CHEMICAL WEATHERING RATES FROM WATERSHEDS IN THE CENTRAL RANGE, TAIWAN

Numerous studies in the past decade have been conducted on the watershed scale in North America and Europe to evaluate chemical weathering rates. However, few data collected at the watershed scale exist from other parts of the world. Weathering calculations from two small subtropical forested mountainous watersheds in northeastern Taiwan reveal high weathering yields compared to available published data from other forested systems in temperate and sub-tropical regions. The Fu-shan Forest is a moist, subtropical mixed evergreen rain forest in I-lan, in northeastern Taiwan (24° 46' N 121° 43' E). The Fu-shan watersheds are drained by small streams which are tributaries of the Nan-she-chi River. Fu-shan watershed elevations are between 670 to 1100 meters. Annual rainfall in 1993 to 1998 ranged between 2900 and 6000 mm. Mean annual temperature is 18° C. The Fu-shan watersheds are composed of Miocene and Eocene slates and quartzites of the Central Range. Rapid uplift rates and abundant mass movement are characteristic of the region.

We performed geochemical analyses of major cations, bicarbonate and silica from stream water samples collected in December 2000 in watersheds 1 and 2 in the Fu-shan Experimental Forest. Watershed 1, of 38 hectares in area, and watershed 2, of 94 hectares in area, had similar weathering yields of silica, major cations and bicarbonate. Silica yields were 7.7 to 7.8 kmol/ha annually. Total cation yields were 6.5 to 8.1 keq/ha annually. Bicarbonate yields were 4.8 to 5.5 keq/ha annually. Comparison of these data with weathering yields calculated using published data from forested watersheds studied as part of the Long Term Ecological Research (LTER) program in North America reveal higher weathering rates in Fu-shan and other sub-tropical sites compared to temperate sites. Lower weathering yields were found at sites with lower mean annual temperatures and with older bedrock. Unlike the comparison data from the LTER sites, our analysis represents only a short term quantification of weathering dynamics in these Taiwanese watersheds. More work should be undertaken to ascertain longer term weathering rates.