternary glacial advances in southern South America. Cosmogenic 10 Be and 26 Al surface exposure ages were obtained from outwash gravels associated with two of the oldest glacial sequences in the Lago Pueyrredon valley (47.5°S), Argentina. The outermost Gorra de Poivre glacial sequence marks the greatest extent of the Patagonian Ice Sheet. A cobble from this surface gives 10 Be and 26 Al surface exposure ages of ca. 1.2 Ma, consistent with bracketing 40 Ar/ 39 Ar age constraints obtained elsewhere in Patagonia. This is the first time early Pleistocene glacial surfaces have been directly dated in Patagonia. Cobbles on a younger Canadon de Caracoles outwash terrace give exposure ages of ca. 600 ka, while 4 of 5 boulders on an associated moraine give exposure ages that are significantly younger. If the demonstrated stability of outwash terraces in the valley is common throughout the region, it will be possible to extend Patagonian glacial chronologies as far back as the early Pleistocene.

Streaming flow in an ice sheet through a glacial cycle

Abstract Geological evidence indicates that the flow of the last European ice sheet was dominated by numerous large ice streams. Although some were ephemeral, most were sustained along well-defined axes at least during the period of retreat after the Last Glacial Maximum. A thermomechanically coupled three-dimensional numerical ice-sheet model has been used to simulate the ice sheet through the whole of the last glacial cycle, but with a spatial resolution that is high enough to capture streaming behaviour. An experiment with a smoothed bed is used to explore the self-organizing behaviour of streams when they are not forced by bed topography. On such a bed, streams typically have a width of 1–10 km, much narrower than the inferred European ice streams. An experiment using a realistic topography suggests that widths of ice streams are strongly influenced by topography, and tend to be of order 100 km. Moreover, even where the topography is muted, it stabilizes the locations of ice streams which, once formed, tend to be sustained along pre-existing axes. The model creates patterns of streaming that are similar to inferred patterns, suggesting strong topographic forcing. In a simulation using a realistic bed in which the ice was very cold and basal melting rarely occurred, streams were again very narrow. Widespread streaming under low driving stresses tends to reduce ice-sheet thicknesses compared with weak streaming or models that do not produce streaming. Consequently, ice thicknesses are smaller and tend to be consistent with the results of sea-level inversions based on geophysical Earth models.

Thermal structure of Svalbard glaciers and implications for thermal switch models of glacier surging

Switches between cold- and warm-based conditions have long been invoked to explain surges of High Arctic glaciers. Here, we compile existing and new data on the thermal regime of six glaciers in Svalbard to test the applicability of thermal switch models. Two of the large glaciers of our sample are water-terminating while one is land-terminating. All three have a well-known surge history. They have a thick basal layer of temperate ice, superimposed by cold ice. A cold terminus forms during quiescence, but is mechanically removed by calving on tidewater glaciers. The other three glaciers are relatively small, and are either entirely cold or have a diminishing warm core. All three bear evidence of former warm-based thermal regimes and, in two cases, surge-like behavior during the Little Ice Age. In Svalbard, therefore, three types of glaciers have switched from slow to fast flow: (1) small glaciers that underwent thermal cycles during and following the LIA (switches between cold- and warm-based conditions), (2) large terrestrial glaciers which remain warm-based throughout the entire surge cycle but develop cold termini during quiescence, and (3) large tidewater glaciers that remain warm-based throughout the surge cycle. Our results demonstrate that thermal switching cannot explain the surges of large glaciers in Svalbard. We apply the concept of enthalpy cycling to the spectrum of surge and surge-like behavior displayed by these glaciers and demonstrate that all Svalbard surge-type glaciers can be understood within a single conceptual framework.

Parallel processing for finite-difference modelling of ice sheets

Improved modelling of ice sheets, by use of high resolution and with representation of more physical processes, is constrained by long run-times even on the latest single-processor workstation. Parallel processing therefore has a role to play. This paper describes techniques for the parallel processing of ice sheet models and presents design approaches for both the Cray T3 series and other parallel architectures. An implementation of a fully coupled, thermodynamic, 3D ice sheet model is described for the Cray T3D and is shown to be scaleable and efficient.

A Sliced-3D FDTD approach as an alternative to 2D Ground Penetrating Radar modelling

Ground penetrating radar (GPR) is a powerful tool for characterisation of the subsurface in a range of applications. Finite-difference time-domain (FDTD) forward modelling is often used to gain a more quantitative understanding of the interaction between GPR systems and the region of interest. This can be undertaken in 2D, where simulations fail to model multiple polarisations and require a number of simplifying assumptions for both electromagnetic fields and for modelling the environment. Alternatively full 3D modelling may be used, but this can be very computationally expensive. Here we present an idea, in contrast to the more formal 2.5D FDTD GPR modelling approach, which is based on using a thin slice of a full 3D model in cases that we want full 3D fields but we have a 2D modelling environment. The key issue with this approach is minimising, if possible, the error introduced by the very close proximity of the perfectly matched layer (PML) absorbing boundaries of the model in the invariant direction of the modelled geometry. We use gprMax, an open source FDTD GPR modelling package, to check the viability of this idea. An assessment of domain size required is made considering the error produced and the computational demands. Optimising the PMLs will be key in making this approach viable for future GPR modelling that provides a full 3D field solution in cases where a 2D geometry is a reasonable assumption.