Chemical and physical weathering in New Zealand’s Southern Alps monitored by bedload sediment major element composition

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The Haast and Clutha rivers drain opposing flanks of New Zealand’s Southern Alps. Major element analyses of grain size fractions (2–1 mm, 1 mm–355 μm, 355–63 μm, and <63 μm) from bedload sediments collected throughout the reach of each river suggest that weathering is strongly partitioned between the chemical weathering of carbonates and the physical weathering of silicates. Sand size fractions from both rivers are depleted in CaO (∼0.2–2.1 wt%) relative to source schists (∼3 wt% CaO), while silt fraction CaO concentrations range from 2–5 wt%. The depletion of CaO in the sediments is interpreted to be due at least in part to removal of carbonate during chemical weathering of the schist protolith in the soil zone. The observed covariance of CaO and P 2O 5 concentrations in all river sediment suggests that most CaO is bound in a combination of phosphate-bearing minerals such as apatite along with other heavy mineral phases with similar hydrodynamic properties (e.g. epidote). Chemical index of alteration (CIA) values for grain size fractions from both rivers are similar (Haast: 54–63, Clutha: 49–61) and do not systematically vary with grain size or sample location. Al 2O 3–CaO ∗ + Na 2O–K 2O (A–CN–K) relationships suggest that CIA values are controlled by albite–muscovite mixing rather than feldspar weathering. Both A–CN–K relationships and modal mineralogical calculations from Clutha river samples indicate progressive downstream attrition of muscovite from coarser to finer grain size fractions. In contrast, Haast river sediments display less variable normative muscovite concentrations and no downstream enrichment/depletion trends. The Haast and Clutha watersheds have drastically different sediment yields, but the similarity of sediments from both rivers indicates that there is minimal climatic control on the weathering intensity of fluvial sediments. Rather, bedload geochemistry is controlled primarily by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz. The geochemical signature of mechanical attrition and hydrodynamic winnowing is more developed in Clutha river samples because of longer sediment residence time within its fluvial system. These findings suggest that high standing island (HSI) fluvial sedimentary evolution is characterized by the dominance of physical weathering processes and the absence of silicate chemical weathering signatures.