Martin S. Brook

Twenty-First Century Calving Retreat of Tasman Glacier, Southern Alps, New Zealand

Abstract Tasman Glacier is the largest glacier in the New Zealand Southern Alps. Despite a century of warming and down-wastage, the glacier remained at its Little Ice Age terminus until the late 20th century. Since then, a proglacial lake formed, and comparatively rapid calving retreat has been initiated. In this paper we use sequential satellite imagery to document terminus retreat, growth of supraglacial ponds, and expansion of the proglacial Tasman Lake. Between 2000 and 2008, the glacier terminus receded a maximum of c. 3.7 km on the western margin, and the ice-contact Tasman Lake expanded concomitantly. This northward expansion of Tasman Lake up-valley proceeded at a mean annual rate of 0.34 × 106 m2 a−1 over 2000–2008, attaining a surface area of 5.96 × 106 m2 in May 2008, with a maximum depth of c. 240 m. Terminus retreat rates (Ur) vary in both space and time, with two distinct periods of calving retreat identified during the study period: 2000–2006 (mean Ur  =  54 m a−1) and 2007–2008 (mean Ur  =  144 m a−1). Terminus retreat can also be categorized into two distinct zones of activity: (1) the main ice cliff (MIC), and (2) the eastern embayment ice cliff (EEIC). During the period 2000–2006, and between 2006 and 2008 for the EEIC, the controlling process of ice loss at the terminus was iceberg calving resulting from thermal undercutting. In contrast, the retreat of the MIC between 2006 and 2008 was controlled by buoyancy-driven iceberg calving caused by decreasing overburden pressure as a result of supraglacial pond growth, increased water depth, and rainfall. The presence of a >130-m-long subaqueous ice ramp projecting from the terminal ice cliff into the lake suggests complex interactions between the glacier and ice-contact lake during the 8–10 km of possible future calving retreat.

Ablation of debris-covered ice: some effects of the 25 September 2007 Mt Ruapehu eruption

Abstract Melt rates of ice surfaces are strongly influenced by the existence of debris-cover. Dependent on thickness, climate and physical properties of debris, a debris layer can enhance or reduce ablation, compared to bare ice conditions. Ice ablation on bare ice and under varying thicknesses of tephra was measured using a network of 12 ablation stakes drilled into the Summit Plateau ice field, Mt Ruapehu, from 17 January to 2 February 2008 in order to study how tephra from the 25 September 2007 eruption was affecting ablation. Observation of air temperature allowed the application of a simple positive-degree-day approach to the calculation of ablation rate factors, which for bare ice was 7.8 mm d−1 °C−1, with a rate of 6.0 mm d−1 °C−1 mm for ice under 10 cm of tephra. Mean daily ablation rates varied from 116 mm d−1 on bare ice to 78 mm d−1 under tephra cover. Debris covers thicker than 70 mm reduced ablation.

STRUCTURAL GLACIOLOGY OF A TEMPERATE MARITIME GLACIER: LOWER FOX GLACIER, NEW ZEALAND

Abstract. This paper describes the structural glaciology of the lower Fox Glacier, a 12.7 km‐long valley glacier draining the western side of the Southern Alps, New Zealand. Field data are combined with analysis of aerial photographs to present a structural interpretation of a 5 km‐long segment covering the lower trunk of the glacier, from the upper icefall down‐glacier to the terminus. The glacier typifies the structural patterns observed in many other alpine glaciers, including: primary stratification visible within crevasse walls in the lower icefall; foliation visible in crevasses below the lower icefall; a complex set of intersecting crevasse traces; splaying and chevron crevasses at the glacier margins; transverse crevasses forming due to longitudinal extension; longitudinal crevasses due to lateral extension near the snout; and, arcuate up‐glacier dipping structures between the foot of the lower icefall and the terminus. The latter are interpreted as crevasse traces that have been reactivated as thrust faults, accommodating longitudinal compression at the glacier snout. Weak band‐ogives are visible below the upper icefall, and these could be formed by multiple shearing zones uplifting basal ice to the glacier surface to produce the darker bands, rather than by discrete fault planes. Many structures such as crevasses traces do not show a clear relationship with measured surface strain‐rates, in which case they may be ‘close to crevassing’, or are undergoing passive transport down‐glacier.

Lateral moraine age in Park Valley, Tararua Range, New Zealand

Abstract Recent geomorphologie and sedimentologic investigations in Park Valley, in the central Tararua Range, have identified several landforms of glacial erosion and deposition, including cirque basins, a U‐shaped glacial valley, and a lateral moraine ridge. The presence of Kawakawa tephra (Aokautere Ash) within loess c. 50 cm beneath the surface of the moraine has indicated that the moraine was formed prior to 27 ka, suggesting late MIS 3 glaciation in this sector of New Zealand. However, recently published 10Be cos‐mogenic dates from glacially‐scoured bedrock and an erratic block on the surface of the moraine indicate that glaciation was much later, the corollary being that the Kawakawa tephra has been re‐distributed post‐eruption, perhaps from the ridgelines surrounding the moraine. Here, two optically‐stimulated luminescence ages on the loess are reported that suggests most of the moraine was formed during late MIS 3 or early MIS 2, and that a readvance at c. 17.7–18.7 ka extended as far as the moraine.

Valley cross‐profile morphology and glaciation in Park Valley, Tararua Range, New Zealand

Abstract Previous anecdotal research by G. L. Adkin over 90 years ago suggested that Park Valley in the Tararua Range was glaciated during the Late Quaternary, on the basis of the “U‐shaped” cross‐profile character of the uppermost parts of the valley. We quantitatively describe the cross‐profile morphology of the upper parts of Park Valley in the Tararua Range, using the power‐law model (y = axb ) and the form ratio model (FR = D/2W). Comparison of these results with morphological data published in global studies of glaciated landscapes suggests the upper parts of Park Valley have indeed been glaciated. Palaeoglacier reconstruction of this area gives a surface area of c. 1.41 km2, a maximum thickness of 130 m and a maximum basal shear stress value of 99 kPa. The equilibrium‐line altitude of the former glacier has been calculated as c. 1210 m, and with the absence of any dated moraines, the glacier is assumed to have formed during the Otira Glaciation.

Recognition and paleoclimatic implications of late-Holocene glaciation on Mt Taranaki, North Island, New Zealand

Evidence for the timings of inter-hemispheric climate fluctuations during the Holocene is important, with mountain glacier moraine systems routinely used as a proxy for climate. In New Zealand such evidence for glacier expansion during the late Holocene is fragmentary and is limited to glaciers in a narrow zone within the Southern Alps. Here, we present the first evidence for late-Holocene glacier expansion on the North Island of New Zealand in the form of two unconsolidated debris ridges on the south side of the stratovolcano, Mt Taranaki/Mt Egmont, at ~1920 m a.s.l. The two ridges are aligned north–south along the western and eastern sides of a small basin (Rangitoto Flat), which is formed between the main Taranaki cone (to the north), and the parasitic cone of Fanthams Peak (to the south). The approximate age of the ridges is constrained by dated eruptive events and the relationship between ridge locations and the spatial positioning of adjacent volcanic landforms. We propose the ridges formed as two lateral moraines on the margins of a cirque glacier during the final construction phase of Fanthams Peak between 3.3 and 0.5 ka BP, during late-Holocene time. This time interval accords with published cosmogenic 10Be dating of moraine-building episodes in the Southern Alps, indicating the Mt Taranaki moraines are a response to the same regional climatic forcings.

GLACIATION OF MT ALLEN, STEWART ISLAND (RAKIURA): THE SOUTHERN MARGIN OF LGM GLACIATION IN NEW ZEALAND

Abstract. The origin of two ridges on the eastern slopes of Mt Allen, southern Stewart Island, has remained equivocal, with differences of opinion over the exact process‐mechanisms of formation. A variety of approaches was used to test a number of possible hypotheses about the origin of the ridges. These include topographic and spatial positioning, geomorphology, sedimentology and palaeoclimatological extrapolations to reconstruct two small former cirque glaciers with equilibrium line altitudes (ELAs) of c. 600 m. It would appear the two ridges reflect a glacial origin, the glaciers interpreted as forming during the Last Glacial Maximum (LGM) in New Zealand. Whilst glaciation during this time (18–19 ka) was extensive in the Southern Alps, the restricted nature of glaciation on Mt Allen suggests the low altitude restricted glaciation to niche sites on the lee side of upland areas.

ABLATION OF ICE‐CORED MORAINE IN A HUMID, MARITIME CLIMATE: FOX GLACIER, NEW ZEALAND

Abstract Depending on thickness, debris‐cover can enhance or reduce ablation, compared to bare‐ice conditions. In the geological record, hummocky moraines often represent the final product of the melt‐out of ice‐cored moraines, and the presence or absence of such moraine deposits can have paleoclimatic implications. To evaluate the effects of varying debris‐cover and climate on ice‐melt in a maritime mid‐latitude setting, an 11‐day ablation stake study was undertaken on ice‐cored moraine at Fox Glacier, on the western flank of the New Zealand Southern Alps. Ablation rates varied from 1.3 to 6.7 cmd−1, with enhancement of melt‐rate under thin debris‐covers. Highest melt‐rates (effective thickness) occurred under debris‐cover of c.2 cm, with ∼3 cm being the debris thickness at which melt‐rates are equal to adjacent bare‐ice (critical thickness). Air temperature from nearby Franz Josef Glacier allowed for a simple degree‐day approach to ablation calculations, with regression relationships indicating air temperature is the key climatic control on melt. Digital elevation models produced from topographic surveys of the ice‐cored moraine over the following 19 months indicated that ablation rates progressively decreased over time, probably due to melt‐out of englacial debris increasing debris‐cover thickness. The morphology of the sandur appears to be strongly determined by episodic high‐magnitude fluvial flows (jökulhlaups), in conjunction with surface melt. Thus, ‘hummocky’ moraine appears to be a transient landform in this climatic setting.

Stable isotope (δD–δ18O) relationships of ice facies and glaciological structures within the mid-latitude maritime Fox Glacier, New Zealand

ABSTRACT Relationships between stable isotopes (δD–δ18O), ice facies and glacier structures have hitherto gone untested in the mid-latitude maritime glaciers of the Southern Hemisphere. Here, we present δD–δ18O values as part of a broader study of the structural glaciology of Fox Glacier, New Zealand. We analyzed 94 samples of δD–δ18O from a range of ice facies to investigate whether isotopes have potential for structural glaciological studies of a rapidly deforming glacier. The δD–δ18O measurements were aided by structural mapping and imagery from terminus time-lapse cameras. The current retreat phase was preceded by an advance of 1 km between 1984 and 2009, with the isotopic sampling and analysis undertaken at the end of that advance (2010/11). Stable isotopes from debris-bearing shear planes near the terminus, interpreted as thrust faults, are isotopically enriched compared with the surrounding ice. When plotted on co-isotopic diagrams (δD–δ18O), ice sampled from the shear planes appears to show a subtle, but distinctive isotopic signal compared with the surrounding clean ice on the lower glacier. Hence, stable isotopes (δD–δ18O) have potential within the structural glaciology field, but larger sample numbers than reported here may be required to establish isotopic contrasts between a broad range of ice facies and glacier structures.

George Leslie Adkin (1888-1964): glaciation and earth movements in the Tararua Range, North Island, New Zealand

Abstract In the northern hemisphere the broad extent of glaciation had been mostly accepted by the start of the twentieth century, but in New Zealand at that time even the general picture of glaciation was uncertain. Julius von Haast had established that in the South Island the glaciers had extended out considerably from the Southern Alps, but the extent of glaciation in the North Island was unknown. Two rival viewpoints were put forward in 1909. James Park thought that there was a widespread ice sheet, which covered the South Island, Cook Strait and much of the North Island; Patrick Marshall thought that the extent of ice was much less, and argued that much of Parks evidence was spurious. The argument was ‘somewhat resolved’ by a young amateur geologist, Leslie Adkin, who showed that glaciation in the North Island was at most modest, and largely confined to the uppermost part of the main axial ranges. Adkin was a farmer and had no university education, but he published nearly 40 articles in scientific journals on topics as varied as Maori archaeology, and glacial and tectonic geomorphology. This article examines the evidence he adduced for the occurrence of a limited Pleistocene glaciation in the Tararua Range in the south of the North Island, and considers the role of the amateur in New Zealand geology.