The influence of sediment budget on geomorphic activity of the Tasman Glacier, Mount Cook National Park, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy (PhD)
Previous studies of sediment transport by valley glaciers have emphasised the dependence of rates of transport on glacier dynamics, in turn a function of climatic environment. Very few studies have considered cases where it is the debris in transport that plays a major role in affecting the dynamics of the glacier. This study of the Tasman Glacier explains the interdependence of ice and debris fluxes in a tectonically-active maritime alpine environment. Glaciological monitoring has allowed the construction of a model of Twentieth-Century glacier behaviour. A model of medial moraine dynamics has been formulated from theoretical and empirical studies of debris in transport. Feedbacks between glacier flow structure, sediment routeways, supraglacial debris accumulation, ablation, glacier thickness and gradient have resulted in a positive sediment budget in the lower glacier and the growth of a 20 km2 debris mantle. Insulation of underlying ice by the debris mantle has led to the preservation of a 7 km long ice tongue which would have ablated away within the last century without the protective mantle. The flow structure of the glacier has been radically affected by debris mantle spread and changes to the ablation gradient, causing slow downwasting and reduced surface gradient with no terminal retreat. Studies of clast shape have revealed that much debris supplied to the terminus of the Tasman Glacier has been modified by water action rather than by glacial action. It is concluded that sediment transfer in the lower glacier is dominantly by fluvial transport in englacial conduits rather than by truely glacial transport. The implication is that much rounded debris found in older moraines was modified during high-level transport through the glacier. Twentieth-Century negative mass balance has resulted in the formation of thermokarst lakes at valley glacier termini in the region. Growth and coalescence of these lakes has heralded the onset of the first phase of rapid terminal retreat for at least 5,000 years in the Godley Valley. Commencement of rapid retreat of the Tasman Glacier is imminent. The two-phase pattern of slow downwasting of debris-mantled glacier tongues followed by rapid retreat of a calving terminus with rapid glacio-lacustrine deposition provides an analogy to the mechanism of retreat from Late Pleistocene and early Holocene ice maxima. The size and persistence of the proglacial lakes allows them to act as major sediment traps. Their rapid formation during deglaciation marked an important transition from net aggradation to net degradation of proglacial outwash plains at the end of the Pleistocene, leading to a phase of terrace-forming incision of rivers downstream of the lakes. The formation of similar lakes in front of the modern glaciers is in progress and may mark a comparable threshold in river regime. Reconstructions of the Tasman Glacier have been made for various stages of the Neoglacial period. The implications of processes of ice ponding by an outwash head and preservation during negative balance phases over a 5,000 year period have been investigated. Neoglacial fluctuations are minor compared to nearby glaciers with no extensive supraglacial debris mantles. The terminus has become ponded behind an aggrading proglacial fan and resulting changes in the flow structure have increased the potential for supraglacial debris accumulation. The glacier terminus may have become progressively less sensitive to climatic oscillations since c.5,000 B.P. It is concluded that there has been a non-climatic evolution of glacier morphology due to feedbacks in the glacier-debris dynamic system. It is concluded that in regions of high debris mass flux, glaciers of this type have complex responses to climatic change governed by lag responses and thresholds which are not controlled directly by climate. The strongly-positive sediment budgets in the lower parts of glaciers and in proglacial areas is a major cause of this complexity. Climatic interpretations of moraine sequences must therefore be made with caution.