THE GEOCHEMISTRY OF RIVERS IN TECTONICALLY ACTIVE AREAS OF TAIWAN AND NEW ZEALAND

Abstract

The geochemistry of river water, river sediments, and suspended matter in three mountainous watersheds in New Zealand and Taiwan is used to determine chemical erosion yields in regions of rapid tectonic uplift. Suspended matter from all three rivers is depleted in soluble alkali metals and alkaline earths compared to upper continental crust material and marine clays, reflecting the bedrocks' origin as marine sediments that had undergone previous weathering cycles prior to uplift and subjection to the current chemical weathering regime. The New Zealand rivers are depleted in Mg2+ and enriched in Ca 2+ and Na+ + K+ compared to global average river water, but the Taiwan river is enriched only in Mg2+ compared to global average. The Haast River, draining the Southern Alps of New Zealand, is depleted in Cl + SO4 compared to the global average, but has higher alkalinity and slightly higher H4SiO4. The chemical weathering yields determined here compose only a small portion (1%-5%) of the total weathering in these systems, but are still among the highest chemical yields ever reported. Our new data, in comparison to previously determined physical erosion yields in these watersheds, show that physical erosion strongly enhances chemical erosion. This work demonstrates the importance of chemical erosion as a process denuding the landscape, especially in high-standing, tectonically active mountainous landscapes. ©2006 Geological Society of America.