Chemical Composition of Alpine Rivers in the Southern Alps, New Zealand
The Southern Alps of New Zealand are an exemplary case environment for studying the nature and relative proportion of chemical and physical denudation because of relatively high precipitation rates coupled with rapid uplift and relatively simple catchment lithology. Two products of weathering, particulate river bed-sand and solute river load, were sampled at 74 locations along the Southern Alps and show some correlations with catchment geology. Particulate (bed-sand) geochemistry was dominated by silica oxide (53–82 wt %) and aluminium oxide (6–19 wt %); whereas calcium (3–14 mg L-1), sulphate (0.4–16 mg L-1), and bicarbonate (10–39 mg L-1) ions dominated the hydrochemistry of the dissolved load, highlighting that different processes control or contribute to the chemical abundances. To discern the influences on bed-sand geochemistry and dissolved load hydrochemistry two approaches were adopted. Firstly, predictive discriminant analysis provided an end-member model to infer whether different lithological units influenced the bed-sand geochemistry and dissolved load hydrochemistry. Characteristic composition signatures indicated that most different lithological units along the Southern Alps could be detected through the relative concentrations of FeOT, Rb, Sr, and Ba in bed-sand. In the dissolved load, the lithological influence on end members was characterised by a combination of, SiT and SO42-, Na+, and the ratio of SO42-:FeT. Hydrochemistry was, however, less responsive to lithological units compared to the bed-sand particulate geochemistry. Secondly, multivariate regression was used to assess the potential intrinsic and extrinsic catchment influences such as soil type, catchment morphometrics, land cover, and glaciation. Hydrochemical constituents could be predicted by combinations of catchment factors; however, particulate geochemistry did not model well. Base cations (Na+, K+, Mg2+, Ca2+) and Sr2+ are relatively enriched in the dissolved load, whereas Al, Ba, Fe, Mn, and Si are enriched in the bed-sand geochemistry. Products of weathering are partitioned between the different dissolved and particulate loads and neither are reflective of the overall catchment lithology, highlighting variations in the importance of physical and climatic conditions as drivers of denudation. Catchment-scale differences reflect specific influences of landslides, sediment stores, glaciation, vegetation, and soil development as key attenuating factors of sediment mobilisation and transport. Multi- year observations of dissolved load concentrations together with modelled discharge values indicate dissolved loads of 24 to 538 t km-2 a-1, which are equivalent to around 4% of the Southern Alps suspended sediment yield. The dissolved yields are among the highest reported globally. Although the material flux via dissolved load is relatively small compared with suspended sediment or bedload, it is an important contributor of elements to the oceans and significant component of Southern Alps.
Advisor: Mager, Sarah; Cox, Simon
Degree Name: Master of Science
Degree Discipline: Geography
Publisher: University of Otago
Keywords: Hydrology; Hydrogeochemistry; Southern Alps; Chemical Weathering; Physical Weathering
Research Type: Thesis