Strontium isotope geochemistry of the Taieri River, South Island, New Zealand

Boparai, Aaspreet

The geochemistry of water samples in the Taieri River basin has been investigated using Sr isotopic ratio, cations and trace elements data. On a basin wide scale, the 87Sr/86Sr ratio in river systems reflects the basin’s lithology and are characterised by mixing of Sr from silicate and carbonate rocks. The results indicate that most of the samples demonstrate 87Sr/86Sr ratios and cations concentrations that are dominated by weathering of silicate rocks, with an increased contribution from carbonate weathering for sites around the Kyeburn area, including Kyeburn and Capburn tributaries, as well as the mainstream location just downstream of the Kyeburn tributary. The contribution from weathering of silicate rocks has reduced at these sites as evident from low 87Sr/86Sr ratios and reduced concentration of elements like sodium, rubidium and potassium, which are sourced from silicate minerals. The Kyeburn River drains bedrocks whose bedrock mineral composition lacks K-feldspars and muscovite, which are minerals characteristic of silicate rocks. With Rb affinity to substitute for K in these minerals, the rocks in this lithological area are not very Rb-rich and therefore have high Sr concentration, low Rb/Sr and being geologically young, the result is low 87Sr/86Sr ratio. At Silverstream and Lee, fluctuations in cations concentration and strontium isotopic ratios are not dependent on the type of weathered rock alone. Silverstream and Lee have a very low Ca/Mg ratio and high concentration of silicate bearing elements – all of which are characteristic of silicate weathering, but this is contradicted by a low 87Sr/86Sr ratio. Both Lee and Silverstream are known to have agriculture activity, which results in contradicting Sr isotopic ratio relative to the cation data. TB08 has a high 87Sr/86Sr ratio and the concentration of cations is higher than other localities by several orders of magnitude. Such elevated concentrations, in particular of Na+, are attributed to the seawater intrusion by tidal mixing, as lower 30 km of the river is tidally influenced. Despite the tidal influence, the 87Sr/86Sr ratio of TB08 is lower than the present day seawater 87Sr/86Sr ratio, which is attributed to the non-radiogenic freshwater influence from the Taieri and its tributaries.

ix, 100 pages A4

2015Boparai

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