An Oxygen Isotopic Study of Soil Water and Pedogenic Clays in Hawaii

Citation

Hsieh, Jean Chia Chin (1997) An Oxygen Isotopic Study of Soil Water and Pedogenic Clays in Hawaii. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xcze-bj98. https://resolver.caltech.edu/CaltechTHESIS:05192023-221323734

Abstract

Soils result from complex interactions amongst the biosphere, atmosphere, hydrosphere, and lithosphere. In this project oxygen isotopes were used to trace the movement of water in soils and to determine the conditions of mineral formation during pedogenesis. Incoming rainwater and soil-water δ¹⁸O values were monitored for two seasonal cycles in a series of soils along an arid-to-humid transect in Hawaii. The δ¹⁸O values of halloysite separated from the soil profiles was related to depth profiles of soil­ water δ¹⁸O values. This is the first oxygen isotopic investigation of soil water and pedogenic clays from the same soil profiles.

A direct CO₂ equilibration method was developed to measure the δ¹⁸O value of soil water. This method also has the advantage of requiring only one step compared with two steps for existing methods. Reproducibility of measurements using this method is as good as existing methods. Tests designed to investigate factors controlling equilibration determined that biological respiration, water content, and soil type were important. Samples should be irradiated to eliminate CO₂-respiring organisms. These results imply that water in soils can be partitioned into compartments such as bulk liquid water and adsorbed water and that these compartments may have unique isotopic compositions.

Application of this method to the soils in Hawaii showed that seasonal wetting and drying cycles affected the δ¹⁸O value of soil water. During the dry season, the soil-water δ¹⁸O values decrease with depth in the soil profile due to evaporation of water from the surface. During the rainy season, they increase with depth as water infiltrates through the surface from storms. The δ¹⁸O values of rainwater and soil-water generally increased as annual rainfall increased. Rainwater δ¹⁸O values were 5%o more negative soil-water δ¹⁸O values at low rainfall sites and about 2‰ to 3‰ more negative at high rainfall sites. These trends are consistent with the current understanding of parameters that influence these δ¹⁸O values.

The chemical treatments used to separate halloysite from bulk soil material did not alter the δ¹⁸O values. Halloysite δ¹⁸O values at low rainfall sites ranged between +20.4‰ to +23.6‰ and the two values at a high rainfall site ranged from +18.0‰ to +18.7‰. The data suggest halloysite formation in isotopic equilibrium with its environment and imply that it forms in a restricted range of conditions. A straightforward comparison of these data to soil-water δ¹⁸O values suggests that halloysite formed between 50°C and 60°C, obviously an unrealistic circumstance. It is possible that independently calibrated mineral-water fractionation factors for low-temperature systems are incorrect or that climatic conditions in Hawaii are poorly constrained.

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