Sedimentation Processes in the Proglacial Tasman Lake

Katrin Roehl

Unpublished Dissertation

University of Otago

Dunedin
New Zealand

July, 1999

Abstract

Studies of physical processes in modern glaciolacustrine environments provide the basis for reconstructing past glacial and climatic episodes from sedimentary evidence. This study aims at improving the understanding of contemporary proglacial sedimentation by conducting a reconnaissance-level investigation on the processes and influencing factors of the Tasman Lake, Southern Alps, New Zealand. It examines whether these processes and factors are similar to those in other proglacial lakes, and whether the short-term variability of the sedimentary processes is directly linked with the climatic environment.

During the four week field study the physical properties of the Tasman Lake exhibited considerable variations that were strongly linked to precipitation and incoming solar radiation. The sediment concentration in the lake increased markedly with depth from 0.2 g/l at the surface to 2 g/l, which accounted for predominant density stratification. Only some areas close to ice cliffs appeared to be fully mixed. The water temperature generally decreased with depth, although temperature inversions throughout the water column were observed that changed with time. The temperature variations (mainly between 1.5 and 3.5 ºC) and the sediment stratification contradict findings of previous studies of the lake that reported lower temperatures and inferred entire mixing. The major sediment input appears to come from the glacier. This is reflected by decreasing grain sizes and sedimentation rates obtained from sediment traps. These ranged from 3.2 mm/d in the ice-proximal deep basin to 0.23 mm/d near the distal margins. The sedimentation processes were found to be essentially similar to those from other proglacial lakes. The main differences are caused by the nature of site-specific controlling factors that include direct meltwater inflow from the glacier, surface inflow of the Murchison River, basin geometry and the weather. The variability of the weather and inflow properties accounted for the dynamic structure of the lake. One major rainstorm during the field study resulted in significant increases of sediment concentration and decreases in temperature of the near-bottom water. This suggests enhanced underflow conditions in the lake. The inflow of the Murchison River shifts between overflows in dry and warm periods and underflows during rain events. The variations in type and direction of inflow, sediment load and transported particle sizes will be reflected in the composition and structure of the deltaic deposits as well as the lake floor deposits.

Current research on the Tasman Glacier