THE GEOCHEMISTRY OF DISSOLVED LOAD AND SEDIMENTS IN HIGH SEDIMENT-YIELDING RIVERS OF TAIWAN

Recent studies estimate that rivers draining high-standing oceanic islands between Australia and Asia may contribute as much as 33% of the total sediment to the oceans. The short length, steep gradient, and mass wasting enable these rivers to transport water and sediment quickly from the mountains to the ocean. Because the residence time of this sediment may be very short compared to larger continental rivers, terrestrial geochemical inputs to the ocean, calculated from larger rivers, may be underestimated.

We collected and analyzed water and river channel sediments from the Nan-she-chi, Hualien and Lanyang Rivers in Taiwan. These rivers flow down the steep eastern flank of the Central Range, which is composed of a metamorphic (schist, slate and marble) complex. Average rainfall in this sub-tropical environment is ~3 m yr^-1, and erosion rates are equal to uplift rates (5 - 10 mm yr^-1). The sediment yields of the Hualien and Lanyang are 13,700 and 8,200 tons km^-2 yr^-1, respectively. These sediment yields are up to 75 times the world average.

Relative to major world rivers, the dissolved load of the Taiwan rivers has similar or greater concentrations of highly mobile ions (Na, Ca, and Sr) and therefore, does not appear to be controlled by the residence time. Less mobile ions, such as rare earth elements, are up to two orders of magnitude lower in the Taiwan rivers and appear to be controlled by stream chemistry (e.g., pH). On the other hand, channel sediments are affected by the short residence time and exhibit a geochemical composition similar to the upper crustal composition. Calculated weathering indices indicate that these river channel sediments are more highly weathered than those found in a watershed of similar lithology and gradient in New Zealand, but much less weathered than suspended load in major world rivers with longer residence times.