Weathering Mechanisms and Mass Balance in a High Sierra Nevada Watershed: Distribution of Alkali and Alkaline Earth Metals in Components of Rock and Soil, Snow, Soil Moisture and Stream Outflow

Citation

Hinkley, Todd King (1975) Weathering Mechanisms and Mass Balance in a High Sierra Nevada Watershed: Distribution of Alkali and Alkaline Earth Metals in Components of Rock and Soil, Snow, Soil Moisture and Stream Outflow. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T1D8-8H84. https://resolver.caltech.edu/CaltechETD:etd-06032004-093611

Abstract

The alkali metals K, Rb and Cs, and the alkaline earth metals Ca, Sr and Ba were analyzed in the rocks, minerals, soils, snow, stream water and soil moisture films of Thompson Canyon, a glacially scoured watershed in the central crest region of the Sierra Nevada in east central California. The stable isotope dilution method was used. 1. The mass balance of metals entering and leaving the watershed, the denudation rate, and chemical and mineralogical details of the weathering process: In the batholithic source rock, two major minerals, microcline and plagioclase, and one minor mineral, biotite, contain most of the mass of the six metals. Snow is the main source of water to the valley, and contains much lower concentrations of metals than has been previously reported. These metals originate from distant sources. The mass of metals leaving the watershed annually in stream outflow indicates that the watershed is being denuded at a rate of less than 1 cm per 1000 yr, and that the principal process is chemical removal. Ca and Sr are being denuded more rapidly from the batholithic rock than the alkalies or Ba. This is shown by the greater proportion of Ca and Sr in stream water in contrast to the greater proportion of alkalies and Ba in the source rock. Rapid breakdown of plagioclase is responsible for this differential removal. In rock and grus, ratios among metals present in plagioclase (Ca and Sr) and those in K-feldspar (K and Ba) show that plagioclase is preferentially removed as rocks weather and crumble to grus. Rivulets running away from melting snow banks in mid summer dissolve large amounts of Ca and Sr at only short distances away from the snow banks, and hold these metals for transport out of the watershed. Particles from the snow banks, rich in Rb and Cs, similar in composition to mica, are deposited in soils by the rivulets. Comparison of metal compositions of rock and grus show that mica particles are freed as the rocks break down. 2. Metals in soils: Masses and distribution of the six metals were determined in the soil reservoir. Three soil fractions were analyzed: (1) organic matter (humus), (2) clay (+ silt) size and (3) igneous minerals (sand size). The Rb/K and Cs/K ratios are greater in humus than in whole soil or in the source rocks. On the other hand, Sr/Ca and Ba/Ca ratios are lower in the humus. The metal ratios in humus are similar to those in soil moisture films in subsaturated soils. Area and humus content of soil types show that about 100,000 tons of humus are present in the watershed. The mass of metals in humus is about 100 times greater than that mobilized annually by weathering or present in soil moisture films. Metal composition of the clay fraction is similar to batholithic rock, but richer in Rb and Cs. The igneous mineral fraction shows that soils are not the residue rich in quartz and alkali feldspar which would be predicted from rock composition and data on metal loss from the watershed. Instead, the soils are enriched in plagioclase and depleted in K-feldspar, necessitating a hypothesis that the weathering products of K-feldspar in soils are periodically swept out of the watershed. Data on metals in turbid streams after violent rains support this. 3. Metals in soil moisture films: Soil moisture films may be regarded as the final geological occurrence of metals in a food chain, because in this form metals are available to the roots of growing plants. A technique was developed for collecting minute samples (300 mg) of moisture from undisturbed soils in the field by rapid blotting onto stacks of cleaned absorbent paper discs. The proportions of metals in soil moisture films are different from those in source rocks. In soil moisture the heavy metal Cs is greatly enriched in the alkali family, and in the alkaline earth family the heavy metal Ba is depleted. Metal concentrations in soil moisture films collected in the field are quite different from those in soil solutions prepared in the laboratory by traditional methods, and from "exchangeable" and "soluble" ions determined in [ammonium] acetate buffered solutions: soil moisture films show K/Ca ratios commonly as high as 4, while other traditional methods give values below unity for the same soils. Compared to other types of water, such as phreatic and stream samples, soil moisture films have much higher concentrations of alkalies relative to alkaline earths. Metal concentrations in soil moisture films vary seasonally, concentrations of alkalies increasing in the autumn near the surface of humus rich soils.

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