Martin P. Kirkbride

The temporal significance of transitions from melting to calving termini at glaciers in the central Southern Alps of New Zealand

Termini of valley glaciers in the central Southern Alps are undergoing a three-phase sequence of retreat that began in the 1950s. An initial phase of melting and slow downwasting under thickening supraglacial debris mantles is associated with stationary or slowly retreating termini. This is followed by a transitional phase of disruption of insulating debris mantles allowing the rapid growth of shallow thermokarst lakes, the shape and location of which vary according to glacier gradient and marginal topography. A third phase of rapid calving retreat of ice cliffs into deepening proglacial lakes develops from the transitional phase. At no stage are terminal responses simply related to climate. Present transitions to calving retreat are related to the morphology of glacier termini, particularly to large outwash heads. Landform evidence indicates that similar calving phases do not appear to have occurred since Late Pleistocene deglaciation, so that present changes to glacier termini are of great significance in the context of the whole Holocene. At millennial timescales, the melting-calving transition is effectively instantaneous and represents a sharp threshold heralding rapid deglaciation. The commencement of calving is imminent at the stationary terminus of Tasman Glacier which will soon retreat rapidly as a result.

Meteorology and surface energy fluxes in the 2005–2007 ablation seasons at the Miage debris-covered glacier, Mont Blanc Massif, Italian Alps

monitored at 25 points with debris thickness of 0.04–0.55 m, spread over 5 km 2 of the glacier. The radiative fluxes were directly measured, and near‐closure of the surface energy balance is achieved, providing support for the bulk aerodynamic calculation of the turbulent fluxes. Surface‐layer meteorology and energy fluxes are dominated by the pattern of incoming solar radiation which heats the debris, driving strong convection. Mean measured subdebris ice melt rates are 6–33 mm d �1 , and mean debris thermal conductivity is 0.96 W m �1 K �1 , displaying a weak positive relationship with debris thickness. Mean seasonal values of the net shortwave, net longwave, and debris heat fluxes show little variation between years, despite contrasting meteorological conditions, while the turbulent latent (evaporative) heat flux was more than twice as large in the wet summer of 2007 compared with 2005. The increase in energy output from the debris surface in response to increasing surface temperature means that subdebris ice melt rates are fairly insensitive to atmospheric temperature variations in contrast to debris‐free glaciers. Improved knowledge of spatial patterns of debris thickness distribution and 2 m air temperature, and the controls on evaporation of rainwater from the surface, are needed for distributed physically based melt modeling of debris‐covered glaciers.

Tasman Glacier, New Zealand: 20th-century thinning and predicted calving retreat

Abstract The nature and rate of the transition from a thinning, melting ablation zone to a retreating, calving terminus is examined at the debris-mantled Tasman Glacier. The debris mantle has existed since the earliest glaciological observations were made in 1863, indicating that debris cover is the normal glaciological state regardless of historic mass-balance change. The relationship between debris thickness and ablation rate has been derived from short-term heat flow calculations. Extrapolation over time and space indicate that the thermal effect of the debris mantle has resulted in a post-1890 reduction in glacier surface gradient which, through positive feedback involving ablation rate, ice velocity and particle emergence paths, has caused upglacier spread of supraglacial debris and upstream migration of the locus of maximum ablation. This has lead to the preservation of a long ice tongue at low gradient while preventing terminus retreat from the outwash head, and has made the glacier vulnerable to calving. Since the late 1970s, thermokarst melting has formed an ice-contact proglacial lake in which water depths now exceed ca. 130 m against the ice front. Since 1994, evidence of extending and accelerating flow may indicate the imminent onset of rapid calving. Predicted retreat scenarios suggest a rapid retreat of at least 10 km will probably cause major (possibly catastrophic) rock and debris avalanches into the enlarging proglacial lake as debuttressing of mountainsides progresses.

The Role of Fluvial and Glacial Erosion in Landscape Evolution: The Ben Ohau Range, New Zealand

A morphometric comparison of valleys has been made for the Ben Ohau Range in the central Southern Alps of New Zealand. The range is undergoing rapid tectonic transport and uplift. The humid north of the range is a glacial trough-and-arete landscape, with a temperate glacial climate. The dry south has rounded divides and plateau remnants dissected by fluvial valleys. Assuming that space–time substitution allows todays spatial valley-form transition to represent evolutionary stages in valley development, the tectonic history allows time constraints to be placed on the rate of transition to an alpine glacial landscape. Morphometric change has been quantified using hypsometric curves, and distance–elevation plots of cirque and valley-floor altitudes. Ancestral fluvial valleys have less concave long profiles but are stepped at altitude owing to the presence of high-level cirques and remnant plateau surfaces, and possess a low proportion of land area at low elevation. Increasing glacial influence is manifest as smoother, more deeply concave long profiles and U-shaped cross-profiles associated with a higher proportion of the land area at lower elevation. The full morphological transition has involved up to 2.4 km of vertical denudation over the 4 Ma lifetime of the mountain range, of which 80 per cent would have occurred by preglacial fluvial erosion. Combining the trajectory of tectonic transport with reconstructed glaciation limits and climatic history, it is indicated that about 200 ka of temperate glacial erosion produces recognizable trough-and areete topography. Mean and modal relief increase where glacial activity is confined to cirques, but decrease when trough incision by ice becomes established as a dominant process in the landscape.

Ice Loss and Slope Stability in High-Mountain Regions

The present time is one significant stage in the adjustment of mountain slopes to climate change, and specifically atmospheric warming. This review examines the state of understanding of the responses of mid-latitude alpine landscapes to recent cryospheric change, and summarizes the variety and complexity of documented landscape responses involving glaciers, moraines, rock and debris slopes, and rock glaciers. These indicate how a common general forcing translates into varied site-specific slope responses according to material structures and properties, thermal and hydrological environments, process rates, and prior slope histories. Warming of permafrost in rock and debris slopes has demonstrably increased instability, manifest as rock glacier acceleration, rock falls, debris flows, and related phenomena. Changes in glacier geometry influence stress fields in rock and debris slopes, and some failures appear to be accelerating toward catastrophic failure. Several sites now require expensive monitoring and modeling to design effective risk-reduction strategies, especially where new lakes as multipliers of hazard potential form, and new activities and infrastructure are developed.

Glaciological response to distal tephra fallout from the 1947 eruption of Hekla, south Iceland

Glacier responses to tephra deposition are shown to be highly variable where wind-transported eruption plumes produce narrow distal fallout zones with steep lateral thickness gradients. Significant but short-lived advances of faster-flowing glaciers can be triggered by deposition from modest eruptions. The 1947 eruption of Hekla, south Iceland, covered nearby glaciers with variable thicknesses of tephra, causing dramatic spatial variations in ablation rate. Relative snow ablation increased by an estimated >80% at the eastern margin of Eyjafjallajokull, but decreased by >54% at the western margin. Relative ice ablation increased by only 4% in the east but decreased by >75% at the western margin, only 15 km distant. The effect on mass balance therefore depends on tephra distribution as well as on the nature of the glacier surface. On Gigjokull, retardation of ablationwas greatest at the terminus, associated with an anomalous advance of ≥328 m between 1947 and 1954. Other glaciers in the fallout zone show either no recognizable response, or accelerated retreat. Advances will be short-lived due to the rapid redistribution of supraglacial tephra in a maritime climate, and the potential for discriminating between volcanically and climatically forced advances in the glacio-geomorphological record is low. The long-term glaciological effect of volcanism is to create thin, low-albedo covers after reworking, which make conditions less favourable for glaciation.

Calving speed and climatic sensitivity of New Zealand lake-calving glaciers

Abstract Calving speeds and calving mechanisms in fresh water contrast with those in tidewater. We obtained calving speeds for six lake-calving glaciers in New Zealand’s Southern Alps, and surveyed the depths and temperatures of their ice-contact lakes. The glaciers are temperate, grounded in shallow (≤20 m) water, and exhibit compressive flow at their termini. These data increase the global dataset of fresh-water calving statistics by 40%, bringing the total to 21 glaciers. For this dataset, calving rates (uc) correlate positively with water depths (hw) (r2 = 0.83), the relationship being expressed by: uc = 17.4 + 2.3 hw. This is an order of magnitude lower than values of uc at temperate tidewater glaciers. For a subset of 10 glaciers for which ice-proximal water temperature (tw) data are available, uc also correlates positively with tw, supporting a physical relation between calving and melting at and below the water-line. Fluctuations of New Zealand lake-calving glaciers in the period 1958–97 show that although the transition from non-calving to calving dramatically increases frontal retreat rates, the onset of calving does not isolate terminus change from climatic forcing. In terms of climatic sensitivity, lake-calving glaciers occupy an intermediate position between tidewater glaciers (least sensitive) and non-calving glaciers (most sensitive).

Quantified time scale for glacial valley cross-profile evolution in alpine mountains

Space is used as a proxy for time in determining the role of glaciation in shaping valley cross profiles in the Two Thumb Range (Southern Alps), New Zealand. The range is undergoing rapid tectonic transport and uplift as it is advected toward the Alpine fault. Repeated cycles of glacial erosion during the Quaternary have fashioned U-shaped valleys in the north of the range, close to the Main Divide, while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles. Tectonic transport and uplift rates, coupled with an offshore oxygen isotope (δ18O) record and glacial- geological reconstructions, allow time constraints to be applied to valley development along the range. Valley width and depth measurements and power-law equations were used to quantify the shape of cross-profile transects in 37 valleys. Valleys evolved into a characteristic U-shaped cross profile formed over ≥400 k.y. of glacial occupancy. This, regardless of other models of glacial flattening of valley long profiles, suggests that alpine glaciation excavates greater volumes of rock, indicating that glaciers are a more effective erosional agent than fluvial activity in alpine mountain belts.

Calving processes and lake evolution at Miage glacier, Mont Blanc, Italian Alps

Abstract A marginal ice-contact lake at Miage glacier, Mont Blanc, Italian Alps, has been studied to reconstruct changes in lake area. Historical sources, comprising sketches, maps, photographs and scientific surveys, have been supplemented by recent field surveys. These include surveys of glacier surface velocity (which varied along the glacier tongue from 70 ma–1 in the upper part to about 6 ma–1 close to the snout, consistent with data in the literature, showing that velocity rates have remained constant during the last 40 years), volumetric ice-cliff loss (–92 000±180m3 in 2002–03), lake temperature and bathymetry, and qualitative observation of calving events, crevassing, and meltwater production. Results indicate that the lake has been stable for the last half-century following a period of enlargement due to ice-marginal retreat. The lake hydrology is complex, with possible reversals of englacial water flow causing infrequent emptying episodes. The debris cover on the glacier and ice-cliff surfaces seems to have played an important role in the ice-cliff evolution and the calving phenomena; calving is driven by undercutting at the water-line aided by the opening of water- and debris-filled crevasses in the glacier surface.

Temperature and bathymetry of ice‐contact lakes in Mount Cook National Park, New Zealand

Abstract Several ice‐contact lakes have formed in conjunction with twentieth century glacier retreat in Mt Cook National Park. They occupy overdeepened glacial valleys and are dammed by terminal moraines and/or outwash heads. During the autumns of 1994 and 1995, the temperature and bathymetry of “Maud lake”, “Godley lake”, and Hooker Lake were surveyed. The near‐glacier vertical water temperature profiles exhibited greater temperature variation than those at the distal ends of the lakes. Thermal stratification existed in Hooker Lake, whereas both Maud and Godley lakes were thoroughly mixed. Water temperatures in the latter were consistently between 3 and 4.5°C, but most parts of Hooker Lake were cooler than 2°C, with a minimum recorded temperature of 0.2°C. These contrasts are important because melting of submerged parts of glacier termini is significant for ablation rates and for the dynamics of calving termini. All the lakes are steep sided and deep. Maud and Godley lakes approach 100 m in depth, wherea...