Abstract

The geologic balance and distribution of two families of metals, K-Rb-Cs and Ca-Sr-Ba, were investigated in Thompson Canyon, an undisturbed watershed in the central crest region of the Sierra Nevada of California. The study supports the following conclusions: (1) In the source rock, two major minerals, microcline and plagioclase, and one minor mineral, biotite, contain most of the mass of these metals. (2) Ca and Sr are being removed from the watershed as stream solute at a higher rate than the alkalies or Ba relative to their abundance in source rocks. Rapid breakdown of plagioclase and exchange mechanisms involving the soil may be responsible for this differential removal. However, no excess of alkalies or of Ba is to be expected in the soil relative to the source rock, because the masses of metals that are mobilized each year are small in relation to the masses in the soil reservoir, and only a short time has been available for accumulation since the end of the last glacial period. (3) Snow, the main source of water to the valley, enters the system having a much lower concentration of metals than has been previously reported, and its slight contamination appears to be from distant sources. As the snowpack ages in the late spring, it accumulates dusts on its surface, with metal ratios indicating that Thompson Canyon rocks and soils are the source of the dusts. (4) Determination of the mass of metals leaving the watershed annually in stream flow indicates that the watershed is being denuded at a rate of less than 1 cm per 1,000 yr and that chemical removal is responsible for the bulk of this rate. The rock type and the precipitation regime in Thompson Canyon are similar to much of the terrain in the glacially scoured central crest region of this part of the Sierra Nevada range.

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