Changes in Landsystem Distribution over Space and Time

Glacial landsystems change over space and time due to climate change. The Ben Ohau Range (New Zealand) demonstrates the response of a range of glaciated valley landsystems as glacier equilibrium altitudes (ELAs) have risen since Late Glacial time (Brazier et al., 1998). A steep precipitation gradient allows a variety of forms associated with humid and arid climates to have developed in close proximity. It is possible to see how landsystems within individual catchments have evolved as debris supply and transport have altered under changing Holocene climates.

Spatial and temporal patterns are revealed by mapping and rock weathering-rind dating of landsystems within single catchments (Fig. 15.24) and along the range. At present, outwash-dominated landsystems of the major Mount Cook valley glaciers occupy large catchments to the north of the range. Relatively clean alpine glaciers occupy the humid north of the range itself, debris-covered cirque glaciers with moraine dams in central catchments, and active talus rock glaciers towards the arid south (Birkeland, 1982; Brazier et al., 1998). The spatial pattern occurs over only ~30 km when plotted normal to the regional isohyets, due to the steepness of the climatic gradient. Variability occurs within single catchments, where forms occupy local topoclimatic 'niches' defined by particular mixes of ice and debris in the transport system (cf. Morris, 1981).

The modern spatial succession demonstrates how limiting factors for landform development change with increasing aridity. At the humid extreme, fluvial competence and capacity limit sediment transfer from debris-rich glaciers to their proglacial zones. With reduced precipitation, 'alpine' glaciers fail to build larger moraines because of more limited sediment supply and dynamically fluctuating ice-margins. At the arid extreme, abundant rockfall supply but precipitation starvation limit the transport of debris largely to gravitational processes, so that uncoupled landsystems dominated by rock glaciers and talus predominate.

The temporal dimension of landsystem development has been combined with the regional spatial pattern in Fig. 15.25. It can be seen how landsystems that occupy particular process-form domains have shifted in space and time as ELAs have risen since the Last Glacial Maximum. Rising ELAs have caused a shift along the spectrum from 'coupled' to 'uncoupled' landsystems, as the ice-covered area and importance of glacial transport and runoff within the catchment has reduced.

MacMillan Creek, Ben Ohau Range (New Zealand). The rock weathering-rind histograms give calibrated ages in l4C years BP. The landform sequence shows a trend to smaller, more debris-rich, and 'decoupled' landsystems through the Neoglacial period. The proportion of ice to debris in transport has declined as equilibrium line altitudes have risen.

Preserved landsystem sequences reflect both the spatial and temporal climate controls on catchment ice and debris fluxes.

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