Year

2012

Degree Name

Bachelor of Science (Honours)

ANZSRC / FoR Code

040202 Inorganic Geochemistry, 040203 Isotope Geochemistry

Department

School of Earth & Environmental Science

Advisor(s)

Tony Dosseto

Abstract

Silicate weathering acts as a global sink for the greenhouse gas carbon dioxide. In order to understand long‐term climate change and the fluctuations in atmospheric carbon dioxide, it is paramount to investigate the regulators of chemical weathering. The controls of rainfall and uplift have been reported to either enhance or inhibit the extent of chemical weathering, and therefore their relationship is still being highly debated in literature. Investigation of this relationship in the Southern Alps of New Zealand has been modest in comparison to the Himalayas. These studies have either focused on solute geochemistry of rivers, and therefore do not integrate weathering history of the catchment, or have failed to investigate the interplay between the controls of uplift and precipitation. Fluvial sediments record conditions of chemical weathering at the catchment scale, whilst integrating its weathering history. Therefore, this study has been undertaken to improve the understanding of the response of chemical weathering to uplift and precipitation in the Southern Alps, utilising the geochemistry and mineralogy of fluvial sediments. Mineralogical analysis was performed through X‐ray diffraction and optical microscopy, while geochemical techniques included X‐ray fluorescence, uranium‐series isotopes and strontium isotopic analysis. Geochemical results suggest that modern chemical weathering is low to moderate and is predominantly occurring within the weathering profile. In contrast, weathering during transport in the fluvial system appears to be dominated by physical abrasion, which contributes to downstream fining. Furthermore, 238U was shown to be leached with increased residence time in the fluvial system and its depletion enhanced by rainfall. Similarly, rainfall amplified Mg loss, while Na was leached regardless. High uplift rates, on the western coast, increase erosion and therefore reduce sediment residence time in the weathering profile. The extent of chemical weathering is consequently inhibited, whereby only the most mobile elements are depleted. Therefore, the Southern Alps are classified as a weathering limited setting, where the intensity of chemical weathering is largely controlled by the residence time of sediments (uplift) and rainfall.

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